Monthly Archives: December 2020

NYC Weather Update – Dec 29, 2020

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Messy weather ahead going into the New Years holiday for NYC. We start with high pressure moving overhead. This then gives way to a frontal system tomorrow night. After a brief lull, another storm, similar in some ways to the Christmas storm comes in Friday. Both these precipitation will be rain in the city. Temperatures during this period will generally be above normal in the 40s, warming into the mid-upper 40s.

Rest of today – partly cloudy with high temperatures around 40ºF. Clouds will be streaming in from the northwest, originating from the Great Lakes as lake effect snow showers. These clouds will tend to thin out as northwest flow downslopes dries out across the higher terrain north and west of the city. This northwest flow could also result in some strong wind gusts. With strong high pressure forecast to move nearly directly overhead tonight, winds should calm with skies becoming mostly clear. This will favor good radiational cooling, leading to cold overnight lows in the mid-20s.

GOES Geostationary Satellite visible imagery this morning showing clouds streaming off the surface of the Great Lakes towards the East Coast.

Wednesday – partly cloudy with high temperatures again around 40ºF. Overnight, a warm front passes, followed by a cold front. This will result in an increase in temperatures overnight from the mid-30s as well as rain.

Thursday – a mild day despite the passage of the cold front mentioned above, with forecasters noting the air mass behind the front being of milder Pacific origin. High temperatures around 50ºF. Rain chances should diminish quickly after the morning passage of the cold front. The respite will not last too long as the next storm system follows up quickly. Overnight temperatures dropping into the low-30s.

  • ECMWF “Euro” model 24 hour quantitative precipitation forecast ending 7AM Saturday, about .70″.
  • GFS model 24 hour quantitative precipitation forecast ending 7AM Saturday, close to 1″.

Friday – high temperatures in the mid-40s with another storm system approaching. Rain chances increase going into the overnight hours. This storm, like the Christmas storm, is expected to deliver some moderate to heavy rain. However, since it’s forecast to be weaker overall, the winds and precipitation totals shouldn’t be nearly as robust as the last storm. Overnight should shape up to be similar to Wednesday, where temperatures may actually increase into the upper-40s with the passage of a warm front and corresponding warm air advection.

NYC Weekend Weather – Dec 18, 2020

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We could see the coldest overnight temperatures this season tonight following the big snowstorm earlier this week. High pressure builds in Saturday, though colder than normal temperatures remain in the low-30s. Temperatures start to rebound to the normal range in the low-40s next week. There’s a chance of stray showers Sun, otherwise a dry forecast. Possibility for more showers Tuesday, though uncertain at this time. Warming trend continues and and chances for precipitation increase later into next week.

Rest of today – partly cloudy with high temperatures in the low-30s. Overnight lows dropping to as low as around 20ºF with high pressure building leading to calm winds and good radiational cooling. This is only further enhanced by the presence of relatively fresh snowpack that helps insulate the ground from radiating heat upwards.

Weather Prediction Center’s surface forecast for 7AM Saturday, depicting high pressure sitting over much of the eastern US.

Saturday – with a cold start to the day, temperatures will only be climbing into the low-30s again, though high pressure should provide us with lots of sunshine. Overnight lows “warm” up to around 30ºF with return flow from the southwest as high pressure starts exiting east.

Sunday – a weak shortwave moves through, and could bring a couple of scattered showers. However, the associated frontal boundary will be weakening as it approaches, so precipitation isn’t all that likely, so we may instead just get a partly sunny day. Temperatures should rise to around 40ºF with warmer southwesterly flow ahead of this disturbance. Overnight lows in the low-30s.

GFS model 500 mb height and relative vorticity at 1PM Monday

Monday – high temperatures rising to the low-40s, back into normal range for this time of year with partly sunny to partly cloudy skies. A stronger mid-level trough will be approaching at this time. This may portend some precipitation going into Tuesday. Overnight lows in the mid-30s.

NYC Weather Update – Dec 14, 2020

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A quick hitting storm passes south of the city today bringing some rain and mixed precipitation, with no accumulating snow expected. Colder temperatures then take hold in the mid-30s behind this storm Tuesday as a strong area of high pressure builds over Quebec. This sets the stage for a potentially significant snowstorm developing Wednesday into Thursday with 8″+ totals possible along the I-95 corridor (with higher amounts north and west). However, there is still a good deal of uncertainty with the eventual track of this storm that could result in large differences in outcomes.

