Extended Forecast Discussion NWS Weather Prediction Center College Park MD 457 PM EDT Mon Jul 26 2021 Valid 12Z Thu Jul 29 2021 - 12Z Mon Aug 02 2021 ...Overview with Weather Highlights/Hazards... Guidance has been increasingly consistent during the last few days and overall agrees that a strong closed upper high over the central U.S. will gradually weaken and shift southward from late this week into Monday while a ridge extends through the Rockies and north-central Intermountain West into western Canada. The northward amplitude and sharpness of this ridge will likely support an unseasonably deep eastern Canada/U.S. upper trough. Impulses and moisture lifting across the West between the ridge/closed high and emerging eastern Pacific troughing will promote widespread chances for monsoonal rainfall with isolated local runoff issues. Highest rainfall totals during the five-day period are likely to be over the Great Basin and northern-central Rockies where there should also be increasing coverage of daytime highs 5-15F below normal. On the other hand the Pacific Northwest may see very warm to hot conditions (highs up to 10-15F above normal), especially Thursday-Saturday, before a moderating trend as heights aloft decline. Some record warm minimum temperature records will be possible to the west of the Rockies, both within the warm pattern over the Northwest and elsewhere due to clouds/moisture. Areas from the southern Plains into Lower Mississippi Valley will be rather hot albeit with mostly single-digit anomalies for daytime highs. Meanwhile, multiple shortwaves will dig downstream from the ridge to amplify the mean upper trough over the northeastern quadrant of the lower 48. Favorable upper support may help to produce strong convection and locally heavy downpours focusing with moisture/instability pooling near wavy fronts that may become slow moving as they reach the southern periphery of the mean trough aloft. Expect convective outflow and reinforcing frontal surges to shift the best focused rainfall chances later this week from the Midwest to Northeast down to the Southeast. Dependent on frontal boundary placement/reinforcement and smaller scale surface lows upstream, focused moisture may then round the top of the central U.S. ridge to fuel periods of renewed convection/local downpours from the northern Plains to the Midwest/Tennessee Valley. The eastern U.S. pattern will be most favorable for below normal temperatures over the Northeast. A few locations with shorter periods of record in the northern Mid-Atlantic could even approach or reach daily record low values on Saturday morning. ...Guidance/Predictability Evaluation... Latest guidance still suggests that the overall pattern evolution during Thursday-Monday offers above average predictability. Primary uncertainties in the forecast involve the specifics of the mean trough's embedded shortwaves that will affect surface low pressure/frontal details and the timing of a Northeast Pacific upper low's arrival into the Alaska Panhandle/western Canada (with some potential effect on the downstream trough). Looking at the past couple days of guidance for the eastern trough, the ensemble means have been fairly steady through the period while the ECMWF has been more consistent in principle than the GFS for the second main feature within the mean trough. The 00Z/06Z GFS runs appeared closer to consensus than some earlier runs but the 00Z run became flatter toward the end of the period while the 06Z run was on the fast/amplified side mid-late period. Both the GFS and ECMWF have been inconsistent for the northeastern Pacific upper low. 06Z GFS/00Z ECMWF trends from their prior 6/12-hourly runs respectively added support for a slower upper low progression. Given the GFS comparisons over the East, the 00Z/06Z operational model composite used early in the period phased out GFS input after mid-period on the way to a model/mean blend that incorporated the 00Z ECMWF and a little 00Z CMC along with 06Z GEFS/00Z ECMWF means. The only noticeable continuity adjustment was a slower trend for the front dropping southeastward from the northern tier Saturday onward--which was plausible given the trend for a more persistent western Canada upper ridge that could hold back the progression of height falls within the eastern trough. 12Z solutions still vary within the eastern trough aloft especially mid-late period and thus for timing of the Saturday-Monday front. The model/mean average appears to have returned to a slightly faster frontal progression. The new CMC is on its own in straying faster with the northeastern Pacific upper low by next Monday. Rausch/Schichtel Hazards: - Heavy rain across portions of the Front Range, as well as from the northern Plains to the mid-Mississippi Valley, Fri-Sun, Jul 30-Aug 1. - Heavy rain across portions of the southern Appalachians, Sun, Aug 1. - Excessive heat across portions of the central/southern Plains to the lower and mid-Mississippi Valley, as well as the interior Pacific Northwest, Thu-Sun, Jul 29-Aug 1. - Excessive heat across portions of the central Plains to the mid-Mississippi Valley, Thu, Jul 29. - Excessive heat across portions of the Carolinas, Fri, Jul 30. 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