Extended Forecast Discussion NWS Weather Prediction Center College Park MD 322 PM EDT Mon Aug 26 2019 Valid 12Z Thu Aug 29 2019 - 12Z Mon Sep 02 2019 1600 UTC Update: A relatively amplified upper level pattern is expected across the northern tier of the U.S. and southern Canada during the Day 3-7 period, with general troughing over the Hudson Bay and Great Lakes region and ridging over the northern Rockies (at least through Day 5). Ridging will persist over the southwest U.S into the southern Plains. A stronger shortwave trough (potentially as a closed low) will approach the northeastern Pacific and British Columbia by Day 6-7. The updated forecast changed minimally from the previous forecast, with the model blend preference primarily a consensus of the operational/deterministic models for Day 3-5 with increasing weight of the 06Z GEFS and 00Z ECENS means by Day 6-7. Taylor ...Expected Weather Pattern... An amplified upper level flow pattern is expected across southern Canada and the northern tier of the continental U.S. through much of the medium range forecast period, with an upper level ridge situated over the Intermountain West, a broad upper level trough over the Great Lakes and north-central plains, and an upper ridge near Bermuda. A large upper low initially over Hudson Bay on Thursday will continue lifting northward with multiple shortwave perturbations pivoting around it, providing cold frontal passages across the northern tier states. A northeast Pacific trough approaches the Pacific Northwest and British Columbia by Sunday and into Monday. ...Model Evaluation... Early in the forecast period, the UKMET is weaker with the trough axis across the Southeast U.S. and stronger with the ridge over the Desert Southwest. The 00Z run of the GFS has backed off some on the idea of a strong shortwave trough crossing southern Canada Friday and Saturday, compared to its 12Z and 18Z runs that appeared to be an outlier solution when compared with the ensemble plots. By next Monday, the CMC becomes much stronger with a northern stream shortwave crossing Ontario compared to the other guidance. Given these considerations, a blend of the deterministic ECMWF/CMC/GFS was used as a forecast starting point for Thursday through Friday, along with some previous WPC continuity and EC mean, and then more weighting towards the 12Z EC mean and some of the 18Z GEFS ensemble mean for the second half of the forecast period. ...Sensible Weather... The cold front approaching the Northeast and Ohio Valley this weekend should have enough moisture channeling northward ahead of it to support a swath of mainly light showers and perhaps a few thunderstorms from the greater Chicago area to southern New England. There will likely be a more noticeable drop in temperature and humidity levels with the passage of this front compared to the previous front, which should be exiting the East Coast by Thursday morning. Attention turns to the central plains and vicinity by Friday and into Saturday as multiple thunderstorm complexes are likely to develop near and to the north of a stationary front. Although the axis of heaviest rainfall remains uncertain at this juncture, the potential exists for several inches of rainfall across parts of Kansas and Oklahoma based on some of the latest deterministic model guidance, and the WPC forecast has also trended slightly higher. Flooding may become an issue if training convection develops near the front, and also strong to severe storms. Numerous showers and storms are also expected for the Florida Peninsula In the temperature department, a high quality airmass will provide a hint of fall across much of the north-central U.S. and the Great Lakes region as a Canadian surface high governs the weather pattern. Expect readings to be on the order of 5 to 15 degrees below late August averages, with the greatest departures from normal extending from the Dakotas to Kansas. The opposite should hold true across much of the western third of the U.S., with high temperatures running about 5 to 15 degrees above average. Highs in the lower 110s are possible across the lower elevations of the Desert Southwest through Saturday, with the potential for excessive heat criteria to be met. D. Hamrick 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, Thu-Fri, Aug 29-Aug 30. - Heavy rain along the Southeast U.S. coast, portions of the Carolina coasts, much of Florida, and along the central and eastern Gulf Coast, Sat-Mon, Aug 31-Sep 2. - Flooding possible across portions of the central Plains, the upper Mississippi Valley, and the northern Plains. - Flooding occurring or imminent across portions of the northern Plains. 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