Wintry mix on Monday Feb 22nd

As a very strong area of high pressure slides eastward, an area of low pressure will move through the Great Lakes region. Our area will remain in the warm sector of the front associated with this low, creating marginal temperatures for snow accumulation across most of the area. Precipitation will begin between 16Z and 18Z. The hi-res and regular NAM suggest the rain/snow line will remain along I-95, with some 1-3″ of snow accumulation possible across areas of Northern New Jersey between 18Z and 22Z on Monday February 22nd. Central New Jersey will remain right on the border of the rain/snow line, and a minor shift will favor a rain-based storm. The GFS is extremely similar, showing the rain/snow line along the I-95 corridor as well. Therefore, there is high confidence in where the rain/snow line will set up, but the ratios are uncertain. A wet snow with low snow to liquid ratios is expected along the I-95 corridor, so expectations for high accumulations should be kept in check south of I-78. Between 0.35″ and 0.5″ of liquid equivalent is expected from this event, as both rain and snow.

Once this storm passes, it will usher in a period of temperatures that are closer to normal for this time of year (44F highs). Tuesday will reach 40F, Wednesday will reach 45F, and so on.

Coupe

Long Range Forecast: Return to Normalcy

Following one of the coldest winter stretches across the continental United States in decades, warmer weather will return across much of the country following the historic Arctic displacement event that crippled Texas’s electrical grid. A ridge will build in the south between days 1-5 and a southeast ridge will become prominent in the 5-11 day range, bringing warm weather to the east. However, a trough in the west will ensure cold and unsettled weather in the intermountain west. A cold west and warm east pattern will take hold between days 5 and 13, which is something the Climate Prediction Center is also forecasting:

Ridging over Canada will bring warm weather to the northern interior of the country in the day 12-16 range. The NAO is expected to remain positive through March 1st, with only a few GEFS ensemble members forecasting a negative NAO into March.

In the 3-4 week time period, the weakened Madden Julian Oscillation (MJO) signal along with more positive trend in the Arctic Oscillation (AO) will allow for the influence of La Nina to dominate over the weather over the continental US for the next month. The only inconsistency with this analog compared to the dynamical models is the extent to which troughing spreads equatorward from Alaska down to the western coast of the US. This creates uncertainty in temperatures over the western United States that creates a fine line between average and below average temperatures. However, over the eastern United States, a southeast ridge will already favor warmer than average temperatures and greater troughing in the west will only increase the probability of warmer temperatures.

Consistent with this, the CPC week 3-4 temperature outlook (March 6 – 19, 2021) shows greater confidence for above average temperatures in the east compared to in the west:

After one possible light snow event on Monday (February 22nd), it appears that, unless we have another historic blocking episode that the dynamical models have not picked up on yet, our chances for another snowstorm are dwindling as we head into March.

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Historic Snowstorm for Central and Northern New Jersey

Although this storm was not particularly strong, the alignment of upper and mid-level features over New Jersey promoted intense mesoscale snowbands. This, in conjunction with the slow-moving nature of the storm, allowed it to dump over two feet of snow across the area. It is still snowing lightly across New Jersey, and another 1-2 of snow accumulation is still possible. After all the snow has fallen, this storm will be comparable to the January 2016 blizzard.

Here are some notable snow reports from New Jersey:

…Middlesex County…
 Metuchen                     19.7 in   0627 AM 02/02   Public
 South Plainfield         19.0 in   0800 AM 02/02   Self-report
 New Brunswick                18.0 in   0426 PM 02/01   Trained Spotter
 …Somerset County…
 Bedminster Twp               19.7 in   1015 PM 02/01   Trained Spotter
 Bridgewater                  19.4 in   1100 PM 02/01   Cocorahs
 …Hunterdon County…
 Byram                        26.0 in   0627 AM 02/02   Public
 …Morris County…
 Mount Arlington              28.5 in   0959 PM 02/01   Trained Spotter
 Randolph                     28.3 in   0600 AM 02/02   Trained Spotter
 Green Pond                   27.1 in   0555 AM 02/02   Trained Spotter
 Lake Hopatcong               25.4 in   0700 AM 02/02   Trained Spotter
 Marcella                     24.0 in   0856 AM 02/02   Trained Spotter
 Millington                   20.0 in   1037 PM 02/01   Public
 …Sussex County…
 Montague                     33.2 in   0709 AM 02/02   Public
 Sparta                       30.3 in   1000 PM 02/01   Trained Spotter
 Stanhope                     28.2 in   0930 AM 02/02   Public
 Andover                      27.0 in   1000 PM 02/01   Trained Spotter
 Byram Twp                    26.0 in   0626 AM 02/02   Public
 Hopatcong                    26.0 in   1155 PM 02/01   Trained Spotter
 1 SW Highland Lakes          25.2 in   0800 AM 02/02   CO-OP Observer
 Vernon                       24.0 in   0640 AM 02/02   Public
 Newton                       22.0 in   0430 AM 02/02   Public
 Stockholm                    20.0 in   0800 AM 02/02   Trained Spotter
 …Bergen County…
 1 ESE Park Ridge             23.0 in   0824 AM 02/02   Trained Spotter
 Closter                      22.4 in   0815 PM 02/01   Public
 Franklin Lakes               20.6 in   1030 PM 02/01   Trained Spotter
 1 S Washington Townshi       20.5 in   1245 AM 02/02   Trained Spotter
 1 SSW Hillsdale              20.5 in   0600 AM 02/02   Public
 Westwood                     20.0 in   0745 PM 02/01   Trained Spotter
 …Passaic County…
 Bloomingdale                 26.2 in   0100 AM 02/02   Trained Spotter

