Moisture plume with subtropical jet continues across the Southern U.S. but really only keeping Florida warm this weekend

Temperature Anomaly | Next 15-days

Flagship European model ensemble solution for temperature anomaly next 15-days shows the cold pool formation and intensification over Alaska and Western Canada — a more typical Polar Vortex dump with strong surface high pressure blasting southward along and east of the Rockies into the central U.S. in the 2-week time frame —> remnants of our recent/ongoing stratospheric warming event —> and then another one in the coming 10-12 days!

Note, there are fast moving disturbances or short-waves riding along the polar front from the Gulf of Alaska through the Pacific Northwest and then in the Midwest and Great Lakes. These features are spaced out in time — so the ensemble mean blends or smooths out 51 solutions that ALL have cold blasts but intermixed with warm periods. The end result is a rather smoothed continuous blob of cold connecting Alaska with the Northeast U.S.

This STILL has potential for VERY cold Arctic blast outbreaks! Example from ECMWF Control (12z)

Massive ridge across the western U.S. —> surface high pressure over central U.S. and a dip in the jet stream w/mid-level trough (associated with tropospheric polar vortex) bringing a chunk of the Alaska/Western Canada cold pool into the Midwest/Great Lakes and then Northeast.

HRRR 18z | Next 48-Hours into Sunday Early Afternoon

Precipitation Type | Weather Next 6-days | ECMWF 18z

Boatloads of moisture coming into the Pacific Northwest with 2 major atmospheric river events in next 6-days … and Alberta Clippers pick up whatever makes it through the Rockies to take into the Midwest and Great Lakes with weak storm system snow events — eventually these start adding up!

Headlines Friday Evening

1. Still not seeing any strong systems across the Eastern U.S. but quick moving “cutter” or Clipper Systems could provide wintry precipitation especially with the cold air in place —> typical with negative AO/negative NAO with help from the Pacific moisture smashing into Northwest

2. Massive ridging across the Western U.S. will completely block any systems from pushing through the central Rockies — instead everything must go up and over including potential shortwave disturbances that will grab lobes of the polar vortex to bring brief but intense cold shots through the northern tier.

ECMWF 18z Precipitable Water Next 6-days | My favorite parameter

Atmospheric river systems into the Pacific Northwest — narrow at first, and then quite a wide swath by next Tues/Wed — see how shortwave disturbances along the polar front (polar vortex edge) pick up the moisture = Alberta clipper systems

White colored precipitable water = absence of moisture = cold/polar air mass

Temperature Analysis | 7:45 PM ET

Temperature Anomaly | 7:45 PM ET

Lower 48 Temperature Anomaly was -2.5°F below normal

NWS Precipitation Next 72-hours

NWS Snowfall Next 72-hours

Blend of Models | 10 Days Precipitation Total (includes liq from snow)

Enormous precipitation totals in the Cascades … upwards of 20” of QPF or more would translate into outrageous snowfall totals

Blend of Models | Total Snowfall Next 10-Days

Saturday Morning Low Temperatures

172 million below freezing (Lower 48 average low is 27.6°F)

Saturday High Temperatures

Still 14 million at/above 80°F in Florida! 36 million don’t get above freezing!

Sunday Morning Low Temperatures

176 million below freezing (Lower 48 average low is 26.9°F)

Sunday High Temperatures

The same 14 million at/above 80°F in Florida! 49 million don’t get above freezing!

AI-GFS 18z | 500 mb Temperatures | Polar Vortex Watch Next 10-days

Fancy new tool from NOAA (AI-GFS) shows the reorientation of the tropospheric polar vortex to ditch the cold pool over Hudson Bay — and instead dip/focus across Western Canada

ECMWF AIFS-Single 18z | Height Anomaly (500 hPa) Next 10-days

Enormous ridge formation across the Western U.S. —> will it build eastward and block off the Siberian / Arctic express? AIFS-Single seems to have a strong ridge bias, but we’ll check back tomorrow.

Integrated Vapor Transport Next 10-days | North America

Parameter used for monitoring “atmospheric rivers” usually along the U.S. west coast, but the idea works well for many other areas — highlighting midlatitude storm systems with warm conveyor belts (moisture plumes)

ECMWF 18z | Snowfall | Next 6-days

ECMWF EPS 12z | Ensemble Median Snowfall Next 15-days

Instead of just the median — let’s look at the 90th Percentile — if the model ensemble was operating in the 1 in 10 worst snowfall category … most places it’s not a major difference BUT it can be —> ECMWF EPS 12z | Ensemble 90th Percentile Snowfall Next 15-days

The difference between the 75th and 25th Percentiles is the interquartile range (IQ) and shows the ensemble spread to a good approximation! Even when we show the median snowfall amounts, there is a +/- range of sometimes a similar magnitude of the median/mean … and that’s why ensemble means of precipitation into the medium range can be misleading during storm events where 100 miles makes the difference between yes/no snowfall/rain — the average is not really a solution

ECMWF EPS 12z | Ensemble Snowfall Probability > 1” | Next 15-days

We’ll be monitoring the 1” chance of snowfall for a “White Christmas” in the coming weeks. This map is through late December 20th.

ECMWF EPS 12z | Ensemble Snowfall Probability > 6” | Next 15-days

6-inches is sufficient for a “big storm” and Lake Effect machine diagnostic but 12” is even better —>

ECMWF EPS 12z | Ensemble Snowfall Probability > 12” | Next 15-days

Next 15-days Temperature Anomaly | Ensemble Mean to December 19

The western ridge is STRONG … and should provide enough Pacific moisture and moderating mild air to temper the longevity and intensity of any Arctic blasts that sink from the massive cold pool over Western Canada.

Temperature Anomaly | Next 15-days | ECMWF EPS 12z

5-Day Chunk #1 | Temperature Anomaly | Dec 5 - Dec 9

5-Day Chunk #2 | Temperature Anomaly | Dec 10 - Dec 14

5-Day Chunk #3 | Temperature Anomaly | Dec 15 - Dec 19

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