After a chilly start to March, things will take a turn towards much more seasonable weather this week. Still, temperatures will be slightly below average on several days. Precipitation chances don’t pop up until Friday when an approaching cold front should bring a warm up and then rain.
Rest of today – sunny, with a high near 50°F. Overnight lows around freezing.
Tuesday – a weak cold front passes through in the morning, but there will not be enough moisture for precipitation. High temperatures will be cooler than today, in the mid-40s with mostly sunny skies. Overnight lows will be cooler around 30°F.
Wednesday – sunny skies and a high temperature again in the mid-40s. Clouds will be on the increase overnight as a warm front approaches from the southwest. Overnight lows will be a touch warmer in the upper-30s.
Weather Prediction Center surface forecast for 8PM EDT Wednesday
Thursday – warmer but mostly cloudy with high temperatures in the mid-50s. Overnight lows expected to be in the mid-40s with the chance of rain increasing.
Last Sunday, while I was preparing my post on the snowstorm that was about to hit NYC and the Northeast, the southern side of this same storm system was starting to produce a serious severe weather event in portions of the Deep South. A large, violent, and ultimately deadly EF4 tornado hit parts of Lee County, AL during the afternoon. The tragic toll of 23 confirmed fatalities due to this tornado was more than double the total deaths due to tornadoes in all of 2018. This was also the deadliest single tornado since the EF5 tornado that hit Moore, OK on May 20, 2013. In this post, I’ll share some thoughts and observations about the meteorology behind this event, and about what made this tornado so powerful.
Storm Prediction Center’s Forecasts
One aspect of the event that impressed me was the prescient, geographically accurate, and timely Mesoscale Discussions and convective outlooks that the Storm Prediction Center issued during the course of the day. The SPC already had a handle on the risk for severe weather in parts of the Deep South as evidenced by the convective outlooks they issued Sunday morning.
Storm Prediction Center’s Day 1 Convective Outlook issued at 7:52AM Sunday Mar 3, 2019. The black dot within the orange “ENH” enhanced risk area shows the approximate location of Lee County, AL
Storm Prediction Center similarly recognized the risk for tornadoes on this day. The black dot indicates the location of Lee County, AL. within the hatched yellow area representing a 10% or greater probability of EF2 – EF5 tornadoes within 25 miles of a point risk area
Regarding the enhanced risk area that the SPC identified as possibly being affected by tornadoes:
The most favorable … space for tornadic potential … still appears to be within the enhanced-risk area, where strong deep shear, large low-level hodographs, and at least low-end surface-based buoyancy will juxtapose. Forecast soundings show rapid prefrontal destabilization …. [a]s that occurs, severe potential will steadily ramp up…. a few tornadoes also are possible. Tornado-event density, and risk of significant tornadoes, still is somewhat unclear — being strongly dependent on existence/number of preceding supercells that can develop…
SPC foresaw that the energy (instability) and spin (shear, imparted by strong winds at different levels of the atmosphere) required for strong tornadoes would have a chance to come together in the enhanced risk area. They also identified that the greatest risk would be with any supercells that could form ahead of the main line of thunderstorms that would accompany the cold front later on.
Weather Prediction Center’s surface analysis valid for 1PM EDT Mar 3, 2019, an hour or so before the EF4 tornado struck Lee County, AL
As it turned out, supercells did form ahead of the cold front – one in particular drew the attention of astute SPC forecasters, and this would end up being the supercell responsible for the tornado that hit Lee County. In follow up Mesoscale Discussions regarding the tornado watches over the enhanced risk area, SPC forecasters were remarkably accurate and timely in identifying the risks associated with this supercell and the favorable conditions it would encounter.
Lee County is circled in green in SPC’s Mesoscale Discussion #0145 surface map.
MCD #0145 was issued at 1PM CDT (local time), and contained the following text. The forecasters cited favorable conditions for a strong tornado to form within 30-60 minutes. Just around 2PM, about 60 minutes after this MCD was issued, the EF4 tornado hit Lee County.