Here is a preliminary plot using NOHRSC analysis data from Jan 31st at 12 UTC through Feb 2nd at 12 UTC. This plot will be updated as NOHRSC analysis is updated, as this is an underestimate that does not include some of the highest totals.

Coupe

Intense Snowfall over Central New Jersey

Synoptic forcing and mid-level frontogenesis is promoting 1-2″ per hour snowfall rates across central and northern New Jersey this morning.

At all levels, there is support for lift, which will continue through the morning. Although intense snow rates will drop off for the afternoon, an area of low pressure will linger off the New Jersey coastline for the next 24 hours at least, providing additional light snow accumulation on top of the 10″ that parts of Central Jersey received overnight. Sleet may mix in with snow during the early afternoon, but snow should dominate again Monday night into Tuesday. This is highly dependent on the track of the meandering low pressure. In the 12 UTC high resolution NAM simulation from this morning, the low stays just offshore and there is steady snowfall through Tuesday afternoon. This results in the accumulation of >12″ of snow on top of the >9″ received across the area. The NWS forecast for this area was 18-24″, which is expected to verify on the high-end if the high resolution NAM is right. If the HRRR model is right, the forecast will still verify on the low end of the NWS range.

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Feb 1st DC-to-Maine Snowstorm

A surface low will move across the eastern United States from the Southern Great Plains across the Ohio River Valley. As it passes over the Appalachians, the surface low will transfer north of Cape Hatteras and develop into a broad surface low with a large precipitation shield that will envelop much of the eastern seaboard with wintry precipitation. An upper level trough and a jet streak south of Nova Scotia supports cyclogenesis off the coast of Maryland. With sufficient cold air in place, there is synoptic support for an area of low pressure to form and move along the eastern coast of the United States. The spatial location of the center of the low pressure is still an area of uncertainty. This can be illustrated by the GEFS ensemble low pressure centers for Tues Feb 2nd at 00 UTC:

Uncertainty with this aspect combined with uncertainty over the spatial orientation of any mesoscale banding creates issues with snow accumulation forecasts. Regardless, large areas of New Jersey will see at least 3-6″ of snow with up to 12″ locally. The 12Z NAM shows the potential for the locally higher amounts:

As certainty increases, forecasted snowfall totals are expected to increase, unless the surface low becomes so close to the New Jersey coast that there is a transition to sleet or rain. This aspect of the forecast will become much clearer 24-48 hours before the storm. The potential for a very large snowstorm exists and one should be prepared for over 12″ of snow over a large area.

Coupe

Mild weather ahead

It is mid January and raining steadily as temperatures rise into the upper 40s. At this point, it is apparent that medium range forecasts of cold were oversold. Yes, an upper level trough and mid-level low will move through the area over the next 48 hours, bringing unsettled weather. Behind this frontal system, 850 mb temperatures look unseasonably cold over the next 5 days. However, a surface high in the southeast will advect warm surface temperatures from the south and west after the cold front moves through on Saturday January 16th. There will be snow showers across the midwest, as far east as central Pennsylvania, but none of that will make it into New Jersey. Instead, temperatures rebound on Sunday into the 40s with southwest winds and clear skies. Notably, low temperatures will remain around 30F for the next 5 days.

An upper level ridge will begin to build in the southeast around Tuesday January 19th. This will battle lower 500 mb geopotential height anomalies to the north in central Canada, where cold air attempts to build up. However, the current GEFS is forecasting a build-up of cold temperatures in Canada that then retrogrades westward, bringing colder than average temperatures to the northwestern United States. Cold looks like it will develop in central or western Canada and shift southwest. Ridging will build south of Alaska and in response, persistent troughing develops off the west coast of the United States. A ridge over the southeast will contribute to a warm southeastern United States from days 3-10. All of these factors, but most importantly this southeast ridging will make it extremely difficult for an east coast snowstorm over the next 7-10 days. Unfortunately, it appears that Greenland blocking and a negative NAO will be wasted over this time period.

Even through forecast hour 384 of the operational GFS, there is virtually no snow for the east coast. The effects of the SSW are currently expected to avoid the east coast of the United States through the end of January. However, a period of west-based blocking in the Atlantic shows promise in the first week of February.

Coupe

Accessible Science: Coupe et al. (2019)

My first published paper on nuclear winter, Coupe et al. (2019), attracted considerable media attention and gave me my first exposure working with science journalists writing about newsworthy papers on short deadlines. I tried my best to explain the implications of my work without fearmongering. In the month after publication, I gave half a dozen phone interviews with radio stations and newspapers across the United States. I watched as my paper’s Altmetric score climbed above 500 within months, which is in the top 5% of all research scored by Altmetric.

The apocalyptic consequences as simulated by the model that we used in this paper were no doubt responsible for the media frenzy, but from what I understand, few members of the public engaged with the manuscript itself. This isn’t surprising. Anyone without science training who earnestly tries to engage with most peer-reviewed publications are met with a wall of terms and jargon that impedes a deeper understanding of what is actually being done. Not before long, I found out that my work was being used in the culture wars when the Daily Mail and Fox News published their articles on my work, and the rigor of what I had done was questioned by hundreds of people who comment on those news sites, assuming the work was part of advancing a political agenda. This was an odd twist, but not too surprising given this was in the second half of the Trump presidency.

So here, I attempt to explain exactly what I did in simpler terms.

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Unseasonably Warm Week Ahead – SSW Impacts Possible in Days 7-10

Over the next week, surface temperatures will grow progressively warmer over New Jersey as an area of high pressure over the southwest Atlantic advects relatively warmer temperatures towards the east coast. An area of low pressure will move to our north between January 14-15th, and we will remain in the warm sector of this cyclone, allowing high temperatures to rise above 50F. Very little precipitation is expected with this system for New Jersey. As the cold front moves through on January 15th, high temperatures will remain in the 40s, which is still unseasonably warm. This cold front is primarily bringing drier air as opposed to very cold air for the northeastern United States at first. The southeast United States will experience a shot of cold air, which begins to moderate over time. Thus, this shot of cold air will likely not generate negative surface temperature anomalies for the northeastern United States despite cold 850 mb temperatures.

All this occurs as the stratospheric polar vortex is displaced over Eurasia. The upper level forcing that is contributing to the southeast cold shot is an additional lobe of the stratospheric polar vortex that will become displaced towards the east coast of the United States over the next 7-10 days. Tropospheric blocking over Greenland intensifies after January 17th while negative 500 mb geopotential height anomalies build over the southeast United States between January 17th and January 20th. Anomalously warm temperatures over Canada, will limit wintry precipitation for much of the eastern United States during this time period and the cold air will become moderated as time moves on. In the Northern Hemisphere, cold air is concentrated over Eurasia during the time period coinciding with the lowest geopotential height anomalies over the eastern United States. Any unsettled weather that develops will likely fall as rain. However, the next 2 weeks look unseasonably dry in the GEFS due to a lack of available moisture.

Around January 24th, surface based Arctic high pressure will descend into Canada and produce cold anomalies that will spread southward into the northern interior of the United States. A robust -NAO is maintained through this time period as cold air begins to filter into the United States. Thus, the best potential for unsettled weather that can produce snow will be after January 24th while the -NAO coincides with ample cold air nearby.

Coupe

Sudden Stratospheric Warming and Stratospheric Polar Vortex Displacement

As 2021 began, enhanced meridional and vertically propagating planetary wave activity increased heat flux into the polar stratosphere and generated sudden warming, leading to the dynamical response of a slowing of the polar stratosphere winds. As is usually the case following a sudden warming and weakening of polar stratosphere winds, a positive feedback loop has caused the winds to weaken further. On January 5th, the zonal mean zonal winds at 10 mb and 60N officially transitioned to easterly, marking the official occurrence of a sudden stratospheric warming (SSW) event. A SSW event occurs on average once every two years, although it has been suggested (with low confidence) that an increase in upward wave activity into the stratosphere in a warmer climate could increase the frequency of these events. When a SSW event occurs, it typically has broad ranging impacts for weather in the mid-latitudes in the winter, often raising the probability of seasonally cold air outbreaks across Eurasia, although there is less certainty surrounding the impacts over North America. Often this depends on whether or not the stratospheric polar vortex has split completely or has simply been displaced towards one hemisphere.

