Category Archives: Snow

NYC Jan 30, 2019 Snow Squall Analysis

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Last Wednesday, a strong Arctic front swept across much of the Northeastern US, impacting many areas with a line of heavy snow showers, then ushering in record-breaking cold. The line of heavy snow immediately preceding the frontal boundary set off Snow Squall Warnings, which many readers would have seen on their mobile devices. The Snow Squall Warning is a new type of warning that went live nationwide on November 1, 2018. I believe that last Wednesday’s event was the first time National Weather Service forecast offices issued this new warning type for a widespread frontal snow squall. In this post, I’ll share my thoughts on the new warning type, and some observations about the event itself.

The New Snow Squall Warning

The relatively new snow squall warning product is, like other existing warnings, an effort by the National Weather Service to inform the public about imminent hazardous weather impacts. The main motivation behind this new warning type is to try and reduce the number of potentially fatal multi-vehicle accidents that can occur in snow squalls. Snow squalls can cause these kinds of accidents because the intense snow and wind in them can rapidly reduce visibility to near whiteout conditions with little advance warning. The heavy snow can also result in quick accumulations that make driving even more dangerous. Snow squalls can occur along frontal boundaries, like what we saw last week, but they can also be isolated or form in conjunction with lake effect snow. Although I haven’t as yet seen a clear-cut definition of what triggers this new warning, the criteria I have seen tie in directly with the hazards mentioned above: visibility less than 1/4 mile (whiteout conditions), strong wind gusts (above 35 mph, it appears), heavy snow, and surface temperatures below freezing.

Archived text of the snow squall warning issued for NYC by the local National Weather Service. Credit: Iowa State University‘s archive tool for NWS warnings.

Snow squall warnings are functionally similar to severe thunderstorm warnings, which makes sense because snow squalls and severe thunderstorms share some sensible weather impacts and meteorological properties. In this case, with a frontal snow squall, my professor and seasoned forecaster Steve Corfidi observed that “Essentially, a winter cold-frontal snow squall band is simply a summer cold frontal squall line with its bottom two-thirds or so chopped off. For all practical purposes today you simply experienced the passage of a narrow, fast-moving band of convective cirrus!”

Snow Squall Event in NYC

The snow squall that hit NYC last week was associated with a strong Arctic front. As I’ll discuss below, this frontal boundary provided the necessary lift to generate a narrow band (along the east-west dimension) of heavy snow along much of its length. Light snow began falling around 3:30PM by my estimate. The intensity of the snow picked up moderately over the next 15 minutes. However, it wasn’t until close to the end of the event that snowfall rates truly kicked into high gear, along with the winds. During a span of about 5-10 minutes, as the snow and wind rapidly picked up, visibility dramatically decreased, with scenes like the one below typical.

Storm Prediction Center’s Mesoscale Discussion graphic showing the general set up of the snow squall event
Image taken looking north along Sixth Avenue from the 12th floor of my office building at approximately 3:50PM. The building that is barely visible as a dark grey blob in the middle is about 400 feet (2 blocks north), showing you just how bad visibility was at the peak of this snow squall.

Once the worst of the snow squall cleared, conditions rapidly improved, with visibility recovering quickly and precipitation ending rather abruptly. Following the passage of the Arctic front, forced subsidence with the much colder and denser air behind the front sinking to the surface helped mix down some very strong wind gusts, and helped usher in some of the coldest air of the season.

Why the Squall Seemed to Peak at the End

The snow squall started off as a few flurries, and for most of the duration of the event, it seemed like that was all we’d get. Then, within a very brief span, the squall peaked in intensity, and as quickly as it had peaked, it was over. So, why did this event appear to unfold this way to us as observers on the ground? It all has to do with the profile of the winds above surface relative to the Arctic front and the squall line.

A sounding collected at KPIT (Pittsburgh) earlier during the day, close to the time that the same snow squall line was progressing through Pennsylvania. This sounding is thus a reasonable approximation for what the atmosphere looked like over NYC later in the afternoon.

In the sounding above, we can observe that the wind barbs on the right side of the sounding are generally increasing in speed up to 600 mb – triangles represent 50 knots, each full tick represents 10 knots, and half a tick is 5 knots. At the surface, winds were west-northwest at 10 knots, but at 600 mb, winds were at 90 knots! Quite a difference. The second thing to note is above 850 mb, the wind barbs are oriented roughly at the same angle, indicating winds from the same direction at these levels (west-southwest) . This is what forecasters refer to as a “unidrectional” wind profile. The result here is that we had a set up where there was significant vertical speed shear. This has tangible effects on the structure of the clouds/convective activity within the snow squall, as shown below.