A mature supercell located near Montgomery is favorably located within a region of maximized surface pressure falls (3-4mb per 2 hours) immediately east/southeast of the surface low. KMXX VAD shows 500 m2/s2 0-1km SRH when accounting for the observed Montgomery County supercell’s storm motion. Given the ample buoyancy and intense shear profile in place, it appears tornadogenesis will likely occur within the next 30-60 minutes with the possibility of a strong tornado occurring.
Why Conditions Were So Favorable for a Strong Tornado
The following analysis about the mesoscale conditions that favored strong tornadoes on this day came about from a discussion I had with Steve Corfidi, my instructor for the class I took on mesoscale forecasting (severe weather forecasting) as part of Penn State’s Undergraduate Certificate in Weather Forecasting. Steve Corfidi also used to be the Lead Forecaster at the SPC. Suffice to say, I am quite privileged to have been able to glean some insights about this storm from him. These observations are related to another MCD from SPC that day, MCD #0147.
Storm Prediction Center’s MCD #0147, issued at 2:06PM CDT, right around the time the EF4 Lee County Tornado was on the ground. Lee County is circled in green.
In this MCD, the SPC highlights an area of localized surface pressure falls in dashed blue. Steve Corfidi commented this effect is related to “rise and fall pressure “waves” that move across the earth twice-daily in response to solar heating”. As the earth heats up, air warms and rises, and this generates a thermal low since there’s less air over a warmed up spot of the earth than surrounding areas. In this case, this resulted in a localized area of surface pressure falls over the area circled in dashed blue as the day progressed. In response, surface winds will have a tendency to deflect towards the center of the lowering pressure. You can see this by looking at the wind barbs in the chart above: those that are closer to the cold front are more southwesterly, but the ones closer to the blue dashed area are actually more southerly, since they are deflecting towards the north and the localized pressure falls. This is known as the isallobaric effect. This had direct impacts on the favorability of the environment for tornadoes, as Steve Corfidi helped me understand.
My illustration of the situation over Lee County during this tornado.
As winds near the localized pressure falls became more southerly in response to isallobaric effect, this actually increased the vertical wind shear values in the area of the pressure falls (green here, blue dashed area in the SPC analysis, the red 300 mb wind profile barbs are approximated from this sounding). Since vertical wind shear is measured by looking at both the difference in direction and speed of winds at different levels, a change in wind direction at the surface, all else being equal, will result in higher wind shear. Relative to other areas in the warm sector of this storm, this produced an even higher value of storm relative helicity (SRH, as alluded to in MCD #0145) as well as the aforementioned vertical wind shear. I don’t have space to elaborate on why SRH and vertical wind shear are important for tornadoes, I will say that it has to do with enhancing storm rotation, and tornadoes are intense, vertical circulations of rotating air.
One other observation worth mentioning is that the “geometry” of the warm sector maximized the amount of time the supercell could spend in an extremely favorable environment. If you look at the large blue arrow in my illustrated diagram, check out how the approximate mean storm motion was largely parallel to the orientation of the warm front and axis of the maximized surface pressure falls. That meant that as the tornado formed, it was able to keep moving through a favorable environment for much longer than if the storm motion had been more northeasterly, or say southeasterly.
As we spring forward this Sunday, our weather pattern finally takes a step forwards towards spring as well. Rain will accompany this warmup, then we start next week with temperatures close to average for this time of year.
Rest of today – Increasing clouds as an area of low pressure moves offshore well to our south. High temperatures in the upper-30s. A 500 mb (upper level) shortwave trough passing through overnight could provide enough lift for some scattered flurries. Lows dipping to around freezing.
Saturday – High temperatures in the mid-40s with lots of sun as high pressure temporarily exerts its influence. Overnight lows in the mid-30s. Rain starts overnight going into Sunday as a storm system draws near.
Weather Prediction surface forecast for 8AM Sunday
Sunday – rain during the morning with improving conditions later. Overcast to start with cloud cover decreasing as the storm departs to the east. High temperatures around 50°F. Overnight lows in around 40°F
Monday – sunny and seasonable with highs in the upper-40s. Overnight lows cooler than Sunday in the mid-30s.
