Tag Archives: snow

December 10, 2018 Southeastern Snowstorm Post-Mortem

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

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

Dry air at the outset of the storm

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

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

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

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

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

Frontogenesis and mesoscale (localized) banding

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

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

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

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

Cold air damming

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

11AM Sunday surface analysis from the Weather Prediction Center

What are some takeaways from this?

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

WxChallenge for Omaha, Nebraska (KOMA) | Oct 9-12

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A couple days into the forecast period for the second WxChallenge city for this fall, and I’ve experienced my worst forecast bust (for yesterday). Highlights in the post-mortem section. It has been a week of transitions for Omaha, which started off with relatively mild conditions but will end the week with possibility for patchy frost!

Synoptic Set Up

A 500 mb longwave trough will with multiple attendant vorticity maxima will pivot through the KOMA region during the first half of the forecast period before flow becomes more zonal. At the 300 mb level, multiple jet streaks could boost divergence with these 500 mb vorticity maxima. This enhanced lift, and strong wind shear profile could lead to some isolated strong-severe thunderstorms. The 850 mb shows evidence of a decently robust LLJ that would should bring a continuous supply of Gulf moisture up the Missouri Valley into the area. At the surface, a warm front looks to lift through Monday, then stall as a cold front in the vicinity of KOMA through late Wednesday due to the deep unidirectional winds parallel to the front boundary.

Notes

The potential for flash flooding, and bouts of heavy rain, is duly noted in the AFD during the first half of the weak. The synoptic ingredients for a long duration rainfall event are certainly evident. Soils in the area are already quite moist, given that flood warnings were present for stretches of the Missouri last week. GFS MOS QPF range on Sunday for 06Z Tuesday to 06Z Wednesday was 1.75-4.46”. NAM MOS QPF range for this time was 1.35-2.71”, consensus was 1.55-3.59”. Records around this time of year for QPF show that 1-2” can easily occur, despite the normal being only a mere 0.07”. SREF probabilities of >= 0.50” were above 90% for this period, and >= 1.00” was 50-70%, but over 2.00” of QPF probabilities were less than 10%. SREF mean was 2.73”, with a pretty large spread, from 0.69-5.54”. GFS mean was 3.93” with spread of 1.82-5.16”. For now, going with a superblend of the low-end of the ranges of MOS and plumes: 1.40”.

As of Monday, SREF and GEFs means had become much better aligned, with SREF mean at 2.83” and GEFs mean 2.95”. SREF spread was 1.34-4.46”, GEFs was 1.43-4.43”. QPF forecast shows KOMA a pretty large QPF gradient, and that uncertainty seems borne out well in the ensemble forecasts. SREF probabilities of >= 2.00” increased to the 10-30% with KOMA lying right on the fringe of 30-50% probabilities of this amount. >= 1.00” probabilities increased to 70-90%. Official NWS forecast as of 10Z Monday called for 1.65” during the forecast period Tuesday. GFS MOS QPF range was 2.10-4.21”, NAM 1.01-2.06”, consensus here is 1.55-3.13”. Based on these updates, I’m increasing my forecast to 1.85” – I’m hedging down below the SREF and GEFS means because I’m still not sure where the stationary/slow-moving surface front will set up in relation to KOMA, and where the best lift will be for heavy rain/thunderstorms as a result. Also, if the 850 mb LLJ fails to materialize close to KOMA, we could be looking at lower overall totals. Low SREF probabilities of over 2.00” also help justify this call. Looking at Wednesday, rain chances diminish considerably after the front moves through, but the story then becomes strong winds possible, with MOS pointing at 17-20 knot synoptic winds.

Ensembles continue to trend upwards as of Monday afternoon with QPF, SREF mean now 2.96” and GEFS mean is 3.16”. SREF probabilities are close to placing KOMA in the 50% >= 2.00” QPF. Will take one more look at 18Z MOS guidance and some other factors before finalizing, but am leaning towards increasing the precip forecast again, this time to maybe 1.90” I’m wary of going for 3” of QPF or higher, given that the entire month’s single-day QPF record is 3.09”. Even beyond 2” still seems unlikely.

