November 21, 2024 Thursday Winter Weather Update

Much colder air on the way into December

Nov 22, 2024

Thursday Headlines

Thursday — Friday: 2 ongoing storms on each coast: (a) atmospheric river w/yet another bomb cyclone in California/Oregon/Washington (b) Mid-Atlantic and Northeast low pressure looping with significant snowfall.
We are flipping to full-tilt winter = much colder conditions across the Lower 48 with significant snowfall threats. I am watching various ensemble and A.I. models to determine how cold it could get in early December.

Thursday Evening Across the Lower 48

Ongoing storm has lowered temperatures dynamically into the 20s and lower-30s across the eastern Great Lakes and Ohio Valley. You can see the cold pool in the anomaly map — plenty cold enough for snowfall.

HRRR over next 18-hours into early afternoon on Friday shows heavy snowfall continuing to wrap around the center of low pressure just west of New York City.

Day 10 Upper Level Pattern

Main question is how much Arctic air can dump onto the Lower 48 east of the Rockies to spur heating demand. LNG markets are betting on the outcome of various forecasts over the next few days.

ECMWF AIFS 12z

EPS Ensemble Mean 12z

Comparing the AIFS and ensemble mean just with a quick eyeball glance suggests that sure, the AIFS could be a member of the ensemble mean, just washed out or smoothed.

Looking at the ECMWF HRES 12z, similar story as AIFS with more highly detailed trough. That’s typical of looking at the ensemble means vs. the individual members or a deterministic forecast.

The ensemble mean is averaging 51 similar forecasts at Day 10 but small deviations in time and space reduce the amplitude of the upper-level wave pattern and therefore reduce / spread out the lowest geopotential height centers.

ECMWF 12z HRES

Finally as an independent option, we can look at the GFS 12z deterministic and see a similar pattern. That should indicate agreement in the general “cold shot” thesis.

Turns out that 5-day skill is insanely high right now. That doesn’t mean that Day 7 or 10 is also high, but it could — more on that later.

ECMWF HRES 12z at 0.98 is outrageous, some of the best forecasts ever produced.

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ECMWF 18z | 48-Hour Precipitation Type

Bomb cyclone #2 is looping its way into the Pacific Northwest offshore Washington with continued “atmospheric river” flood of moisture on shore, eventually extending southward into Central California, and even SoCal.

Not a bomb cyclone, but plenty of moisture with a low that drops into the mid-980s doing a counter-clockwise track loop around New York City.

In another few weeks, the cold air in place north of the system in Canada would be blockbuster snowfall amounts for all of New England. Is this a sign of what’s to come this winter?