Extended Forecast Discussion NWS Weather Prediction Center College Park MD 148 AM EDT Sun May 30 2021 Valid 12Z Wed Jun 02 2021 - 12Z Sun Jun 06 2021 ...Record warmth likely Wednesday and Thursday across parts of the West... ...Overview and Guidance Evaluation/Preferences... The medium range period starts Tuesday as a rather amplified pattern with strong ridging over much of the West and troughing over the Central/Midwest states. After Thursday though, the pattern should become less amplified as a couple of shortwaves into the Pacific Northwest will erode the upper ridge and push the central U.S. troughing eastward as it also weakens. The latest guidance also continues to indicate the possibility that some combination of energy from northwestern Mexico and what may filter through/around the western U.S. ridge could yield a weak trough/upper low over the southern High Plains. Models and ensembles continue to show sufficient agreement for an operational model blend to serve as the starting point for the forecast for days 3 and 4. After this, increasing shortwave detail uncertainties begin to arise, particularly regarding Northwest U.S. shortwave timing/amplitude as well as whatever energy hangs around the Southern Plains late in the period. The ensemble means however do show good agreement through the entire week, so the WPC forecast is heavily skewed towards the means by day 7. Minor contributions from the deterministic GFS/ECMWF were continued through the entire period to help maintain some system definition and strength. ...Weather Highlights/Hazards... The main focus for rainfall will be ahead of the Central U.S. upper trough as moisture pools along a wavy and weakening frontal system. The activity should shift eastward with time into the Tennessee/Ohio Valleys and the Northeast, with pockets of moderate to heavy rainfall likely somewhere, but impossible to pin down those kinds of details at this time. Even with a lack of frontal forcing by the end of the week, lingering shortwave energy and moisture may be sufficient to also produce showers/thunderstorms of varying intensity into the southern Appalachians and Southeast. Meanwhile the West should initially be dry, but trend more unsettled later in the week as incoming shortwave energy may produce areas of scattered precipitation, mainly from the Northwest into portions of the Rockies. The most extreme temperature anomalies during the medium range period will be from Wednesday-Thursday from California and the Northwest into the Great Basin where daytime highs of 15-25F above normal and possibly locally higher may meet or exceed daily records. Morning lows may be warm enough to challenge daily records across this same region into Thursday as well. Weakening/progression of the western upper ridge will yield a moderating trend over the Northwest in particular late in the week along with a warming trend farther east across the northern tier and eventually the Northeast. Portions of the Pacific Northwest may actually drop a couple degrees below normal by next weekend as lowered heights move through. The southern half of the Plains will be moderately below normal under cloudy and stormy skies on Wednesday followed by some lingering below normal readings over Texas. Much of the eastern U.S. should see near-normal temperatures through the period aside from the warming trend in the north by the end of the week. 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