Extended Forecast Discussion NWS Weather Prediction Center College Park MD 221 AM EDT Tue Jun 01 2021 Valid 12Z Fri Jun 04 2021 - 12Z Tue Jun 08 2021 ...Record warmth likely through Friday from parts of the West to the northern Plains... ...Heavy rainfall possible for parts of the Southern Plains and Lower Mississippi Valley... ...Overview and Guidance Evaluation/Preferences... The guidance continues to advertise a substantial upper level pattern change over the course of the medium range period (Friday-Tuesday). This means strong ridging over much of the West/Rockies will be replaced by a mean trough spreading into the West this weekend. Meanwhile, heights are forecast to rise across the East as mean ridging prevails over the western Atlantic and some sort of trough/upper low lingers over the Southern Plains. While the latest suite of models and ensembles show generally good consensus on the large scale set up, there remains considerable uncertainty in the details of individual systems. The model agreement the first half of the period though was sufficient enough to warrant a purely operational model blend through the weekend (between the ECMWF, GFS, and UKMET). The CMC continues to be a bit out of phase with the Southern Plains system, wanting to maintain troughing (or maybe a closed low) over the Mississippi Valley, while the ECMWF/GFS/UKMET are more consistent with the ensemble means. After day 5, models also begin to diverge on what happens to a cutoff low out of the base of the initial shortwave into the Northwest this weekend. The latest runs of the GFS/ECMWF (and UKMET through day 5 at least) drag this feature south off the California coast, before eventually ejecting back into the Southwest early next week, possibly getting absorbed back into the Western mean trough. The CMC however, has been consistent in showing this low drifting west into the Pacific. Notably though, a few past operational GFS/ECMWF runs have also showed this possibility so there's plenty of uncertainty remaining. Thus, increasing weighting of the ensemble means was used days 6-7 to help mitigate some detail differences, while also being able to maintain at least a 50% combo of the ECMWF/GFS which are the deterministic guidance pieces closest to that of the means right now. ...Weather Highlights/Hazards... The biggest concern for heavy rainfall potential will be with the southern Plains trough/low which should promote daily episodes of rain and thunderstorms across parts of Texas into the Lower Mississippi Valley much of the period. The guidance continues to show the potential for heavy rainfall, across south Texas on Friday, shifting into East Texas and Louisiana this weekend, with multi-day rain totals of several inches or more. Although models continue to show plenty of uncertainty in the details of where and how much, this region is already highly susceptible to any kind of heavy rainfall given much above normal precipitation over recent weeks. Elsewhere, conditions should dry out across the East into the weekend, with showers lingering over parts of the Southeast. Out West, a gradual increase of moisture over the Pacific Northwest is expected, with some activity also possible into the northern Rockies and eventually the northern tier. Much above normal temperatures (+15-25F) from the central Great Basin into the Northern Plains will continue into Friday with daytime highs near or exceeding record values for some locations. The trend toward cyclonic flow aloft over the Northwest by next weekend and rising heights over eastern North America will gradually suppress the heat across the West and spread above normal readings into the Great Lakes/Northeast this weekend, with near record warm morning lows possible for some locations from the Great Basin into the northern tier states, and the Northeast. Elsewhere, the Northwest and West Coast may dip marginally below normal by early next week, while the southern Plains will tend to see highs up to 5-10F below normal due to clouds and showers/thunderstorms this weekend. Southern half of the East should be near normal for much of the period. Santorelli 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