Mild weather ahead

It is mid January and rain is bouncing off my window 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.

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.

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

Hurricane Sally Poised to Weaken

This morning’s visible satellite images were promising and Sally did intensify rapidly over the course of the day. However, the storm is now taking on shear and dry air is poised to infiltrate its entire core. This should halt strengthening and could even bring weakening over the next 6-12 hours.

Visible satellite image over the Gulf of Mexico showing Hurricane Sally with dry air wrapping around the SE quadrant of the inner core of the storm. Image valid for 9/14 at 21:15 UTC

Notably, however, the storm has another 24 hours over the warm waters of the Gulf of Mexico and could shrug off the dry air and shear and intensify again in the 12-24 hour period. Still, because of this dry air I am bringing down my maximum potential intensity to 110 mph, 970 mb (down from 115 mph, 965 mb). No changes to storm surge and freshwater flooding, which will be devastating across large portions of Mississippi and Alabama.

Coupe

Update: Hurricane Sally

16 UTC UPDATE: Aircraft reconnaissance has indicated that Sally is intensifying quite quickly. This was anticipated, given this morning’s visible satellite images, but central pressure has suddenly dropped to 985 mb with maximum sustained winds up to 85 mph.

The forward motion of this storm (6 mph) is extremely slow and indicates a long-duration water threat along the Gulf Coast. Depending on how much additional strengthening can occur, the wind threat will continue to grow along the Gulf Coast east of New Orleans.

Be sure to use National Hurricane Center forecast products to make important decisions regarding life and property.

Coupe

The Intensification of Tropical Cyclone Sally

Sally remains a tropical storm with maximum sustained winds of only 65 mph and and a minimum central pressure of 994 mb. While it currently resides in the bathwaters of the Gulf of Mexico, it has clearly been impacted by moderate shear over the last 24 hours. This hindered the anticipated steady intensification to a hurricane.

That is, until this morning. The first visible satellite images from Sally this morning strongly indicate that Sally will undergo its most steady period of intensification yet. Maybe not quite rapid intensification, but this morning will mark a turning point in the transition of Sally from a high-end tropical storm to a formidable hurricane that will inundate portions of the Gulf Coast east of New Orleans.

Visible satellite image of tropical cyclone Sally in the Gulf of Mexico

The forward motion of Sally is expected to stall as it approaches the Gulf Coast. Precipitation forecasts indicate a high likelihood of flash flooding across a broad area:

The maximum windspeed attained by Sally is not as relevant as the storm surge and precipitation, but for what it’s worth, I would cap Sally’s maximum potential intensity at 115 mph, 965 mb.

Coupe

Sample 5k Forecast

Unfortunately, the COVID-19 pandemic continues to cause race cancellations deep into the Fall 2020 racing season. As a demonstration of the product I can deliver to improve a race director’s situational awareness of weather factors potentially hazardous to volunteers and participants alike, I will post a series of forecasts, assuming a 5k will take place this Saturday August 22, 2020 starting at 9 AM. The location of the race will be in Three Bridges, like for the annual Three Bridges 5 miler race that we time.
I will provide a 5-day forecast on Monday morning with a big picture forecast of the general conditions to be expected for that morning. On Wednesday I will provide a 3-day forecast with more explicit timing of any potential weather hazards along with temperature, winds, and dewpoint, and any other variable necessary to assess racer comfort levels to inform the quantity of things the race often provides such as ice, water, shade, or on the other hand, cover from rain or shelter from cold. Finally, on Friday I will send out a 1-day forecast. A day-of forecast (0-day lead time) will only be sent out if high uncertainty was a factor in the 1-day forecast such as thunderstorms or a mesoscale convective system moving through the midwest overnight. In 2017, Tropical Storm Cindy moved through New Jersey and spawned a tornado in Monmouth County, only miles away from a race at CBA, causing a delay in the start of the race. This would require a 0-day lead time forecast, possibly in the form of a live phone call briefing with the race director within three hours before the race. A 0-day lead time forecast is only issued in these rare circumstances.

