Category Archives: Storms

NYC Snowfall Forecast – Mar 3, 2019

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A winter storm warning is in effect for NYC and the surrounding metropolitan region. This isn’t exactly how we’d all want to start March off! This storm is anticipated to bring some travel impacts to the area, however, for reasons discussed below, this won’t be a blockbuster snowmaker. Watch out for a slog of a morning commute tomorrow. This snow may also stick for a while – a frigid continental polar air mass from Canada will sweep in behind this storm bringing temperatures generally 10-15°F below normal for this time of year. High temperatures in the mid-30s should limit melting.

Headlines

Snowfall totals: I’m forecasting 4-6″ in parts of eastern Queens, southeastern Brooklyn, and lower totals further east into Long Island. Higher totals of 6-8″ are more likely to occur in Manhattan, the Bronx, and points further inland, particularly interior regions of Connecticut. Below are probabilistic forecast maps of various amounts of snow (>= 2″, >= 6″, and >= 8″).

Timing: Precipitation starting in earnest around 8PM. Starting out as a mix of rain/snow near the coast, but transitioning over to all snow later in the evening. The heaviest snow will happen overnight. Because of the fast-moving nature of this storm, precipitation is expected to end rather quickly between 4-6AM Monday morning in the city.

Uncertainties: There is still potential for a wobble in the storm track, further east and south would result in higher snow totals near the coast. Further west and closer to the coast would mean more mixing/rain at the coast and lower snow totals. There will be a rather sharp gradient of increasing snowfall totals spreading across the region (as seen in the previous probabilistic snowfall total forecasts). Mesoscale heavy snow bands will be difficult to pinpoint ahead of time. Some areas could see several inches more than neighboring areas just a few miles south and east.

Synoptic Set Up (The Big Picture)

A storm that’s currently unleashing severe storms with tornadoes across the Deep South now will slide up along the Mid-Atlantic and Northeastern coast of the US. As this storm progresses, it will move close to the 40°N/70°W benchmark, a spot that’s climatologically correlated to heavy snow events along the heavily populated I-95 corridor during the winter. This storm will continue to strengthen as it moves offshore. Snow is expected to develop ahead of the advancing warm front associated with this storm as its precipitation shield advances. Heavier snow is forecast to develop later on as strong isentropic lift associated with the warm front occurs, creating the potential for frontogenesis and some mesoscale bands of very heavy snow. The storm is expected to move quickly along the Northeast coast, such that the duration of precipitation in any one spot is expected to be less than 12 hours.

At the 500 mb level, a shortwave trough will provide positive vorticity and some additional lift/divergence, allowing the storm to continue strengthening. Finally, at the 300 mb level, the surface low will be close to the entrance region of a curved 300 mb jet streak. This will provide yet more divergence and lift, if only for a brief period.

  • Weather Prediction Center’s surface forecast for 1AM Monday morning
  • GFS model 500 mb vorticity and height for 1AM Monday
  • GFS 300 mb height, wind forecast for 1AM Monday, note the brighter red, orange, and white streak over the Northeast, indicating a jet streak.

Evolution of the Storm Track

Over the course of the last three days, forecast models have come into better agreement with this storm tracking close to the 40°N/70°W benchmark (circled in red in the images below). Note the increasingly tight clustering of storm center locations around the benchmark in progressive storm track forecasts from the Weather Prediction Center.

  • WPC storm track forecast issued 7AM Saturday, the brown spots indicate forecast storm center locations around the benchmark
  • WPC storm track forecast issued 7AM Sunday, the green spots indicate forecast storm center locations around the benchmark

The tightening clustering of these forecast storm center locations lends greater confidence to the idea that the storm will track very close to the benchmark.

Ensemble Snowfall Totals

The two primary model ensembles (GEFS and SREF) have been edging ever so slightly upwards in their forecast mean snowfall totals, while the model spread has decreased over the weekend

  • Latest GEFS ensemble plumes for snowfall totals at KLGA
  • Latest SREF ensemble plumes for snowfall totals at KLGA

These means/spreads were part of what informed my own forecast snowfall totals at the top of this post.

