Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 AM EDT Sun May 24 2020 Valid 12Z Wed May 27 2020 - 12Z Sun May 31 2020 ...Heavy rainfall possible over the Lower Mississippi Valley and possibly Southern Plains... ...Multiple days of much above normal temperatures over the West... ...Overview... An upper low expected to linger over the Southern Plains for most if not all of the period should help to generate periods of locally heavy rainfall, most likely to its east over parts of the Mississippi Valley/central Gulf Coast but perhaps at times closer to the upper center. An upper ridge over the West will drift eastward with time as an approaching low-latitude upper low tracks close to northern California and the Pacific Northwest. This evolution will slowly shift the core of much above normal temperatures (and numerous record highs/warm lows) over the region from the West Coast states and interior areas toward the Rockies. Meanwhile over the past day or so guidance has adjusted East Coast upper ridging more into the western Atlantic with another ridge possibly extending into the Southeast during the latter half of the period. This change in mean flow could spread more moisture over parts of the eastern U.S. For the time being guidance appears to have stabilized regarding North Pacific through southern Canada/northern tier U.S. flow--with the primary upper trough pushing a midweek Northern Plains front into the East by late week. Areas from the Great Lakes into Northeast will be quite warm ahead of this front with some daily record warm lows and an isolated record high possible. ...Weather/Hazard Highlights... The most common signal for significant rainfall associated with the persistent Southern Plains upper low exists over parts of the Lower Mississippi Valley and central Gulf Coast, but also with secondary potential closer to the upper low itself. Detail confidence continues to be tempered by the fact that mesoscale processes will likely play an important role and will not be resolved very far out in time. Continued northward progression of moisture reaching Florida in the short range time frame could lead to some areas of organized rainfall along and inland from portions of the East Coast. Meanwhile a dissipating Midwest front early in the period and then the Northern Plains front pushing into the East later in the week will likely produce other areas of rainfall with varying intensity. The upper low forecast to brush the West Coast next weekend may bring some rain to portions of northern California and the Pacific Northwest. Within the large area of much above normal temperatures over the West, the core of highest anomalies will start the period on Wed over the West Coast states and Interior West, followed by a gradual drift toward and into the northern-central Rockies and northern High Plains. Some locations should see highs up to 20-25F above normal for one or more days during Wed-Fri from the Central Valley of California through parts of Oregon/Nevada/Idaho. Then the upper low forecast to lift northward near the West Coast by next weekend should bring readings down to near normal over and north of California. At that time highs of 10-20F or so above normal should be common from Idaho and the Great Basin through the northern-central Rockies and northern High Plains. Expect numerous daily record highs and warm lows within the area of highest anomalies through the period. Great Lakes/Northeast areas should see temperatures 10-20F above normal from Wed into late week, with some daily record warm lows and perhaps an isolated record high. A cold front will then bring temperatures closer to normal, over the Great Lakes starting by Fri and remaining locations during the weekend. Areas of clouds and rainfall will tend to keep highs near to moderately below normal across the southern tier of the central-eastern U.S. ...Guidance/Predictability Assessment... The updated forecast started with a blend of mostly 12Z GFS/ECMWF with minor input from the 18Z GEFS/12Z ECMWF means on day 3 Wed followed by a gradual increase of ensemble weight, reaching 60 percent by day 7 Sun. The 12Z CMC/UKMET each differed meaningfully from consensus for one or more important features while the 18Z GFS was quicker than other models to weaken the Southern Plains upper low plus became the most amplified solution for the upper trough reaching the northeastern quadrant of the lower 48 during the latter half of the period. For the most part the 00Z GFS has returned to a more palatable solution. Recent adjustments for upper ridging over the eastern U.S./western Atlantic likely played a role in the latest consensus nudging the Southern Plains upper low a bit farther east for at least part of the period. This is still well within typical variability for such a feature though. Uncertainty over the Plains in general increases next weekend with spread developing for exactly how the northern part of the western ridge may extend into the Plains with a corresponding effect on path of the upper low. Farther west, general trends over the past 12-24 hours favor slower timing of the low-latitude upper low approaching the West Coast. Timing shows sensitivity to uncertain upstream flow with some recent GFS runs and 12Z CMC faster due to arrival of upstream energy, in contrast to the slow 12Z ECMWF which held the upstream energy well westward (perhaps too far back per the ensemble means). A solution close to the ECMWF mean seemed most reasonable and the 00Z GEFS mean has come in very similar to that. The 00Z GFS is still a little faster than the means but slower than most prior runs, while the 00Z ECMWF catches up by early Sun. Northern stream flow and surface features from the Northeast Pacific through southern Canada and northern U.S. are starting to cluster better, with a model-mean consensus providing a reasonable way to resolve existing differences/variability in upper trough amplitude affecting southward extent of the cold front reaching the East. 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