A quick hitting storm system brings possible light snow to the region overnight into Wednesday. High pressure builds behind this storm, with winds shifting to the northwest bringing a cool down midweek. Another storm approaches towards the end of the week. Ahead of this, temperatures surge back above normal, approaching 60°F, as southwesterly winds instigate warm air advection.
Rest of today – increasing clouds with high temperatures in the mid-40s. A coastal low passes east of the area, bringing some potential for light snow accumulations (1″ or so). Precipitation will start off as rain, though, then transition to snow briefly overnight. Overnight lows drop to around 30°F.
Weather Prediction Center surface forecast for 7PM Tuesday
Wednesday – snowfall may be enhanced by a second fast-moving arctic front. Behind this front, high pressure builds. Decreasing clouds, with winds picking up from the west then veering northwest towards overnight hours. High temperatures in the low-40s. Overnight lows will plummet quickly into the upper-20s with an Arctic air mass and northwesterly flow. Winds may gust above 30 mph during the overnight hours.
Thursday – expecting a calm, bright, sunny day with high temperatures in the low-30s, below normal for this time of year. High pressure will be moving right over us during this time. Overnight lows in the low-30s.
GFS model 500 mb height anomalies for 1AM Friday. Note the strong positive height anomaly over the Northeast, with a corresponding strong negative height anomaly over the Rockies.
Friday – high pressure will move offshore to the east, setting up warm southwesterly return flow between the western periphery of the high and an incoming storm system. High temperatures expected in the upper-40s. Temperatures should in fact warm up overnight, through the upper-40s.
A soggy and messy start to the week will give way to a blast of Arctic air, leading to an extended period of below normal high temperatures only in the 30s for the most part. Temperatures do look to rebound over the weekend towards more seasonable levels. The next chance for precipitation happens Wednesday as an arctic front, harbinger of these much colder temperatures, crosses the area and brings possible snow squalls.
Rest of today – periods of rain during the day with high temperatures in the mid-30s. Rain tapers off and ends early this evening as the low pressure responsible for the stormy conditions moves off to the east. Overnight lows drop to around 30°F.
GFS model 500 mb height anomalies for Wednesday at 7PM, note the strong negative height anomaly over the Northeastern US and St. Lawrence Valley
Wednesday – should be a mostly sunny day, though as an arctic front hits us late in the day and in the early evening, a few snow squalls/snow bands could hit. These would be brief in nature but could impinge on visibility, making travel difficult. High temperatures should be similar to today in the mid-30s. Overnight lows will plummet quickly into the upper-teens with an Arctic air mass and northwesterly flow.
Thursday – northwesterly flow continues as strong Arctic high pressure builds. This will keep temperatures only in the upper-20s for highs under mostly sunny skies. Overnight lows in the low-20s.
Weather Prediction Center surface forecast for 7AM Friday
Friday – temperatures moderate a little bit, with highs in the low-30s as the high starts moving east. Overnight lows in the low-20s.
A quiet, though colder than normal week lies ahead after the departure of Monday’s storm. This storm ended up not bringing the city much of any accumulating snow, though the story was quite different further north. High pressure settles in briefly before a weaker disturbance affects the area later in the week. An arctic front passes Friday, bringing a reinforcing shot of cold. Temperatures during this time will range in the low-40s.
Rest of today – mostly sunny with high temperatures in the low-40s. Winds diminishing as the low that hit us yesterday continues to pull away to the northeast. Overnight lows around the freezing mark.
Weather Prediction Center surface forecast for 7PM Wednesday
Wednesday – partly sunny with high temperatures in the low-40s. Overnight lows in the mid-30s.
Thursday – mostly sunny with high temperatures in the low-40s, and overnight lows in the low-30s.
Climate Prediction Center temperature outlook for next week. A warm up is coming after this week of below normal cold.
Friday – mostly sunny with high temperatures in the low-40s and overnight lows in the mid-20s behind the passage of a dry Arctic front.
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.
Get ready for record-breaking cold and a blast of deep winter tonight. The cold spell slowly abates over the course of the weekend, but even by Monday, high temperatures are still anticipated to be below normal for this time of year. There is a chance for precipitation on Sunday night with the passage of a weak disturbance.
Rest of today – we’ve already reached today’s daytime high of about 40°F and temperatures will be falling throughout the rest of the day. An arctic front is passing over us now, in its wake, strong cold advection (transport) is ongoing as a continental polar airmass from northern Canada dives down into the area. A high pressure center over Minnesota is also creating a fairly strong pressure gradient with a low pressure system in central Quebec. This will result in breezy conditions with winds from the northwest at around 25 mph and gusts over 40 mph. Wind chill values will fall from the 30s into the 20s.
