Extended Forecast Discussion NWS Weather Prediction Center College Park MD 234 PM EST Fri Dec 20 2019 Valid 12Z Mon Dec 23 2019 - 12Z Fri Dec 27 2019 ...Heavy rain and gusty winds possible along parts of the Southeast U.S. coast on Monday... ...Overview and Guidance/Predictability Assessment... Models and ensembles show a persistent upper-level mean trough generally across the West Coast and Great Basin through late next week. Moderately progressive flow will persist downstream across the central and eastern U.S. as shortwave energy ejects from the western trough and encounters broad anticyclonic flow across the south central and southeastern states. A persistent upper-level low across the Arctic Ocean well north of Alaska along with weakly positive height anomalies across Canada will minimize the risk of any significant outbreaks of cold air across the CONUS next week. Guidance expects a relatively deep upper low and associated low pressure system to begin pulling away from the Southeast Mon-Tue (days 3-4). Timing spread is gradually narrowing but most recent solutions have maintained their place in the envelope--GFS/CMC fastest and other solutions slower by varying degrees with the UKMET tending to be slowest. ECMWF runs have waffled somewhat but slower GFS trend over the past couple days (and continuing in the 12Z run) plus the overall guidance majority favor leaning somewhat more in the ECMWF direction. Within the western mean trough aloft, there are still some modest timing/amplitude differences for a fairly vigorous shortwave reaching California/Southwest states Mon night-Tue. This feature rapidly deamplifies across the central/eastern U.S. Wed-Thu (days 5-6), reflecting at the surface as a front likely to have multiple weak waves from the central U.S. to the Ohio Valley. Compared to yesterday guidance appears to have settled down somewhat with respect to northern stream flow and frontal timing over the East mid-late week though some differences remain. Returning farther upstream, by the latter half of the period recent GFS runs have been more amplified than most other guidance with a fast-moving North Pacific storm--ultimately leading to faster ejection of trough energy which consensus (including the GEFS/ECMWF/CMC means) shows reaching the West Coast around late Wed-Thu and Great Basin/Southwest by early Fri. The 12Z GFS is less extreme with the North Pacific storm but still pushes troughing into the western-central states faster than consensus late next week. As for the mid-late week trough, guidance continues to vary on existence/time frame of an embedded closed low. The 06Z GEFS mean trend toward the ECMWF mean recommended at least a strong hint of flow separation in the manual deterministic forecast into Thu. In a theme similar to 24 hours ago the 12Z CMC adjusted close to the 00Z ECMWF after being slower/more closed in the 00Z cycle. A starting blend consisting of about two-thirds 00Z ECMWF to one-third 06Z GFS represented forecast preferences well for about the first half of the period. Increasingly questionable aspects of the GFS from the Pacific into the lower 48 after midweek led to transitioning the forecast to a mix of the 06Z GEFS mean and 00Z ECMWF/ECMWF mean (with 60-75 percent weight on the means) for days 6-7 Thu-Fri. ...Weather Highlights/Threats... On Mon there will likely be a band of moderate to heavy rain on the north side of the surface low/frontal system about to progress off the Southeast U.S. coast. Model consensus suggests up to 1-3 inch totals within the most concentrated band of rainfall. This system may also produce areas of strong winds due to a rather intense pressure gradient north of the surface low. Meanwhile a leading shortwave aloft will bring areas of rain and higher elevation snow to a portion of the West primarily Mon-Tue. Best focus should be over favored terrain from southern California through Arizona and parts of the Great Basin/central Rockies, with a brief period of moderate to locally heavy activity. East of the Rockies the weakening shortwave and associated wavy front may produce an area of lighter precipitation (potentially of a wintry variety on the northern side of the moisture shield) around midweek. The next system(s) affecting the West Coast should bring more widespread rain and mountain snow mid-late week. Thus far the uncertainty in exact character of the upper trough energy has tempered confidence in location/duration/intensity of heaviest precip. At the moment the full array of guidance provides a signal for parts of California to see the highest totals with the Pacific Northwest seeing a lower probability. Toward the end of the week this West Coast trough may reach far enough eastward to promote increasing moisture return into the southern half of the Plains with an expanding area of rain in an overrunning-type setup north of a frontal boundary. Timing the onset of this event is a challenge given current model spread for the western trough, but consensus suggests it will be later than seen in GFS runs. Expect temperatures to be above average across much of the central and eastern U.S. next week. Greatest temperature anomalies are likely from the Central Plains to the Midwest Mon-Tue with high temperatures at least 10-20F above average. Some plus 10F or greater anomalies will probably persist over the eastern half of the country into Wed-Thu. For morning lows anticipate anomalies to be similar to or greater than those for highs. Meanwhile the persistent mean trough aloft affecting the West will promote near to slightly below average highs and near to above average morning lows. Rausch/Ryan 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 California, Thu, Dec 26. - Heavy rain across portions of the Central Plains, the Middle Mississippi Valley, the Lower Mississippi Valley, and the Southern Plains, Fri, Dec 27. - Heavy rain across portions of the Southeast and the Southern Mid-Atlantic, Mon-Tue, Dec 23-Dec 24. - Heavy snow across portions of California, the Central Great Basin, the Pacific Northwest, the Northern Great Basin, and the Southwest, Mon, Dec 23 and Thu-Fri, Dec 26-Dec 27. - Heavy snow across portions of the Southern Rockies, the Central Rockies, the Central Great Basin, and the Southwest, Tue-Wed, Dec 24-Dec 25. - Flooding possible across portions of the Southeast and the Northern Plains. - Flooding occurring or imminent across portions of the Pacific Northwest and the Northern Plains. - Flooding likely across portions of the Pacific Northwest and the Northern Great Basin. - High winds across portions of the Southeast, Mon-Tue, Dec 23-Dec 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