Extended Forecast Discussion NWS Weather Prediction Center College Park MD 316 PM EST Thu Nov 19 2020 Valid 12Z Sun Nov 22 2020 - 12Z Thu Nov 26 2020 ...Overview and Guidance Evaluation... The medium range forecast pattern continues to feature multiple embedded shortwaves providing unsettled weather across the Pacific Northwest and northern Rockies while uncertainty remains quite high from the central to the eastern U.S. for the middle of next week as Gulf moisture returns but there remains a great deal of run-to-run model variability regarding the intensity and forward speed of a low pressure system. Global models continue to show good indications of cyclogenesis from the central to eastern U.S. in their deterministic runs but their ensemble mean solutions remain much less robust. Thus, a conservative approach was adopted to generate this morning's WPC prognostic charts and weather grids. An even blend of the 00Z ECMWF, 00Z EC mean, 06Z GFS, 06Z GEFS together with a small portion from the 00Z CMC made up the Days 3 to 5 forecasts, switching to mainly using the 00Z EC mean and 06Z GEFS for Days 6 and 7. This yielded a weaker low pressure system tracking toward the Great Lakes for the middle of next week than in the previous WPC package. ...Sensible Weather and Hazards... From Sun into Mon, expect the wave tracking northeastward from the Mid-Mississippi Valley into southeast Canada to bring an area of organized precipitation from the south-central Plains into southeastern Great Lakes and Northeast. Most of the precipitation should fall as rain but parts of the moisture shield's northern periphery should contain some snow and wintry mix. The gradual southward trend for the precip axis southwest of the Great Lakes has been continuing. For next Tue-Thu, confidence in the specifics remains low but there is an increasing signal for a low pressure system to produce another episode of meaningful precipitation from the Plains into the East, with some snow possible in the upper Midwest to the upper Great Lakes. Over the Northwest, progressive mean flow aloft will promote a wet period as a series of fronts approach/reach the region. Some of the fine details are fluctuating as would be expected for this pattern but there is good continuity for persistence of rain and higher elevation snow with highest totals over Coastal and Cascade Ranges of the Pacific Northwest and a secondary maximum over northern parts of the Rockies. Lighter precipitation may extend as far south as northern California and into the central Rockies. The progressive pattern will keep temperatures variable over a majority of the lower 48. Most areas should remain within 10 degrees on either side of normal for highs but there are enough uncertainties with embedded system details that there may end up being some pockets of more anomalous highs. Larger areas of morning lows 10F or more above normal are possible ahead of the front pushing into the South/East Sun into Mon and another front reaching the Plains by Tue. Kong/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 Lower Great Lakes, Sun, Nov 22. - Heavy rain across portions of the Lower/Middle Mississippi Valley, the Tennessee/Ohio Valleys, and the Great Lakes, Tue, Nov 24. - Heavy snow across portions of the Pacific Northwest and the Northern Great Basin, Tue, Nov 24. - Flooding occurring or imminent across portions of the Southeast and the Mid-Atlantic. - Heavy precipitation across portions of mainland Alaska, Wed, Nov 25. - Heavy precipitation across portions of the Alaska Panhandle, Mon, Nov 23. - High winds across portions of the Alaska Panhandle and mainland Alaska, Mon-Tue, Nov 23-Nov 24. - High significant wave heights for coastal portions of the Alaska Panhandle, Tue, Nov 24. 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