Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Sat Dec 10 2022 Valid 12Z Tue Dec 13 2022 - 12Z Sat Dec 17 2022 ...Major storm system to bring heavy snow/blizzard conditions to northern-central Plains with a heavy rain/severe weather threat for southern areas next week... ...Overview... A major storm system will be exiting the Rockies by the start of the period on Tuesday, with high confidence for a deep low pressure system to emerge into the Plains. This system should bring a wide array of hazards including heavy rain/severe weather to the south and heavy snow/blizzard conditions across parts of the north-central Plains and into the Upper Midwest around midweek with some chance of freezing rain for some as well. This storm system should track steadily eastward bringing a heavy rain threat to parts of the Southeast and Mid-Atlantic (with some more uncertain snow potential farther north) later next week. ...Guidance/Predictability Assessment... Through around midweek, model guidance has been and continues to be well clustered for the overall pattern setup involving a vigorous surface low consolidating in the central Plains and shifting toward the Midwest. There remain some wiggles in the timing of the surface low track and its associated fronts, with recent CMC runs on the faster side and UKMET runs a little slower. After Wednesday/day 4, recent runs of the GFS have been a very notable outlier with wanting to weaken and elongate the upper low with an overall much less amplified pattern later next week. The remainder of the models, with support from the ensemble means (including the GEFS) still show a distinct upper low progressing into the Ohio Valley and eventually the Northeast. The GFS was not included in tonights blend past day 4 at all. There is also plenty of uncertainty still on a potential coastal low/nor'easter development off the Mid-Atlantic Coast by Friday. The CMC (including the new 00z run) is significantly faster with the low compared to the ECMWF, but both take a similar track. The GFS is shunted well to the south. The ensemble means again support low development most consistent with the ECMWF. While the guidance is showing better agreement on a low development, the exact track remains highly uncertain at this time and would have huge implications for any kind of wintry weather across the Northeast later in the period. Out West, an upper low drops south off the West Coast, and as early as day 3, there's a lot of uncertainty. The ECMWF and CMC are deeper and maintain a low well off the coast, while the GFS and UKMET (GFS faster) drop it down along the Coast. The GFS spins it near the coast and eventually offshore. Lots of uncertainty there lended to favoring the ensemble means. The WPC blend for tonight relied heavily on the ECMWF which seemed the most stable and agreeable to the better model ensemble consensus. Days 3 and 4 was able to incorporate smaller amounts of the CMC and GFS, but after that, used increasing parts of the ensemble means with the ECMWF given the issues addressed above with the CMC and GFS. The UKMET was too different across the East Pacific early in the period to favor not using it at all in the blend. This approach maintained good continuity with the previous shift as well. Any one model solution seemed plausible (some more than others), but with no clear trend, keeping with WPC continuity seemed safer at this point. ...Weather/Hazard Highlights... As the upper trough emerges into the Plains and a surface low consolidates and deepens, deep ample moisture will spread ahead of a cold front to produce significant rainfall across the Lower/Middle Mississippi Valley into the Tennessee Valley/Appalachians and vicinity, steadily shifting east with time. The latest WPC experimental excessive rainfall outlooks show slight risks across eastern Arkansas, far west Tennessee, and southeast Missouri on Tuesday, with the slight shifting east into the Tennessee Valley/southern Appalachians and northern parts of the Southeast where stream flows and soil moisture are above normal. The Storm Prediction Center continues to monitor and highlight the potential for strong to severe convection across the lower Mississippi Valley and central Gulf Coast Tuesday and Wednesday as well. On the backside of the low, cold temperatures and wrap-back moisture will support significant snow/winds in the Northern Plains/Upper Midwest as shown by steadily increasing WPC Winter Weather Outlook probabilities, with some mixed precip/freezing rain potential in the transition zone and also along the eastern edge of the precip shield near/north of the warm front. By the latter half of the week, current forecasts show precipitation is likely to spread across the East Coast along and ahead of the frontal system, with potential for wintry weather spreading across the Great Lakes region and Northeast. But precipitation types and areas of heaviest precipitation have a great deal of uncertainty especially considering possible coastal low development off the Mid-Atlantic coast late week. The West should see below normal conditions through the period, with the most widespread readings of at least 10-15F below normal for high temperatures as the deep upper trough crosses the region for the first half of next week. Some of this colder air should spill into the High Plains by next Tuesday-Thursday behind the expected Plains storm. On the other hand, southern/eastern areas will be near to above normal for temperatures. Anomalies will be greatest for morning lows, with some plus 15-25F anomalies across the southern tier through Thursday. Highs are forecast to be around 10-15F above normal for the central and then east-central U.S., locally higher ahead of the storm system. Much of the country looks to be near or below normal by next Friday and Saturday after the cold frontal passage as high pressure overspreads much of the country. Santorelli 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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