Factors Supporting Heavy Snow

  • Storm track over or very near the benchmark
  • Strong isentropic lift and possible frontogenesis (see images below). Strong lift is a critical ingredient for generating heavy precipitation
  • Possibility of mesoscale bands as a result of this lift, generating heavy snowfall rates
  • Temperatures probably supporting frozen precipitation through the atmosphere
  • GFS forecast for 850 mb frontogenesis
  • NAM 12km forecast for 850 mb frontogenesis and temperature advection (red shades indicate warm advection)
  • NAM forecast sounding for 1AM Monday
  • GFS sounding for 1AM Monday

Factors Suggesting Lower Snow Totals

  • Possibility still remains for storm track to shift further inland, introducing more warm air off the ocean, more rain than snow at the coast
  • Warm advection associated with the storm’s warm front possibly also affecting snow development. Note how close the overlapping temperature and dew point profiles in the forecast soundings above are to the freezing mark, the dashed blue line the middle of the image that is angled to the right at 45°. Evaporational cooling should help somewhat in staving off warming but if temperatures warm more than forecast, we could see more mixing
  • Mesoscale bands of heavy snow may not push far enough onshore
  • Surface temperatures ahead of the storm in the upper-30s near the coast, urban heat island effect could retard snow accumulation
  • Fast moving nature of the storm, total precipitation window only 12 hours
  • Small window for the best moisture support at the 850 mb level. No real evidence to suggest a low-level jet carrying a ton of moisture into the region.
NAM model forecast of 850 mb relative humidity and winds. There’s not too big of an area of completely saturated air at this level, and winds are not strong at this level either.

NYC Lunar New Year Weather Update, Feb 4, 2019

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What a difference a week makes. Last week, we saw some of the coldest air to hit NYC in quite a while. This week, we’ll have the chance to hit 60°F. Two low pressure systems will impact the area, the first is not forecast to produce precipitation, while the second will likely bring rain. Both of these lows will bring a surge of warm air in their warm sectors prior to the passage of the trailing cold front accompanying them. Enjoy the balmy February weather while you can, because the warmth won’t last.

Rest of today – mostly sunny with high temperatures reaching into the low-50s. Patchy fog developing overnight with low temperatures near 40°F.

Tuesday (Lunar New Year)– a mild start to the day gives us a shot to reach into the upper-50s. We’ll be in the warm sector of the first low. If a deck of low clouds builds in and persists, we might be a bit cooler in the mid-50s. On the other hand, if we get a relatively sunny day instead we could see high temperatures around 60°F. Overnight lows considerably cooler around 30°F with a dry cold frontal passage from this first low pressure system.

Weather Prediction Center’s surface forecast for 7PM Tuesday, showing the “dry” passage of a cold front

Wednesday – a cooler start to the day should result in temperatures only reaching the low-40s. Clouds increasing, and rain expected to develop overnight into Thursday with low temperatures in the mid-30s.

Thursday – a few degrees warmer with high temperatures in the mid-40s and rain. Rain expected to taper off a bit after the warm front associated with the second low to hit us moves north of the area. This will set us up for a warm day on Friday as well. Overnight lows going into Friday not much cooler than day time highs, in the low-40s.

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 Weather Update – Jan 21, 2019

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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.

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 New Year’s Day Detailed Forecast – Jan 1, 2019

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A wet, windy New Year’s Eve in NYC will give way to a very mild day to start 2019. However, for reasons outlined below, I don’t believe that record warm temperatures will be broken tomorrow. Strong winds will be a distinct possibility, though, and as such the National Weather Service has issued a wind advisory for the area. I think sustained winds could reach 30 mph, especially early in the day.

My Forecast

High: 57°F | Low: 36°F | Max sustained winds: 30 mph | Total precipitation: 0.04″ –
verification for temperatures and precipitation will come from METAR data for the period between 1AM Tuesday and 1AM Tuesday (06Z Tuesday to 06Z Wednesday) at LGA. Wind speed verification will draw on the daily climate summary from the National Weather Service.

Verification

High: 60°F | Low: 40°F | Max sustained winds: 36 mph | Total precipitation: 0.02″ – this wasn’t a great forecast, aside from the precipitation. I was once again overly conservative on max wind speeds. Following layer mean wind analysis results would have yielded an almost perfect forecast here. For temperatures, I think the main issue was that the low level clouds I thought would materialize based on the forecast soundings available the day before just never came to be. This allowed high temperatures (and hence low temperatures) to become warmer than I forecast. In this case, perhaps it would have been prudent to consider that winds would be downsloping from the northwest, hence drying out somewhat at lower levels. This wind direction also favors clearing conditions following a cold front passage from the west.

Weather Prediction Center surface forecast for 7AM Tuesday

Synoptic Set Up

NYC will start the day off in the warm sector of a low pressure center tracking northeast through New England. This low will continue to deepen as it progresses east. The tightening pressure gradient will induce strong winds, and a trailing cold front will cross the region bringing about much colder air in its wake. Temperatures will peak early in the day and drop steadily through the day. The low temperature will occur late in the forecast period.