Rest of today – temperatures dropping through the low-40s into the 30s with mostly cloudy skies and a steady, chilly rain, possibly mixing with some wintry precipitation. No accumulation is expected. Overnight lows around the freezing mark with precipitation ending as high pressure begins building in.

Weather Prediction Center surface forecast for 1PM Monday. An area of low pressure is seen exiting the east coast, bringing us a period of rain and possible mixed precipitation, with snow further inland.

Tuesday – proverbial calm before the storm as high pressure dominates, giving us a mostly sunny day with high temperatures around 40ºF. Overnight lows again in the upper-20s with cold northerly winds flowing around the east side of this high.

Wednesday – high pressure yields to a developing coastal storm. Partly sunny to start with increasing clouds. High temperatures in the mid-30s. Overnight lows drop into the mid-20s. The big story is the potential for a significant snowstorm with accumulations possibly over 8″. GFS and the Euro model have come into agreement for much higher totals, 14-15″. However, NAM shows a warmer scenario with warmer air working further north, thus cutting into overall totals (only 7″). It’s also possible that the low tracks close enough to shore that we end up with a good deal of rain mixing in at the coast, cutting snow totals even further.

Part of the energy fueling this storm is coming all the way from a shortwave with origins over the Pacific Ocean. This has only just made landfall in the US today. As this energy gets sampled by upstream soundings (weather balloon launches) and other means, the forecast models should get a better sense for how this storm will evolve. At this time, there’s enough uncertainty in the forecast to still not buy into any specific totals. If you look below, you can see there’s a lot of spread in the models in terms of the track of this storm. This then translates to a wide range in ensemble forecast totals. In addition, with a coastal storm like this, where there’s a lack of deep, Arctic cold air ahead of the storm, there’s a sharp gradient between areas where the event would be all snow vs. a mixed event. If you see the 24 hour probabilities of > 8″ accumulation, you can see that a 50 mile shift in the storm track further south and east would give us much higher chances for big snowfall totals, while a shift the other way could result in a bust on snow totals. Suffice to say, this storm will bear close monitoring today and tomorrow as more data comes in and models have a chance to digest it.

  • ECMWF “Euro” model surface pressure, precipitation and type for 1AM Thursday
  • ECMWF total snow accumulation at 7AM Thursday, showing 14″ of snow in NYC.
  • GFS model surface pressure, precipitation and type for 1AM Thursday
  • GFS total snow accumulation at 7AM Thursday, showing 15″ of snow in NYC.
  • NAM model surface pressure, precipitation and type for 1AM Thursday
  • NAM total snow accumulation at 7AM Thursday, showing 7″ of snow in NYC.
  • Weather Prediction Center probabilities of > 8″ total accumulation in 24 hours leading up to 7AM Thursday
  • Weather Prediction Center storm track forecasts, note the cluster of blue crosses associated with a surface low near the Delmarva Peninsula. This is the storm that could bring us snow.
  • GEFs ensemble model plumes showing a huge range of possible snowfall totals at LGA during this storm. The mean is 7.83″.

Thursday – depending on the speed and track of the storm, we could see precipitation lingering into the morning hours, with additional accumulation possible. Highs should hover around the freezing mark. Overnight lows drop into the mid-20s with reinforcing cold air advection on the backside of the departing storm.

NYC Weekend Weather – Dec 11, 2020

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A mostly sunny start to the weekend gives way to rain Saturday as a storm center passes north of us. A warm front pushes through then becomes stationary, allowing for a period of mild temperatures and light rain. The mild weather lasts through Sunday with high temperatures in the mid-upper 50s possible before a cold front sweeps through to start next week. Another storm passing off to our south cloud clip the region with rain Monday. Looking ahead, there’s potential for a coastal storm next week that could bring some snow/wintry precipitation to the area.

Rest of today – mostly sunny with clouds on the increase later, high temperatures in the low-50s with southwesterly winds circulating as a high pressure center slides off to the southeast. Overnight lows in the mid-40s with fog possible as moisture increases ahead of the next storm.

GFS model 10 meter above ground winds and surface pressure at 1PM Saturday. Note that winds will continue to be southerly, bringing us some milder air in stark contrast to the chilly weather earlier this past week.

Saturday – high temperatures in the mid-50s with warm, southerly winds continuing. Periods of showers likely especially during the afternoon, though a dry slot may work in and put a pause on rain before another round possibly overnight. Overnight lows will be in the low-50s, barely budging due to the ongoing moderating influence of southerly winds.

