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.
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:
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.
A quiet, and mostly dry weekend is on tap after a wet and windy day yesterday. This will be a welcome change of pace from the active weather pattern we’ve seen on prior weekends. There isn’t any precipitation expected and temperatures are forecast to alternate between the low-40s and low-30s. The next chance for meaningful precipitation comes Tuesday.
Rest of today – high temperatures near 40ºF, partly sunny with winds picking up in the afternoon. Overnight lows will be much cooler in the wake of a dry cold front passing through around 20ºF.
Saturday – high pressure will be in control for the day, but it’ll be a cold day with high temperatures topping out in the low-30s under partly sunny skies. Overnight lows warmer in the upper-20s as the high pressure center moves off to the east and winds shift to the south.
Sunday – temperatures warm into the low-40s with return flow from the south on the west side of the departing high. Overnight lows will be similar to Saturday in the upper-20s.
Weather Prediction Center surface forecast for 7AM Sunday
Monday – things turn colder again in the wake of another cold front passing through with high temperatures in the low-30s with partly cloudy skies.
What ended up being a rain storm for NYC yesterday has brought along the coldest air of the year in its wake. The bitter Arctic chill won’t last long, with temperatures on an upwards trend through the mid-week period as another storm approaches. By Thursday, temperatures in the upper-40s will feel like summer compared to the frigid start today. Beyond that, temperatures look to cool off again into next weekend, but it appears to be a dry weekend ahead.
Rest of today (MLK Jr. Day) – overnight lows in the city were in the single digits, and despite lots of sun, the Arctic high in control over the area will limit any significant warming. High temperatures are expected to be in the mid-teens with blustery northwest winds making wind chills feel below zero. Overnight lows dip back into the low-teens, but winds should abate somewhat as the core of the Arctic high moves closer to us and the pressure gradient relaxes.
Tuesday – much warmer than today, by about double, but still cold with high temperatures in the upper-20s. Sunny skies expected with high pressure still in control. Not much change in temperatures overnight into Wednesday as the next storm system brings a warm front to the area.
Weather Prediction Center surface forecast valid 7AM Wednesday
Wednesday – temperatures in the low-40s as we get in the warm sector of an approaching low centered over the Great Lakes. Temperature profiles should support all rain, though there could be some mixing early Wednesday as precipitation starts. Overnight lows again don’t budge much. More rain is expected going into Thursday.
Thursday – long duration rain event continues, with temperatures in the upper-40s and mostly cloudy skies when it’s not raining.
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.
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.
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.
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.
The latest seasonal forecast from the Climate Prediction Center suggests a 90% chance of an El Niño forming during this winter. Because El Niño (and its opposite, La Niña) occurs when there are sea surface temperature anomalies over large portions of the equatorial Pacific, it can affect sensible weather across the world. However, even if an El Niño does form, and is potentially strong, it doesn’t mean it’s the only determining factor for climate outlooks in our region.
Climate Predicition Center’s latest ENSO Outlook as of December 13, 2018. The CPC’s forecast probability that El Niño will form and persist through April 2019 exceeds 80%.
Definition: What is El Niño?
The term El Niño refers to the large-scale ocean-atmosphere climate phenomenon linked to a periodic warming in sea-surface temperatures across the central and east-central equatorial Pacific (between approximately the date line and 120oW)… [CPC] declares the onset of an El Niño episode when the 3-month average sea-surface temperature departure exceeds 0.5oC in the east-central equatorial Pacific [between 5oN-5oS and 170oW-120oW].
There are links between a pattern of weakening trade winds and the onset of El Niño, though there’s no conclusive understanding of the mechanics that lead to the formation of this effect. Either way, this post will focus more on possible effects of El Niño. The key lies in the geographic extent of El Niño, impacting much of the central and east-central Pacific. Since the oceans play a pivotal role in governing global atmospheric patterns, it’s no surprise that El Niño can have global weather impacts.
As you can see, impacts from a classic El Niño bring warmer than normal weather to the northern part of the western US, and cooler and wetter conditions to the Gulf Coast/Deep South. Though not official yet, it does appear an El Niño was already in progress September-November, and possibly into December. This has already brought copious rains to the Southeastern US.
Green and blue hues indicate areas that received above normal precipitation.
One of the primary ways that El Niño affects global weather is by altering the intensity, orientation, and physical extent of the subtropical jet at the 200 mb level. Over the southeastern US, El Niño promotes a stronger subtropical jet streak – this can lead to the formation of stronger than usual storms over this portion of the country, bringing above normal precipitation patterns we see above.
CPC’s analysis of atmospheric anomalies, in this sequence of images, you can see the elongation of the subtropical jet (area of yellows, oranges, reds) flowing east from Asia. Similarly, a stronger subtropical jet streak is seen over parts of the US.
El Niño doesn’t have particularly strong impacts on our area, and this is borne out by the CPC’s seasonal 3-month outlook for this winter. It appears we may see slight chances for above normal precipitation here, but about equal chances of temperature anomalies.
CPC 3-month temperature outlook
CPC 3-month precipitation outlook
Notice, however, that some of the areas forecast to experience above normal temperatures do map well with a classic El Niño’s impacts (Alaska, parts of the Pacific Northwest, extreme Northern Plains), as do parts of the southern tier (Texas, Gulf Coast).
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.