Strong cold advection is ongoing behind the arctic front. Temperatures are dropping by as much as 2.5-3°F per hour in the Northeast.
Saturday (Veterans Day) – overnight lows going into Saturday are forecast to plummet to record-breaking levels in the low-20s. The high pressure pictured above will be moving closer to us, decreasing the pressure gradient and reducing the winds here. Temperatures should top out around 40°F with lots of sun. It will be a chilly, but sunny day for the Veterans Day Parade.
Sunday – temperatures continue to warm into the upper-40 with increasing clouds.
Monday – some uncertainty exists about the possibility of precipitation overnight into Monday. High temperatures will finally be approaching normal for this time of year with highs in the low-50s and mostly cloudy skies.
The record breaking warmth of only a couple days ago seems like ancient history. We could see some snow flurries to kick off the weekend, then a very cold airmass takes hold with temperatures well below normal. Another warm up is in store to start next week.
Rest of today – mostly sunny to clear skies to start. High temperatures near 40°F. Later in the day, and into the evening hours, an arctic front is forecast to pass through. Snow showers are anticipated to accompany this frontal boundary, though they should not result in any appreciable accumulation.
Saturday – behind the passage of this arctic front, winds shift to the northwest and pick up into the 15-25mph range, with stronger gusts. High temperatures will struggle to top freezing even with lots of sun, as overnight lows will be around 20°F. Wind chills will be in the teens or lower.
Sunday – overnight into Sunday, low temperatures are expected to drop into the teens. Sunny conditions prevail, with slightly warmer temperatures in the mid-30s.
Monday – a warm front will push through, finally breaking the spell of very cold weather. Temperatures should warm up into the upper-40s to near 50°F as clouds increase.
The weather headline for this weekend will be bitterly cold temperatures that may have you reconsidering any plans to be outdoors during Valentine’s Day. The incoming airmass will definitely be the coldest of the year and the season thus far. The good news for us is that this Arctic airmass does not stick around for long, with temperatures expected to rebound nicely by early-mid week next week.
Rest of today – will be a preview of the cold coming our way with temperatures only topping out in the upper-20s. Scattered snow showers are possible (some of these snow showers are actually remnants of lake effect snows occurring hundreds of miles to our north and west). With west winds blowing in the 20-30mph range, wind chill will make it feel like 15-20ºF out there.
Friday – an Arctic cold front swings through overnight into Friday. Overnight lows will be in the upper-teens in the city and in the low-teens further north and west. High temperatures should rest in the mid-20s with increasing cloud cover as another impulse of energy makes its way in for Saturday.
Saturday – there will be a slight chance for snow showers overnight Friday into Saturday. Low temperatures will be only in the mid-teens, and with winds expected to pick up, wind chill values will hover in the single digits. Daytime highs Saturday will struggle to break 20ºF in the city. Northwest winds 25-30mph will lead to wind chills again only in the teens. As an impulse of energy pushes through Saturday morning, there may be some scattered snow showers before skies clear and things dry out.
Sunday – overnight lows Saturday into Sunday are going to be the coldest we’ve seen all year, with a low temperature in the city forecast at a mere 2ºF! Wind chill advisories will likely be posted for this period, with widespread below zero wind chills likely. Temperatures don’t rebound much during the day Sunday, with high temperatures only in the upper-teens.
Monday (President’s Day) – this is when the recovery in temperatures begins with high temperatures expected to go back up to around freezing. The next storm system looks like it will be moving through Tuesday, but it will also be accompanied by a much warmer airmass.
Warming Up Next Week
40ºF doesn’t sound all that warm, but you’ll welcome that next week as we see a return to above normal temperatures.
The new year starts off with a blast of Arctic cold that will plunge the mercury down into numbers we haven’t seen since last winter. We will be seeing the coldest weather thus far this winter to start this week, but temperatures will rebound back into above average range mid-week. We won’t be getting any precipitation to go along with this cold air, so no snow still for us this winter.
Rest of today – parts of the area have likely already seen their high temperatures for the day. In the city, we’ll struggle to get above freezing today. An arctic front moved through, and is allowing for much colder air to enter the region. Some parts of eastern Long Island may actually see snow flurries today, but the rest of the region should see mostly sunny skies.
