Extended Forecast Discussion NWS Weather Prediction Center College Park MD 418 PM EDT Tue May 25 2021 Valid 12Z Fri May 28 2021 - 12Z Tue Jun 01 2021 ...Record highs possible in the Pacific Northwest next week... ...Heavy rain potential over portions of the Plains... ...Overview... Upper ridging will likely build into the Pacific Northwest early next week while a trough takes shape over the Great Basin/Southwest. Farther east, Upper Midwest shortwave energy will push along eastern U.S. low pressure Friday into Saturday with the trailing front sinking southward over the eastern half of the country. The western end of the front should linger over the Plains while another front moves in from the west/northwest, helping to promote episodes of showers and storms which may contain locally heavy rainfall. The initial Upper Midwest shortwave may combine with some energy over eastern Canada to form an upper low over the Northeast for a brief time before lifting northeastward. Precise effect on weather over the Northeast is somewhat uncertain thus far. After a recent spell of warm weather in the East and cool temperatures in the West/Northwest, the opposite will manifest itself through the weekend into next week. ...Guidance Evaluation and Preferences... Recent trends in some guidance generally improved clustering through the 00Z/06Z cycle but typical detail uncertainties remain. Latest runs reinforce the idea that some combination of initial Upper Mississippi Valley shortwave energy and northern stream flow from Canada may form an upper low over the Northeast during the weekend followed by northeastward progression as upstream flow approaches. The 06Z GFS became a southwestern extreme for this feature (12Z run adjusted toward consensus) while the new 12Z UKMET is a deep extreme, corresponding to a deeper/northwest surface wave. Trends for the trailing cold front (into the northern Gulf of Mexico) during the weekend appear to have stabilized for the time being. While the great majority of guidance keeps western Atlantic frontal waviness offshore, the lower-probability 12Z UKMET does illustrate how sensitive the forecast for the surface evolution and amount of clouds/rain will be to the exact strength/shape of the upper feature. The new 12Z ECMWF adds to the uncertainty, depicting more north-south elongation of shortwave energy near the East Coast. Over the West, ensembles show good agreement on an upper ridge (to at least +2 sigma) building into the Pacific Northwest, downstream of equally-as-deep troughing south of Alaska. Meanwhile for the trough to the southeast of this ridge, recent GFS/CMC runs have gravitated away from a West Coast closed low to the weaker/eastward trough of ECMWF/ECMWF mean runs. The 12Z CMC is a bit deeper than consensus though, a slight reversal from its 00Z run. The early portion of the updated forecast started with a blend of the 00Z ECMWF, 06Z-00Z GFS, 00Z UKMET, and 00Z CMC in order from more to less weight. The blend transitioned toward a model/mean mix later in the period--by day 7 Tuesday incorporating the 00Z ECMWF/00Z GFS (due to the 06Z GFS being slow with the Northeast upper low) along with the 06Z GEFS/00Z ECMWF means. This approach yielded good continuity in principle with only typical run-to-run refinements in details. ...Weather Highlights/Hazards... The two primary areas of focus for rainfall during the period will be near the exiting low pressure in the East on Friday (highest totals likely in the northern half of the Mid-Atlantic) and then back over the central/southern Plains to the Corn Belt along the west end of the wavy frontal boundary and ahead of a weaker northern stream cold front. High pressure that builds in behind the latter front could lead to some areas of low level upslope flow that would help to enhance convective development over the High Plains. It will take additional time to monitor potential overlap of heavy rainfall potential with areas that have seen above normal rainfall over the past week or so. The West Coast will see little to no precipitation but areas near and east of the Great Basin to the Divide may see at least scattered showers as the upper trough moves in and a weak cold front sinks through. Temperatures will be cool in the Upper Midwest/Great Lakes/Northeast Friday-Saturday, expanding south down the Mid-Atlantic Saturday with a cold air damming setup. Expect readings to be 10-15F below normal in general but with some pockets of highs 15-20F below normal possible from the Midwest into the Lower Great Lakes. Warm to hot temperatures in the Southeast on Friday (reaching 10-12F or so above normal) will steadily cool as the cold front works down to the Gulf. The Pacific Northwest will see a warming trend with temperatures 10-25F above normal by Memorial Day into next Tuesday. This could approach or exceed record levels for parts of Washington/Oregon into northern California and may continue into the week beyond this period. See the latest CPC day 6-10 forecast and 8-14 day U.S. Hazards Outlook for more information. On the other hand the cloudy/stormy pattern setting up over the central/southern Plains and eastern slopes of the Rockies will bring highs down to 10-15F below normal by next Monday-Tuesday. Rausch/Fracasso 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 Plains, and the Mississippi Valley, Sun-Tue, May 30-Jun 1. - Heavy rain across portions of the Mid-Atlantic, the Central Appalachians, and the Ohio Valley, Fri, May 28. - Flooding possible across portions of the Middle Mississippi Valley. - Flooding occurring or imminent across portions of the Central Plains, the Middle Mississippi Valley, the Lower Mississippi Valley, and the Southern Plains. - Flooding likely across portions of the Northern Rockies. - Much above normal temperatures across portions of California, the Central Great Basin, the Pacific Northwest, and the Northern Great Basin, Mon-Tue, May 31-Jun 1. 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