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.
During my time taking classes as part of Penn State University’s Undergraduate Certificate in Weather Forecasting, we were taught that understanding the climatology of the location you are interested in is an important prerequisite for making accurate forecasts. As such, I’ve decided to post some critical components of climatology for the closest station to me, LaGuardia Airport (KLGA). Below, I’ve posted some general climate data, and also specific data for the month of March.
City Name / Station ID: New York, NY (LaGuardia Airport – KLGA)
Local Geography and Topography
Station Elevation: 10 feet above sea level.
Station Location: LaGuardia Airport (KLGA) is situated on the north shore of Queens along the East River, approximately 6 miles east-northeast of Midtown Manhattan.
KLGA’s location within the broader NYC area, as seen in a Google Maps terrain view
Important Topographical Features: New York City is located in the extreme southeastern corner of New York State, bordering suburban New Jersey and Connecticut. These suburban regions combined with those in Long Island comprise the Greater New York City Metropolitan Area, which is the most populous urban agglomeration in the United States and one of the populous urbanized areas in the world with an estimated population of 18 million. New York City itself sprawls across the coastal plain around the Hudson River estuary. The terminal moraine formed by glaciers of the last Ice Age result in a ridge of higher terrain that cuts a swath from southwest to northeast across the boroughs from northern Staten Island, northern Brooklyn, southwestern through central and northeastern Queens. Otherwise, the city itself is low lying. This ridge varies in height between 200-400 feet, rising sharply from south to north, but tapering more gently north. North and west of the city (about 30-50 miles away), lie significant elevations of the Catskills (north), Poconos (west), Taconics that are part of the broader Appalachian Mountain Range. The elevations of the lower foothills can range from 1000-1500 feet. Some of the elevations in the Poconos and Catskills, west and north of KLGA respectively, peak between 2000-3000 feet. The open expanse of the Atlantic Ocean lies south of KLGA and New York City. Long Island Sound also lies east-northeast. The vast urbanized area of the NYC metropolitan region has significant effects on local microclimates via differential heating (urban heat island effect). KLGA is in a low-lying area sensitive to UHI effects and marine influences.
On winter mornings, ocean temperatures which are warm relative to the land reinforce the effect of the city heat island and low temperatures are often 10-20 degrees lower in the inland suburbs than in the central city. The relatively warm water temperatures also delay the advent of winter snows. Conversely, the lag in warming of water temperatures keeps spring temperatures relatively cool. One year-round measure of the ocean influence is the small average daily variation in temperature.
National Weather Service – NYC Office
Wind Patterns
Below is a wind rose – you can read more about how to interpret this chart here.
Frequency (percentage) of the single most common wind direction: Due northwest occuring about 14% of the time.
Directions that are most and least common: Most common wind directions following due northwest are due northeast (11.75%), west-northwest (10.25%), north-northwest (8.5%). Least common wind directions are east-southeast (1%), due southeast (2%), and due east (2.25%).
Direction(s) most likely to produce the fastest winds: The most common wind directions are also the ones most likely to produce the fastest winds.
Direction(s) least likely to produce the fastest winds: Again, the least common wind directions are also least likely to produce the fastest winds.
Impacts of wind direction on local weather: Prevalent northwesterly winds during this month generally follow in the wake of cold fronts and coastal storms. These winds can lead to substantial cold air advection (transport of cold, dry continental polar air mass from interior Canada), often because of subsidence in the wake cold fronts mixing down very fast winds to the surface. These winds will downslope and warm slightly as they approach the coast though. A secondary maximum of winds from the northeast can be attributed to backdoor cold fronts arriving from the Canadian Maritimes, bringing a moist, cool maritime polar air mass, or in conjunction with the advance of coastal Nor’easter type storms. During March, sea surface temperatures in the vicinity of NYC are above freezing, but by no means warm. If a warmer air mass is in place prior to winds shifting to the northeast, cooler, more moist conditions will result. If a colder, below freezing air mass is present, northeasterly winds can exert moderating influence on temperatures. Persistent northeasterly winds can also lead to the potential for coastal flooding given the shape of local coastline.
