Extended Forecast Discussion NWS Weather Prediction Center College Park MD 342 PM EDT Fri Apr 23 2021 Valid 12Z Mon Apr 26 2021 - 12Z Fri Apr 30 2021 ...Heavy rain threat from the South-Central Plains into the Midwest next week... ...Guidance/Predictability Assessment and Preferences... Latest guidance continues to show an amplified upper pattern over the eastern Pacific and lower 48, with an upper trough continuing to spread rain/mountain snow over the West early in the week and then likely generating a threat for heavy rainfall and in some cases severe weather over portions of the Plains/east-central U.S. as it pushes eastward. The leading wavy surface front will separate well above normal temperatures that will extend from the southern three-fourths of the Plains into the East next week from below normal readings crossing the West early in the period. Western temperatures will likely surge to well above normal levels by late week as a strong upper ridge builds over the region. Recent trends have continued into the 00Z/06Z model cycles, with a slower/stronger ridge building into the West ahead of a slower and more amplified upstream Pacific trough--leading to less precipitation reaching the Northwest and slower progression of the initial western trough that moves into the central U.S. Most guidance has displayed these trends but over recent days the ECMWF/ECMWF mean have generally been somewhat ahead of the GFS/GEFS mean in reflecting them. Thus prefer to continue weighting the forecast more in the ECMWF/ECMWF mean direction overall. The 06Z GFS/GEFS mean did adjust back from their 00Z runs enough to allow for minority blend inclusion. UKMET/CMC runs are also consistent among themselves in advertising a slower and southward upper low track near the Mexico border. Their upstream pattern clusters fairly well with the ECMWF/ECMWF mean in principle, so it remains to be seen if the western trough/upper low ends up being quite as slow/separated as forecast by the UKMET/CMC. The new 12Z ECMWF adjusted a little slower for a time but then remains faster/more open than the UKMET/CMC by early Thursday. Toward the end of the week there is added uncertainty over how trough energy may evolve over the eastern U.S. with sporadic operational model runs hinting at the possibility of another upper low closing off. For low pressure expected to track into the Canadian Maritimes early in the week, 00Z/06Z GFS runs were the only remaining solutions to pull the system southeast of Nova Scotia after early Tuesday instead of continuing to the north. The 12Z GFS adjusted a bit north of previous runs but is still south of consensus. ...Weather/Hazards Highlights... The system crossing the West early in the week will provide the best lower elevation rain/mountain snow focus over favored terrain in the Great Basin and Arizona through north-central/central Rockies from Monday into Tuesday. Some leading precipitation may also stream across the extreme northern tier into the Upper Great Lakes. By midweek the system should reach far enough east for the leading wavy front to begin interacting with Gulf of Mexico moisture to produce increasing coverage/intensity of rain and thunderstorms over central/southern parts of the Plains. Moderate to heavy rainfall should extend eastward with time into the Mississippi Valley and east-central U.S. See Storm Prediction Center outlooks for the latest details on severe weather threats with this event. At this time the best signal for highest rainfall totals extends from parts of Texas and Oklahoma into the Middle Mississippi Valley/Lower Ohio Valley and vicinity. Guidance is still in the process of resolving important details of this system so the expected focus for heaviest rainfall may continue to show some variation in coming days. Elsewhere, flow around the upper ridge building into the West may bring some moisture into the Pacific Northwest and possibly extreme northern Rockies. Heaviest precipitation should remain over Vancouver Island with light to moderate totals extending into western Washington, but confidence in specifics is not great due to guidance spread and variability over recent days. Low pressure tracking into the Canadian Maritimes by Monday may produce breezy conditions over the Northeast early in the week but recent trends have been a little weaker for wind speeds and more scattered for any wrap-around precipitation. The amplified upper pattern will support a succession of anomalous temperature areas. Warmth ahead of the system initially crossing the western U.S. will spread from the central/southern portions of the Plains into the East, with max/min readings most likely to be 10-20F above normal from the southern three-fourths of the Plains into the Ohio Valley/Great Lakes and Mid-Atlantic/Northeast. In the cooler airmass behind this system expect highs of at least 10-15F below normal to progress across the southern half or two-thirds of the West during the first half of the week. This air will modify as it reaches the Plains/Mississippi Valley mid-late week with highs more modestly below normal. Meanwhile temperatures over the West will rebound strongly as an upper ridge builds over the region after midweek. Expect highs to reach 10-20F above normal over much of California/Oregon/Nevada and possibly some surrounding areas by Thursday-Friday. The Southwest could even see highs approach daily record values. Rausch/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 rain across portions of the Middle/Lower Mississippi Valley, the Tennessee/Ohio Valley, the Great Lakes, the Mid-Atlantic, the Southern Appalachians, the Southeast, the Southern Plains, and the Ohio Valley, Tue-Thu, Apr 27-Apr 29. - Severe weather across portions of the Central Plains and the Southern Plains, Tue, Apr 27. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - High winds across portions of the Central Plains, and the Central Rockies, Mon, Apr 26. - Much above normal temperatures across portions of the Northern/Central Great Basin, California, and the Southwest, Wed-Fri, Apr 28-Apr 30. - Much above normal temperatures across portions of the Mid-Atlantic, the Northeast, and the Central Appalachians, Tue-Wed, Apr 27-Apr 28. - Enhanced wildfire risk across portions of the Southern Rockies, the Southern Plains, and the Southwest, Mon, Apr 26. - Heavy precipitation across portions of the Alaska Panhandle, Wed-Thu, Apr 28-Apr 29. 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