Extended Forecast Discussion NWS Weather Prediction Center College Park MD 202 AM EDT Mon May 31 2021 Valid 12Z Thu Jun 03 2021 - 12Z Mon Jun 07 2021 ...Record warmth likely through Thursday across parts of the West... ...Another round of heavy rainfall possible for parts of the Southern Plains next weekend... ...Overview and Guidance Evaluation/Preferences... Most of the guidance shares the theme of a significant upper level pattern change over the course of the medium range period (Thursday-Monday). This means strong ridging over the West to start the period Thursday will be replaced by a mean trough reaching the Northwest U.S. by the weekend. Meanwhile, heights are forecast to rise across the East as mean ridging prevails over the western Atlantic. Between this however, there remains considerable spread on what becomes of an initial Plains/Mississippi Valley trough and a High Plains upper trough (or low). The specifics of these features will likely have considerable influence on the distribution and intensity of rainfall over the eastern two-thirds of the lower 48. The models and ensembles show good agreement the first couple of days to warrant a purely operational model blend for days 3 and 4. After this, the guidance cannot seem to agree on what to do with the troughing across the Midwest. The most outlier solution is still the 12z CMC which keeps an upper level low over the Mississippi Valley through Monday. Meanwhile, the GFS is the quickest to race the northern stream energy into the Northeast on Saturday, while the ECMWF hangs the energy back across the Mid-Atlantic, but in a much weaker fashion than the CMC would suggest. The ECMWF is however the solution closest to that of the ensemble means. There is also plenty of questions on leading shortwave energy reaching the West Coast next Thursday-Friday, with the latest runs of the CMC/UKMET continuing to pull off a closed low farther offshore California. The GFS and ECMWF also have this detaching energy but to a much weaker degree. Finally, details of the possible trough/upper low over the High Plains next weekend continue to be problematic but at least the GEFS/ECENS means and most of the GFS/ECMWF runs have maintained this feature in some way for several cycles now. The blend for this forecast was quickly skewed towards the ensemble means for days 5-7 to help smooth out all the lower predictability details of the various systems while maintaining smaller parts of the ECMWF for added feature definition. This approach also maintains good continuity with the previous WPC forecast. ...Weather Highlights/Hazards... The best focus for rainfall to begin the period will be ahead of the Central U.S. upper trough. There is some signal for at least locally moderate-heavy rainfall from parts of the Tennessee Valley into the Mid-Atlantic/Southeast. Over the southern Plains and vicinity, a lingering front early in the period may promote episodes of showers/storms Thursday and Friday. The guidance is beginning to trend wetter across parts of southern/eastern Texas and into southern Louisiana in response to likely trough/upper low across the High Plains next weekend. Even the ensemble means (and the NBM) are showing at least an inch or two of rainfall possible next weekend, with some deterministic model runs hinting at the potential for much more than that. This of course is highly dependent on flow details regarding the High Plains energy, but this region is already highly susceptible to any kind of heavy rainfall given much above normal precipitation over the past few weeks. Out West, a gradual increase of moisture over the Northwest is expected as flow aloft trends toward a broad trough. Some light to moderate totals will be possible over the Pacific Northwest with some activity also possible over the northern Rockies and eventually spreading eastward across the northern tier. Much above normal temperatures (+15-25F) across the central and northern Great Basin will continue into Thursday with both daytime highs and warm morning lows near or exceeding record values. The trend toward cyclonic flow aloft over the Northwest by next weekend and rising heights over eastern North America will gradually suppress the heat over the West and spread above normal readings across the northern tier into the Northeast. A handful of locations in the Northern Plains/Upper Mississippi Valley may also challenge daily records into the weekend. Parts of the Northwest may actually dip slightly below normal by next Sunday-Monday as well. Elsewhere, the southern Plains will tend to see highs up to 5-10F below normal due to clouds and showers/thunderstorms while the southern half of the East should see near-normal readings for most of the period. 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