Maximum observed two-minute wind speed for the month in knots: 40 knots (46 mph).
Temperature and Precipitation Averages/Records
Temperature units are in Fahrenheit and precipitation is in inches.
Worth Noting: Average temperatures in March rise above 50°F for the first month since November. However, March can certainly still produce cold days – many of the record lowest max temperatures are below freezing, with record lows still in the teens and single digits. It is a month indicative of spring when large temperature swings are possible.
Although the calendar has flipped to March and we’re less than 3 weeks from the official start of spring, the weather has taken a decidedly winter-like turn as of late. This pattern will continue over the weekend and into next week. Initially, we’ll contend with 2 coastal storms that will bring chances for snow, and then enter next week with temperatures well below normal for this time of year. Sadly, this colder than normal pattern looks locked in for the next week or more
Rest of today – overcast conditions with temperatures hovering in the mid-30s. Precipitation will move back into the area by later this evening. Thermal profiles overnight point to a mix of snow/sleet at the coast with lows just around freezing. The local forecast office calls for an accumulation of 1-2″ of this wintry mess.
Weather Prediction Center surface forecast for 7AM Saturday
Saturday – the wintry precipitation continues the first half of the day as the first of two coastal storms continues to impact the region. High temperatures should warm up to around 40ºF with mostly cloudy skies, and precipitation should die off later in the day. Overnight lows should be around freezing.
Sunday – starts off as a decent enough day, but a second storm will be brewing and moving offshore, impacting the area overnight into Monday. High temperatures should be similar to Saturday in the low-40s with partly sunny skies. At this point, the primary precipitation modes at the coast appears to be a rain/snow mix. Thermal profiles don’t appear cold enough to support all snow with overnight lows expected to be above freezing around 35ºF.
Global Forecast System 1000 mb – 500 mb height, pressure, thickness foreast for 6AM Monday
Monday – precipitation tapering off early in the morning as this storm is a fast-mover. Temperatures topping out in the upper-30s with mostly cloudy skies.
Colder than Normal Temperatures Ahead
Climate Prediction Center has a 6-10 day temperature outlook suggesting a colder than normal pattern for much of the country, including our area. During this time, the polar jet stream is expected to dip further south allowing colder air to penetrate into the Continental US.
In lieu of an early week forecast this week, I’m opting to share some observations about weather I experienced in Mexico last week while on vacation. I stayed in Isla Mujeres, a small island located about 13 miles off the coast of Cancun. Temperatures were of course quite warm. It was also unusually windy for this time of year down there, though nothing quite like the windy weather NYC experienced today with some peak winds recorded at over 50 mph nearing 60 mph.
METAR READINGS FROM JFK and LGA showing peak winds of 56 mph and 58 mph respectively recorded at 12:04AM and 8:31AM respectively todaY
I’m only just getting adjusted back to cold temperatures, and am not looking forward to snow possibly falling Wednesday night and another storm bringing wintry precipitation Friday. On the bright side, we are now only about 3 weeks off from the vernal equinox and the start of spring!
On the Servicio Meteorológico Nacional (SMN) – National Meterological Service of Mexico
Since I was going to be in Mexico, I started checking out the Mexican government’s weather service page. Check out the surface analysis below that’s overlaid on what looks like a GOES East image from last Thursday (02/21/2019).
Click to enlarge this surface analysis from the Mexican SMN
Unlike our own National Weather Service, the SMN numbers frontal systems that move through Mexico. Notice the stationary front in the center of the image is labeled as “Frente No. 38” (Front #38) and you can see “Frente Frio No. 40” (Cold Front #40) crossing from southern California into northern Baja California in the upper left corner of the image. They also number their winter storms. The “B” (representing a low pressure center) over Nevada is labeled as “Octava Tormenta Invernal” (Eighth Winter Storm). “Corriente en Chorro Polar” (polar jet stream), “Corriente en Chorro Subtropical” (subtropical jet stream) are familiar features to us, which we seen streaking across the northwest and central portions of Mexico respectively. A “Corriente de Bajo Nivel” (low-level jet) is seen flowing from the east towards the Yucatan. Here’s a translation of the text in the lower left panel:
Systems affecting Mexico The Eighth Winter Storm over the southwestern US combined with cold front #40 in northwestern Mexico will favor showers with some strong storms, very cold temperatures, and wind gusts over 60 km/h in the northwest and northern Republic, and also the potential for snow or sleet in mountainous areas of Baja California, Sonora, and Chihuahua, extending gradually towards Durango. Front #38 with stationary characteristics extends over the western Gulf of Mexico and will generate clouds with isolated rain in the eastern and northeastern parts of the country.