Simplified illustration showing the scenario during the snow squall

In the diagram above, as the leading edge of the Arctic front progresses, the air ahead of it is mechanically lifted above the dome of cold air behind the frontal boundary. Once the air reaches the LCL (lifting condensation level), it’s saturated and clouds begin to form. In this case, there’s enough lift and available moisture that precipitation begins to fall. Temperatures at the time supported all snow. Lift provided by the front would continue allowing the clouds to grow until they hit a stable layer – I won’t go into specifics about this but suffice to say that at this point, the cloud can’t keep growing vertically. This results in the cloud spreading out horizontally, creating an “anvil”. Because the wind speed is so much faster at this level, the anvil is sheared away from the direction of oncoming wind producing a “leaning” effect.

This radar image from the Newark TDWR (Terminal Doppler Weather Radar) at 2:58PM last Wednesday has a good depiction of the light snow falling ahead of the main squall line that’s coming from the sheared anvils of the main convective line. Note the scattered lobe of light snow ahead of the solid band of darker blue hues indicating heavy snow.

This is a diagram showing the effect of the speed shear on the evolution of the snow squall line’s cloud structure.

Because the winds were coming from the west-southwest, the anvils leaned in the opposite direction to the east-northeast. For us on the ground, that meant the light snow preceded the heart of the action, “the worst of the storm” that was closer to the leading edge of the Arctic front itself. Had winds aloft been weaker, or from a different direction, suppose more parallel to the frontal boundary itself, the contrast between the light and intense snow wouldn’t have been as dramatic.

Thoughts on Improving the Snow Squall Warning

This was the first widespread use of this new warning product, and it’s not surprising that this led to some confusion. I had several people ask me when the warning was issued and the snow started “How long is this going to last?”. Some people even did the exact opposite of what the warning is intended to prevent: they rushed out to “beat the snow” since it started off light and they didn’t realize it would be over in a short span of time.

I think that these warnings could be improved if a specific duration of the event were mentioned in the warning text, something along the lines of “Expect snow squall conditions to last between 30-45 minutes”. Some other weather forecast offices issue warnings with such text. As discussed above, frontal snow squalls are similar in nature to their warm season relatives. While people are used warm season convective activity ending pretty quickly, intense snow squalls here are often caused by the mesoscale bands accompanying Nor’easters. These can last several hours. In general, many winter weather warnings are long duration, which I believe contributed to some of the confusion that people had about this new type of warning.

NYC Weather Update – Jan 28, 2019

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This week starts off quiet, and cold with sunny skies today. Tomorrow brings a mess of rain/snow mix, rain, then possibly ending with snow leading into Wednesday. A blast of Arctic air lies behind the storm system bringing this precipitation, with low temperatures plummeting into the single digits overnight into Thursday. The Arctic chill stay in place until the weekend.

Rest of today – sunny, with some scattered high clouds. Temperatures around freezing. Overnight lows in the upper-20s.

Tuesday – the timing of the next storm system to affect us, at this time, points to a period of rain/snow mix starting in the late morning Tuesday. A warm front should induce enough warm advection to raise temperatures into the low-40s. That will allow for a transition to all rain in the city until the overnight hours when temperatures are forecast to fall below freezing into the upper-20s. A period of all snow should follow.

Wednesday – periods of heavy snow squalls are possible before the storm finally clears out. At this time, forecasters are calling for about 1″ of snow accumulation starting Tuesday night for NYC. If the forecast trends colder, we could see some more snow, but no blockbuster amounts either way. High temperatures Wednesday should rise to around freezing. On the cold back side of the passing storm, Arctic air will swing into the region. Overnight lows are forecast to only be in the single digits going into Thursday morning.

Weather Prediction Center surface forecast for 7PM Wednesday

Thursday – frigid day in store with Arctic high pressure in control. Sunny skies but temperatures only expected to reach into the mid-teens. Overnight lows into Friday will stay chilly, only around 10°F.

January 20, 2019 KLGA Forecast Post-Mortem

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For reference, here’s the post that triggered the following forecast post-mortem analysis. To start, here’s my forecast and the verified totals.

My Forecast
High: 48°F | Low: 17°F | Max sustained winds: 35 mph | Total QPF: 1.40″ | Total snow accumulation: 1.00″

Verification
High: 40°F | Low: 15°F | Max sustained wind: 38 mph | Total precipitation: 0.80″ | Total snow: 0.00″

Since I did decently at forecasting maximum sustained winds and the low temperature, this analysis will focus primarily on why I missed the mark on both total precipitation and the high temperature.