Weather Prediction Center’s surface forecast for 7AM Sunday.

Sunday – high temperatures rising potentially into the upper-50s ahead of the cold front accompanying this storm. Rain should end in the morning and give way to a partly sunny afternoon. Overnight lows dropping quite a bit into the low-40s behind the cold front.

Monday – high temperatures in the upper-40s with the chance for showers as a weak low passes off to our south. Overnight lows in the low-30s.

Coastal storm possible next week

GFS model runs suggest the possibility of a coastal storm that could bring some accumulating snow to the area mid-week next week. It’s far too early to tell if this scenario will pan out, as Euro models depict a warmer solution that would result in rain at the coast. We’ll see how subsequent model runs come together. Without a deep reservoir of cold air to draw on ahead of the storm, significant snowfall is less likely.

GFS model 24 hour snowfall accumulation ending 7AM next Thursday, with 10:1 snow to liquid ratio. Decent totals shown here are not a sure thing, especially this far out from the event.

NYC Weather Update – Dec 7, 2020

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A week of contrasts is in store for us with colder than average temps in the low-40s to start things off. However, a warming trend will take hold as a more zonal flow evolves at the upper levels, moderating heights and temperatures. This will result in milder conditions later on in the low-50s.High pressure and dry northerly flow should keep things rain-free until closer to the weekend.

Rest of today – below normal high temperatures around 40ºF with increasing clouds as a low pressure center passes well off to our southeast over the Atlantic. The presence of this storm offshore will result in persistent northerly winds, though less potent than over the weekend. Overnight lows in the low-30s.

Weather Prediction Center surface forecast for 7PM Monday. Note the presence of a low pressure center east of North Carolina.

Tuesday – high pressure briefly builds in and provides a mostly sunny day but with temperatures still only peaking around 40ºF. Overnight lows again in the low-30s.

Wednesday – high temperature start to rebound towards normal in the mid-40s with partly sunny skies as a storm passes well to our north, and we end up in the storm’s warm sector. Overnight lows drop into the upper-30s.

GFS model 500 mb temperature, height and winds forecast for 1PM Thursday. Flow at this level is forecast to become more zonal (flatter) than earlier this week.

Thursday – temperatures jump into the low-50s with sunny skies as high pressure briefly moves in again and upper level flow starts to flatten out resulting in the colder air mass in place to start the week getting pushed north. Overnight lows in upper-30s.

NYC Weekend Weather – Dec 4, 2020

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A strong nor’easter/coastal low will impact NYC tonight into Saturday. Conditions will deteriorate with the worst weather overnight into tomorrow morning. Gusty winds, soaking rains will accompany this storm. Temperatures will also drop as the storm moves off northeast and winds turn to the north and northwest, however, cold air wrapping around the low will not arrive in time to set the stage for accumulating snow here. Storm clears out by later Saturday. Afterwards, calm, seasonably cool temperatures (low-40s) are in store for next week.

Rest of today – temperatures topping out in the upper-40s with warm air advection/warm front moving up ahead of the main storm. Cloudy, with chances for rain increasing through the day, mostly on the lighter side. Overnight lows largely holding steady in the mid-40s again due to the influence of the incoming warm front. Rain expected to increase in intensity overnight going into Saturday morning.

  • ECMWF hi-res model surface precipitation/precipitation type valid 7AM Saturday
  • ECMWF hi-res model surface precipitation/precipitation type valid 1PM Saturday
  • GFS model surface precipitation/precipitation type valid 7AM Saturday
  • GFS model surface precipitation/precipitation type valid 1PM Saturday

Saturday – models are in pretty good agreement (see above images from GFS and ECMWF models depicting evolution of this storm during the day Saturday) that this coastal storm will evolve quickly and intensify as it tracks offshore southeast of the region. This will result in periods of rain throughout the first half of Saturday, along with increasingly strong winds as the pressure gradient tightens around the deepening low. As the low tracks northeast, winds will be turning from the south to the north and northwest. This will bring in much cooler air, although not cold enough to support accumulating snow at the coast. As a result, temperatures will be dropping during the day Saturday into the low-40s. Precipitation should end rather quickly in the afternoon. Overnight lows will be in the low-30s wit winds easing.

Weather Prediction Center’s forecast storm track based on recent ensemble model runs.

Sunday – sunny with high temperatures in the low-40s. Overnight lows around 30ºF.

Monday – similar day to Sunday with sunny weather and high temperatures in the low-40s. Overnight lows again around 30ºF.