Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Sun Dec 10 2023 Valid 12Z Wed Dec 13 2023 - 12Z Sun Dec 17 2023 ...An emerging wet pattern over the South/Southeast... ...Pattern Overview and Guidance/Predictability Assessment... Multiple aspects of the forecast continue to be in a state of flux, with guidance varying considerably for particular features/evolutions or rapidly shifting their clustering. Ahead of the systems leading to the ongoing uncertainties, a fairly strong upper trough crossing eastern Canada and the northeastern quadrant of the lower 48 Wednesday-Thursday will push a leading cold front into the Atlantic. Then a prominent feature of interest through the period will be a closed low expected to form over the Southwest by the start of the period early Wednesday and most likely track into the southern High Plains by Friday. Although clustering has been improving over the past 24-36 hours, there is still quite a bit of spread by early Saturday with the 12Z UKMET quite slow and ECMWF runs alternating between the southern (00Z run) and northern (new 12Z run) side of the spread. Implied ensemble mean upper low tracks would also be on the northern side. Meanwhile farther upstream the guidance has rapidly consolidated toward separation of eastern Pacific upper troughing that was first advertised by the operational GFS. This has led to an eastern Pacific upper low far enough offshore to bring minimal precipitation to California, while latest trends with more progressive northern stream flow are for the shortwave energy to put less of a dent in the northern part of the western U.S. mean ridge than the first solutions that depicted the splitting trough. Sheared shortwave energy reaching the Pacific Northwest and southwestern Canada by Thursday, which has been a problem in the forecast for days, continues to play a potential role in a minority of guidance. Now it's the GFS/GEFS that are generally on their own with the degree to which this energy amplifies over/east of the Mississippi Valley by next weekend. The GFS/GEFS evolution leads to northern stream dynamics pulling Gulf of Mexico/Florida low pressure well northward near the East Coast compared to the remaining majority that is more suppressed due to the slower southern stream feature providing primary dynamic support for low pressure. 00Z ECMWF-initialized machine learning models also favor a fairly suppressed solution, with potential low pressure positions as of early next Sunday ranging between the central Gulf and just east of the Florida Peninsula. GFS trends over the past couple days have been slower with northward progression of the surface low though. Based on the latest array of guidance and trends, an operational blend of 00Z/12Z model guidance provided a reasonable starting point early in the period. Then preference adjusted to a model/mean blend that tilted considerably more to the ECMWF/CMC and their ensemble means relative to the GFS/GEFS (with the 00Z GFS a little less extreme than the 06Z run). Note that NBM QPF totals were considerably reduced late in the period along parts of the East Coast, though northern extent of light measurable totals is likely still too great. Meanwhile adjustments were also made to slow the advance of rainfall toward California, in a partial trend to operational model clustering. ...Weather/Hazards Highlights... The upper low expected to form over the Southwest by early Wednesday and then track into the southern High Plains, combined with the inland return of Gulf moisture interacting with a wavy lead front, will likely generate an area of heavy precipitation over the southern Rockies/High Plains mid-late week. Currently the best potential for heaviest snow extends from northern-eastern New Mexico/southeastern Colorado into far northwestern Texas. Within the area forecast to see rain, most of the southern High Plains region has fairly dry antecedent conditions and short-term rainfall rates do not appear to be high enough to merit significant runoff concerns through the Wednesday. Thus the Day 4/Wednesday Excessive Rainfall Outlook continues to show no risk areas. However, by Day 5/Thursday the guidance signals for heavy rainfall potential have become more pronounced with improved (though not yet ideal) clustering for upper low track. The combination of potentially wetter ground conditions to start Day 5 along with the 00Z ECMWF showing some instability and guidance overall showing some longevity of low level upslope flow, the Day 5 ERO update proposes an expanded Marginal Risk area centered over west-central Texas. Depending on the path of the upper low, heavy rainfall could extend farther east over Texas into Friday-Friday night. Across Florida, expect easterly flow to set up south of strong eastern U.S. high pressure while moisture and instability increase for midweek and beyond. The Days 4-5 Excessive Rainfall Outlooks for Wednesday-Thursday maintain a Marginal Risk area for southeastern Florida given these ingredients for heavy rainfall are in place across the sensitive urban corridor. There is still uncertainty over the magnitude and northward extent of instability/deep moisture though. The sensible weather forecast over a broader portion of the South/East becomes increasingly uncertain toward Friday and next weekend, given the combined spread for the ultimate path of the upper low over the southern High Plains on Friday and evolution of northern stream flow. The most likely scenario would have moisture spreading farther north over the eastern U.S. but with low confidence regarding northward extent and totals. At the very least, trends continue to favor a slower northward progression of moisture than recent GFS/GEFS runs. Over the West, a weakening front may bring some light precipitation to the Pacific Northwest around midweek or so, with some light/scattered activity possibly extending into Thursday. Thereafter, northern parts of the Pacific Northwest could see additional precipitation with another front while latest trends are gradually increasing confidence in the idea that the upper low over the eastern Pacific should remain far enough offshore to keep most of California dry through next Sunday. Much of the northern tier of the lower 48 east of the Rockies should see above normal temperatures during the period. Expect fairly broad coverage of plus 10F or greater anomalies which could spread gradually farther south by the weekend, with warmest readings (15F or more above normal) tending to be centered over the Dakotas into Upper Midwest Thursday-Saturday. Also mean ridging aloft over the West will favor moderately above normal temperatures and a very gradual warming trend with time. In contrast, clouds/precipitation during the latter half of the week will produce chilly highs over the southern High Plains during the latter half of the week. Temperatures there should rebound closer to normal next weekend. Areas near the East Coast should see a day or so of below normal highs late this week as strong and cool high pressure builds in behind a cold front. Highs over northern parts of the East should trend above normal Friday into the weekend while the South stays near normal (but with Florida above normal for lows due to persistent clouds/rainfall). Rausch/Schichtel Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw