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″.
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.
The last weekend of the year gets off to a mild, rainy start as a storm system moves through. Things cool off and dry out the remainder of the weekend but it looks like another rain event and warm spell hits for New Year’s Eve.
Rest of today – rain through most of the day. Gusty winds around 30 mph possible. High temperatures well above normal in the mid-50s due to a surge of warm advection accompanying the warm front initially responsible for widespread rain. At the 850 mb level, a low-level jet develops, helping enhance moisture convergence and allow for heavier rains. A flood watch is actually in effect for areas of New Jersey in the metro NYC region. Lows going into Saturday will be quite warm in the mid-40s.
Storm Prediction Center mesoanalysis of 850 mb moisture transport. The maroon vectors indicate the magnitude and direction of moisture transport. The long vectors over the NYC region show efficient moisture transport due fast moving winds of a low-level jet at the 850 mb level. The shaded contours show values of theta-e (equivalent potential temperature), while the details of this are quite complex, the easiest way to understand theta-e is that high values indicate very moist airmasses that can be prime for development of convective activity, as an example. In this case, we can tell that the potential for heavy rain exists within areas of high theta-e (dark-green to red shaded areas).
Saturday – a strong cold front will move through during the afternoon which should result in high temperatures peaking early in the day around 50°F, even though it will remain mostly sunny all day. Behind this cold front, Canadian high pressure builds yielding much colder overnight lows into Sunday in the low-30s.
Sunday – much colder day on tap with Canadian high pressure in control. High temperatures around normal for this time of year near 40ºF even with plenty of sun. Overnight lows into Monday are forecast in the mid-30s.
Monday (New Year’s Eve) – clockwise return flow from the southwest on the western side of the exiting high pressure warms temperatures up into the upper-40s ahead of the next storm. The timing of this storm looks like it will result in a rainy New Year’s Eve in the city. This may sound less than ideal, but recall that we rang 2018 in with near record cold. It’ll be downright balmy in comparison this time with overnight lows forecast to barely budge in the mid-40s.
We won’t have a white Christmas in NYC this year, but it will feel like winter. The weather this week is shaping up to be pretty average. Like last week, a notable warm up appears to be on the way for the end of the week, which will also be accompanied by rain.
Rest of today – light rain should be ending within the next hour or so in NYC. After that, expect mostly cloudy conditions this Christmas Eve with high temperatures in the mid-40s. Lows going into Christmas will be cold, around freezing.
Tuesday (Christmas Day) – decent day with increasing clouds as a cold front draws near but dissipates. High temperatures in the low-40s. Lows overnight will once again be around freezing.
Wednesday – high pressure will build in and provide for a mostly sunny day, with temperatures remaining in the low-40s. Cold overnight lows around 30°F.
Thursday – the high pressure center mentioned above continues to build, and is forecast to be quite strong at 1040 mb by Thursday. That’ll all but ensure sunny skies with high temperatures around 40°F.
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/s
Speed in knots
997
8.20
15.94
975
12.90
25.08
950
14.60
28.38
925
15.90
30.91
900
17.10
33.24
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.
The official start of winter hits at 5:23PM EST, though you’d be forgiven for thinking it were a spring day instead, with temperatures well above normal and a driving rainstorm to end the week. The storm tapers off rather quickly. Cold air wrapping around the exiting storm will drop temperatures right back into normal ranges for the rest of the weekend and going into the week of Christmas.
Rest of today – bouts of heavy rain, which have already induced flash flood and flood warnings, will continue through the early afternoon hours. There should be a lull in activity as the first cold front that’s been bringing this heavy rain exits east. More rain is possible with a secondary cold front set to pass through later this evening and tonight. Strong winds are set to continue throughout the day as two areas of low pressure responsible for this storm consolidate and continue to deepen (hence inducing a large pressure gradient). High temperatures likely have already occurred today in the low-60s, and temperatures will slowly drop into the upper-50s. Overnight lows should still be quite warm, in the upper-40s.