How I Verify Forecasts
I haven’t explained in previous posts like this how I go about verifying the results of my own forecasts, though I do talk about METARs (hourly weather reports) and daily climate summaries from the National Weather Service as sources for verification data. There’s a reason why I choose to use the 06Z Day 1 to 06Z Day 2 (1AM/2AM Day 1 to 1AM/2AM Day 2 depending on Daylight Saving Time) time window to forecast, and that’s because this lines up well with METAR synoptic reports that occur every 6 hours (00Z, 06Z, 12Z, 18Z). This is something I picked up from my Weather Forecasting Certificate Program at Penn State World Campus. So, when I’m looking at the METAR data, I’m looking for specific data points at these synoptic times:

KLGA 210551Z 32023G36KT 10SM SCT037 M09/M16 A2980 RMK AO2 PK WND 31037/0500 SLP091 T10941161 11067 21094 51032 $
KLGA 202351Z 32027G35KT 10SM SCT035 M07/M13 A2960 RMK AO2 PK WND 31041/2323 SLP024 T10671133 10033 21067 51063 $
KLGA 201751Z 34012G22KT 10SM FEW008 SCT012 BKN024 BKN190 03/00 A2927 RMK AO2 RAE09 SLP911 P0000 60020 T00280000 10044 20022 55002 $
KLGA 201151Z 06016KT 6SM RA BR BKN006 BKN010 OVC028 02/01 A2944 RMK AO2 PK WND 05028/1102 WSHFT 1059 PRESFR SLP970 P0003 60060 70114 T00220011 10028 20017 56055 $

I won’t bore you with details of how to read METARs, which you can learn about here, but from these entries, I can get the maximum temperature from the highest value 1 group, low temperature from the lowest 2 group, and total precipitation from summing up the 6 groups for these synoptic times. So in this case, “10044” indicates a maximum temperature of 4.4C, which is converted from 40F. “21094” shows a minimum temperature of -9.4C, converted from -15F. “60060” translates to 0.60″ and “60020” likewise is 0.20″, and the sum gives us 0.80″ total precipitation.

Last, with max sustained wind, and in this case snow, I checked the NWS daily climate reports for LGA (see red outlined boxes).

Post-Mortem Analysis
On this forecast, I ended up handling the low temperature and max sustained winds well, however, I was much too high on both the high temperature and total precipitation. So what happened here?

High Temperature
In my forecast, I had confidence based on various model data that NYC would spend a decent amount of time in the warm sector of the low that would be responsible for the storm. Unfortunately, this simply just did not happen, and as a result, we never got into that warm southerly/southwesterly flow that would have propelled temperatures into the upper-40s. Instead, looking at the METARs for that day reveals that winds stayed consistently east-northeast to north-northeast overnight into the early morning hours before almost immediately shifting to the northwest by 11AM. This makes sense, given the orientation and location of the warm front just to our south to start.

Click the images below to see the Weather Prediction Center’s surface analyses at 7AM, 10AM, 1PM respectively for Sunday, January 20.

In the end, I should have heeded some signals that there was enough uncertainty in the storm track even on Saturday that we could miss the warm sector. The local forecast office for the NWS also indicated that there was a potential for this, which would keep temperatures suppressed due to persistent, cool, northeasterly flow. Their forecast high, which I believe was 42°F, factored this in, and ended up being a lot more accurate. The takeaway here for me is to not completely buy into model consensus even if there’s good agreement, when there’s a possibility of storm tracks shifting. I don’t think I would have gone as low as 40°F even with this in mind, but I might have forecast something like 44°F, which would have been closer.

Total Precipitation
I missed the total precipitation forecast by more than 0.50″ – objectively a bad outcome. In this case, I think there were a couple reasons behind my own forecast bust. First, the storm progressed faster than data on Saturday suggested, resulting in heavier precipitation earlier in the overnight period, also meaning that the strongest frontogenesis/isentropic lift moved through quicker than anticipated. Secondly, the best moisture convergence stayed just offshore, leading us to miss out on some of the heavier rain.

Click the images below for enlarged versions of the archived radar image for 8AM Sunday, January 20, and the Storm Prediction Center’s moisture convergence analysis for the same time.

The fact that we never ended up in the warm sector for too long during the day Sunday also meant that the best moisture didn’t quite make it up to NYC. Note how the areas of strongest moisture convergence are also coincide well with the most intense radar echos. For precipitation with strong storms like this, it can always be a hit-or-miss proposition to pinpoint precipitation totals for one spot. My own personal forecast bias leads me to over forecast precipitation quite often. I should have consulted the daily average for precipitation to factor climatology into this as well before doing the forecast. For reference, the record precipitation total for KLGA on January 20th was 1.41″ – so I was, in essence, forecasting a near record-breaking precipitation event. That usually doesn’t pan out, as you see.

NYC Detailed Forecast for Jan 20, 2019

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A major winter storm is set to bring immediate impacts to the entire Tri-State region with a mixed bag of wintry precipitation ranging from snow to sleet, freezing rain, and rain. Coastal flooding and even urban street flooding could be a concern. Temperatures will plummet in the wake of this storm, with Arctic winds blowing from the northwest bringing the coldest air of the season and the potential for a flash freeze. The track of this storm has bounced around a bit throughout the week, though at the time of this writing, a warmer scenario with a storm track further north closer to the coast looks more likely.

My Forecast
High: 48°F | Low: 17°F | Max sustained winds: 35 mph | Total QPF: 1.40″ | Total snow accumulation: 1.00″ – verification for temperatures and precipitation will come from METAR data for the period between 1AM Sunday and 1AM Monday (06Z Sunday to 06Z Monday) at LGA. Wind speed verification will draw on the daily climate summary from the National Weather Service.

