Extended Forecast Discussion NWS Weather Prediction Center College Park MD 232 PM 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 due to a weak but defined surface low just off northeastern Florida Wednesday could lead to some areas of organized rainfall along and inland from portions of the Southeast 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. 80s and 90s will be common across lower elevations with near 100F readings possible even into the northern Great Basin and Snake River Valley Fri-Sat. 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., focused over the Texas Hill Country. ...Guidance/Predictability Assessment... The forecast started with a blend of the 06/00Z GFS and 00Z ECMWF/UKMET/Canadian Wed-Thu as the guidance was in good agreement overall with expected detail differences. For Fri-Sun, preferred the 00Z ECMWF for some detail added to the 06Z GEFS/00Z ECMWF ensemble means along with a small percentage of the Canadian. The GFS appeared too quick to lift the Pacific upper low to the northeast though it was not unreasonably fast (but was dislodged from the agreeable ensemble consensus). Ended at a 50/50 deterministic/ensemble blend by the end of the period due to lingering uncertainty in northern stream flow across southern Canada with which that guidance has had trouble. Recent adjustments for upper ridging over the eastern U.S./western Atlantic as well a a more defined but compact surface low off northeast Florida 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 before perhaps moving back southwestward in light steering flow. This is still well within typical variability for such a feature though and more small shifts are likely. 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. 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, which is expected to settle no farther south than about the northern part of the Southeast given the upper ridging just to the south. Fracasso/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