Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1130 AM EDT Sun Aug 25 2019 Valid 12Z Wed Aug 28 2019 - 12Z Sun Sep 01 2019 1530 UTC Update... Overall, no major changes were made to the ongoing forecast in this update. A blend of the 00Z ECMWF and 00Z GFS served as a basis to the forecast through much of the forecast period, with more emphasis on the GFS, especially during days 3-5. The 00Z GFS was preferred over the 06Z for a couple reasons. The 00Z GFS track was very close to the ensemble consensus with respect to the area of low pressure expected to track off the Eastern Seaboard during days 3-4 (Wed-Thu), while the 06Z run seemed to race the area of low pressure northward more quickly than most other guidance. Additionally, the 06Z GFS was much stronger than consensus with a northern stream shortwave crossing moving southeastward from the Gulf of Alaska across western Canada and into the north central U.S. Thu-Sun, resulting in a relatively deep area of low pressure that has little support among other guidance or ensemble members. The most significant change was to slow down the system approaching the Pacific Northwest next weekend, as a trend toward a slower solution was noted among a majority of the model and ensemble guidance. Weighting of ECENS/GEFS ensemble means was increased during days 6-7 to account for increasing spread/uncertainty among the guidance. Ryan Previous Discussion (issued at 0653 UTC)... ...Expected Weather Pattern... An amplified upper level flow pattern is expected across much of North America 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 Ontario on Wednesday lifts northward across Hudson Bay by the weekend 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 late in the forecast period. ...Model Evaluation... There is above average model agreement on the synoptic scale across the continental U.S. through Thursday with the broad trough over the eastern U.S. and the ridge across the West, with the UKMET being the least amplified with the trough over the Southeast U.S. By Friday, more noteworthy model differences become apparent across the eastern Pacific and West Coast region. The CMC/ECMWF/UKMET are farther east with the ridge axis compared to the GFS and its ensemble mean, and this also holds true for the upstream trough near 140 degrees W Friday morning, and this continues going forward into the weekend. This difference is also apparent between the GEFS and EC means, whilst remaining similar with the eastern U.S. trough. Given these considerations, a blend of the 12Z ECMWF/UKMET/CMC and some of the 18Z GFS was used as a forecast starting point for Wednesday through Friday, with more emphasis on the ECMWF/CMC/EC mean by the end of the week with their more favored northern stream solutions, and then more weighting towards the 12Z EC mean and some of the 18Z GEFS ensemble mean by Saturday and Sunday. ...Sensible Weather... The cold front approaching the East Coast mid week should have enough moisture channeling northward ahead of it to support a swath of showers and thunderstorms from the Southeast U.S. to New England, with the best prospects for 1 inch or greater rainfall from the Mid-Atlantic region to Upstate New York. The majority of this rainfall should exit the East Coast by Friday with some lingering showers across New England. The airmass behind the front will be modified quite a bit by the time it reaches the Eastern U.S., so temperature changes should be rather minimal behind it. Attention turns to the central plains and vicinity by Friday and into the weekend 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. Flooding may become an issue if training convection develops near the front, and also strong to severe storms. 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. The opposite should hold true across the southern plains and extending westward to the Desert Southwest, where excessive heat could make weather headlines with highs exceeding 100 degrees and potentially above 110 degrees for the normally hotter locations. 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 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