Verification
High: 40°F | Low: 15°F | Max sustained wind: 38 mph | Total precipitation: 0.80″ | Total snow: 0.00″ – Decent job of handling max sustained wind and the low, however, I missed the mark on high temperatures and overall precipitation by a large margin. I’ll post a more in-depth post-mortem analysis on this later this weekend. However, I do want to provide an initial assessment of the reasons behind my forecast bust on high temperatures and precipitation. These both have to do with the progression of the coastal and warm fronts associated with this storm. The warm front never made it further north than offshore of Long Island. As a result, we didn’t really get into the warm sector as I thought we would. This resulted in more northeasterly flow, keeping temperatures suppressed. The timing of the heaviest precipitation happened to be in the immediate six hour period preceding the start of my forecast window, when another 0.64″ of precipitation fell. I’ll look a bit more into this in the post-mortem, but I suspect the storm progressed a bit faster than forecast model data suggested on Saturday afternoon.

Weather Prediction Center surface forecast valid 7AM Sunday.

Synoptic Set Up
A large scale storm system with origins from the Pacific has been marching through the Ohio Valley and will eventually move offshore right in our vicinity. As this surface low continues to intensify, a warm front will first push into the coastal areas of the region. Ahead of this, snow should develop late Saturday, heavy at times. Eventually, as the warm front moves north of us, we’ll experience a transition over to rain. As the surface low progresses east, precipitation will end in the afternoon Sunday. This low will then continue to deepen, causing strengthening winds from the northwest to usher in an Arctic airmass and the coldest temperatures of the year.

Weather Prediction Center’s most recent storm track map. There’s still some uncertainty at this point with the track. However, trends have been pushing the storm track north since yesterday and even within the cluster of orange dots near us denoting the storm’s forecast position at 7AM Sunday, not too many are far south enough to see a colder scenario pan out.

High Temperatures
National Weather Service only recently released a warmer forecast in the mid-40s, before which they were showing highs in the mid-30s. However, most other sources are suggesting a substantially warmer high temperature in the upper-40s to near 50°F. In this case, I tend to agree with the consensus shaping up on the warmer side, with MOS guidance being on this warm side, and the generally reliable National Blend of Models similarly warm. The current most likely storm track places the area in the warm sector of this low, with MOS guidance showing an extended period of east-southeasterly winds veering to the south and southwest and blowing at a good clip, I can buy into the idea there will be decent warm advection and a surge of warm air to push temperatures up in to the upper-40s. Furthermore, I don’t see a lot of potential for evaporational cooling occurring, since the atmosphere will start off saturated during the morning hours (it’ll be raining).

  • NAM MOS for KLGA
  • GFS MOS for KLGA

Low Temperatures
With strong northwest winds expected to develop on the cold flank of the surface low as it exits the area, low temperatures are likely to actually occur late overnight going into Monday. This is because warm advection will proceed the bulk of the rain Sunday morning. I’m siding with a blend of low temperatures from NAM, GFS MOS, and National Blend of Models.

NAM forecast sounding for 7PM Sunday, note the environmental temperature line (red/yellow) from the surface upwards to about 875 mb is well-mixed, parallel to the nearest line of dry adiabatic ascent (grey lines slanted to the left at ~45 ° . This is a set up that favors strong winds.

Max Sustained Winds
Strong northwest winds are expected to develop behind the exiting low. A well-mixed layer is visible in forecast soundings, and this suggests the potential for the strong winds aloft at 850 mb to mix down. Sustained wind speeds could easily be in the 25-30 mph range with higher gusts. The intensifying low should also lead to an isallobaric component (winds becoming stronger in response to a deepening low) to the wind, which I think could push wind speeds to 35 mph. I believe the strongest winds of the day are actually coming after the precipitation since there’s going to be a warm nose above the surface, putting an inversion in place that would diminish downward momentum transfer while rain is coming down.

NAM forecast model showing 850 mb frontogenesis early Sunday morning at 4AM. These values of frontogenesis entail potential for bands of heavy rain and snow, with plenty of lift possible to propel precipitation.

Total Precipitation
Several of the key ingredients needed for a heavy precipitation look to be coming together for this storm. This includes several strong vorticity maxima at 500 mb moving through. At the 300 mb level, a jet streak is forecast to be in the vicinity, but I don’t see this area sitting under a favorable jet streak quadrant that would support additional lift. Most of the lift here I think will come from the combination of 500 mb vorticity maxima and 850 mb isentropic lift depicted above. I actually see some low potential for elevated convection that could spur bouts of this heavy precipitation to fall. Moisture looks robust, with an 850 mb low-level jet forecast to develop as well. This should support deep moisture and lends further credence to the potential for heavy rain. Given the temperature profiles, it appears the risk of freezing rain and sleet is low at the coast, though more significant north of the city.

