Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM 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 models still show minor detail/track differences but their overall average looks consistent. 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. The adjustment made by the 12Z ECMWF to amplify all of the trough energy inland and not pull off a closed low brought it close to the ensemble means which have consistently been advertising a fairly open trough. The GFS remains a bit faster but not to the extent of the faster/flatter CMC. The UKMET continues to drop a closed low southward along the California coast, and now the new 00Z ECMWF is back to pulling a closed low offshore Baja California ultimately leading to a flatter/faster shortwave to the north. Model inconsistency and such a broad ensemble spread continued to favor a conservative/consensus approach for the overnight update, emphasizing the 18Z GEFS/12Z ECMWF ensemble means and 12Z ECMWF/12Z-18Z GFS runs. 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. The preferred guidance blend started with an emphasis on the 12Z-18Z GFS and 12Z ECMWF/CMC Thu-Fri. The forecast phased out the CMC after Fri and started to increase ensemble mean weight, reaching 60 percent by day 7 Mon. ...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. The storm tracking through the Northeast on Thu will bring one more day of above normal temperatures to the East Coast then, while bringing much colder Plains air into the East by Fri-Sat. 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 to above normal values by Sun-Mon. 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. 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 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