GaWx

Which are y’all going with for Saturday’s (12/13) low? The GFS sticks out as much colder than the others at ATL with upper teens there, which tells me it’s likely too cold there. It has had a cold bias over snowcover in recent years (which is well above average in the Midwest). That’s probably a factor as I don’t know that that has been fixed. 6Z GFS: pipe busting cold with upper teens ATL/RDU: likely too cold 0Z Euro: slower with the cold and thus is much warmer on Sat AM with upper 30s: likely too warm though it has 20s the subsequent few days 0Z ICON: mid 20s RDU to upper 20s ATL: looks like a reasonable middle-ground 0Z CMC: coldest at RDU with mid teens (CMC has a well-known cold bias) though only down to mid 20s ATL 0Z UKMET: only down to low 30s though it has a warm bias/very likely too warm though it is headed for 20s on Sunday

I’m not using Dec 2022 as a heavily weighted analog because of snowcover. On this date in 2022, there was only 1% snowcover in the Midwest: Compare that to today’s 46%, the 3rd highest only to 2013’s and 2007’s 53% and just ahead of 2005’s 45%. The 2003-24 average is only ~15% meaning today’s is ~3 times the average for Dec 7th: @donsutherland1you may want to keep this in mind for your forecasts. https://www.nohrsc.noaa.gov/nsa/index.html?region=Midwest&year=2025&month=12&day=7&units=e

I created a new thread for tomorrow’s wintry event in mainly NC/VA:

I created this for the Monday (12/8) wintry threat for mainly NC and VA so as to leave the longer term for the main Dec thread. @buckeyefan1please pin this. Thank you.

Regarding the Januaries overall in the NE US following the last 11 -ENSO -PNA Decembers, only 2 were mild (2017 and 2023) while 3 were within a few degrees of normal and 6 were cold (1984, 1985, 2009, 2011, 2014, and 2022). So, 3 times as many mild were cold. *Edited for correction: 6 Jans were cold, not just 5.

But the idea of a big Arctic plunge 12/12-17 deep down into the US (possibly from two highs per GFS) is becoming increasingly likely based on model guidance. This is very believable considering it would be during an EPS progged longest phase 8 in 50 years along with the highest % Dec 5th snowcover in the Midwest since at least 2002. This is also setting the table for a potential quite cold Dec/Jan considering the -PNA Dec -ENSO analogs suggesting a +PNA Jan, which would make it likely that DJF would end up colder than normal for much of the E US even if we get the favored mild Feb. The Euro Weeklies would have to bust badly to allow for a +PNA, but it’s been too strong with the SER since just before Thanksgiving.

18Z GFS: 2m anomalies for 0Z on 12/14:

Highest US snow-cover as of Dec 5th 2003-25 (%)(2003-24 avg 27%): ‘05 48 ‘25 45 ‘18 43 ‘10 39 ‘13 38 ‘19 37 ‘06 33 ‘16 32 But more importantly for E US cold prospects is the Midwest snowcover: highest % as of Dec 5th (2003-24 avg 12%): ‘25 65 ‘05 45 ‘18 32 ‘06 31 ‘10 27 ‘16 25 ‘03 23 ‘08 20 https://www.nohrsc.noaa.gov/nsa/index.html?region=Midwest&year=2025&month=12&day=5&units=e

US snowcover as of Dec 5th is way higher now than it was in ‘16 and even moreso vs ‘22.

Followup: this is only 8 days out 12Z GFS

All 0Z ensembles are much colder in the E US 12/13-17 vs earlier runs.

New weeks 3-4 outlook makes sense since it’s similar to the Euro Weeklies. But hopefully they will end up too warm:

Chicago per model consensus is on track for a top tier cold Dec 1-20. Here are the 15 coldest Dec 1-20 at Chicago of the last 75 years (top 20 %) in reverse chronological order: 2016, 13, 10, 08, 05, 00, 89, 85, 83, 76, 72, 63, 58, 56, 50 2025 appears at this time like it might compete with many of these.

At 1PM, there was a whopping 26F diff. between Valdosta’s 76 and Tifton’s 50. Tifton is a mere 45 miles N of Valdosta!

The yucky rainy pattern continues here. Luckily during a short break in the light rains yesterday evening, I squeezed in a walk. Today so far has been different. The rains at times have been heavy, with my measurements suggesting ~0.75”. There’s been enough to be wary of hydroplaning if going too fast. This is the heaviest I’ve seen in several months. Thus, it’s the first minor test of my completed yard drainage projects as well as the city’s redug nearby street drainage ditch project. But it’s still not yet a complete tear as I need even heavier rains. Regardless, there will be more on and off rains through Sunday. My temps have been mainly in the mid to upper 50s after dipping to the rain cooled upper 40s last night. Temps are 12 warmer in Brunswick 75 miles to the S.