NYC MLK Jr. Weekend Weather – Jan 18, 2019

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The dusting of snow we got last night into today is just a small preview of what lies ahead this weekend. A major storm, that first brought heavy rain to the west coast, has been tracking across the nation and will impact the region late Saturday through Sunday with potentially a mixed bag of everything from snow, sleet, freezing rain, rain, urban street flooding, coastal flooding, and strong winds that could result in power outages and tree damage (esp. if ice accumulates). Behind this storm, the coldest air of the new year surges in, bringing a bitterly cold start to next week. I’ll post an in-depth forecast of the incoming winter storm tomorrow.

Rest of today – the veritable calm before the storm. High pressure briefly establishes itself behind the weaker disturbance that brought us light snow overnight. Skies slowly becoming partly cloudy. High temperatures in the low-40s. Overnight lows colder around 30°F.

Saturday – should start off as a decent day with partly cloudy skies. Cloud cover increases through the day, and snow starts to spread from southwest to northeast during the late afternoon hours. High temperatures in the mid-30s. Overnight lows right around freezing will critically impact how much snow accumulation we get before what is widely expected to be a transition over to wintry mix of sleet and freezing rain, then finally bouts of heavy rain leading into Sunday. If a warmer scenario plays out, we’ll get less snow and ice, and more rain. Various forecast sources call for between 1-4″ of snow and sleet accumulations, with up to as much as 0.10″ of ice.

Weather Prediction Center surface forecast for 7AM Sunday. A real mixed bag of various precipitation types all along the Mid-Atlantic and Northeast

Sunday – high temperatures will continue to play a pivotal role Sunday in determining precipitation type. Some forecast sources suggest colder high temperatures in the mid-30s, while others remain warmer, in the upper-30s to even 40°F. There is broad agreement that the day starts off with plain rain but then transitions back to wintry mix of sleet, freezing rain, and finally snow. The differences in high temperature will be pivotal to the timing of this transition, with a warmer scenario keeping things all rain longer. As the center of the low responsible for the storm moves off to the north and east, it will continue intensifying and the pressure gradient around it will increase quite dramatically. Robust northwest winds are expected to develop, rapidly advecting cold, Arctic air into the area. The intensity of this cold advection will govern how quickly temperatures start to plummet during the second half of Sunday. It would appear that temperatures will be well below freezing by the late afternoon hours. This prompts concerns for a flash freeze of any standing water or untreated surfaces, leading to dangerous travel conditions. Temperatures will continue dropping overnight Sunday, with overnight lows going into Monday only around 10°F. Overall, this forecast shapes up to be a slushy mess, with a layer of snow coated by ice and sleet, melted by rain, but then re-frozen with an additional light layer of sleet and ice before all is said and done.

Weather Prediction Center’s plot of ensemble forecast low tracks/positions. Note that there’s still decent uncertainty even at this stage approaching the storm. The pink cluster denotes storm center positions by 7AM Sunday. Though they seem closely clustered, even a shift of the storm track 25-50 miles north or south could result in big differences in terms of whether locations at the coast receive more snow and frozen precipitation (southerly track) versus more of a rain event (northerly track).

Monday (MLK Day) – if you have the day off, count yourself lucky, because this is shaping up to be a brutally cold day where temperatures topping out in the upper-teens despite ample sun. Continued windy conditions will produce wind chill values below zero. The bitter cold continues into Tuesday morning with overnight lows again in the low-teens.

NYC Weather Update – Jan 14, 2019

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We will get a slight warm up going into the mid-week period this week. Precipitation will hold off until late in the week, when two storm systems are forecast to push through. Temperatures could be cold enough to support some snow, though rain should also mix in at some point.

Rest of today – mostly sunny with high temperatures in the mid-30s. Lows overnight will again be quite cold, in the mid-20s with mostly clear skies helping aid radiational cooling.

Tuesday – warmer with temperatures in the upper-30s and mostly sunny skies. Lows going into Wednesday also a touch warmer, in the upper-20s.

Wednesday – temperatures finally forecast to break above 40°F. This is because we’ll be in the warm sector ahead of a cold front attached to an Alberta Clipper type low pressure system. Overnight lows behind this cold front will return to being cold, in the mid-20s.

Weather Prediction Center surface forecast for 7AM Wednesday.

Thursday – a sunny day with high pressure in control briefly behind the cold front above, temperatures in the low-30s though. Overnight going into Friday, a disturbance is forecast to move towards the region and could touch off some snow flurries with temperatures in the upper-20s.

NYC Weekend Weather – Jan 11, 2019

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We’ll have a cold weekend ahead, however, we will dodge a bullet with a major winter storm tracking to our south. The northern fringes of this system could still bring some light snow to the city, though. The cold temperatures stay in place through the weekend and into early next week. Looking ahead, we should see a pattern change that leads to some warmer temperatures by mid-week next week

Rest of today – mostly sunny, cold, with high temperatures hovering around 30°F. Gusty northwest winds steadily subsiding as a strong Arctic high pressure center moves closer to us. Clear, crisp night with overnight lows around 20°F.