MONDAY MORNING FORECAST [5-day lead time]

The current forecast for Monday morning is dry and warm. A 5-day forecast will require the use of global ensemble numerical weather prediction models such as the GEFS and ECMWF and signals in 24-hour precipitation are the most useful at this leadtime to detect areas of disturbed weather.

Precipitation
Between 2am Saturday and 2am Sunday, there is a low probability of precipitation (<10%) accumulations greater than 0.01″ across most of New Jersey. However, there is a large precipitation signal around Florida which may indicate a stalled tropical wave or depression in the area. In the upper levels, an anomalously strong digging trough into the Gulf of Mexico will favor cyclogenesis to the east of Florida and an intensification of a SW to NE oriented upper level jet along the southeast coast. This moves north towards North Carolina through Tuesday but current steering patterns prevent it from moving towards New Jersey. There does not appear to be a major rain threat for Saturday morning from this system. The ECMWF indicates lower pressure over the Great Lakes region running up against the subtropical high pressure system over the southeast, but any precipitation from this system will lift north into Canada. The red polygon in the figure below shows the area with the lowest probability of precipitation on Saturday. The green colors over the area indicate the ensemble mean value of only 0.01 to 0.05″ of precipitation in the 24 hours from 2 am Saturday to 2 am Sunday, which is negligible and indicates an extremely low probability when all of the ensembles are assessed.

In terms of temperature, there is a high confidence of above average temperatures. All ensembles suggest warm temperatures in the northeast, cool temperatures in the southeast, a dipole with a high likelihood of occurring given the upper level pattern. Dewpoints will remain consistent around 65F throughout most of the day, which isn’t as dangerous as it has been for a lot of the summer but with temperatures nearing 80F towards the end of the race, expect runners to be sweating out a lot of water and electrolytes. Above average temperatures during a morning race in August may pose heat related race hazards. Here are how temperatures will look as a function of time throughout the morning:

6am: 74F
7am: 75F
8am: 75F
*** race start ***
9am: 78F , sunny with a very light wind from the southwest.
*** first runner finishes ***
9:30am: 79F, sunny with a very light wind and will feel HOT.
10am: 81F, sunny. Most runners will have finished or will be finishing and it will feel HOT. Ensure water remains to help bring runners’ body temperatures down.
11am: 84F

Air temperatures nearing 80F, dewpoints in the mid-60s, and a negligible wind all contribute to more sweat and electrolytes that runners will lose during the race that will have to be replaced. Offering cold water and potentially cups filled with a sports drink with electrolytes at mile 3, or somewhere else near the halfway point may help combat runner lightheadedness and the need to vomit from rapid loss of fluid and/or electrolytes. Offering this at the finish line would also benefit runners and prevent post-race cramping, vomiting, and lightheadedness.

WEDNESDAY MORNING FORECAST [3-day lead time]

No update to the 3-day forecast is necessary from the 5-day forecast. All heat related hazards remain highly likely. and the likelihood of precipitation is again very low.

Coupe

September Hurricane Forecast

The Colorado State and NOAA hurricane forecasts call for near record number of tropical cyclones in the North Atlantic for the duration of the hurricane season (through November). Based on the August NMME forecast for global SSTs in September, broad La Nina-like cooling will be present along with near-record North Atlantic tropical SSTs, including into the subtropical North Atlantic. Both factors will bring about favorable environmental conditions for any TCs that do form. The unresolved question is how many situations for TC genesis arrive? Environmental conditions are only one factor – one needs sufficient convergence or simply a train of easterly wave disturbances. Some suggest a standing wave pattern over east Africa will generate these easterly waves through time and generate a number of TCs between August 20th and September 15th.

The week 3-4 CPC precipitation forecast (August 29th – September 11th, 2020) includes an above average precipitation signal for much of the southeast and Gulf Coast. This is undoubtedly a part of the signal for TCs impacting the US coast in this time frame. For forecasts into October, the signal of warming only gets worse. If an active period TC genesis does occur into October, it could be a long fall for the east coast.

Coupe