Servicio Meteorológico Nacional of mexico
Synotpic Conditions – the Tropical Big Picture
The consistent breezy south-southeasterly winds I felt on Isla Mujeres were tied to that low-level jet (LLJ) pictured above. This LLJ enhanced the general easterly trade winds in the area. This was a result of the influence of a high anchored over the Western Atlantic, and a low over northern Colombia pictured in the OPC surface analysis below (issued Friday 2/22 02:35Z) “funneling” the winds.
In this analysis of the Western Atlantic from NOAA’s Ocean Prediction Center, you can see that a broad high pressure center was anchored near Bermuda. Meanwhile, a low sat over northern Colombia. The clockwise flow around the high and counterclockwise flow around the low in proximity to each other act to enhance the easterly trade winds found in the tropics.
A
sounding from Philip Goldson International Airport near Belize City
(the closest sounding station I could find to Cancun) showed clear
evidence of a well-mixed layer from the surface to just about 900 mb. It
felt like in Cancun, this mixed layer extended a bit further up into
the 850 mb level where the LLJ sat because the winds were stronger.
Sounding taken above Philip Goldson International Airport near Belize City at 7PM Feb. 20, 2019, showing a well-mixed boundary layer representative of the area around the Yucatan Peninsula during these few days
By
way of brief explanation, well-mixed layers like the one shown above
provide favorable conditions for faster moving winds aloft to transfer
their momemtum downwards, in this case all the way to the surface. It
shouldn’t be a surprise that a deep well-mixed layer also existed today
over NYC – enabling the strong winds aloft to mix down, leading to some
very strong winds and gusts.
Sounding from Upton, NY for at 7AM Feb. 25, 2019, showing a deep well-mixed layer down to the surface from around 800 mb with strong winds 35-50 knots through most of this layer
Aside
from the winds, the weather followed a pretty standard tropical pattern
with clouds building in the afternoon and isolated showers. Despite how
flat the Yucatan Peninusla is, it nevertheless provides at least some
small potential for lift and convergence for air flowing off the
Caribbean Sea. This is because there’s actually a significant difference
in frictional properties of land and water, which makes sense since the
surface of the ocean is considerably “smoother” than the corresponding
forested Yucatan. One other notable trait was that the base of rain
clouds in the area took on a distinctively blue hue, which I imagine was
a reflection of the characteristically blue waters of the Caribbean
Sea.
A weak cold front passes through to start the weekend. a decent weekend is in store with a chance for snow showers on Sunday night. Temperatures will be about average for this time of year through Presidents Day. We have a chance for more significant winter storm mid-week next week although details are fuzzy. At this time, thermal profiles at the coast suggest a mixed event like this past week with snow transitioning to sleet, freezing rain and plain rain. This event looks longer in duration due to a slow moving high east of the incoming storm. I will be on vacation next week and won’t be posting until the following week.
Rest of today – overcast with temperatures in the low-50s. Slight chance for rain with a weak cold front passing later in the afternoon. Overnight lows in the mid-30s.
Weather Prediction surface forecast for 7PM Friday
Saturday – mostly sunny with highs in the low-40s. Overnight lows around 30°F.
Sunday – mostly sunny during the day with highs around 40°F. Overnight lows in the low-30s will support some light snow associated with a broad area of low pressure. Not anticipating much accumulation beyond a coating.
Monday (President’s Day) – imoroving conditions. Partly sunny with highs in the low-40s and overnight lows dropping into the low-20s.
