Extended Forecast Discussion NWS Weather Prediction Center College Park MD 454 PM EDT Fri Oct 22 2021 Valid 12Z Mon Oct 25 2021 - 12Z Fri Oct 29 2021 ...A Stormy Pattern Continues Next Week for the Nation... ...Overview... It will be quite the stormy weather pattern for the nation next week. A lead Midwest upper trough/low and uncertain phasing with northern stream trough energies digging to the lee of an amplified central Canadian upper ridge Monday may forge a closed low to work slowly across the Northeast to the Canadian Maritimes Tuesday-Thursday with a main low/frontal system whose deep low and enhanced moisture/rainfall now seems likely to focus further organizing with development of a deepening coastal low. Well upstream, multiple rounds of impactful weather for the West Coast this weekend/Monday will work inland across the Intermountain West/Rockies and then force Plains cyclo/frontogenesis by midweek as an amplified upper trough tracks eastward. Flow amplification from the Pacific to the West should also meanwhile refocus enhanced periods of precipitation back into the Pacific Northwest and also quite an amplified and slowing downstream upper trough over the central to eastern U.S. into later next week, along with a deepening low and an increasingly active/wet wrapping frontal system. ...Guidance Evaluation/Preferences... The WPC medium range product suite was primarily derived from a blend of the 00 UTC ECMWF and ECMWF ensemble mean and the 13 UTC National Blend of Models days 3-7 with a focus increasingly on the ensemble mean over time amid growing run to run model variance due to phasing and system evolution uncertainties/increasing forecast spread. These guidance pieces seemed to best cluster with WPC continuity and be best in line with guidance trends toward amplification during this forecast period, but still in a pattern with average to below normal predictability. The 12 UTC GFS/GEFS have trended more amplified with time over the lower 48 than prior runs whose uptream flow over the upstream Pacific also tended to be an outlier solution vs the multi-model ensemble guidance envelope. However, the 12 UTC ECMWF does still does offer some less than stellar run to run variance in an albeit still reasonably similar larger scale flow pattern evolution with a trend twoard slowing the flow. ...Weather/Hazards Highlights... A lead Midwest upper trough/low and uncertain phasing with northern stream trough energies digging to the lee of an amplified central Canadian upper ridge Monday may forge a closed low to work slowly across the Northeast to the Canadian Maritimes Tuesday-Thursday with a main low/frontal system whose deep low and enhanced moisture/rainfall threat now seems likely to focus further organizing with development of a deepening coastal low. Meanwhile across the West Coast region, multiple rounds of enhanced rains are expected from the Pacific Northwest through especially California that will likely be heavy at times with breezy conditions as well. This will especially hold true into Monday as a deep and long fetch moisture plume (Atmospheric River) shifts southward from the Pacific Northwest into California. Several inches of rain are possible over the multi-day period (which begins during the short range/this weekend), particularly across favored windward terrain which may result in some instances of runoff issues and/or flooding especially over already vulnerable burn scar areas. Heavy snows are likely for the higher elevations of the Cascades and especially Sierra, with multiple feet possible at the highest elevations. Snow levels are expected to lower Sunday into Monday as the core of the upper trough moves inland along with the surface cold front. Strong winds are also possible for the coast and inland along mountain ridges. Some of this moisture is expected to reach the Intermountain West and the central/northern Rockies with significant snowfall. The main associated cold front reaching the Plains by Tuesday and onward across the east-central U.S. Wednesday/Thursday is expected to present an emerging and expanding lead rainfall/convective focus with an influx of Gulf moisture. Closed upper trough development and slowed frontal system progression and rainfall/moisture focus then suggests a later period enhanced qpf/weather potential over the East and perhaps in particular the Mid-Atlantic and Northeast. Meanwhile, additional upstream Pacific systems are forecast to maintain the active pattern out West with periods with enhanced precipitation chances going into the Pacific Northwest well into next week. In terms of temperature highlights, it will feel more like late September across much of the central and southern Plains as the upper level ridge remains in place across this region. High temperatures are forecast to be 10-15 degrees above normal ahead of the cold front and possibly near record temperatures over parts of Texas and Louisiana. Cooler than normal temperatures are forecast for much of the West, influenced by the upper trough and mainly overcast skies. High temperatures may be 10-20+ degrees below normal early in the week but will moderate by the middle of next week. These may challenge daily record cool max values over parts of California into early next week. Schichtel/Santorelli Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Wed-Thu, Oct 27-Oct 28. - Heavy precipitation across portions of California, the Pacific Northwest, and the Northern Great Basin, Mon-Tue, Oct 25-Oct 26. - Heavy precipitation across portions of the Great Basin, the Northern/Central Rockies, California, and the Southwest, Mon, Oct 25. - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, the Southern Appalachians, the Ohio Valley, and the Tennessee Valley, Wed, Oct 27. - Heavy rain across portions of the Great Lakes, the Mid-Atlantic, the Northeast, and the Central Appalachians, Tue-Fri, Oct 26-Oct 29. - Severe weather across portions of the Central/Southern Plains, and the Middle/Lower Mississippi Valley, Tue, Oct 26. - Severe weather across portions of the Southeast, the Lower Mississippi Valley, and the Tennessee Valley, Wed, Oct 27. - Flooding occurring or imminent across portions of the Southern Plains. - High significant wave heights for coastal portions of California and the Pacific Northwest, Mon, Oct 25. 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