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NYC Weather Bulletin – Jan 25, 2014 – Major Snowstorm Mon-Tue

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Yesterday’s Nor’easter will seem downright quaint by the time Tuesday morning rolls around. Just a couple of days ago, forecasters were not even clear if we’d get any precipitation Monday going into Tuesday. Now, words like “historic” are being used to describe what will undoubtedly be the biggest snowstorm of the season so far. When all is said and done, we could be looking at widespread snow totals on Tuesday of 18″ to as much as two feet of snow. During the peak of this much stronger Nor’easter, high winds and blowing snow could lead to an extended period of blizzard conditions near the coast.

The Setup

Forecast models that up to just 2 days ago were displaying significant differences as to how this system would transpire are now in close agreement on a classic setup for a strong Nor’easter, and a powerful, high-impact blizzard for the Northeast.

A relatively weak clipper-type low will be moving across the Midwest during the day today. By Monday evening, a secondary low will form from the remnants of this first system. This second low will rapidly intensify as it tracks on or just west of the 40ºN 70ºW benchmark early Tuesday morning.

NAM model output for the next couple of days
NAM model output for the next couple of days

lowtrack_ensembles

As the low bombs out (dropping more 30mb in less than a day), its forward progress will be slowed by a high pressure to the north. This high pressure over Quebec is a key difference between the setup of the storm on Saturday and this coming Nor’easter. In contrast to Saturday’s storm, this high pressure will allow a relatively deep layer of cold air to take hold prior to the arrival of this storm. As the low pressure continues moving northeast, the pressure gradient between the rapidly intensifying low and this high pressure will allow for strong winds to affect the area from first from the east, then northeast and finally north.

gfs_namer_042_1000_850_thickThis combination of a slow moving, strong low pressure with ample cold air and moisture available will translate to widespread snowfall totals of over a foot, and close to 2 feet, primarily for the Hudson Valley and points east. This will primarily be due to the influence of a mesoscale deformation banding feature is expected to form north of the storm center, then slowly wrap around the storm as it moves northeast. Because the forward motion of the storm will be so slow, the banding feature (which could contain snowfall rates of 2-4″ per hour) could be sitting nearly stationary over large portions of the region for a prolonged period of time – thus leading to what could be record breaking totals.

With the wind expected to pick up during this event, there is a blizzard watch in effect for coastal areas of the region, where wind sustained winds (25-35mph) could lead to near zero visibilities while heavy snow falls.

Snowfall Totals

As you can see from below, almost the entire region is expected to pick up more than 12″, with eastern areas possibly picking up near 2 feet!

StormTotalSnowFcst

Looking at probabilities of snowfall totals below, you can see that there’s now at least a 70-80% chance a good portion of the NYC region receiving more than 8″ of snow.

Screen Shot 2015-01-25 at 1.38.03 PM

This storm will also have ample moisture to draw off of, with the liquid water equivalent of between 1.5″ to over 2″ across a good part of the region. Using the standard 10-1 ratio of snow to liquid water, this would translate to between 15-20″.

Quantitative precipitation total forecasts from the Weather Prediction Center
Quantitative precipitation total forecasts from the Weather Prediction Center

Timing

Snow will begin Monday, with increasing intensity through the day. Winds  from the northeast will also pick up during the day Monday with high temperatures only in the mid-upper 20s. Anywhere between 1-3″ of snow could fall across NYC.

The heaviest snow will be taking place overnight Monday into Tuesday morning, where parts of the area could pick up another 9-13″. Again, the heaviest snow will be occurring within the mesoscale banding feature, where intense snowfall rates of 2-4″ an hour for a prolonged period of time could even be accompanied by thundersnow. Winds will probably peak during this overnight period.

Snow will continue to fall during the day Tuesday, with another 4-8″ of snow possible. High temperatures will again only be in the upper 20s.

Rest of the Week

Wednesday – snow finally ends Tuesday night with conditions clearing by Wednesday, but high temperatures again struggling to reach the upper 20s.

Thursday – a slightly warmer day with high temperatures in the low 30s.

Friday – there’s a chance at some more precipitation (but much lighter) from a weak low pressure system moving through Friday.