High Resolution Rapid Refresh model simulated radar for 3PM
Saturday – the first full day of winter will see a return to more seasonable temperatures with highs in the mid-40s under mostly cloudy skies. Clouds will be slowly dissipating as high pressure builds in behind the exiting storm. Overnight lows will get back into the mid-30s.
Weather Prediction Center forecast surface map for 7AM Saturday
Sunday – mostly sunny day with high temperatures in the low-40s and overnight lows into Monday in the mid-30s.
Monday – another mostly sunny day anticipated with a high in the low-40s.
This week starts off with a couple windy days, with Tuesday seeing a blast of Arctic air. Temperatures moderate Wednesday then start warming up on Thursday to above normal levels as a high pressure center slides off to our southeast and return flow from the southwest brings warmer air up our way. This will set the stage for a bout of possibly heavy rain to end the week.
Rest of today – partly to mostly cloudy skies. High temperatures in the mid-40s. Clouds are forecast to increase during the day as a potent 500 mb shortwave brings ample vorticity and divergence aloft. This will help induce some instability in the lower atmosphere and lead to more clouds. The other story will be blustery winds in the 15-20 mph range due to the development of a well-mixed boundary layer, allowing for effective downward momentum transfer of faster winds above to the surface.
GFS 500 mb forecast valid for 5PM today. The oranges and dark purples indicate an area of very strong positive vorticity associated with a 500 mb shortwave pushing through the area. It may be difficult to make out, but the bottom layer of the atmosphere, starting from about 850 mb down to the surface, shows evidence of becoming well mixed by this afternoon. This is indicated by the angle of the environmental temperature profile (red) paralleling or being even more acute than the light grey lines that slope to the left at about a 45 degree angle. These are the lines of dry adiabatic lapse rates.
Tuesday – an Arctic front passes through the area early during the day. There isn’t sufficient moisture in the atmosphere to bring about any precipitation. The tangible impact of this frontal passage will instead be below normal temperatures in the mid-30s despite a sunny day as a high pressure builds. This building high pressure should maintain stiff winds as the pressure gradient stays pretty steep.
Wednesday – sunny conditions with temperatures right around normal 40°F.
Thursday – clouds increase as the high pressure responsible for sunny weather mid-week starts moving off to the southeast. As this happens, clockwise return flow on the western side of the exiting high pressure will bring some warmer air our way. High temperatures are expected to be warmer than average, in the upper-40s.
Saturday looks to be a dreary start to the weekend, however, it appears at this time that it’ll actually end up being the better half of the weekend. While rain chances exist for Saturday, the best conditions for heavier rain don’t look to materialize until Sunday. Even with clouds, rain, and winds shifting onshore from the east and northeast, Saturday afternoon may end up being the warmest day we’ve seen for quite some time. Below, I will provide a detailed forecast using KLGA (LaGuardia Airport) as a reference point. This is because KLGA will be easy to verify forecast outcomes for.
My Forecast High: 51ºF | Low: 41ºF | Max sustained winds: 17 mph | Total precipitation: 0.13″ – verification for temperatures and precipitation will come from METAR data for the period between 1AM Saturday and 1AM Sunday (06Z Saturday to 06Z Sunday). Wind speed verification will draw on the daily climate summary.
Verification High: 50ºF | Low: 45ºF | Max sustained winds: 24 mph | Total precipitation: 0.11″. Updated Sunday, December 16, 2018 1PM EST. Assessment: this ended up being a good forecast on precipitation and high temperature. However, I was too low on wind and low temperatures. Because of the light precipitation, little evaporational cooling occurred at the surface, where the air was nearly saturated most of the day, leading to warmer low temperatures. For winds, I may need to adjust my forecasts upwards for LGA when winds are out of the northeast because winds from this direction will flow over the open waters of the East River, with very little frictional drag.
Synoptic Set Up (The Big Picture)
A slow moving storm system with origins in the Deep South will make its way into the Northeast over the weekend. This storm will eventually transition to a coastal storm as it moves offshore of the Northeast. At the 500 mb level, this storm is driven by a closed low, which will almost nearly become completely cut off from the primary wind pattern (steering currents) at this level. This is what explains the slow forward progress of the storm. The result of this is that we’ll experience an extended period of cloudy and rainy weather this weekend. Low-level moisture appears to be maximized on Sunday, though, so Saturday may end up being a better weekend day.