On Tuesday, looking ahead at Wednesday and Thursday, the passage of a cold front is quite evident. Temperatures are forecast to drop considerably compared with the beginning of the weak, and precipitation will end, too. The forecast for Wednesday still retains a slight chance for some measurable QPF on the tail end of the main precipitation event Tuesday. Models have backed down from up to 0.25” of precip to less than 0.10”. SREF and GEFS means show approximately 0.04” on Wednesday. SREF probabilities showed that KOMA had a greater than 90% chance of receiving precipitation >= 0.01”, but less than 10% of >= 0.10”, going to go straight up with the ensemble means here. The other story for Wednesday will be stiff winds from the northwest, with MOS synoptic winds of 17-18 knots. This suggests even stronger sustained winds, and will aid in cold advection, keeping high temperatures suppressed. These blustery conditions are forecast to gradually abate by late in the day Thursday, such that the strongest winds Thursday should occur early in the overnight hours. Cold temperatures overnight into Thursday should be somewhat tempered by decent vertical mixing and overcast skies.

Forecasts Submitted

This only reflects finalized forecasts, I’ve made small adjustments to the forecast based on checking MOS daily.

Forecast Day Max Temp (°F) Min Temp (°F) Max Sustained Wind (kts) Precipitation (inches)
10/09/2018 62 57 14 1.91
10/10/2018 48 44 20 0.02
10/11/2018 48 34 17 0.00
10/12/2018 48 35 9 0.10

Post-Mortem

Day 1: Major bust across the board. Was off by more 5 degrees or more on both the high and low. This made up about half of the error points. The rest came from being off on max precipitation by more than 1.20”. MOS was considerably off the mark with temperatures – though NAM was closer. This was all due to the fact that a stationary frontal boundary never made it close enough to KOMA to realize warmer temperatures, and heavier precip. The axis of the 850 mb LLJ stayed south and east, along with the best surface convergence. As a result, nearly all of the model guidance was horribly wrong. The closest forecast in the class was still nearly 0.50” too high. I put too much credence into the upwards trends on the models – there were some indications already that KOMA was trending to be on the drier side of this event. Still, even if I had nailed the precipitation forecast, I would have still busted hard on the temperatures. Need to be a lot more cautious going forward about situations where a frontal boundary and precipitation are around, and probably hedge down on temperatures during transitional season when a forecast site could be on the cold side of a front with rain.

NYC Weather Update – Apr 2, 2018

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I had trouble typing the date in the title of this post, given that there’s snow falling as I write this! Unfortunately, this snow is not a belated April Fool’s joke. Didn’t have time to post an update about this yesterday, sorry about that! I’ll also be out this weekend for weddings. Now on to the weather: look ahead to a week of up and downs in temperatures and active weather, but no more snow. The weekend coming up… well, there could be some chance for snow again Saturday.

Rest of today – snow departs the city by the lunch hour. Around 3″ of total accumulation, mostly on untreated surfaces, before the sun breaks out and starts immediately melting the stuff. High temperatures in the low-40s with decreasing clouds.

Tuesday – rain associated with an approaching warm front. Cloudy, with temperatures in the mid-40s. Overnight temperatures actually don’t go down much before warm air surges as we enter the warm sector of this second storm of the week.

Wednesday – more rain, this time with a chance for a rumble of thunder with decent lifting associated with the cold front that’s part of this storm. Significantly warmer, cloudy, with high temperatures around 60°F.

Thursday – much cooler behind the cold front, temperatures only in the upper-40s with mostly sunny skies.

NYC Weekend Weather – Mar 23, 2018

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Now that we have a record-breaking, snow-making, spring nor’easter behind us, we can finally start to look ahead to better, calmer weather. This weekend may not be entirely free of precipitation and will be colder than average, but it will still be a welcome reprieve. Looking ahead, there’s some chance we could see a true taste of spring with temps nearing the 60s late next week!

Rest of today – decreasing clouds as an area of high pressure starts to make its presence felt. High temperatures in the mid-40s.

Saturday – sunny, high temperatures in the mid-40s but with a chilly feel due to some northerly winds.

Sunday – high pressure breaks down and a storm system passes to our south but results in increased clouds. Temperatures in the low-40s as a result. Small chance for rain/snow possible due to the influence of the storm that hit us this week still stuck spinning out in the western North Atlantic.

Monday – high pressure builds again, yielding a sunny day with high temperatures in the mid-40s.

 

Warmer than Average April?

It’s been a rollercoaster temperature-wise in the Northeast to say the least over the last 3 months. January was bitterly cold and much below average. Then we experienced some record-breaking warmth in February, only to revert back to a colder than average month of March. Looking ahead, there are some signs from the Climate Prediction Center that we could experience a warmer than average April!