Tuesday – overnight lows will be downright frigid with temperatures at daybreak only in the teens for the city and single digits elsewhere. Couple that with stiff north winds and we’re looking at wind chill values in the single digits in the city. During the day Tuesday, north winds should slowly subside, but high temperatures will again only be near freezing.
Wednesday – the center of the high pressure responsible for bringing in the cold air will be close to us, meaning winds will become minimal, and will begin to shift from north to southwest as the high pressure continues moving to our east. This will allow for daytime highs to reach back up into the low-40s, which is 6-8ºF above normal.
Thursday – looking like a very similar day to Wednesday, with high temperatures again in the low-40s and with sunny skies.
The low overall snowfall totals for NYC yesterday wasn’t so much a problem as the significant icing that resulted from an extended period of freezing rain. Ironically, a week the city government took criticism for shutting down the transit system for blockbuster blizzard that did not come to pass here, it was 1/4″ coating of ice that wreaked havoc on the entire transit system (the hours long complete suspension of the 7 train was an extreme example of this). We do get a couple more shots at snow, once tomorrow night into Thursday, and then again during the weekend, but at this time neither event looks to be nearly as high impact as what we’ve experienced the last two weeks.
Wednesday – will be comparatively mild in the context of the rest of the week and the weather we’ve had recently. High temperatures may actually even hit 40 (slightly above normal). However, as a cold front approaches from the west, we’ll have an increasing chance for snow showers, which maybe mixed with rain at the coast during the earlier part of the evening.
Thursday – chance for snow showers continues along the vicinity of the frontal boundary as it passes over us. Following the passage of that cold front, temperatures take a serious drop as northwest winds allow for Arctic air to make a comeback into the area. Lows overnight Thursday could be as low as the single digits for the city and below zero for outlying suburbs. Coupled with a northwest wind of 15-20mph, this would produce dangerously cold wind chill values.
Generally low snowfall totals expected across the region from this event
Friday – high pressure builds in briefly for Friday, giving us the only guaranteed dry day through the forecast period. It will still be cold with highs only in the low 20s.
Saturday – an arctic front will be approaching during the day Saturday, and ahead of the front, we’ll actually about normal temperatures in the upper 30s due to southwest flow. Clouds build in and the chance for snow showers builds during the later part of the day into the overnight.
Sunday – the arctic front mentioned above moves over us but then it looks like it will stall in our vicinity for an extended period from Sunday into Monday. High temperatures will cool to around freezing in the wake of that frontal passage.
At this time, it’s too early to know exactly how much snow we would pick up. It does appear that the precipitation with this system will be generally on the lighter side, except as the low pressure center depicted over southern Illinois in the graphic below approaches the coast. Once it arrives just west of the coast, it could draw in some moisture off the ocean as the counterclockwise cyclonic flow around the eastern periphery of the low induces an extended period of onshore flow (like a sea breeze, except in this case, the air over the water is actually warmer than it is over the snowpack on the land). If this onshore flow is robust enough, it could warm coastal areas up enough to get some rain out of the storm.
Stationary arctic front
You can see the distinct separation between warm and cold airmasses demarcated by the stationary front depicted in the image above – the front separates the areas with the blue dotted lines from the areas of the warm dotted lines.
On the heels of the nor’easter earlier this week, we get another chance at snow tomorrow night into Friday, albeit of a much lighter intensity. Things get downright frigid after that with low temperatures Friday night in the single digits in the city, and wind chills below zero! The cold locks in over the weekend before we get another chance at precipitation to start off next week, continuing what has been an active weather pattern.
Thursday – clouds increase from west to east, with high temperatures hovering just above freezing. Precipitation begins overnight, and may include some rain/snow mix at the coast (all snow inland), but with minimal accumulations.
We get clipped by a clipper system Thursday night into Friday, with both NAM and GFS models pointing to us being on the periphery of the heavier precipitation.
Friday – light flurries continue into the morning Friday before ending, skies clear quickly while temperatures begin nosediving from a high in the mid-30s during the morning hours to the upper teens by sunset. Winds also pick up from the north and northwest as the departing clipper system from above deepens and the pressure gradient increases between it and a high pressure center over the Midwest. The combination of the cold temperatures and winds will likely induce wind chills below zero, so bundle up before you go out there Friday night.
Saturday – the deep freeze continues, with high temperatures only in the low 20s Saturday despite plenty of sunshine.
Sunday – another day of decent conditions, with clouds increase, but still cold with highs in the upper 20s. Sunday night into Monday, we get another shot at precipitation, although it’s a bit too early to nail down the specifics of this upcoming storm (whether it’ll be rain/snow, rain, or just snow, and how much).