Based on GEFS guidance, the disruption to the stratospheric polar vortex can best be characterized as a displacement over Europe and Eurasia. Ridging over Alaska and western Siberia has forced the stratospheric polar vortex to become displaced in the opposite direction over the next 2 weeks in the GEFS. This will lead to below normal temperatures over much of Europe in the next two weeks. However, this will coincide with riding in the far east Pacific Ocean and North Atlantic blocking in the troposphere, setting the stage for increased probability of mid-latitude shortwaves and troughiness across the interior United States. An upper level low over the southeast currently also will help to build blocking in the North Atlantic, working together with the displacement of the stratospheric polar vortex to increase the probability of a cold and potentially stormy mid-January.

GEFS 10 hPa temperature and height for Jan 11th to Jan 18th, depicting a stratospheric polar vortex displaced towards Eurasia.

Although the stratospheric polar vortex displacement would not have typically favored North American impacts, the GEFS 850 mb temperature anomalies in the day 11-15 range have that “look”, meaning a potential cold air outbreak across the northern interior of the country.

GEFS 850 hPa temperature anomalies for Jan 17 to Jan 21st.

Cold air combined with a -NAO (due to North Atlantic blocking) suggests increased potential for cold, stormy weather across the eastern seaboard between January 18th and 22nd. More details will be posted in the 5-7 day range if a storm does materialize from these favorable conditions.

Coupe

Snow Potential on 12/14 and 12/16-17

The NAO is expected to dip negative for the next 6 days before becoming positive again, providing two opportunities in the next week for accumulating snowfall across the NJ I-95 corridor as two shortwave troughs trigger intense cyclogenesis along the eastern coast of the United States.

The evolution of this in the GEFS initialized Dec 12 2020 at 06 UTC shows these two shortwaves moving through, with the latter one becoming more amplified. At 300 mb, event one benefits from the right entrance region of a strong 100 kt jet stream. Event two intensifies rapidly due to a 120 kt jet upstream of the surface low.

Evolution of 500 mb geopotential height and normalized anomalies from GEFS initialized at Dec 12 2020 at 06 UTC. Animation from FH+48 to FH+132.

Dec 14
For event one, the surface manifestation of the upper-level forcing looks like the plot below in the same GEFS ensembles. There is still significant spread at FH+60 that determines the position of the rain/snow line for this event, but the GEFS mean has the low taking a path slightly inside 70N/40W. This suggests mixing issues will limit the amount of accumulating snowfall.

GEFS mean ensemble member pressure centers for GEFS init at Dec 12 2020 at 06 UTC valid at Dec 14 2020 at 18 UTC.

The operational run of the GFS (init Dec 12 2020 at 06 UTC) shows a narrow band of accumulating snowfall across the northeast on the edge of the rain/snow line (figure below). Up to 6″ (using Kuchera method) of snow accumulates in some parts of New Jersey. But with the large spread in the low pressure center in the GEFS, the exact areas that see snow accumulations are uncertain and much of central Jersey and North Jersey are in play.

24 hr snow accumulation using Kuchera method for GEFS init Dec 12 2020 at 06 UTC, valid at Dec 15 2020 at 18 UTC.

Early forecast: 1-3″ of accumulating snow on Dec 14 2020 starting around 16 UTC.
Confidence: Medium.

Dec 16-17
For event two, a stronger area of surface low pressure is expected to form but there is more significant spread in the positioning of the low, creating far more uncertainty in this forecast. However, the consensus is still for a stronger and larger storm. This is illustrated based on the operational GFS (init Dec 12 2020 at 06 UTC) snowfall accumulation map using the Kuchera accumulation method. More than 20″ of snow is simulated to accumulate in parts of New Jersey with this operational run, but run to run variability of the placement of these totals is high, creating uncertainty. However, the thermal environment appears favorable for snow early on the event in far northern New Jersey because there is sufficient cold air to the north that will favor widespread snowfall, even if the low pressure comes closer to the coast.

24 hr snow accumulation using Kuchera method for GEFS init Dec 12 2020 at 06 UTC, valid at Dec 17 2020 at 12 UTC.

There is high confidence of an area of low pressure forming based on confidence in the synoptic set-up, but the details of the surface are still uncertain. Therefore:

Early forecast: 3-6″ of total snow accumulation starting on Dec 16 2020 at 21 UTC. There is an unusually large probability of significant snowfall (<12″) at this early stage, which suggests this initial forecast will only increase as lead time to the event decreases.
Confidence: Low.

Coupe