Extended Forecast Discussion NWS Weather Prediction Center College Park MD 342 PM EST Tue Feb 25 2020 Valid 12Z Fri Feb 28 2020 - 12Z Tue Mar 03 2020 ...Overview... Guidance continues to advertise a reversal in the large scale upper pattern over the course of the period. An amplified western ridge/eastern trough late this week will progress eastward while a building ridge to the north/northeast of Hawai'i and south of the Gulf of Alaska will favor the development of a sharp trough digging into California/southwestern U.S. by Sun-Mon. This trough could contain an embedded upper low for a time. Northern stream flow will continue across the northern tier U.S./southern Canada, with a gradual northward trend east of 100W as a subtropical ridge rebuilds over the western Caribbean by late in the period. ...Guidance/Predictability Assessment... Within the fairly agreeable mean pattern evolution, the most prominent issues involve specifics of the North Pacific flow that splits as it heads into North America during the weekend and then the shape of upstream flow that reaches the Northwest by day 7 Tue. Of particular note the 12Z GFS has backed away from 00Z/06Z runs that had closed off an upper low offshore the West Coast, versus the majority cluster that shows the core of the upper trough/possible upper low center along or a little inland from the coast. The past couple days of ensemble mean runs have been showing a moderate westward trend for the digging trough so there was some support for partial 06Z GFS input for the updated blend to nudge the overall trough a tad westward. Meanwhile guidance has been inconsistent/diverse with specifics of northern tier U.S./southern Canada flow. The past 12 hours of GFS runs exhibit a pronounced slower trend from a fast 00Z GFS solution, while the 00Z UKMET/12Z CMC agree poorly with the means. Behind this feature the forecast preference sides with an intermediate solution for flow reaching the Northwest by day 7 Tue. Recent ECMWF/CMC runs and their means favor some degree of shortwave energy versus the higher heights in the GFS/GEFS mean, though the 00Z ECMWF could be a little strong with its shortwave. The 12Z GFS/GEFS cycle extends this difference, now showing a ridge coming into the Northwest next Tue. Current consensus still favors a modest shortwave. This flow may eventually have some influence on the forecast for the leading system reaching the southern Rockies and central U.S. toward the end of the period. Forecast preferences through the 06Z cycle led to a starting blend that consisted of mostly the 00Z ECMWF/06Z GFS/00Z CMC early in the period followed by a trend to a model-mean blend (06Z GEFS/00Z ECMWF mean for the ensemble input) by the latter part of the period. ...Weather/Hazard Highlights... The North Pacific energy digging into the western U.S. will initially bring rain and high elevation snow into the Northwest in the late week/early weekend time frame with a fairly broad moisture shield spreading south-southeast over the West into early next week. Most activity should be light to moderate though with some terrain enhancement possible. Expect another precipitation episode over the northwestern states next Mon-Tue but with lower confidence regarding intensity and southward extent. To the east of the Rockies precipitation should be limited in coverage Fri-Sun. Some lake-effect snow will extend into the weekend while a weak wave and associated front may bring a band of light rain/snow to parts of the Ohio/Tennessee Valleys into southern Appalachians late this week. Parts of the Plains and east-central U.S. should see increasing coverage/intensity of precipitation by next Mon or Tue due to the combination of upper trough with possible embedded low approaching from the southern portions of the West and strengthening low level flow of moisture from the Gulf. Currently expect highest totals within an area between the southern half of the Plains and Tennessee-Ohio Valleys/Lower Great Lakes. Any snow will likely be confined to fairly northern latitudes. Areas of above/below normal temperatures will follow system progression during the period. The East will see a decent area of highs 10-15F below normal Fri-Sat. A few locations, including southern Florida, may be close to daily records for cold max readings. Eastern U.S. temperatures will moderate after Sat. Above normal readings over the West (some 10F or greater anomalies) late this week will continue eastward into the Plains by Sat-Sun and expand toward the East thereafter. Min temperatures may be as high as 15-20F above normal from the Southern Plains northeastward into the Ohio Valley by next Mon or Tue. 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 azards: - Heavy precipitation across portions of the Pacific Northwest, Mon-Tue, Mar 2-Mar 3. - Heavy rain across portions of the Central Plains, the Lower Mississippi Valley, the Tennessee Valley, the Great Lakes, the Middle Mississippi Valley, the Southeast, the Southern Plains, and the Ohio Valley, Mon-Tue, Mar 2-Mar 3. - Heavy snow across portions of the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Mon-Tue, Mar 2-Mar 3. - Heavy snow across portions of the Southern Rockies, the Central Rockies, and the Central Great Basin, Sun-Tue, Mar 1-Mar 3. - Flooding possible across portions of the Southeast, the Middle Mississippi Valley, the Great Lakes, and the Ohio Valley. - Flooding occurring or imminent across portions of the Lower Mississippi Valley, the Tennessee Valley, the Middle Mississippi Valley, the Southeast, the Southern Plains, and the Ohio Valley. - Flooding likely across portions of the Central Plains, the Southeast, and the Southern Plains. - Much below normal temperatures across portions of the Southeast, the Great Lakes, the Ohio Valley, and the Upper Mississippi Valley, Fri-Sun, Feb 28-Mar 1. 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