Saturday – partly sunny, high temperatures topping out around the freezing mark. Snow flurries are possible overnight into Sunday. Overnight lows in the mid-20s.

Weather Prediction Center surface forecast for Saturday at 7AM. Note the strong Arctic high pressure to our north, 1040 mb, which will effectively block the winter storm to the south from heavily impacting this region.

Sunday – partly sunny with clouds possibly breaking from north to south later in the day. Some flurries still possible early in the day. High temperatures similar to Saturday around freezing. Overnight lows in the low-20s.

Monday – mostly sunny, with high pressure building again, temperatures still cold in the mid-30s.

NYC Weather Update – Jan 7, 2019

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We get a quick passing storm tomorrow that will bring a shot of rain/snow mix. A couple windy days will follow this before our attention turns towards a possible weekend nor’easter. As of now, temperature profiles appear to support the possibility of an accumulating snow event, if the storm track supports precipitation. The picture will become more clear later in the week as more forecast data comes in.

Rest of today – increasing clouds with temperatures in the low-30s. Temperatures will increase overnight as a warm front attached to an incoming low pressure system approaches from the southwest. Moisture will also increase and precipitation could begin falling overnight. Forecast soundings suggest temperature profiles will support a period of snow/sleet when precipitation does fall.

GFS forecast sounding for 1AM Tuesday morning, the atmosphere is beginning to saturate even at low levels. Temperature profiles remain below freezing through most of the atmosphere at this time, so if/when the atmosphere saturates, precipitation would be mostly frozen until the very near surface layer, suggesting sleet. See this post for how to read a Skew-T.

Tuesday – mixed precipitation could linger into the morning hours. Temperatures should continue to warm with southwest flow behind the warm front (warm sector) and ahead of an approaching cold front. High temperatures should reach into the upper-40s. Thing should dry out for a bit, but a second round of rain may accompany the passage of this trailing cold front. This cold frontal passage will be accompanied by strong lift, enough to even support a couple thunderstorms. Overnight lows will remain in the low-40s.

Weather Predicition Center surface forecast for 7PM Tuesday.

Wednesday – temperatures in the mid-40s. Winds will pick up into the 20-25 mph range with stronger gusts in the 30 mph range as the low pressure continuing to strengthen as it moves east. Partly sunny skies for the most part this day. Overnight lows will be much cooler, in the low-30s in the wake of the cold front passage.

GFS surface pressure and wind forecast for Thursday at 7AM. Note the dense packing of isobars (black lines indicating areas of the same pressure) over the northeast. This is due to the strengthening low exiting east over Nova Scotia and high pressure building from the west. The result will be strong winds impacting the area.

Thursday – windy conditions continue with high temperatures significantly cooler in the upper-30s under clearing skies.

NYC Detailed Forecast for Sunday Dec 23, 2018

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On Friday, I forecast that Sunday and Monday would be mostly sunny and that we wouldn’t see any precipitation on either day. Updated forecast data shows that there is potential for some light mixed precipitation overnight Sunday into Monday. Below, I’ll provide a detailed forecast update for Sunday

My Forecast
High: 42ºF | Low: 33ºF | Max sustained winds: 25 mph | Total precipitation: 0.02″ – verification for temperatures and precipitation will come from METAR data for the period between 1AM Saturday and 1AM Sunday (06Z Sunday to 06Z Monday). Wind speed verification will draw on the daily climate summary from the National Weather Service.

Verification

High: 44ºF | Low: 36ºF | Max sustained winds: 24 mph | Total precipitation: 0.00″ – this was a decent forecast in terms of wind speed, and total precipitation. Precipitation didn’t start falling until late in the forecast period, so it ended up being a good call to hedge down below some of the more aggressive model output that suggested earlier/heavier precipitation. Layer mean wind analysis aided my forecast, though again, noting a possible small surface inversion in the NAM forecast sounding led me to make a wise decision to hedge down on max sustained winds. On temperatures, the overnight lows going into Sunday ended up being on the warmer side of the forecast envelope. Here, I mixing and downsloping played a bigger role than I anticipated.

Synoptic Set Up (The Big Picture)

An occluded low pressure center tracking over the Great Lakes will bring a weak cold front through the area Sunday night into Monday. Above the surface, a limiting factor will be the lack of deep moisture or strong winds at the 850 mb level. At the 500 mb level, a negatively tilted shortwave trough will supply good positive vorticity and divergence. However, there isn’t a clear signal for favorable entrance/exit regions of a jet streak at the 300 mb level to provide additional lift.

High Temperature

MOS forecasts show high temperatures Sunday in the mid-40s, with NAM (North American Model) coming in at 45ºF and GFS (Global Forecast System) pointing to 46ºF. NBM (National Blend of Models) – a consensus based forecast model, showed cooler highs at 42ºF. I tend to side with high temperatures on the cooler side due to persistent low overcast clouds during much of the day. Low overcast clouds are a known bias for statistical forecast models like NAM and GFS because they can effectively block out solar radiation, keeping things cooler than otherwise expected.

