Extended Forecast Discussion NWS Weather Prediction Center College Park MD 352 PM EST Fri Dec 09 2022 Valid 12Z Mon Dec 12 2022 - 12Z Fri Dec 16 2022 ...Mountain heavy snow/wind threat into the Rockies on Monday... ...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 is expected to cross much of the U.S. next week. As the period begins Monday, a deep upper trough and embedded upper low will be crossing the Intermountain West, producing widespread and potentially significant mountain snows across the higher elevations including the Rockies. By Tuesday and beyond, intense cyclogenesis is expected to occur and confidence remains high for a deep low pressure system emerging into the Plains. This system should bring a wide array of hazards including heavy rain/severe weather to the south and heavy snow with high winds creating potential blizzard conditions across parts of the north-central Plains and into the Upper Midwest around midweek with some chance of freezing rain in the transition zone. 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 including an upper low closing off as it moves from the West into the Plains, with 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. But a multi-model deterministic blend weighted toward the GFS and ECMWF worked well for a compromise solution on timing, which was a bit faster than the previous forecast. Model differences increase later into the week with how the central U.S. upper low evolves, which are due in part to systems to the east and west. To the east, a deep compact upper low over the Canadian Maritimes (with a shortwave extending into the Northeast early Monday) is likely to drift gradually southward through midweek, but then the timing of its ejection east (generally faster in the new 12Z guidance) plays a role in how much the central U.S. upper low can move eastward or stall. Then to the west, ample model differences have been seen over the past few days in the evolution/placement of a closed upper low forming across the Pacific and then potentially additional energy spilling into the West by the latter half of the week dependent on many factors including the first upper low. With these differences among others, GFS runs including the new 12Z have trended toward faster weakening of the central U.S. upper low and a quicker shunt eastward compared to other guidance that maintains it longer. These differences affect the potential for a developing surface low that could turn into a nor'easter late next week (ECMWF/CMC solutions) or get shunted eastward over the Atlantic (GFS deterministic run with support from many GEFS members). With these differences, confidence in the pattern dips by the later part of the period especially across the East. The latter part of the medium range forecast blend favored increasing proportions of the EC and GEFS ensemble means given this increasing uncertainty. ...Weather/Hazard Highlights... As the main upper trough digs across the West on Monday and shifts eastward the remainder of the week, a wide variety of hazards are likely across the country. Heavy precipitation is imminent across the West through Monday, with heavy snow especially into higher elevations of the Great Basin and Rockies. Then as the upper trough/low approaches the Plains and a surface low consolidates and deepens, ample moisture will spread ahead of a cold front to produce significant rainfall across the Lower/Middle Mississippi Valley and vicinity beginning Monday but with higher amounts by Tuesday, which should shift east with time. The Storm Prediction Center continues to monitor and highlight the potential for strong to severe convection across this region 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 as well as areas of ice/freezing rain within the transition zone. 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 while heavy rain could be a threat in the Southeast. 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. 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 much of the period expanding northward at times across the Mississippi Valley. 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 after the cold frontal passage. Tate/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 Hazards: - Heavy precipitation across portions of the Northern/Central Plains, the Great Lakes, and the Upper Mississippi Valley, Tue-Wed, Dec 13-Dec 14. - Heavy precipitation across portions of the Mid-Atlantic, the Northeast, and the Great Lakes, Thu, Dec 15. - Heavy rain across portions of the Central/Southern Plains, the Lower Mississippi Valley, the Tennessee Valley, the Upper/Middle Mississippi Valley, and the Ohio Valley, Tue, Dec 13. - Heavy rain across portions of the Lower Mississippi Valley, the Central/Southern Appalachians, the Tennessee Valley, the southern Mid-Atlantic, the Southern Appalachians, the Southeast, and the Ohio Valley, Wed, Dec 14. - Heavy snow across portions of the Central/Southern Rockies, the Central Great Basin, and the Southwest, Mon, Dec 12. - Heavy snow across portions of the Northern/Central Plains, the Great Lakes, and the Upper Mississippi Valley, Mon-Wed, Dec 12-Dec 14. - Severe weather across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, and the Southern Plains, Tue, Dec 13. - Severe weather across portions of the Southeast and the Central Gulf Coast, Wed, Dec 14. - High winds across portions of the Northern/Central Plains and the Upper/Middle Mississippi Valley, Tue-Wed, Dec 13-Dec 14. 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