Extended Forecast Discussion NWS Weather Prediction Center College Park MD 409 PM EDT Mon Aug 05 2019 Valid 12Z Thu Aug 08 2019 - 12Z Mon Aug 12 2019 ...16Z Update... Overall, no major changes were needed to the medium range suite for the day shift update. Some differences remain in the newer model guidance with the shortwave in the northwest to north-central U.S. mentioned in the previous discussion. The 00Z ECMWF and even its ensemble mean is faster with this feature than the recent GFS/GEFS runs (the 06Z deterministic GFS and GEFS mean are almost perfectly collocated), but may be coming into better alignment. The shortwave positioning does affect the low position at the surface, and did back up the low and frontal system in the Plains westward on Mon compared to the previous forecast. The only other main problem with the models was that the 00Z CMC strengthened and dropped an upper low southward into west central Canada on Fri/Sat, which was an outlier, so excluded that from the blend. Tate ...Previous Discussion... ...Overview and Guidance/Predictability Assessment... An upper low is expected to drift slowly eastward across James Bay and northeastern Canada through the medium range, with broad cyclonic flow south of the low across the eastern third of the CONUS. Several shortwaves will traverse this zone of cyclonic flow, the most pronounced of which should move a cold front from the Midwest/Great Lakes to the Eastern Seaboard Thu-Fri (days 3-4), with the trailing end of the front lingering from the Southern/Central Plains to the Southeast into the weekend. Farther west, an upper low/trough initially off the West Coast is forecast to push inland by Sat, moving into the northern Great Basin and northern Rockies by Sun, and the Northern Plains by Mon. Meanwhile, broad upper ridging is expected to remain centered across the south central U.S. Model consensus through much of the forecast period was sufficient to use a multi-model blend (12Z ECMWF/UKMET/CMC/18Z GFS) as a basis for the forecast. The most significant differences arose by days 6-7 (Sun-Mon) with the shortwave energy moving from the western into the north central U.S. The ECMWF has consistently been on the faster side of the guidance with the timing of this feature by early next week, and the GFS was actually a bit closer to being a middle-of-the-road solution in terms of timing based on the ensemble spread. Thus, the forecast was trended heavily toward ECENS/GEFS ensemble means by days 6-7, with some continued weight on the GFS as well. ...Weather Highlights/Threats... Models continue to show a signal for an area of organized convection with heavy rainfall along the lingering surface front across portions of the Central Plains and Mid-Mississippi Valley Thu-Fri, with multi-inch rainfall totals possible. Scattered showers and storms will also be possible along the northern extent of the cold front from the Ohio Valley to the Northeast. The shortwave moving into the western U.S. will bring scattered showers and storms from the interior Northwest to the northern Rockies late in the week into the weekend. The system should bring an increase in convection to portions of the Northern Plains and Upper Midwest by Sat-Sun. Monsoonal moisture will keep diurnal showers and thunderstorms active across the Four Corners region and the southern/central Rockies through the forecast period, with a daily threat for locally heavy rainfall. The upper ridge will keep hot conditions in place for the Southern Plains through much of the period, with high temperatures forecast to be 5 to 10 deg F above average into early next week. Cooler temperatures will accompany the Pacific shortwave inland, with highs forecast to be 5 to 10 deg below average from the Pacific Northwest to the northern Rockies Sat-Sun, spreading into the Northern Plains by Mon. Ryan 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 Hazards: - Heavy rain across portions of the Central Plains, the Lower Mississippi Valley, the Tennessee Valley, the Middle Mississippi Valley, the Southern Plains, and the Ohio Valley, Thu-Fri, Aug 8-Aug 9. - Heavy rain across portions of the Central Plains, the Middle Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains, Sun-Mon, Aug 11-Aug 12. - Heavy rain across portions of the Northern Plains, Fri, Aug 9. - Flooding occurring or imminent across portions of the Northern Plains. - Flooding likely across portions of the Northern Plains. - Excessive heat across portions of the Lower Mississippi Valley and the Southern Plains, Thu-Mon, Aug 8-Aug 12. 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