Extended Forecast Discussion NWS Weather Prediction Center College Park MD 518 PM EDT Tue Jul 27 2021 Valid 12Z Fri Jul 30 2021 - 12Z Tue Aug 03 2021 ...Widespread Monsoonal Moisture and Rainfall Pattern spreads over the West... 18Z Update: Overall, the models and ensemble means are in above average agreement on the synoptic scale features through at least Sunday, with an eastern U.S. trough and a central/western U.S. ridge resulting in a fairly amplified pattern. For the fronts/pressures, the CMC becomes a strong outlier with an upper trough/low over the eastern Pacific, and the 00/06Z runs of the GFS became more progressive with shortwave energy across the Great Lakes and Northeast by the weekend when compared to the ensemble guidance. For the Friday through Sunday time period, a general blend of the UKMET/GFS/ECMWF/some CMC was used as a forecast baseline, and then maintaining some of the GFS/ECMWF along with their ensemble means for next Monday and Tuesday, while keeping some previous WPC continuity. The previous discussion is appended below for reference. /Hamrick ...Overview with Weather Highlights/Hazards... Latest guidance remains consistent and agrees that a strong closed upper high over the central U.S. will gradually weaken and shift southward from late this week into next as a ridge extends through the Rockies and north-central Intermountain West to western Canada. The northward amplitude and sharpness of the 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 Friday-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. Many record warm lows are also likely across the moist South. Meanwhile, multiple shortwaves rounding the aforementioned upper ridge will dig downstream 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. Convective outflow and reinforcing frontal surges will act to gradually shift best focused rainfall chances later this week into next down over the Plains and from the Mid-MS/TN Valleys to the Carolinas/Southeast, with guidance trends forcing the main axis of precipitation slightly southward compared to continuity. The eastern U.S. pattern will be most favorable for below normal temperatures from the Great Lakes/Northeast to the Mid-Atlantic where a few locations with shorter periods of record may approach record low values on Saturday morning. ...Guidance/Predictability Evaluation... The medium range pattern offers above average predictability Days 3-7. A trend to develop an unseasonably amplified eastern U.S. upper trough seems reasonable considering the strength of the upstream ridge. The WPC medium range product suite was derived from a blend of well clustered guidance of the 18 UTC GFS and 12 UTC ECMWF/UKMET/Canadian along with the 01 UTC National Blend of Models Friday into the weekend before switching to a blend of the ECENS/GEFS ensemble means and 18 UTC GFS/12 UTC ECMWF by early next week. This strategy maintained excellent WPC product continuity. The latest 00 UTC GFS/ECMWF maintain good run to run continuity despite continued differences with shortwaves in the large scale eastern U.S. upper trough. The blend mitigates these differences. The 12 UTC UKMET/Canadian were outliers into Sunday-Tuesday by bringing northeast Pacific upper trough energy more robustly inland into western Canada than the GFS/ECMWF and GEFS/ECMWF ensemble means. The 00 UTC UKMET/Canadian have trended in line with other guidance, bolstering forecast confidence. Schichtel 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 Hazards: - Heavy rain across portions of the Upper and Middle Missouri Valley, Fri, Jul 30. - Heavy rain across portions of the Lower Missouri Valley and Middle Mississippi Valley, Sat, Jul 31. - Heavy rain across portions of the Front Range and Central High Plains, Sat-Sun, Jul 31-Aug 1. - Heavy rain across portions of the Southern Plains, Lower Mississippi Valley, and Tennessee Valley, Sun-Mon, Aug 1-Aug 2. - Heavy rain across portions of the Lower Mississippi Valley and Southeast, Sun-Tue, Aug 1-Aug 3. - Excessive heat across portions of the Southeast, Fri, Jul 30. - Excessive heat across portions of the interior Pacific Northwest, Fri-Sat, Jul 30-Jul 31. - Excessive heat across portions of the Southern Plains and Lower Mississippi Valley, Fri-Sun, Jul 30-Aug 1. - Excessive heat across portions of the Southern Plains, Sat-Mon, Jul 31-Aug 2.