Extended Forecast Discussion NWS Weather Prediction Center College Park MD 105 PM 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... ...Heavy rainfall possible for parts of the Southern Plains Saturday and Sunday... ...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 and southern portions of the lower 48. The models and ensembles show good agreement the first couple of days to warrant a purely operational model blend for Thursday into early Friday. After this, the guidance shows general agreement, outside the GFS (more progressive with systems in the Westerlies and weaker with an upper low over TX) and the Canadian late (which becomes somewhat deep/out of phase late with the trough stretching across the Mississippi Valley). The ECMWF/Canadian/UKMET camp are closest to, yet not weaker than, the ensemble means. For the pressures, 500 hPa heights and QPF, stuck closer to this grouping early (with some GFS inclusion Thursday into early Friday), with slowly increasing ensemble mean contribution (ECMWF/NAEFS) from this weekend onward. The grids will have a more even blend of the deterministic and ensemble mean guidance, as usual. ...Weather Highlights/Hazards... The best focus for rainfall to begin the period will be ahead of the Central U.S. upper trough across the South and East. 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 and trough late in the period should promote episodes of showers/storms with heavy rainfall. The guidance remains wet near the western Gulf coast in response to likely trough/upper low across TX next weekend, with the GFS showing a multi-day trend in that direction as it figures out the details of the northern and southern stream portions of the upper level trough. TX and the northwest Gulf Coast is highly susceptible to any kind of heavy rainfall given much above normal precipitation over recent weeks (TX) and months (LA). 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 of the Lower 48. 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. 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; record high minima are in the cards for portions of New England this weekend. Roth/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