Extended Forecast Discussion NWS Weather Prediction Center College Park MD 237 AM EDT Thu Aug 20 2020 Valid 12Z Sun Aug 23 2020 - 12Z Thu Aug 27 2020 ...Hot temperatures to continue for much of the West into next week... ...Overview and Weather/Hazard Highlights... A strong upper ridge centered over the Four Corners region should remain in place through the period. It will likely be strongest around Sun-Mon followed by modest erosion of its northern periphery as some energy ejects from the Northeast Pacific across the northern tier U.S./southern Canada. The ridge will maintain well above normal temperatures over many locations from the West into the Rockies and parts of the Plains eastward to the Northeast atop another upper high in the western Atlantic. Best potential for record highs with readings up to 10-12F above normal will be Sunday from the Great Basin/Southwest into the lee of the Rockies and western High Plains. Expect morning lows to be somewhat more extreme Sun-Thu with broader coverage of plus 10-15F anomalies and potential daily record warm values over California and the Southwest/Great Basin. There should be isolated to scattered showers/storms over the West each day, aided by shortwaves flowing around the upper high center. The most prominent focus for rainfall will be over the Gulf of Mexico and adjacent areas. First there will be lingering moisture associated with an east-central U.S. upper trough (and old surface boundary) that will be weakening on Sunday. The best moisture and highest rainfall totals should be over/near the northern Gulf with more modest moisture and generally lighter rain extending into the eastern states Sunday ahead of the upper trough. Then two separate tropical features may enhance rainfall: one over parts of the western half of the Gulf/Gulf Coast and another (now T.D. 13) that may affect Florida and portions of the eastern Gulf. Clouds and rain should lead to somewhat below normal highs over the Southeast and vicinity especially on Sun-Mon then perhaps Tue-Thu depending on the track of T.D. 13 as a potentially stronger system. Low amplitude upper flow across southern Canada and northern U.S. will support multiple waves/fronts across the northern states and episodes of showers and thunderstorms between the Northern Plains and Northeast. Most features will be fairly transient so rainfall totals should generally be on the moderate side, though locally more intense activity could be possible at times. Locations to the north of a front on a particular day may see below normal temperatures while above normal readings (up to plus 5-10F anomalies) are possible over the north-central latitudes that will tend to be in the warm sector ahead/south of the fronts. ...Guidance/Predictability Assessment... The latest forecast started with a blend of the 12Z-18Z deterministic models and ensembles to account for uncertainty in the progression of height falls into the Pac NW and track/strength of T.D. 13 toward the Florida Straits. The low amplitude of northern tier flow aloft continues to provide challenges in resolving northern U.S.-southern Canada wave/front specifics. No single solution was preferred CONUS-wide, but rather aspects of each were deemed more or less plausible. Blending the several solutions formed a good starting point for the pressures forecast. For T.D. 13, this was represented best by the 12Z Canadian and many of the 12Z ECMWF ensembles and some 18Z GEFS ensembles (perhaps more from the parallel GEFS). Still quite uncertain of its track/intensity by next Wed/Thu as it may lift northward or more northwestward around the upper and sfc high to the east/northeast. Off the west coast, the southern part of the upper trough may try to move eastward into northern CA/southern OR but ensemble agreement was poor. 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