Extended Forecast Discussion NWS Weather Prediction Center College Park MD 343 PM EST Thu Jan 07 2021 Valid 12Z Sun Jan 10 2021 - 12Z Thu Jan 14 2021 ...Overview... The guidance for flow aloft continues to show the establishment of an amplified western ridge/eastern trough pattern with various uncertainties for embedded features within multiple streams. Consensus shows the upper ridge strengthening during Tue-Thu while it will likely take until next Thu and beyond for the eastern trough to deepen as a result. The two main areas of notable precipitation will be over the Northwest in association with multiple shortwaves/frontal systems and over portions of the South/East with a system tracking from the northern Gulf of Mexico into the western Atlantic. Parts of the northern tier will see well above normal temperatures through the period while the South will be below normal from the weekend into early next week. ...Guidance/Predictability Assessment... Dependence on shortwaves in multiple streams has been causing significant difficulty in forecasting the details of the northern Gulf/western Atlantic system. At least early in the period guidance has developed better clustering in resolving the shortwave aloft crossing the southern Plains/Lower Mississippi Valley Sun into Mon. Stronger signal for a potential compact upper low (though likely to open up after Mon) seems to be supporting recent trends for a slightly slower and farther northwest surface low over/near the Southeast. After Mon there is still a lot of spread in model/ensemble spaghetti plots for northern stream flow details from the Plains into the Northeast. Thus confidence remains low for specifics along the East Coast. However trends of the 06Z GFS/GEFS mean closer to the 00Z ECMWF/ECMWF mean (ECMWF mean generally the most stable individual guidance source aside from the modest slower trend over the past couple days) do provide some modest improvement to confidence compared to the past couple days. The 00Z CMC was a part of this cluster for about the first half of the period before straying slow/westward versus other guidance, while the new 12Z run has adjusted favorably eastward by day 6 Wed. Most differences over the West either have low predictability or are within typical model error for the time frame of interest. The one more questionable solution is the new 12Z CMC which brings a suspiciously strong shortwave through the Northwest Tue into Wed. Guidance comparisons led to a 00Z ECMWF/06Z GFS/00Z CMC emphasis for the first half of the period followed by a transition to the ECMWF/GFS and their ensemble means (more ECMWF mean versus GEFS) toward Wed-Thu. ...Weather/Threats Highlights... Rain and high elevation snow over the Pacific Northwest should be heaviest early next week (Mon-Tue), with highest totals over Washington and along favored terrain of the coastal ranges/Cascades. Some of the moisture should eventually reach the northern Rockies. Expect precipitation to trend lighter by Wed-Thu as the upper ridge near the West Coast strengthens. The system emerging into the western Gulf on Sun may produce locally moderate to heavy precipitation over portions of Texas and farther east along the Gulf Coast. Also parts of Texas (west-central in particular) could see a significant snowfall from this system. Consult Weather Prediction Center's winter weather products for more information on where this potential is greatest. There is still considerable uncertainty for the exact northward/westward coverage and intensity of precipitation farther east, corresponding to guidance spread for the strength/track/timing of low pressure tracking into the Atlantic. The northwestern part of the moisture shield may contain some snow. General lack of cold air in place may lead to somewhat higher potential to be over higher elevations and/or at night. Taken as a whole, new 12Z models including the ECMWF suggest decreasing precipitation potential for the Northeast. Highest temperature anomalies in the northern tier should extend from Montana through the Dakotas into Minnesota. Morning lows will likely be at least 10-20F above normal most days and possibly 20-30F above normal by Wed-Thu. Highs may not be as extreme versus average but plus 20-30F readings are still possibly mainly on Wed. A Pacific cold front should start a cooling trend on Thu. Some plus 10F or greater anomalies may extend into the central Plains next week. Meanwhile warmth over the West will likely become more pronounced by midweek as the upper ridge strengthens, leading to decent coverage of plus 10F or greater anomalies for highs by next Thu. Within the cool pattern over the South, areas from the southern Plains into Lower Mississippi Valley should see highs 10-20F below normal Sun-Mon. Coldest anomalies are likely over the parts of Texas that see snow on Sun. Expect a moderating trend over the South after Mon. 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 Hazards: - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Mon-Tue, Jan 11-Jan 12. - Heavy rain across portions of the Southern Plains, Sun, Jan 10. - Heavy snow across portions of the Southern Plains, Sun, Jan 10. - Flooding occurring or imminent across portions of the Southeast, the Southern Plains, the Mid-Atlantic, and the Pacific Northwest. - Flooding likely across portions of the Mid-Atlantic. - High winds across portions of the Central Rockies, and the Northern/Central Plains, Thu, Jan 14. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml