Extended Forecast Discussion NWS Weather Prediction Center College Park MD 344 PM EST Thu Nov 14 2019 Valid 12Z Sun Nov 17 2019 - 12Z Thu Nov 21 2019 ...Pattern Overview and Guidance/Predictability Assessment... Latest guidance expects a fairly amplified western ridge/eastern trough pattern aloft into the first half of next week. Pacific energy moving into western North America from Tue onward should eventually eject the eastern trough. The primary forecast problems involve specifics of two possible western Atlantic systems associated with energy in the eastern mean trough, and the ultimate evolution of energy that arrives at the West Coast around Tue-Wed. Over the East the majority of guidance shows one system off the southern Mid-Atlantic coast on Sun lifting north-northeast as upstream troughing approaches. The 00Z CMC strayed to the slow/western side of the envelope by day 5 Tue though a number of CMC ensembles were also westward but faster like remaining guidance. There is a decent signal that the eastern trough energy will generate another area of low pressure off the East Coast by Tue-Wed. Especially by late Wed the 00Z ECMWF was among the most western solutions. Confidence in specifics is low given various model/ensemble ideas for whether upper energy will close off and where an embedded closed low will track. In both cases prefer the best model-ensemble consensus that maintains reasonable continuity. For the energy nearing the West Coast by Tue there are already meaningful differences in timing and whether there will be a closed low within the upper trough, with the scale sufficiently small to lower predictability several days out in time. Toward Wed-Thu the most prominent difference is that GFS runs have tended to be more progressive with the southern part of the trough versus the ECMWF/CMC and their ensemble means as well as other operational models. The 06Z GEFS mean has also adjusted somewhat toward the ECMWF cluster's idea. This is connected to the GFS being fastest to bring height falls into the upstream ridge. The majority consensus of guidance and recent difficulties that GFS runs have sometimes had in properly amplifying energy into closed lows would recommend leaning toward the ECMWF cluster by late in the forecast. Based on the above considerations the updated forecast incorporated ideas from the 00Z GFS/CMC and past two ECMWF runs for about the first half of the period and then trended toward a blend among the ECMWF runs/00Z CMC and 06Z GEFS/00Z ECMWF means. This helped to downplay lower confidence details late in the period. ...Weather Highlights/Threats... Expect chilly weather over the East into the start of next week, with coolest readings of 10-20F below normal likely on Sun. Temperatures should moderate toward normal by the middle of next week. On the other hand areas from the West into parts of the northern-central Plains will see well above normal temperatures (up to plus 10-20F anomalies) especially Sun-Mon and to some extent Tue, followed by mostly single-digit anomalies above or below normal. The lead system off the Mid-Atlantic coast should keep the heaviest rains offshore but could be close enough to produce some rain along the coast. If any moisture extends farther inland it is most likely to be over the Northeast. In that case the western periphery of the moisture shield could contain some snow. The strong pressure gradient between the low and the surface high exiting New England will lead to windy conditions along the coast and especially over the nearshore waters. Please see the Ocean Prediction Center's products for more information. Precipitation coverage and intensity with the second area of low pressure evolving Tue-Thu are highly uncertain, with the Northeast again the most likely area to see some rain and inland/higher elevation snow if the moisture shield extends far enough westward. Favored terrain in the Pacific Northwest/northern Rockies will see one brief episode of light-moderate precipitation Sun-Sun night. Another area of moisture should cross the region around late Mon through Tue or so with similar or slightly higher totals. The majority scenario of an upper low settling over or near southern California mid-late week would increase the potential for significant precipitation (mostly rain) over the Southwest and in particular central Arizona. Still, guidance spread is sufficiently great to temper confidence regarding precip coverage/intensity. Remaining western energy aloft progressing across the northern half of the lower 48 may bring a front and accompanying precipitation into the east-central U.S. by next Thu. 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 rain across portions of the Southwest, Wed-Thu, Nov 20-Nov 21. - Flooding possible across portions of the Northern Plains. - Flooding occurring or imminent across portions of the Northern Plains. - High winds across portions of the Mid-Atlantic and the Northeast, Sun-Mon, Nov 17-Nov 18. 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