Extended Forecast Discussion NWS Weather Prediction Center College Park MD 512 PM EST Thu Nov 21 2019 Valid 12Z Sun Nov 24 2019 - 12Z Thu Nov 28 2019 ...Guidance/Predictability Assessment and Weather Highlights/Threats... The first system of concern in the medium range is a surface low pressure system that is expected to track near the Atlantic coast of the Northeast on day 3/Sun. Overnight model runs, as well as the new 12Z guidance, continued to show the low near the coast, spreading precipitation into the Northeast. As a general rule, the precipitation should fall more as rain near the coast and more as snow for interior New England (following the usual pattern). Probabilities for snow were increased in the Winter Weather Outlook since model guidance (GFS, EC, and UKMET) has been consistent for several runs now. Broad troughing over the CONUS is expected for day 4-5/Mon-Tue with embedded shortwaves with low predictability. Mon to Tue, energy should drop through the western side of the broad trough into the West Coast and then into the central U.S. The question remains how much this energy spins up in the central U.S. on Tue and northeastward on Wed. Deterministic GFS runs have been more bullish with this energy than the deterministic ECMWF runs especially Tue and even into Wed, which affects the depth of the surface low that could spin up just ahead. The 00Z ECMWF was even weaker than its 00Z mean with that surface low, but note that the new 12Z EC has a much stronger low. Overall, with inconsistencies from run to run in the deterministic guidance, mainly went with a blend of the 00Z EC and 06Z GEFS means. Uncertainty remains fairly high regarding the impacts of this system. This surface low will sweep a front across the Plains, Mississippi Valley, and into the Appalachians Mon-Wed. Models were much lower with precipitation in the Mississippi, Tennessee, and Ohio Valleys along this front compared to previous runs, so this forecast package trended down with the QPF there, but the pattern still looks favorable for precipitation. Additionally, behind the low some wintry precipitation is possible, but ensemble probabilities for snow are low and broad without much focus across the Upper Midwest. Thus the areal extent and amount of wintry weather remains in question. Additional energy drops southward again on Wednesday, deepening the trough in the eastern Pacific. With these multiple rounds of trough reinforcement, cooler than average temperatures should be present in the West for midweek with ample snowfall in favored higher elevations. Tate 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 Northeast, Wed-Thu, Nov 27-Nov 28. - Heavy precipitation across portions of the Upper Midwest into the central Great Lakes, and for parts of the Pacific Northwest into northwest California, Tue-Wed, Nov 26-Nov 27. - Heavy snow across much of the Sierra Nevada, Wed-Thu, Nov 27-Nov 28. - Heavy snow across portions of northern New England, Sun, Nov 24. - Heavy snow across portions of northern Idaho, Mon-Tue, Nov 25-Nov 26. - Heavy rain across portions of the Southeast, the Southern Appalachians, the Lower Mississippi Valley and the Ohio Valley, as well as the Tennessee Valley, Tue-Wed, Nov 26-Nov 27. - Flooding occurring or imminent across portions of the Northern Plains. 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