 

NYC Weekend Weather – Nor’easter Update – Jan 23, 2015

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The details are starting to come together for tomorrow’s Nor’easter. Over the past day, forecast models have been pointing to a faster moving system, such that the onset of precipitation is expected to begin overnight tonight, with the bulk of the precipitation ending by early afternoon tomorrow. This storm is expected to throw a little bit of everything our way tomorrow, making for a nasty day overall, with snow changing over to freezing rain or sleet, then perhaps a period of plain rain, followed by a possibility of a rain snow mix.

What to Expect

Snow, moderate to heavy at times, beginning overnight tonight. Winds picking up from southeast, then eventually moving to the northeast and north during the day Saturday. Snow transitioning to a period of possibly freezing rain or sleet during the morning hours, then a period of rain around midday and a possible rain/snow mix towards late afternoon before precipitation gradually tapers off later in the evening. High temperature right around or slightly above freezing.

Forecast Storm Track

Forecast models are in generally good agreement that the center of this Nor’easter will be passing just inside the 40ºN 70ºW benchmark sometime in the late afternoon Saturday (approx 4-5PM).

Without a high pressure center to the north supplying cold air to the area, this setup will allow for enough warm, moist air to circulate around the eastern side of the low pressure center to give us a mix of precipitation throughout the day. The extent to which snow can be sustained will be strongly dependent on dynamical cooling, and how quickly cold air can be pulled in on the western side of the low pressure center.

Storm track forecast from the Weather Prediction Center, with clusters indicating forecast positions of individual ensemble members.
Storm track forecast from the Weather Prediction Center, with clusters indicating forecast positions of individual ensemble members.

gfs_namer_027_1000_850_thick

Snowfall Forecast

As you can see below, the probability of more than 4″ of snow is quite low for coastal areas, especially Eastern Long Island, where mainly rain is forecast. The heaviest totals (no more than 5″, most likely) will be concentrated well north and east of NYC where temperatures are expected to be cold enough to sustain all snow for longer. Because the forward speed of the storm is now predicted to be faster, the chances of NYC getting any significant snowfall (even 2″) is considerably lower. Most of the city is in a swath of between 30-40% chance of getting more than 2″ of snow.

Probability of greater than 2" of snowfall
Probability of greater than 2″ of snowfall
Probability of greater than 4" of snowfall
Probability of greater than 4″ of snowfall

Ice Accumulation

The probability of us receiving a trace, but a measurable amount of ice from freezing rain is quite a bit higher than the probability of receiving snow, with most of the city in the 40-60% range.

Probability of ice accumulation greater than .01"
Probability of ice accumulation greater than .01″

Sunday – the storm will be long gone by Sunday, and we’re expecting to hit highs in the mid-upper 30s.

Monday – there is some potential for a round of all snow on Monday, but forecast models have not been in good consensus on how this will play out. A clipper system is expected to pass through, though it is unclear whether this system will spawn a secondary low offshore. In the latter case, we could see some significant snow, while if it’s just a clipper, we probably won’t see more than a couple inches.

NYC Weekend Weather – Jan 22, 2015 – Nor’easter Coming

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All eyes turn towards the imminent Nor’easter that will be bringing significant winter weather impacts to the NYC metro region. Many variables remain unresolved that could affect the eventual outcome, but at this time, it looks like we’re in for a nasty combination of freezing rain, rain, and snow during the day Saturday. At present, forecasts call for somewhere between 2-4″ of snow in the city along with some ice accumulation. Precipitation should fall as all snow north and west of NYC, however, overall accumulation should be slightly less than in the city itself since the most intense precipitation will remain near coastal areas.

Friday – we get a pleasant, sunny, and slightly above average day with high temperatures in the mid-upper 30s.

Saturday – a tricky forecast for Saturday. What we know for sure, with different forecast models in relatively close agreement, is that an area of low pressure now forming over the Texas Gulf Coast will track over the Southeast, and then offshore of North Carolina by Friday night/early Saturday. Once the low moves offshore, it will rapidly intensify, and then track right on or very close to the 40ºN 70ºW benchmark by Saturday afternoon/early evening.

nam-hires_namer_057_1000_850_thick

The uncertain part about this scenario has to do with the thermal profiles in place. With no high pressure over us or to the north pumping in cold Canadian air, the depth of cold (sub-freezing) air will be relatively shallow.

