Extended Forecast Discussion NWS Weather Prediction Center College Park MD 104 PM EST Sat Nov 26 2022 Valid 12Z Tue Nov 29 2022 - 12Z Sat Dec 3 2022 ...Wet/snowy conditions expanding across the West with below normal temperatures... ...Heavy Rain and severe weather possible across parts of the South and into the Ohio Valley Tuesday-Wednesday... 18Z Update: An analysis of the latest model guidance reveals good overall synoptic scale agreement at the beginning of the forecast period on Tuesday. However, timing differences emerge with the timing of the developing storm system over the Plains and its trailing cold front as early as Tuesday night. The past few runs of the GFS appear to be too slow in the progression of the cold front across the Southeast and then across the East Coast, with the parent surface low positioned to the southwest of the model consensus. The CMC/ECMWF/ECENS are relatively close and this also has some support from the ICON and JMA models, so the front/pressures forecast was hedged in this direction with a faster frontal progression. The 12Z GFS did trend a little faster compared to its 6Z run, but still slower compared to the consensus. The UKMET also strayed from consensus near the Pacific Northwest going into Thursday and beyond, so it was only used through Wednesday. For the QPF forecast, about half of the NBM and 1/6 each of the ECMWF, CMC, and ECENS was used as a baseline through Thursday, and then gradually increased NBM proportion going into Friday and Saturday. A Slight Risk of excessive rainfall was introduced for the Day 4 period from southern Mississippi to south-central Tennessee to account for a better model signal of 1-3 inch rainfall totals with locally higher amounts. Local increases were made downwind of the Great Lakes on Thursday to account for lake effect snow potential. Across the western U.S., there has been an upward trend in expected overall QPF for the latter half of the forecast period, although precipitation amounts were trimmed back slightly from the aforementioned model blend. The previous forecast discussion follows below for reference. /Hamrick --------------------- ...Overview... The models and ensemble means generally agree from the large-scale perspective, showing a mean trough setting up near the West Coast while an upper ridge over the Bahamas/Cuba as of early Tuesday steadily retrogrades into the Gulf of Mexico. A leading strong upper trough emerging from the West should support deepening surface low pressure that tracks from the central Plains into the Great Lakes Tuesday-Wednesday. The trailing cold front may focus heavy rainfall from the South into the Ohio Valley, with severe weather also possible over southern areas, with some heavy rainfall also potentially reaching the Northeast. Some snow will be possible north/west of the low track. The first system dropping into the developing West Coast trough should affect the Northwest Wednesday-Thursday with another system possibly approaching by next Saturday. Expect precipitation to push southeastward across the West with time as the overall upper trough amplifies. By late in the week the details of individual features affecting the West and ejecting downstream to affect areas east of the Rockies become increasingly uncertain. The overall pattern will favor below normal temperatures over much of the West and parts of the High Plains, with Montana and nearby areas likely to see the most persistent and coldest anomalies. ...Guidance and Predictability Assessment... Guidance is still in the process of refining the forecast for the developing storm system expected to track from the central Plains through the Great Lakes and into Canada Tuesday-Wednesday. Track spread has narrowed noticeably but meaningful strength differences remain. 12Z/18Z GFS runs were on the fast and weak side of the spread (the weak character being a persistent trait for multiple runs). The new 00Z GFS has slowed down some but is still among the weaker solutions, albeit now joined by the 00Z CMC. The 00Z GFS parallel is deeper and in line with the operational model average. On the other hand the 00Z UKMET extends a recent trend away from its deep solutions. An intermediate solution close to continuity continues to look reasonable for the surface low given the behavior of latest guidance. By early Thursday there is an increasing signal in the models for a fairly strong frontal wave to reach a position close to northern New England. While loosely fitting within the expected mean pattern, the 12Z/18Z GFS and GEFS means in their own different ways differed somewhat from most other guidance for the West Coast upper trough. They all were flatter with energy nearing the Northwest by midweek and thus much weaker with the surface reflection. Then the GEFS mean became less amplified with the upper trough versus other solutions, as well as what would be suggested by teleconnections relative to the core of positive height anomalies associated with the upstream ridge. Meanwhile those two GFS runs used some of the trough energy to pull of an upper low that ultimately ended up offshore southern California by day 7 Saturday. The 00Z GFS/GEFS mean have adjusted much closer to the majority scenario but with the GFS farther offshore than the GEFS or other guidance for the system affecting the Northwest by Wednesday-Thursday. There is moderate clustering for the next system that may reach near the Pacific Northwest by next Saturday. Also late in the period the forecast becomes increasingly uncertain over how ejecting West Coast energy may influence the surface pattern to the east of the Rockies. There has been some semblance of clustering and continuity for a wavy front to reach the north-central Plains/Upper Midwest by Friday but then models and even to some degree the means rapidly diverge for surface details from the Plains eastward. The updated forecast maintained a solution fairly close to continuity by way of an operational model blend for about the first half of the period, with ECMWF input split between the past two runs through 12Z and a little less 18Z GFS weight than would typically be the case, and then a model/mean mix that tilted somewhat more toward the ECMWF/ECens/CMC than the 18Z GFS/GEFS mean. ...Weather/Hazard Highlights... Over the West, a departing system may produce lingering light snow over the north-central/central Rockies on Tuesday. A trailing system nearing the Northwest should spread moisture into the region Tuesday-Wednesday with some locally moderate to heavy rain and higher elevation snow possible. Gradual amplification of upper troughing should expand the precipitation shield southeastward through the rest of the week, possibly reaching into central or southern California. Also worth monitoring during the latter half of the week is the potential for the upper trough to draw in some lower latitude moisture/surface waviness, which would enhance rain/snow totals over some areas. However uncertainty in important upper trough details keeps confidence fairly low for precise southeastward extent and rain/snow intensity. Snow levels in the Northwest could decline to fairly low elevations toward the end of the week. During Tuesday-Wednesday, strengthening central Plains into Great Lakes low pressure as well as its trailing cold front plus a separate warm front lifting through Gulf of Mexico will likely bring precipitation to much of the eastern half of the lower 48. Widespread and potentially heavy rainfall is possible from the lower Mississippi Valley into the southern Appalachians, and then the Northeast could also see a period of intense rainfall given the strong dynamics and potential wave with the approaching cold front. Sufficient moisture and instability in the warm sector could also lead to severe weather, with the Storm Prediction Center highlighting the chance in their outlook for next Tuesday across parts of eastern Texas into the lower Mississippi Valley. Meanwhile there may be a band of meaningful snow to the northwest of the surface low track with a possible focus over the Upper Midwest. Finally, this storm may produce strong winds over some areas of the northeast quadrant of the lower 48. Some rainfall may develop over the east-central/south-central U.S. late in the week, depending on how the exact pattern at the surface and aloft allows for any incorporation of Gulf moisture. At this time confidence is very low with respect to potential coverage and amounts. Much of the West and northern High Plains should see below normal temperatures for most of next week. Expect the coldest anomalies to be over and near the northern half of the High Plains on Tuesday with highs 20-30F below normal. Highs of 10-20F below normal will be common over the Interior West on Tuesday and the Northwest late in the week. Some daily records for cold lows/daytime highs may be possible around Friday. Meanwhile, the south-central and east-central U.S. will likely see a brief period of above normal temperatures (up to plus 10-15F anomalies for highs and slightly warmer for morning lows) on Tuesday-Wednesday ahead of the system emerging from the Plains. Then the cold air behind the front will drop readings over most of these areas to moderately below normal by next Thursday. Rausch 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