NAM forecast sounding valid 1PM Sunday, showing multiple layers in the atmosphere where the environmental temperature profile (red) and dew point (green) approach each other, which would suggest multiple layers of clouds hindering solar heating.

Low Temperature

The same MOS data above has lows on Saturday of 33ºF for NAM, 34ºF for GFS and 32ºF for NBM. I’ve gone with the middle of the road, in line with NAM at 33ºF because despite relatively clear skies overnight, strong winds are forecast to be blowing from the west to west-northwest, resulting in some downsloping and staving off any radiational cooling.

Max Sustained Winds

Overnight, forecast soundings indicate the presence of a well-mixed near surface layer. A well-mixed layer allows for faster winds aloft to transfer down to the surface relatively efficiently, and as such, the layer mean wind method can be applied to forecast maximum sustained winds in these cases. We can calculate layer mean by averaging wind speeds at every pressure level within the well-mixed layer shown in forecast soundings that are available in forecast model output. Performing a layer mean wind analysis for NAM and GFS produces an average of about 28 mph. However, noticing that there is a possibility for a small surface based inversion in the NAM forecast sounding, I’ve cut this down to 25 mph. In the chart below, the layer mean wind ends up being 26.71 knots (31 mph).

Pressure (mb)Speed in m/sSpeed in knots
9978.2015.94
97512.9025.08
95014.6028.38
92515.9030.91
90017.1033.24
GFS forecast sounding valid for 1AM Sunday, the black circled section shows a well mixed layer where the environmental temperature profile (red line) largely parallels the nearest dry adiabatic lapse rate line (light blue). Note, at the top of this circled area, the environmental temperature profile almost touches the dew point temperature, indicating a layer of thin clouds.

Total Precipitation

An approaching cold front will provide convergence and lift at the surface. However, at the 850 mb level, conditions look pretty dry through much of the day Sunday. 500 mb level divergence and lift looks decent, but the 300 mb level support doesn’t look great. Precipitation, if it occurs, won’t happen until the overnight hours going into Monday, and if it does happen, signs point to no more than a light event. I’m forecasting 0.02″, or just a trace of precipitation by 1AM Monday. Temperature profiles aloft support snow, though it appears that near the surface, temperatures will be above freezing, so it’s more likely that we’ll see a mix of rain and snow.

GFS forecast for 500 mb vorticity and height valid 10PM Sunday. The light blue line shows the axis of a negatively tilted shortwave trough west of the NYC area. This supports divergence downstream (east) of the axis which induces surface pressure drops and growing storms.
GFS 850 mb forecast for relative humidity and wind. The NYC area at this time is far from being saturated at this level.
By 1AM Sunday, the atmosphere starts to become saturated to basically the surface, indicating ongoing precipitation. Most of the precipitation looks to fall in a zone of the atmosphere that’s below freezing, but temperatures at the surface are above freezing.


December 10, 2018 Southeastern Snowstorm Post-Mortem

Outside of NYC, Snow, , , , , , , , , ,

A major news story unfolded over the weekend as the Southeastern US got slammed with a snowstorm that dropped uncommon snow totals over the area, causing widespread travel disruptions. This region of the country is not accustomed to snowstorms of this scale and many municipalities were not prepared for it. Making matters worse, there was a major forecast bust in this storm, which shared key characteristics with a similar forecast bust that led to a high impact snowstorm hitting NYC a few weeks ago on November 15th (and may have prompted the ouster of the director of NYC Office of Emergency Management). For example, Richmond, VA had a forecast going into Sunday for only 1″ of accumulating snow, but in fact received 11.5″ when all was said and done – a near record-breaking storm.

Below, I’ll provide a “post-mortem” analysis of why forecasters missed the mark so badly in this case. The overall lesson here underscores the difficulty of forecasting snow when temperatures are expected to be hovering close to freezing, especially in coastal storms where the precipitation gradient can be quite sharp.

Dry air at the outset of the storm

Soundings from KWAL (Wallops Island NASA Launch Facility, which we can use as a reasonable proxy for areas in Virginia heavily impacted by snow) at the outset of this storm showed very dry air at the low levels of the atmosphere. This is indicated by the large gap between dew points (green line) and the environmental temperature (red line) on the Skew-T diagram below.

Since I think most people reading this are probably not familiar with Skew-Ts, let me provide a brief exposition. These charts are densely packed with data and can be difficult to read. To orient yourself, know that the y axis on these represents pressure levels from the surface (~1000 mb) all the way up to almost the very limit of the atmosphere at 100 mb. Pressure levels are also related to altitude, though this relationship is not linear because it depends on temperature. The x axis on these charts shows temperature in degrees Celsius. However, note that the lines of temperature are actually slanted at a 45 degree angle and not straight up. The dotted blue line to the right marks the 0 degree mark, critical for determining whether precipitation is frozen or not.

