Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Fri Dec 09 2022 Valid 12Z Mon Dec 12 2022 - 12Z Fri Dec 16 2022 ...Mountain/inland 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... Rain and thunderstorms will be likely across parts of the Southeast along a wavy frontal boundary, while period of rain/snow move through the Northeast as a deep upper low rotates offshore near the Canadian Maritimes. The main story however during the medium range period continues to feature a major storm system tracking across the West as the period begins Monday, supporting widespread and potentially significant mountain snows across parts of the interior West and the Rockies. By Tuesday and beyond, intense cyclogenesis is expected to occur with increasing confidence in a major storm 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 blizzard conditions across parts of the north-central Plains and into the Upper Midwest around mid week next week 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 snow potential farther north) later next week. ...Guidance/Predictability Assessment... A deep upper low and shortwave tracking through the Canadian Maritimes and the Northeast on Monday-Tuesday shows good clustering in the guidance, though with some lingering questions on details. Upstream, guidance continues to agree for an amplified trough/closed low over the West to deepen with vigorous surface cyclogenesis into the Plains midweek. The guidance remains well clustered on the parent deep upper low tracking across the central Plains on Wednesday to the Midwest by Friday. For this mid to deep medium range time frame, there's fairly remarkable agreement in the setup and general track, with some lingering questions on timing (CMC seems too quick for this wrapped up/deep of a system, UKMET a little slower). Either way, a general model compromise weighted towards the agreeable GFS and ECMWF seems reasonable through day 5. By later next week, there are some more differences especially regarding track/timing of possible coastal low development off the Mid-Atlantic as the main upper low shifts into the Midwest. Increasing contributions from the ensemble means seemed reasonable by this point, though still maintained at least 50 percent deterministic models for added system depth, which majority of models continue to depict a fairly deep system even through day 7. Compared to previous WPC continuity, this resulted in a slightly slower frontal progression across the central and eastern U.S., though that is consistent with the latest model trends. A trend towards the ensemble means later next week also helped resolve some larger uncertainties with another upper low off the West Coast. ...Weather/Hazard Highlights... The system rotating over the Northeast should continue to bring some rain/snow to parts of the region into Monday, while farther south, a lingering wavy frontal boundary results in scattered rain and thunderstorms. 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, 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 and especially by Tuesday and shifting 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 in a transition zone. This also shows some wintry weather potential spreading across the Great Lakes region and Northeast in favorable downstream theta-e advection over a pre-cooled airmass. 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. Schichtel/Tate 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 Great Lakes and the Upper Mississippi Valley, Tue-Wed, Dec 13-Dec 14. - Heavy precipitation across portions of the Mid-Atlantic and the Northeast, Sun, Dec 11. - Heavy rain across portions of the Central Great Basin, California, and the Southwest, Sun, Dec 11. - Heavy rain across portions of the Mississippi Valley, the Central/Southern Appalachians, the Tennessee Valley, the Great Lakes, the Mid-Atlantic, the Southeast, the Southern Plains, and the Ohio Valley, Tue-Thu, Dec 13-Dec 15. - Heavy snow across portions of the Central Great Basin, California, the Northern Rockies, the Northern Great Basin, and the Southwest, Sun, Dec 11. - Heavy snow across portions of the Southern Rockies, the Central Rockies, the Central Great Basin, and the Southwest, Mon, Dec 12. - Heavy snow across portions of the Central Plains, the Great Lakes, the Upper Mississippi Valley, and the Northern Plains, Mon-Wed, Dec 12-Dec 14. - Severe weather across portions of the Lower Mississippi Valley and the Southern Plains, Mon, Dec 12. - Severe weather across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Tue, Dec 13. 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