NYC Spring Nor’easter Update – Mar 20, 2018

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This is a special bulletin regarding the nor’easter expected to impact the region tomorrow. This may end up being one of the biggest snowstorms ever to hit the NYC Tri-State this late in March. On to the forecast details:

Impacts: snow, heavy at times, with some thundersnow possible. Blizzard conditions possible. Steady north to north-northeasterly winds in the 20-30 mph range with gusts 35 mph+. Coastal flooding during high tide cycles with storm tides 2-3′ above normal. Widespread snowfall accumulations 8″+ in NYC, Central NJ, Lower Hudson Valley, southwestern CT. The possibility exists for more than a foot of heavy, wet snow area-wide. I think the best bet is the low-end 10″ of the official forecast for NYC that calls for anywhere between 10-18″. Expect disruptions to both commutes, in particular the PM commute. Power outages likely due to the combination of accumulating heavy wet snow and windy conditions downing trees and/or power lines.

Timing: conditions deteriorating overnight. Snow beginning early in the morning in NYC (as early as 5-6AM possibly), later in the morning north of I-287. Heaviest snow during the day, especially in afternoon hours when lift is maximized. Snowfall rates of 1-2″/hr or more possible during the afternoon. Snow tapering off overnight, probably ending by midnight.

Discussion: this storm is unlike the previous two that struck the area. The surface and near-surface is colder than what it was at the outset of the last two storms. This will aid in snow sticking quickly, despite the high sun angle this late in March. The storm track has been trending westward but right now looks to be directly over the 40°N/70°W benchmark, a track that favors heavy snow over the area. The bust potential for a snow total below 8″ is still there, but even a snowfall of 5-6″ would still be substantial for this time of the year!

Focus on the storm track of the deeper low, the one that has the orange and brown plotted storm center positions.

NY Weather Update – Mar 19, 2018

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It sounds like a broken record at this point, but another week, yet another nor’easter with a chance of snow, arriving on the second day of spring no less! As with previous storms, how much snow we get will hinge critically on the storm’s track and proximity to shore. Looking ahead past the storm, temperatures remain subdued and below normal for this time of year.

Rest of today – sunny, high temperatures in the low-40s.

Tuesday – increasing clouds, with a chance for rain snow mix later in the day. High temperatures in the upper-30s.

Wednesday – snow developing overnight and continuing through the day. Accumulating snow currently forecast to be light in the 1-3″ range. This could increase though, based on current forecast ensemble totals ranging 5-6″. The caveat is that blacktop will probably be largely snow free due to the high sun angle.

Thursday – conditions improving overnight and mostly sunny skies with highs in the mid-40s. Snow should melt quickly with the high sun angle and temperatures.

NYC Weather Update – Mar 12, 2018

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Another week, another nor’easter! Luckily, this time around, the storm will stay outside of the notorious 40ºN 70ºW benchmark, meaning that impacts for NYC should be relatively light. On the backside of this storm, weather should be fairly innocuous and average for this time of the year.

Rest of today – increasing clouds ahead of this incoming storm. High temperatures in the mid-40s. Temperatures drop to near freezing overnight in the city, setting the stage for snow overnight as the storm approaches and rapidly strengthens.

Tuesday – lingering snow from overnight ends quickly in the morning or transitions to rain. When all is said and done, we might pick up 2-4″ of snow in the city. Temperatures will be marginal at best and the snow will probably have difficulty sticking to blacktop. There is an outside chance of more than 6″ but this probability is low. As you can see from the preceding graphics, the bulk of the storm, and its center will be tracking quite far southeast of the area. Winds will be shifting towards the northwest around 20 mph and high temperatures will only be around 40ºF due to the combination of wind, clouds, and precipitation.

Wednesday – an unsettled regime continues into Wednesday with some slight chance for a couple snow showers. Mostly cloudy with high temperature in the mid-40s.

Thursday – partly sunny, high in the mid-40s.

NYC Weekend Weather – Mar 9, 2018

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This weekend, we’ll be watching for development of yet another nor’easter that could impact the region to start off next week. This would mark the third nor’easter in as many weeks, and put an exclamation point on a very active weather pattern. We also set our clocks forward with daylight saving time overnight between Saturday and Sunday. Enjoy more sun in the evenings!

Rest of today – clods and sun with a possibility for a spot flurry later in the day. High temperatures in the mid-40s.

Saturday – mostly sunny with high temperatures around average for this time of year in the mid-40s.

