Extended Forecast Discussion NWS Weather Prediction Center College Park MD 306 PM EST Mon Feb 10 2020 Valid 12Z Thu Feb 13 2020 - 12Z Mon Feb 17 2020 ...Overview and Guidance/Predictability Assessment... Most guidance maintains a pattern of shortwaves rounding an eastern Pacific ridge aloft and feeding into/ejecting from a long-term western U.S. mean trough. Another ridge expected to remain anchored near the Bahamas and Cuba should maintain a core of positive height anomalies near the East Coast, also helping to define the trough to the west. This pattern continues to favor periodic episodes of enhanced precipitation centered over the east-central U.S., while some flattening/east-west elongation of the Pacific ridge will likely promote a stronger feed of moisture into the Northwest. In spite of the decent agreement for the overall pattern and general weather tendencies, unresolved details for embedded shortwaves will continue to produce challenges in determining precipitation coverage over parts of the West and to an even greater extent for coverage/intensity over the eastern half of the country. Model consensus has held up well over the past day for the details of the system expected to track rapidly through the Northeast on Thu. Latest 00Z/06Z models still show minor detail/track differences but their overall average looks consistent, considering some conflicting signals in the northern/southern stream suggesting both a quicker and slower progression. This consensus proved to be a useful starting point elsewhere in the CONUS Thu-Sat. For the trough reaching the Northwest by late Thu, there is still a wide spread among the full range of model and ensemble guidance for what happens to the embedded energy after that time. There appear to be 3 camps of solutions, bookended by the slower and closed off 00Z ECMWF/Canadian/UKMET and the quicker and open GFS runs. The 00Z NAVGEM continued to be squarely in the middle near a larger majority of ensemble members, and this continues to be a favored position given the large run-to-run swings in the deterministic models. This conservative/consensus approach emphasized the 06Z GEFS/00Z ECMWF ensemble means along with the previous shift for the Sun-Mon period. Ejection of this western shortwave energy would produce weak fast-moving surface waviness through the central/eastern U.S. during the latter half of the period and allow for a return of Gulf moisture that would contribute to another precipitation episode. While confidence is low for the specifics, at least this scenario is consistent with the expected large scale pattern. Behind this feature there is decent consensus for a somewhat flatter shortwave to enter the West around day 6 Sun and then amplify gradually as it heads into the Rockies, supporting Plains low pressure by day 7 Mon. Yet another shortwave/weak frontal system may reach the Northwest on Mon as well. ...Weather/Hazard Highlights... The series of frontal systems expected to move into the Northwest should spread areas of rain and mountain snow eastward/southeastward across the West during the period. Expect highest totals during the period to be over favored Cascades/coastal terrain over Washington/Oregon with a lesser maximum over the northern Rockies. Best consensus of the guidance is pointing to the system arriving on Sat-Sat night as having the heaviest activity for the Pacific Northwest. Expect some moisture to extend into central latitudes of the West but with lower confidence for specifics due to guidance spread for amplitude and timing of shortwave energy. The system tracking through the Northeast on Thu will bring some wintry weather to areas from the central Great Lakes into New England. Meaningful snow accumulations are possible but fast motion of the storm should be a limiting factor for amounts. In the wake of the system, expect a brief period of cold/windy conditions that will produce some lake effect snow. The cold front trailing from the surface low will bring rainfall of varying intensity to the Mid-Atlantic and Southeast. From the weekend into early next week there should be an increase of precipitation coverage and intensity over the eastern half of the country with the ejection of western U.S. shortwave energy and associated surface feature(s). Best ensemble signals for highest totals extend across the Lower Mississippi Valley into the Tennessee Valley which is consistent with the large-scale pattern. Thus the manual deterministic forecast represents this scenario. However the full model/ensemble spread shows such a sensitivity to details aloft that precipitation could range between fairly light and limited in coverage to some heavy rain areas with broad coverage of at least light-moderate activity. Any precipitation in about the northern third of the eastern half of the lower 48 could be in wintry form (generally around the Great Lakes into the central Appalachians). The storm tracking through the Northeast on Thu will bring one more day of above normal temperatures to the East Coast with some record highs possible from Florida northward through the Carolina coasts. Much colder Plains air will rush into the East by Fri-Sat with widespread sub-freezing highs along and north of I-70 east of the Mississippi. Some parts of the Midwest/Upper Mississippi Valley may see temperatures 15-30F below normal Thu into Fri while 10-20F below normal readings extend into the Ohio Valley/Great Lakes/Northeast by Fri into Sat. Eastern U.S. temperatures will likely rebound fairly quickly to above normal values by Sun-Mon as southwesterly flow aloft arrives. System progression may lead to more temperatures variability over parts of the West though most of the Rockies should tend to be modestly below normal on most days. Fracasso/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 and the Northern Great Basin, Sat-Sun, Feb 15-Feb 16. - Heavy rain across portions of the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Mon, Feb 17. - Heavy rain across portions of the Southeast, the Mid-Atlantic, the Lower Mississippi Valley, the Southern Appalachians, and the Tennessee Valley, Sun, Feb 16. - Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Sat-Sun, Feb 15-Feb 16. - Flooding possible across portions of the Lower Mississippi Valley, the Central Appalachians, the Tennessee Valley, the Southern Appalachians, the Southeast, and the Ohio Valley. - Flooding occurring or imminent across portions of the Southeast, the Mid-Atlantic, the Lower Mississippi Valley, the Pacific Northwest, and the Ohio Valley. - Flooding likely across portions of the Southeast and the Mid-Atlantic. - Much below normal temperatures across portions of the Middle Mississippi Valley, the Great Lakes, the Upper Mississippi Valley, the Northern Plains, and the Ohio Valley, Thu-Fri, Feb 13-Feb 14. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Mon, Feb 17. - Much below normal temperatures across portions of mainland Alaska, Thu-Sun, Feb 13-Feb 16. 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