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What the satellite images and upper air forecasts are telling us about temperatures and precipitation for the upcoming holiday period.
Since early autumn I have been upfront with you about the likely lack of sustained cold and snow/ice in the lower 48 states until after the January Thaw. There have been cold intrusions, but mostly of the polar variety, but these chilled periods have always been swept away by air masses of Pacific Ocean origin. This is courtesy of a semizonal flow aloft (no entrenched blocking in the higher latitudes).
What has been different about the El Nino episode this year is that the southern branch jet stream has stayed away from California and the Southwest and passed through Mexico into the Gulf Coast, Florida, and through the western Atlantic Ocean into the British Isles. There has been interaction with the polar jet stream (like this past weekend, with tragic results). It is only then when temperatures feel more like December.
The model ensemble forecasts have some changes during the December 24 – January 7 period. Once again, a broad closed low aloft stalls/breaks up off of California, regroups across northern Mexico and the Lone Star State, and then progresses through the Dixie states and up along the Eastern Seaboard. While snow is a good possibility in Appalachia and the St. Lawrence Valley, the Interstate 95 cities are likely looking at all or mostly rain. The CFS series suggests two more large storms of this type in the first week of 2024. But the January Thaw should arrive in the middle of the month, a national warm-up that will outrage snow lovers.
Only in the second half of January and the month of February, as the basin-wide El Nino weakens and -EPO, -AO, and -NAO signatures start to expand will we see a good chance for “real winter” in the U.S.
The winter discussions sometimes overly emphasize on either El Nino as a warm influence for the USA, or warming and vortex displacement at 10MB as reasoning for Arctic air mass generation and displacement. However, neither of these narratives discuss how one may cancel out the other. Or. for that matter, other features which can break a pattern.
With the subtropical jet stream being very strong and far displaced to the south (Mexico to Florida), all it would take to draw colder values down into the U.S. would be a lower-latitude strong storm. Which I must point out, all of the forecast guidance ensemble packages show happening in an arc from Texas to Georgia and on to the Massachusetts Islands December 23 – 26. This feature may mean a White Christmas in Appalachia and interior NY/New England (little chance of that along the Interstate 95 corridor…sorry…). More importantly, with a giant sub-Aleutian (mAk) vortex pumping up 500MB heights in northern Canada and Greenland, there will be further incentive for cold air drainage from the snowpack above the International Border. This may not be the Arctic intrusion of your dream or nightmare. But the chance for more cold shorts east of the Rocky Mountains will be in play through the first week of 2024.
It is not giving too much away by saying that the January Thaw will bring the lower 48 states back into mild values between the 7th and 18th of the next month. But as El Nino weakens, the analog forecasts for more widespread cold from later January through February from the Great Plains to the East Coast look reasonable.
While the national map looks disorganized now, rapid intensification and deepening of low pressure over West Texas on Saturday morning will upend temperatures and raise the threat for severe weather and gusty winds. Not only in the Lone Star State, but across the South, Appalachia and the Eastern Seaboard as well.
The new GraphCast AI modeling of this system seems to be working out well. A sudden southward shift of a cold air pool from the Prairie Provinces enhances surface low pressure that speeds across the Arklatex into West Virginia, then up through the Quebec Eastern Townships by Monday. This is NOT a major snow producer, although some lake effect plumes could form in the cold advection behind the disturbance. Rather, with strongly sheared wind profiles, surface convergence, and cooling aloft this feature could bring some rather intense gusts and large hail from SE TX into AR on Saturday afternoon.
There may be some postfrontal precipitation Saturday night as the cold intrusion pushes rapidly south into Mexico. But given the rapid motion of the storm and attendant frontal structure, a sharp clearing line will be seen through the middle of the USA on Sunday.
To repeat, I am not expecting any lasting cold or major winter storm threats until the weekend of Christmas Eve. Until then, shop your heart’s content, have a wonderful time. Happy Hanukkah to all!