High Temperatures

Steady warm advection overnight with a warm front and warm sector associated with the parent low above will mean that temperatures increase to the mid-50s by day break. Forecast soundings from both GFS and MOS indicate, persistent, low overcast that I believe will be pivotal to keeping temperatures from going above 60°F. These low overcast clouds should block out enough sun to keep things a bit cooler than otherwise would be. Forecast soundings also show some upper level clouds that would help block additional sun. For these reasons, I’m siding with cooler NAM MOS guidance that calls for 55°F, and bumping up a couple degrees. Cold advection should take hold quickly in the afternoon in the wake of the trailing cold front. This will usher in westerly and northwesterly winds, which will downslope and warm a touch but will still overall bring colder air into the area. Once this cold advection takes place, temperatures should start to fall.

GFS forecast sounding for 9AM Tuesday, showing a well mixed layer up to about 825 mb and low clouds around 800 mb.

Low Temperatures

Because of the factors above, the low temperature should end up coming in late in the evening/overnight period going into Wednesday. I believe there’s not too much reason to deviate from MOS guidance which averages out to be about 35°F.

Max Sustained Winds

A well-mixed boundary layer is forecast to develop with pretty strong winds aloft in the late morning hours. A period of efficient downward momentum transfer should allow fast winds from above to mix down to the surface. Layer mean wind analysis suggests that winds could be as strong as 35 mph, however, I’m going a bit lower with sustained winds topping out at 30 mph since neither set of MOS guidance goes over 20 mph.

Total Precipitation

It’s clear at this point that the bulk of precipitation will fall before the beginning of the forecast period. However, both sets of MOS guidance, SREF and GEFS means all suggest some precipitation early in the overnight period. I’m going with a composite average of various sources at 0.04″.

NYC New Year’s Eve Weather – Dec 31, 2018

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New Year’s Eve this year looks like a washout, but temperatures will be very mild. In fact temperatures will maintain a non-diurnal trend, warming through the night. New Year’s Day could see the possibility of record warmth before a return to more normal conditions mid-week. Another storm system approaches for the end of the week.

Rest of today – cloudy with high temperatures in the upper-40s. A warm front approaching from the south and west will start to bring steady rain to the area by late afternoon. A favorable low-level jet, along with robust isentropic lift (overrunning of warm air above cold air along the warm front) should enable plenty of moisture and enough instability to trigger bouts of heavy rain later in the evening and overnight. Unfortunately, the bulk of rain looks to fall right during periods when revelers will be out on the town celebrating the new year. Winds will pick up as well with the pressure gradient tightening and efficient downward momentum transfer of low-level jet winds during periods of precipitation. As mentioned above, temperatures will follow a non-diurnal trend, with the warm front and warm sector of this storm advecting milder air from the southeast. Temperatures will actually rise into the mid-50s overnight.

NAM (North American Model) forecast of 850 mb relative humidity and winds for 10PM tonight. The blue hues indicate saturated air, while we can see some wind barbs showing winds > 60 knots. These are classic signs of a low-level jet that is often implicated in producing heavy precipitation events.
Weather Prediction Center surface forecast for 7AM Tuesday. Note the tight packing of isobars (black lines) around the parent low that’s going to bring us rain. This indicates a tight pressure gradient that will drive strong winds. This surface forecast also suggests that our time in the warm sector of this low (outlined in light orange above) will be brief. Warm advection (transport of warm air) won’t last long into Tuesday.

Tuesday (New Year’s Day) – I’ll follow up with a detailed forecast, the start of 2019 will be quite interesting for a couple reasons, one being possible record warmth and the other being strong, gusty winds. Temperatures will be rising throughout the night into Tuesday such that high temperatures will peak early in the day. Depending on exactly when this happens, temperatures could push above 60ºF, which would be nearing or exceeding records. The record high for January 1st at Central Park is 62ºF set in 1966. A lot will hinge on whether clouds clear up early enough to allow for a few hours of solid sunshine. This warmth will be short-lived though, because we won’t spend that much time in the warm sector of this parent low. As a trailing cold front moves through, winds will shift to the northwest and result in cold advection. As a result, temperatures will be falling during the day into the upper-40s by sunset. Temperatures are forecast to continue dropping overnight into Wednesday with lows bottoming out around freezing in the city.

Wednesday – a rude awakening to normal temperatures for this time of year. There will be a cold start to the day with high temperatures 20ºF cooler than Tuesday, in the upper-30s. Partly cloudy skies. Overnight lows in the mid-30s.

Thursday – warmer than Wednesday with another storm system approaching. So far, looking at a mostly cloudy day with highs in the low-40s. Rain may start as early as the overnight hours. A lot of uncertainty still at this time with the timing, scope, and precipitation type of this next storm though.

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.