MOS forecasts show high temperatures Saturday in the low-mid 50s, with NAM (North American Model) coming in at 53ºF and GFS (Global Forecast System) pointing to 56ºF. NBM (National Blend of Models) – a consensus based forecast model, showed highs of only 49ºF. I tend to side with high temperatures on the cooler side due to persistent low overcast clouds (discussed below), and an onshore northeasterly wind later in the day.
Low Temperature
The same MOS data above has lows on Saturday of 44ºF for NAM, 46ºF for GFS and 43ºF for NBM. I’m siding with the cooler side of the forecast envelope again due to the fact that precipitation will be ongoing overnight and there could be some evaporational cooling impacts to account for. Also, with the forecast period ending 06Z Sunday, I actually think the low for this period could be at the tail end, based on MOS. This would be after a prolonged period of onshore winds that I think could bring temperatures into the low-40s.
Max Sustained Winds
Although there will be precipitation during Saturday, as cited below, there looks to be a stable layer with a surface inversion during much of the day on Saturday (see the second Skew-T below). This will make downward momentum transfer during precipitation periods rather inefficient. The pressure gradient isn’t looking particularly strong either, since the low won’t be deepening until Sunday when it moves offshore. With these factors in mind, I think that a max sustained wind of 15 knots (17.25 mph) is reasonable.
Total Precipitation
This will be the trickiest part of the forecast. As is often the case with coastal storms, the gradient of precipitation totals is quite steep. There’s also considerable spread in ensemble models for total precipitation, leading to a low confidence in this forecast. A key factor here is that low-level moisture doesn’t look to be particularly robust as the day progresses (see graphic below). There’s also no clear lifting mechanism at the surface.
850 mb relative humidity, and wind at 1PM Saturday. Areas in blue are saturated, browns indicate very dry air, note how close that dry air is to NYC
While it’s clear that the day starts off with rain overnight, both GFS and NAM suggest that things will dry out a bit in the afternoon hours. That could translate to a lull in precipitation during that time frame.
NAM forecast sounding for 4AM at KLGA. Refer to my earlier post for a guide on how to interpret this Skew-T. By 1PM, though, not that there’s a marked drying of the low levels above 850 mb. This could certainly cut into precipitation and result in light rain or even a lull in rainfall.
SREF probabilities of total precipitation and the best lift show that these remain south of the area, over the open waters of the Atlantic. However, the gradient is quite sharp for this, so any deviation northward on the storm track could throw a lot more rain our way.
SREF (Short Range Ensemble Forecast) forecast of strong lift (omega >= -9 microbar/sec). Lift is a critical ingredient in producing precipitation. I’ll explain in a later post 🙂
A slow moving storm system will bring a prolonged period of unsettled weather to the NYC region this weekend. The upside of this storm is that it will usher in some warmer air from the south, giving us some above average temperatures for a brief spell. Behind this storm system a cold front is forecast to pass through early next week and return us to below normal cold for this time of year. I will be following up on this post with a detailed forecast for Saturday, as I attempt to keep my forecasting skills sharp.
Rest of today – overcast, with high temperatures around 50ºF. Increasing chances for rain going into this evening and overnight. Low temperatures in the mid-40s overnight.
Surface forecast for Saturday morning, 7AM
Saturday – similar day to today with a generally gloomy feel, overcast skies with a chance for rain, though looks like precipitation will be on the light side. High temperatures in the low-50s. Overnight lows into Sunday cooler than Friday, in the low-40s.
Sunday – yet more rain possible with the storm slowly making its way east and transitioning to a coastal storm. High temperatures are expected to be cooler because winds will shift onshore from the northeast – highs probably in the mid-40s. Overnight into Monday, temperatures should hold in the upper-30s with rain chances winding down.
Surface forecast for Sunday at 7AM
Monday – skies slowly clearing with high temperatures in the mid-40s.
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.
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.