As the Nor’easter approaches us, the cyclonic flow around its center will wrap in some relatively warmer air in from the ocean and points south. This makes for a complicated picture – with warmer air aloft (see animation below) during parts of the day, and below freezing air nearer to the surface throughout points in the day, the recipe is set for at least some periods of freezing rain – precipitation that falls as rain, and then freezes on contact with a sub-freezing surface.

During the middle portion of the day, temperatures should be warm enough at the surface to support plain rain for some time, before transitioning back into freezing rain, rain/snow mix, perhaps with a period of all snow before tapering off Saturday evening.

output_QGEuaq
NAM output of temperature profiles at about 4,800 feet between 10AM and 7PM Saturday. Notice the light pink indicating temperatures between 0ºC and 10ºC, which only gives way to below freezing temperatures (light blue, indicating 0ºC to -10ºC) later in the time frame. The center of the Nor’easter is located within the middle of the set of concentric rings you see near NYC.

nam-hires_namer_057_925_temp_htDeviations in the storm track to the east would result in more cold air being pulled in from the action of northerly winds on the western side of the low pressure center’s cyclonic flow (less freezing rain/rain, more snow), while deviations of the track west would result in warmer air being in place (less snow, more rain). So, even a slight adjustment in the storm track could alter the thermal profiles, and subsequently what kind of precipitation we see.

Sunday – temperatures overnight Saturday will be below freezing, which would allow anything that fell and melted to refreeze. Things do clear up Sunday, with highs about average in the mid-30s, and with clouds diminishing.

Monday – there is a chance for yet another round of mainly snow, potentially heavy, as another coastal storm is possible. Highs will be close to the freezing mark, making this an all snow event if it should pan out.

 

NYC Weather – Pre-Thanksgiving Storm Back On

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It’s hard to believe now, with air temperatures still hovering around 60 at this hour on Monday, that by Wednesday, our entire region could be looking at accumulating snow! Over the weekend, it has become clear through progressive model runs that the mid-week storm that on Friday looked like it would miss us, is tracking to hit us on Wednesday, leading to a high impact storm just in time for the busiest travel day of the year. We’re talking the whole package: wind, rain, and snow.

Tuesday – we continue to stay in the warm sector of a low pressure system that will be exiting to our northeast through Quebec. Cloud cover will be minimal and we should see temperatures hold around the 60 degree mark.

Wednesday – a complex weather situation develops as a coastal low that forms offshore of the Georgia/Florida border Tuesday makes its way up the Northeast coast. Temperature profiles seem to be pointing towards a mostly snow event (the first big snow storm for the region). With the cold front pushing through late Tuesday, temperatures Wednesday will likely peak in the morning around 40 and start to drop slowly through the day as precipitation moves in across the area, generally from southwest to northeast. Wind will start to pick up, first from the southeast, then east and gradually becoming northeast between 25-35mph.nam-hires_namer_048_1000_850_thick

The tricky part about this forecast is that there is still enough uncertainty in the storm track for there to be some last minute changes. As it currently stands, the center of the strengthening coast low is forecast to move on top of or just west of the 40ºN 70ºW benchmark, a scenario that favors a high-impact nor’easter type event for our region. Winter storm watches are already up over much of the interior Northeast. In these areas, we are looking at anywhere from 4-10″+ (depending on where the heaviest band sets up). Along the coast, under the current forecast track and methodology, it is looking like enough rain mixes in during the day Wednesday to keep snow accumulations from topping 5-6″.

Again the storm track could change, and if the track moves slightly to the east, the heavier snow could hit the city, along with some colder air. Regardless, this does not bode well for people who are doing last-minute traveling for Thanksgiving. I would expect widespread flight delays, while traffic conditions on the road could deteriorate quickly especially later in the day with temperatures dropping and snow beginning to stick.

Thursday – Thanksgiving Day itself looks like it will be OK for the parade, and other activities. The coastal storm will have pulled to our northeast, and any lingering effects should clear up early. Temperatures will be around the 40 degree mark in the city, which means that snow which accumulates will not melt all that quickly.

Friday – will be the coldest day of the holiday stretch, with temperatures only in the mid-30s. Areas of snow that have melted during Thursday could re-freeze.

The remainder of the holiday weekend looks good so far, with no major precipitation anticipated until Sunday/Monday.