So back to the Skew-T at hand – notice that above the 700 mb layer, the dew point (green) and environmental temperature (red) lines were essentially overlapping. This indicates a layer of air that’s reached saturation since by definition, dew point is the temperature to which the air would need to be cooled to be saturated. When you see a thick layer of dew points and temperatures meeting, it generally indicates ongoing precipitation (thinner layers like this can indicate clouds). In this case, what’s happening is that precipitation is falling from about 400 mb down, but from 700 mb and below, the air is very dry.

With this set up in place, we have excellent conditions for evaporational cooling. As precipitation from above starts to saturate the layers below (some of the precipitation evaporates into the dry layer), the temperature actually cools because evaporation is a phase change of water that requires an input of energy (heat). This is exactly the same mechanism that occurs when you exercise and sweat, or when you step out of a shower (even a cold one) and feel cooler. The net effect of the evaporational cooling in this case, like in the storm that hit NYC in November, was to keep environmental temperatures below freezing for longer than expected (shifting the red environmental temperature line to the left on a Skew-T), allowing snow to fall and accumulate for a longer period as well.

The issue for forecasters here, and for NYC on November 15, was that the models were not all in agreement about how dry the low levels of the atmosphere would be at the outset of the storm. Forecasters are trained not to rely solely on just one model’s depiction of upcoming events, even though in this case, some models had what turned out to be a much more accurate take on dry air. As we’ve seen, the difference of a degree or two when temperatures in the atmosphere are close to the freezing line can have serious consequences for tangible weather impacts.

Frontogenesis and mesoscale (localized) banding

When coastal storms form off the East Coast during the winter, the temperature differential between the warmer air south of the storm’s core and the colder air to the north can lead to frontogenesis, which is the process of the formation of a frontal boundary. In these storms, the result is a coastal front. During this process, a mesoscale circulation forms as atmospheric dynamics attempt to restore equilibrium between cold and warm airmasses. This circulation can greatly enhance lift, a critical ingredient for heavy precipitation, as well as helping cool the air columns. For coastal storms during the winter, the result of strong frontogenesis is the development of narrow, but intense localized bands of heavy precipitation. The difference between an area impacted by a band like this can easily be more than 0.50″ of liquid equivalent, which if you convert to snow using a standard 10:1 snow-to-liquid ratio is 5″! The trouble with these mesoscale features, as is the case with thunderstorms, is that even the most advanced forecast models do not have sufficient resolution to accurately capture features on these scales. That means it’s often difficult to know for certain if/where/when one of these bands sets up and for how long – a critical, high impact detail that can make or break any forecast.

Analysis of frontogensis at 1PM on Sunday – the blue-green hues over Virginia indicate areas of strong frontogenesis

As it happened, with this storm, stronger frontogenesis than forecast took shape. The North American Model (NAM) actually had a pretty good handle on this, but as with the NYC storm, forecasters didn’t put all their eggs in one basket and side with this solution.

NAM’s forecast for frontogenesis valid at 3PM Sunday – the very tightly packed purple lines are an indication of intense frontogenesis

Cold air damming

Along the Eastern Seaboard, certain orientations of high pressure systems can lead to an effect known as cold air damming. This occurs when high pressure centers of Canadian origin set up northeast of the mid-Atlantic and Southeast. Anti-cyclonic clockwise flow around these highs brings cold air around the core of this high into the East Coast with easterly winds. At some point, these winds start to hit the eastern flank of the Appalachian mountains. Because cold air has higher density, the mountains provide an effective barrier to the westward (and upward) progress of this cold air. This then leads the air to gradually turn to the left (south) and progress further south than would otherwise be possible without the cold air damming effect. This is visible from the following surface analysis where you can see surface isobars linked to the high pressure center “sagging” south along the eastern edge of the Appalachians. This phenomenon can provide a critical shot of cold air in advance of a storm that can tip the balance from a rain event to a snow/mixed/frozen event. Forecasters probably did have a decent handle on this, but I mention it because it would have helped in keeping cold air in place prior to and during the beginning of the event.

11AM Sunday surface analysis from the Weather Prediction Center

What are some takeaways from this?

Given that this scenario has unfolded twice this season, a key takeaway for forecasters should be to have heightened awareness of snowfall totals exceeding model consensus when one or more of those models is indicating the possibility for both strong frontogenesis with a coastal storm like this and very dry air preceding such a storm. Ideally, forecasters and emergency managers should be in close communication about probabilities of exceeding forecast totals as soon as evidence and observations show a colder scenario unfolding. If possible, these details should be passed on to the general public by highlighting the uncertainty that exists and probabilities, even if they’re not high, of exceeding forecast totals dramatically. Municipalities should have a fallback plan for fast mobilization of personnel and equipment for snow removal in the event that a forecast bust of this magnitude starts to look more likely during the early onset of a storm when we can verify things like dew points, and observe trends of mesoscale bands on radar.