Extended Forecast Discussion NWS Weather Prediction Center College Park MD 122 AM EST Sat Feb 13 2021 Valid 12Z Tue Feb 16 2021 - 12Z Sat Feb 20 2021 ...Widespread record cold temperatures over the Plains and Mississippi Valley into early next week followed by only gradual moderation... ...Stormy pattern will bring multiple wintry precipitation events from coast to coast, with heavy rainfall potential across parts of the East late next week... ...Overview and Guidance Evaluation... The medium range continues to feature a very active synoptic pattern through much of the period, with a series of systems digging into and ejecting from a long-term mean trough aloft anchored over the contiguous U.S.. These systems will bring periods of active weather for the Western states, as well as parts of the Southern Plains and through a large part of the East. Across the Plains and into the Mississippi Valley, record cold is especially likely through Tuesday and may extend through midweek at a few locations across the South. As upper troughing heads into the East by late next week and upstream troughing holds back over the eastern Pacific, models indicate an evolution toward higher heights aloft over the western/central U.S., possibly to the point of a flat ridge building across those areas. Agreement is pretty good on the large scale the first half of the period, but some questions remain on the details of the first system exiting the East on Tuesday. The latest runs of the ECMWF/ECENS mean/UKMET continue to be a bit deeper aloft with slower timing for the main surface low off the Northeast coast Tuesday evening versus the GFS/GEFS mean. In addition, the CMC/CMC mean are significantly deeper with an upper low north of Lake Superior with faster leading flow on day 3, which offers a surface low evolution even faster/north than that of the GFS/GEFS. At this point, a solution somewhere between that of the ECMWF/GFS seems to be the best location and maintains good continuity. Things are a little bit more uncertain concerning the next shortwave to drop through the West and amplify into the mean trough early-mid week, continuing towards the Eastern U.S. thereafter. Timing differences really become apparent by next Friday as the trough enters the East, with the last few runs of the GFS/GEFS solutions maintaining a noticeably slower progression of the system, while the ECMWF (and CMC) have trended faster. The ECENS/CMC means generally support their deterministic counterparts, with support from the NAEFS as well. Given the day 6-7 time frame, the resulting WPC blend favors a more intermediate solution towards the ECENS/NAEFS means with the result a slightly faster Eastern U.S. cold front than depicted by the previous shift. This approach also serves well for resolving differences for late-week Northeast Pacific low pressure and the trailing front likely to push into the Northwest. ...Weather/Threats Highlights... The period starts with a surface low deepening as it exits the Mid-Atlantic on Tuesday. The main threat for wintry weather by this point will be across much of the Northeast, while the heaviest rain farther south should be off the coast. Some moderate rain should linger across Florida along the trailing boundary on Tuesday. The next system moving through the West on day 3 should bring a couple of waves of precipitation into the Pacific Northwest/northern California with snow mainly confined to the mountains. The moisture will continue southeastward through the Great Basin and the Rockies, followed by increasing coverage and intensity of precipitation over the Southern Plains/Lower Mississippi Valley into the East from Wednesday through Friday. The models continue to show a signal for heavy precipitation from the lower Mississippi Valley into much of the East. Moderate to heavy rainfall is likely farther south for the Gulf Coast/Tennessee Valley/southern Mid-Atlantic, with a broad area of winter weather impacts on the northern edge of the precipitation shield from the ArkLaTex region, northeastward into the Northeast. Also to note, the Storm Prediction Center continues to mention severe weather potential within the warm sector of this system (and the preceding system), with details to be better resolved over the coming days. Out West, another cold front pushes into the Northwest bringing with it the potential for another round of heavy mountain snows across the Washington/Oregon Cascades and into the Northern Rockies next Thursday and Friday. In terms of temperatures, still anticipate a widespread area of record lows/cold highs over the Plains and lower/mid Mississippi Valley into Tuesday, with temperatures at least 30-45 degrees below normal. Although the central U.S. will see moderating temperatures by mid-late week, this may involve anomalies recovering to "only" 20-30F below normal. Highs could moderate a little more by Friday afternoon. Some of this cold air will extend in modified fashion into the Ohio Valley and vicinity, and at times farther eastward. Temperatures in the West will be near to moderately below normal early-mid week followed by a warming trend as heights aloft increase. By the end of the week the Southwest may trend above normal and the northern Plains could reach above normal values as well. The extreme Southeast/Florida will be the one area of persistent warmth, with lows in particular expected to be well above normal most days. However by Friday into Saturday, below normal temperatures may creep into the Sunshine State as a front crosses the area late in the week. Santorelli/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, 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