Extended Forecast Discussion NWS Weather Prediction Center College Park MD 324 PM EST Mon Feb 17 2020 Valid 12Z Thu Feb 20 2020 - 12Z Mon Feb 24 2020 ...Guidance/Predictability Assessment... Today's guidance shows a fairly progressive pattern overall. Upper low/trough energy that separates from northern stream flow off California on Thu will be somewhat slower for a time, before being pushed along by northeastern Pacific energy that leads to western U.S. trough amplification by the start of next week. Most solutions agree in principle but with meaningful timing differences in recent runs, while a smaller number of models/ensembles suggest potential for some alternative scenarios depending on southern stream system timing and northern stream amplitude/timing. Based on data available through the 06Z cycle, the greatest forecast problem involved the system evolving off California on Thu and then progressing through the Southwest into the Plains. The 00Z CMC was a fast extreme, which combined with a more amplified northern stream shortwave led to more stream interaction than seen in other guidance. On the other hand 00Z/06Z GFS runs were on the slower side. Remaining guidance clustered close to or slightly slower than the ECMWF. A starting blend among non-CMC guidance (80 percent models/20 percent total GEFS-ECMWF means) represented the preferred intermediate solution well, keeping good continuity in concept though with slightly faster progression late in the period. New 12Z runs provide much better agreement as the CMC has adjusted slower and the GFS faster. Model/ensemble agreement was fairly good for the upper trough that amplifies into the West early next week. As with the leading southern stream system, the 00Z CMC was somewhat on the faster side of the spread--and slightly less amplified--while the GFS/ECMWF and their means were within typical guidance error ranges for 6/7-day forecasts. This favored a blend of that model/mean majority, with the means reaching 50 percent weight by day 7 Mon to account for gradually increasing detail uncertainty. 12Z guidance has done little to change this preference. Ahead of these features, the predictability is lower for specifics of shortwave energy that starts the period from the Great Lakes-Midwest into the central/north-central Rockies. Operational runs have had various ideas for how this energy will evolve/progress, though the Appalachians/Mid-Atlantic upper low in latest CMC runs has lower confidence given the flattening of upstream mean flow late this week. ...Weather Pattern and Hazard Highlights... Strong high pressure will prevail over most of the lower 48 late this week and lead to fairly low precipitation coverage. The most organized area of precipitation during this time will extend from the Southern Plains/western Gulf Coast through the Southeast/southern Mid-Atlantic in a band of post-frontal moisture. Uncertain details of shortwave energy aloft will affect some specifics including location of highest totals and northward extent of the precip shield. Parts of the extreme northern fringe of this moisture may contain a little snow. The system evolving off California late this week and then progressing eastward will bring moisture into parts of California and the Four Corners states from about late Fri into the weekend. There may be a brief period of enhanced rain and high elevation snow with this system. As height falls and the surface system continue eastward expect precipitation to develop over the Plains/Mississippi Valley this weekend and spread into the eastern states through early next week. Gulf of Mexico return flow may help to promote locally moderate to heavy rainfall over parts of the mid-lower Mississippi and Tennessee Valleys/southern Appalachians, areas that will be sensitive to additional rainfall given ongoing runoff issues. This will be a fairly warm system so expect any winter weather to be at higher latitudes and that is if moisture extends sufficiently far north. The upper trough/surface system moving into the West by late weekend/early next week will bring an episode of enhanced rain and higher elevation snow to the Pacific Northwest with somewhat lighter activity extending into the northern/central Rockies. Late this week expect below to much below normal temperatures from the north-central Rockies into the Southern Plains as well as Upper Midwest into New England with some locations as low as 15-25F below normal. Much of the remaining eastern two-thirds of the U.S. will be below normal to a less extreme degree. Temperatures will moderate into the weekend though with some lingering coolness over the South/Southeast. The system moving into the West late in the period will bring below normal temperatures to most of the region by the start of next week. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy precipitation across portions of the Pacific Northwest, Sat-Sun, Feb 22-Feb 23. - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Sat-Sun, Feb 22-Feb 23. - Heavy rain across portions of the Southeast, the Mid-Atlantic, the Southern Appalachians, and the Tennessee Valley, Sun-Mon, Feb 23-Feb 24. - Heavy snow across portions of the Pacific Northwest and the Northern Great Basin, Sat-Mon, Feb 22-Feb 24. - Heavy snow across portions of the Central Rockies, the Central Great Basin, the Northern Rockies, the Northern Great Basin, and the Northern Plains, Sun-Mon, Feb 23-Feb 24. - Heavy snow across portions of the Central Rockies, the Central Plains, and the Central Great Basin, Mon, Feb 24. - Flooding possible across portions of the Southeast. - Flooding occurring or imminent across portions of the Lower Mississippi Valley, the Tennessee Valley, the Middle Mississippi Valley, the Mid-Atlantic, the Southeast, and the Ohio Valley. - Flooding likely across portions of the Southeast and the Ohio Valley. - Much below normal temperatures across portions of the Central Plains, the Northeast, the Central Great Basin, the Northern Plains, the Great Lakes, the Central Rockies, the Southern Plains, and the Northeast, Thu-Fri, Feb 20-Feb 21. - Much below normal temperatures across portions of the Middle Mississippi Valley, the Great Lakes, the Upper Mississippi Valley, and the Northern Plains, Thu, Feb 20. - Much below normal temperatures across portions of the Southeast and the Mid-Atlantic, Sat-Sun, Feb 22-Feb 23. - Heavy precipitation across portions of the Alaska Panhandle, Thu-Fri, Feb 20-Feb 21, and portions of mainland Alaska, Mon, Feb 24. - Heavy snow across portions of the Alaska Panhandle and mainland Alaska, Thu-Fri, Feb 20-Feb 21. - Much below normal temperatures across portions of mainland Alaska, Thu-Sun, Feb 20-Feb 23. 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