Extended Forecast Discussion NWS Weather Prediction Center College Park MD 251 AM EDT Mon May 25 2020 Valid 12Z Thu May 28 2020 - 12Z Mon Jun 01 2020 ...Much above normal temperatures over the West into the Rockies/northern High Plains... ...Areas of heavy rainfall possible over portions of the South/Southern Plains... ...Overview... An upper ridge over the West will drift eastward with time as a low-latitude upper low likely brushes the central/northern West Coast during the weekend, immediately followed by an amplifying mean trough offshore the West Coast. From late this week through the weekend the upper ridge progression will gradually shift emphasis for the most extreme temperature anomalies (and numerous daily record highs/warm lows) from the West Coast states/Interior West toward the northern-central Rockies and northern High Plains. Farther east there will still be the potential for areas of moderate/heavy rainfall near and to the east of an upper low that lingers for a while over the Southern Plains/Lower Mississippi Valley. Uncertainty over evolution/path of this feature lowers confidence in details though. Meanwhile expect western Canada upper ridging to support amplification of a downstream trough expected to progress from the Northern Plains toward the East Coast. This evolution will push a northern tier cold front fairly far southward over the eastern U.S. by Sun-Mon. Before the frontal passage a broad portion of the eastern U.S. may see one or more episodes of rainfall with varying intensity. ...Weather/Hazard Highlights... Increasing guidance spread for the exact evolution of the upper low forecast to start the period over the Southern Plains is lowering confidence for rainfall specifics across the southern tier of the U.S. during the period. Flow ahead of the upper low early in the period when there is better agreement may support a somewhat higher potential for locally heavier amounts over and near the lower half of the Mississippi Valley and central Gulf Coast. Otherwise there is still some lingering potential for areas of moderate to heavy activity over parts of the Southern Plains, and through the Southeast and vicinity in moist flow around an initial Atlantic ridge aloft. The cold front pushing southward through the East from late this week onward will provide a focus for showers/thunderstorms of varying intensity. The upper low brushing the West Coast during the weekend may bring an episode of rainfall that could be moderate/heavy at some locations between extreme northern California and the Pacific Northwest. Lighter and more scattered activity is possible over the southern half of the Rockies. Over the West the forecast is consistent in advertising a broad area of temperatures 10-25F above normal through the period with numerous daily records for highs/warm lows. During the Thu-Mon period the emphasis should gradually shift with time from the West Coast states and Interior West to the northern-central Rockies and High Plains. The axis of greatest anomalies for highs will likely extend from the Central Valley of California across parts of Oregon/Nevada/Idaho and then into Wyoming/Montana. The Great Lakes and Northeast will see above normal temperatures Thu into Fri (plus 10-20F anomalies with some record warm lows possible) followed by a return to normal or a fraction below by the weekend after passage of a cold front. Areas of clouds and rainfall as well as the continued southward progress of the eastern U.S. cold front should lead to more below normal than above normal highs across the southern half of the central/eastern U.S. The Southern Plains will see the best chance for some highs 5-10F below normal. ...Guidance/Predictability Assessment... This forecast cycle started with an operational model blend during the first half of the period and transitioned toward an even weight of models/means by day 7 Mon, as models varied for which one(s) compared better to consensus for important features and typically increasing spread developed for some details mid-late period. Over the eastern Pacific/western U.S., guidance continues to show some timing/track spread for the upper low forecast to track northward around the western ridge. The 12Z/18Z GFS appeared to carry the upper low/associated trough too far east into the ridge, leading to an emphasis on the 12Z ECMWF/CMC and 18Z GEFS/12Z ECMWF means. Confidence is lower on precise timing as most models have been variable over the past day. 12Z/18Z guidance was fairly well clustered for the trailing upper trough by next Mon. The new 00Z GFS provides a more palatable solution for the upper low track but may bring the trailing trough too far eastward. Guidance seems to be making some progress in resolving the northern stream flow that has been problematic over recent days. The past 12-24 hours of runs have settled on Northeast Pacific low pressure stalling a ways off the Alaska Panhandle, allowing a more persistent western Canada upper ridge to support amplification of the Northern Plains to East Coast trough. There is still some degree of spread over exactly how amplified the trough will become, favoring the model/mean blend approach for the time being. Over the past day the models have diverged somewhat for the ultimate evolution and path of the upper low expected to be over the northeast corner of Texas to start the period early Thu. The rapid shearing/eastward progression of the 12Z ECMWF represented one extreme while the GFS runs/12Z CMC kept an identifiable feature over Texas into the start of next week. This area appears increasingly sensitive with dependence on strength/evolution of the western ridge (GFS weaker than consensus by Sat) while the northern stream trough becomes tantalizingly close to having some possible influence as well. 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 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