Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EST Fri Feb 12 2021 Valid 12Z Mon Feb 15 2021 - 12Z Fri Feb 19 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... Today's guidance continues to show a very active synoptic pattern through much of the medium range 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. 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. At least through the 06Z cycle the expected pattern and most of the spread/uncertainties involved did not change much over the past day, so the forecast approach likewise maintained continuity. This focused on a composite of 06Z GFS and 00Z ECMWF/UKMET ideas for the first half of the period followed by a transition toward the ECMWF/GFS and their respective ensemble means. Also toward the latter part of the week the ECMWF component included some of the prior 12Z/11 run. The exact structure of the system affecting the East early-mid week remains uncertain and clustering is still about the same--ECMWF/ECMWF mean/UKMET a little deeper aloft with slower timing and somewhat more inland low emphasis versus the GFS/GEFS mean. In addition the CMC/CMC mean continue to differ from other guidance in what happens to the upper low that tracks north of Lake Superior in the short range, offering a lurking risk of adjustments to a solution somewhere between the GFS and ECMWF ideas. Sure enough the 12Z ECMWF holds back the upper low with leading flow yielding a faster eastern system. Concerning the next shortwave to drop through the West and amplify into the mean trough early-mid week and continue into toward the eastern U.S. thereafter, the spread remains moderate relative to typical disagreements/model errors at extended time frames. Developing timing differences for the upper trough and associated surface system are due in part to issues with flow upstream. By late in the week the 00Z ECMWF/CMC and their means bring somewhat more energy into the West, leading to relatively flat flow versus the GFS/GEFS mean. New 12Z runs of the GFS/GEFS/CMC show similar shapes to their previous run but the CMC has raised its western heights. The upstream flow in the ECMWF leads to a farther eastward East Coast system versus some of its prior runs (though the ECMWF mean has been more consistent with this idea). On the other hand operational GFS runs are a bit on the slower/amplified side with the upper trough heading into the East. Prefer to maintain an intermediate solution for this system, which happens to be similar to where the new 12Z CMC has settled by early Friday. This intermediate 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 out with a surface low tracking from the Gulf into the Eastern U.S. Monday-Tuesday accompanied by a broad area of precipitation. The greatest winter weather threats will extend from the Lower Mississippi Valley, across the Ohio Valley, and into the Northeast. Snow will fall sufficiently far into the cold air, but there will be potential for areas of significant sleet/freezing rain south of the snow area. Wintry weather may extend unusually far southward over the southeastern Plains/Lower Mississippi Valley given the extremely cold air in place over the central U.S. early in the week. Meanwhile rain may be locally moderate to heavy at times across the Tennessee Valley/Southeast/southern Mid-Atlantic. The next system that reaches the Northwest early next week 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 to Friday. As upper ridging from the Atlantic builds toward southern Florida and the upper trough heads into the Plains/northern Mexico, combined with a more favorable surface evolution, the system may feature a stronger flow of moisture from the Gulf than the preceding system early in the week. Therefore the heavy precipitation potential will require monitoring over the coming days. As with the previous system, expect a broad area of winter weather impacts from the Southern Plains northeastward. The upstream Pacific front heading into the Northwest late in the week will likely bring moisture into the Pacific Northwest around Wednesday night, with rain and mountain snow continuing for the next couple days while moisture also spreads farther inland. The combination of the above systems should generate one area of significant precipitation totals extending from near the Gulf Coast into the Mid-Atlantic/Northeast, with the exact axis of highest totals to be determined by each system's track/timing. Also note that the Storm Prediction Center is monitoring severe potential within the warm sector of both eastern systems, with further details to be better resolved over coming days. Over the West expect favored terrain of the coastal ranges from Washington into extreme northern California as well as the Cascades to see the highest liquid equivalents. The northern Rockies will see significant precipitation but with somewhat lesser totals, with more moderate amounts extending farther south over the Sierra Nevada, Great Basin, and central/southern Rockies. Still anticipate a widespread area of record lows/cold highs over the Plains and lower/mid Mississippi Valley into Tuesday with temperatures of at least 30-45F below normal. A few locations in Texas/Oklahoma or nearby could even see daytime highs 50F or so below normal on Monday. 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 below normal temperatures may creep into the Sunshine State as a front crosses the area late in the week. Rausch/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 California and the Pacific Northwest, Mon-Tue, Feb 15-Feb 16 and Thu-Fri, Feb 18-Feb 19. - Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Mon-Tue, Feb 15-Feb 16 and Thu-Fri, Feb 18-Feb 19. - Heavy snow across portions of the Northern Rockies, Mon, Feb 15 and Thu, Feb 18. - Heavy snow across portions of the Central Rockies, Mon, Feb 15 - Heavy snow across portions of the Central Great Basin, Mon-Tue, Feb 15-Feb 16. - Heavy snow across portions of the Southern Rockies and the Southwest, Tue, Feb 16. - Heavy snow across portions of the Middle/Lower Mississippi Valley, the Tennessee Valley, the Ohio Valley, the Great Lakes, and the Central Appalachians, Mon, Feb 15. - Heavy snow across portions of the Northeast, and the Great Lakes, Tue, Feb 16. - Heavy precipitation across portions of the Lower Mississippi Valley, the Tennessee Valley, the Ohio Valley, the Central Appalachians, the Mid-Atlantic, and the Northeast, Mon-Tue, Feb 15-Feb 16. - Heavy rain across portions of the Southern Appalachians and the Mid-Atlantic, Mon-Tue, Feb 15-Feb 16. - Heavy precipitation across portions of the Lower Mississippi Valley, the Tennessee Valley, the Ohio Valley, the Central Appalachians, the Great Lakes, the Mid-Atlantic, and the Northeast, Wed-Fri, Feb 17-Feb 19. - Heavy rain across portions of the Lower Mississippi Valley, the Central/Southern Appalachians, the Tennessee Valley, the Mid-Atlantic, and the Southeast, Wed-Fri, Feb 17-Feb 19. - Much below normal temperatures across portions of the Northern/Central/Southern Rockies, the Northern/Central Plains, and the Southwest, Mon-Tue, Feb 15-Feb 16. - Much below normal temperatures across portions of the Northern/Central/Southern Plains, the Upper/Middle/Lower Mississippi Valley, the Tennessee Valley, the Ohio Valley, and the Great Lakes, Mon-Fri, Feb 15-Feb 19. 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