A mild start to this weekend will transition quickly to colder temperatures more typical of this the of year. A windy day in store on Saturday in the wake of a cold front bringing us rain today. cold temperatures remain in place into the beginning of next week. No more 60°F+ days in store for us!
Rest of today – rain tapering off by around lunchtime. Mild temperatures with highs in the mid-50s with clearing conditions. Once the cold front sweeps through, winds will pick up from west into the 15-20 mph range. This will advect cold air into the area and overnight lows are forecast in the mid-20s. By midnight, temperatures will be around the freezing mark.
Weather Prediction Center’s surface forecast for 7PM tonight
Saturday – a windy day on tap with much colder high temperatures in the mid-30s. Winds expected in the 15-20 mph range with strong gusts around 35 mph. Overnight lows again in the mid-20s.
Sunday – winds abate as the low responsible for the rain today moves further off. High temperatures in the mid-30s with mostly sunny skies. Overnight low around 30°F, with a small chance of snow showers.
Monday – increasing clouds ahead of the next storm system that could bring us some wintry weather Tuesday. High temperatures in the low-40s. Overnight lows in the upper-30s.
Last Wednesday, a strong Arctic front swept across much of the Northeastern US, impacting many areas with a line of heavy snow showers, then ushering in record-breaking cold. The line of heavy snow immediately preceding the frontal boundary set off Snow Squall Warnings, which many readers would have seen on their mobile devices. The Snow Squall Warning is a new type of warning that went live nationwide on November 1, 2018. I believe that last Wednesday’s event was the first time National Weather Service forecast offices issued this new warning type for a widespread frontal snow squall. In this post, I’ll share my thoughts on the new warning type, and some observations about the event itself.
The New Snow Squall Warning
The relatively new snow squall warning product is, like other existing warnings, an effort by the National Weather Service to inform the public about imminent hazardous weather impacts. The main motivation behind this new warning type is to try and reduce the number of potentially fatal multi-vehicle accidents that can occur in snow squalls. Snow squalls can cause these kinds of accidents because the intense snow and wind in them can rapidly reduce visibility to near whiteout conditions with little advance warning. The heavy snow can also result in quick accumulations that make driving even more dangerous. Snow squalls can occur along frontal boundaries, like what we saw last week, but they can also be isolated or form in conjunction with lake effect snow. Although I haven’t as yet seen a clear-cut definition of what triggers this new warning, the criteria I have seen tie in directly with the hazards mentioned above: visibility less than 1/4 mile (whiteout conditions), strong wind gusts (above 35 mph, it appears), heavy snow, and surface temperatures below freezing.
Archived text of the snow squall warning issued for NYC by the local National Weather Service. Credit: Iowa State University‘s archive tool for NWS warnings.
Snow squall warnings are functionally similar to severe thunderstorm warnings, which makes sense because snow squalls and severe thunderstorms share some sensible weather impacts and meteorological properties. In this case, with a frontal snow squall, my professor and seasoned forecaster Steve Corfidi observed that “Essentially, a winter cold-frontal snow squall band is simply a summer cold frontal squall line with its bottom two-thirds or so chopped off. For all practical purposes today you simply experienced the passage of a narrow, fast-moving band of convective cirrus!”
Snow Squall Event in NYC
The snow squall that hit NYC last week was associated with a strong Arctic front. As I’ll discuss below, this frontal boundary provided the necessary lift to generate a narrow band (along the east-west dimension) of heavy snow along much of its length. Light snow began falling around 3:30PM by my estimate. The intensity of the snow picked up moderately over the next 15 minutes. However, it wasn’t until close to the end of the event that snowfall rates truly kicked into high gear, along with the winds. During a span of about 5-10 minutes, as the snow and wind rapidly picked up, visibility dramatically decreased, with scenes like the one below typical.
Storm Prediction Center’s Mesoscale Discussion graphic showing the general set up of the snow squall event
Image taken looking north along Sixth Avenue from the 12th floor of my office building at approximately 3:50PM. The building that is barely visible as a dark grey blob in the middle is about 400 feet (2 blocks north), showing you just how bad visibility was at the peak of this snow squall.
Once the worst of the snow squall cleared, conditions rapidly improved, with visibility recovering quickly and precipitation ending rather abruptly. Following the passage of the Arctic front, forced subsidence with the much colder and denser air behind the front sinking to the surface helped mix down some very strong wind gusts, and helped usher in some of the coldest air of the season.
Why the Squall Seemed to Peak at the End
The snow squall started off as a few flurries, and for most of the duration of the event, it seemed like that was all we’d get. Then, within a very brief span, the squall peaked in intensity, and as quickly as it had peaked, it was over. So, why did this event appear to unfold this way to us as observers on the ground? It all has to do with the profile of the winds above surface relative to the Arctic front and the squall line.
A sounding collected at KPIT (Pittsburgh) earlier during the day, close to the time that the same snow squall line was progressing through Pennsylvania. This sounding is thus a reasonable approximation for what the atmosphere looked like over NYC later in the afternoon.
In the sounding above, we can observe that the wind barbs on the right side of the sounding are generally increasing in speed up to 600 mb – triangles represent 50 knots, each full tick represents 10 knots, and half a tick is 5 knots. At the surface, winds were west-northwest at 10 knots, but at 600 mb, winds were at 90 knots! Quite a difference. The second thing to note is above 850 mb, the wind barbs are oriented roughly at the same angle, indicating winds from the same direction at these levels (west-southwest) . This is what forecasters refer to as a “unidrectional” wind profile. The result here is that we had a set up where there was significant vertical speed shear. This has tangible effects on the structure of the clouds/convective activity within the snow squall, as shown below.
Simplified illustration showing the scenario during the snow squall
In the diagram above, as the leading edge of the Arctic front progresses, the air ahead of it is mechanically lifted above the dome of cold air behind the frontal boundary. Once the air reaches the LCL (lifting condensation level), it’s saturated and clouds begin to form. In this case, there’s enough lift and available moisture that precipitation begins to fall. Temperatures at the time supported all snow. Lift provided by the front would continue allowing the clouds to grow until they hit a stable layer – I won’t go into specifics about this but suffice to say that at this point, the cloud can’t keep growing vertically. This results in the cloud spreading out horizontally, creating an “anvil”. Because the wind speed is so much faster at this level, the anvil is sheared away from the direction of oncoming wind producing a “leaning” effect.
This radar image from the Newark TDWR (Terminal Doppler Weather Radar) at 2:58PM last Wednesday has a good depiction of the light snow falling ahead of the main squall line that’s coming from the sheared anvils of the main convective line. Note the scattered lobe of light snow ahead of the solid band of darker blue hues indicating heavy snow.
This is a diagram showing the effect of the speed shear on the evolution of the snow squall line’s cloud structure.
Because the winds were coming from the west-southwest, the anvils leaned in the opposite direction to the east-northeast. For us on the ground, that meant the light snow preceded the heart of the action, “the worst of the storm” that was closer to the leading edge of the Arctic front itself. Had winds aloft been weaker, or from a different direction, suppose more parallel to the frontal boundary itself, the contrast between the light and intense snow wouldn’t have been as dramatic.
Thoughts on Improving the Snow Squall Warning
This was the first widespread use of this new warning product, and it’s not surprising that this led to some confusion. I had several people ask me when the warning was issued and the snow started “How long is this going to last?”. Some people even did the exact opposite of what the warning is intended to prevent: they rushed out to “beat the snow” since it started off light and they didn’t realize it would be over in a short span of time.
I think that these warnings could be improved if a specific duration of the event were mentioned in the warning text, something along the lines of “Expect snow squall conditions to last between 30-45 minutes”. Some other weather forecast offices issue warnings with such text. As discussed above, frontal snow squalls are similar in nature to their warm season relatives. While people are used warm season convective activity ending pretty quickly, intense snow squalls here are often caused by the mesoscale bands accompanying Nor’easters. These can last several hours. In general, many winter weather warnings are long duration, which I believe contributed to some of the confusion that people had about this new type of warning.
