Extended Forecast Discussion NWS Weather Prediction Center College Park MD 210 AM EDT Sat Apr 24 2021 Valid 12Z Tue Apr 27 2021 - 12Z Sat May 01 2021 ...Heavy rain threat from the South-Central Plains into the Midwest next week... ...Guidance/Predictability Assessment and Preferences... The medium range period (Tues-Sat) continues to show a rather amplified upper level pattern over the Eastern Pacific and lower 48. The main feature of interest will be an upper level trough initially over the Western U.S. which should steadily progress eastward with time, reaching the East by the start of next weekend. In its wake, a strong upper ridge will build centered over the Southwest/Four Corners region through the week, with another shortwave possibly impacting the Northwest Coast by Saturday. The latest guidance continues to trend slower with the main upper trough, with the 18z/Apr 23 GFS still on the fast side of the solutions (although slower than it's previous 12z run). The ECMWF/CMC/UKMET and the ensemble means would support something a little slower than the GFS. Late in the period, uncertainties begin to arise in the evolution of the trough and whether it remains an open amplified trough, or sporadic model runs (both the GFS and ECMWF) have hinted at the possibility of an upper low closing off over the Southeast or Mid-Atlantic states. The WPC forecast favored a blend (leaning heavily towards the ECMWF) of deterministic solutions for days 3-5, trending quickly towards a 50/50 blend of ensembles and deterministic (ECMWF/CMC) by day 7. This approach helps mitigate late period details which have yet to be resolved and also maintains good continuity with the previous shift. ...Weather/Hazards Highlights... The system crossing the West early in the week will initially bring some lower elevation rain/mountain snow with the best focus over favored terrain in Arizona through the north-central Rockies on Tuesday. Meanwhile, the leading cold front should reach far enough east to begin interacting with Gulf of Mexico moisture to produce increasing coverage/intensity of rain and thunderstorms over the central/southern parts of the Plains. Moderate to heavy rainfall should extend east and northeast with time into the Mississippi Valley and the east-central U.S.. At this time, the best signal for highest rainfall totals extends from parts of Texas/Oklahoma into the Mid-Mississippi Valley and western Ohio Valley. Confidence is high in heavy rainfall potential with this system, although the guidance is still in the process of resolving important details so the expected focus for heaviest rainfall may continue to show some variation in the coming days. Severe weather may also be a threat with this system, especially for parts of the southern/central Plains on Tuesday (see Storm Prediction Center outlooks for latest information). By Friday into Saturday, moisture should shift into the East/Southeast with moderate to locally heavy rainfall possible. Elsewhere, flow around the upper ridge building into the West and ahead of the next shortwave may bring some moisture into the Pacific Northwest and possibly extreme northern Rockies. Heaviest precipitation should remain over western British Columbia/Vancouver island, with light to moderate totals extending into western Washington by the end of the week. Warmth ahead of the system initially crossing the western U.S. will spread above normal temperatures (anomalies on the order of +10-20F) from the parts of the central Plains into the Ohio Valley/Great Lakes and Mid-Atlantic/Northeast through midweek. In the cooler airmass behind this system expect highs of at least 10-15F below normal to progress across the southern half of the West during the first half of the week, with moderation as it reaches the Plains and points east. 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. Santorelli/Rausch 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 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