Sunday – another average day with sunny skies and high temperatures in the mid-40s.

Monday – things could get interesting as a developing low off the North Carolina coast works its way up the Eastern Seaboard. There’s a good deal of uncertainty at this time about the eventual storm track. Some models take this storm too far offshore to affect us, but notably the GFS has been on the western end of the forecast envelope. Some of its solutions would bring another significant storm with the possibility for more snow to the area Monday. Needless to say, this is situation that will bear monitoring over the weekend. High temperatures Monday will be impacted by the proximity of this storm. For now, forecasts call for a high around 40ºF with a chance of rain/snow.

NYC Weather Update – Mar 5, 2018

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The second week of March brings another nor’easter. The week will start off calmly with temperatures about average for this time of year. Tuesday night into Wednesday, a nor’easter will impact the region. This time around, the primary threat will be possible heavy snow rather than high winds. There are signals for yet another nor’easter next Tuesday as well!

Rest of today – decreasing clouds, high temperatures in the mid-40s with winds diminishing.

Tuesday – the veritable calm before the storm, a mostly sunny day with high temperatures in the mid-40s again.

Wednesday – a low pressure system that tracks east from the Great Lakes will spawn a secondary coastal low offshore of North Carolina. This coastal low will steadily gather strength, aided in part by the presence of an upper-level jet streak and the divergence provided within the exit region of this jet. At this time, two main camps of models have the storm tracking either close to the coast or over the 40N 70W benchmark. The former solution would mean higher precipitation amounts but more potential for mixing at the coast due to the influence of warmer marine air wrapping in. The latter solution would likely yield all snow, but with less total precipitable water. Currently, forecasters are calling for between 5-10″ of snow and a winter weather advisory is in effect.

Primary low moving east from the Great Lakes region, secondary low formation off of the coast of the NC/VA border

High temperatures should top out in the upper-30s. The main snow event would occur Wednesday afternoon and overnight. Based on the NAM and GFS ensembles, it appears as of now that the highest confidence is for a 5-6″ event. Again, this could change with a shift in the storm track, and we won’t know where the heaviest bands of snow set up until the event begins.

NAM ensemble plume forecasts for snowfall totals in LGA

GFS ensemble plumes forecast for snowfall totals at LGA

Thursday – unlike the last storm, this one will exit relatively quickly. This reduces the overall impacts for wind and coastal flooding. Thursday should be a day of improving conditions with partly sunny skies and high temperatures back in the mid-40s.

NYC Weekend Weather – Mar 2, 2018

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March arrives as a roaring lion this year with a powerful nor’easter impacting the region during the first half of the weekend. A variety of hazards from snow to high winds to coastal flooding will affect the area before this nor’easter meanders its way out to the open ocean. Behind this storm, the large scale pattern shifts and much cooler air rushes in, displacing unseasonable warmth with more average temperatures for the week to come.

Rest of today – rain and snow mixing. Little to no accumulation expected. Winds increasing from north to northeast winds 20-25 mph to 30-35 mph with gusts as high as 60 mph near the coast possible later today. Moderate coastal flooding possible over multiple tide cycles tonight, and Saturday. These tides will be higher than normal since they are falling on the full moon (spring tides). Temperatures will hold steady in the upper-30s.

Overnight, an inland low and coastal low consolidated offshore of the Northeast and the coastal low has already rapidly strengthened to a minimum pressure of 984 mb this morning. Two branches of the polar jet have phased together, providing the energy for this robust storm. As the storm continues to deepen, the pressure gradient between it and an area of high pressure over the Upper Midwest will continue to increase, leading to stronger winds. Onshore flow of these winds from the north and northeast over an extended period of time are expected to bring coastal flooding. Periods of moderate to heavy rain could lead to flooding in general. Although the cold, northwest flank of the storm as it exits to the east will support snow (with its cold conveyor belt), accumulations at the coast should be limited. This is another story inland where heavy snow is expected in the Lower Hudson Valley.

Saturday – periods of moderate to heavy rain with snow mixing in continue overnight, but should taper off by the morning. Winds will diminish from the 30 mph range with gusts up to 55 mph overnight but will remain stiff in the 20-25 mph range from the north. Much colder feel with highs in the mid-40s under partly sunny skies.

Sunday – much nicer day, winds finally abating with high temperatures in the mid-40s and mostly sunny skies.

Monday – sunny, with high temperatures in the mid-40s.