Extended Forecast Discussion NWS Weather Prediction Center College Park MD 252 AM EDT Sun Sep 26 2021 Valid 12Z Wed Sep 29 2021 - 12Z Sun Oct 03 2021 ...Overview... An amplified upper pattern is expected especially early in the medium range period, as ridging in the east-central U.S. separates a likely closed low over the Northeast and mean troughing over the West. Southern stream troughing/energy over the Four Corners states and moist inflow will lead to heavy rainfall potential over southern portions of the Rockies and Plains toward the end of the week. ...Guidance Evaluation/Preferences... Model guidance from the 12/18Z cycle begins the medium range period Wednesday fairly well clustered with the pattern described above, and has trended more agreeable than earlier model cycles with the formation and placement of a closed low in the East around Wednesday-Thursday. Most deterministic guidance produces the closed low over the Northeast U.S., continuing the general northward trend. The main outlier by early Friday was the 12Z UKMET, which splits the energy and closes off the low centered around Cape Hatteras, while other recent guidance has trended away from the placement that far south. On Friday and beyond, quite a bit of model variability arises quickly with the upper/low trough feature. Trough energy moving southward spins southwest in the 12Z ECMWF, creating a farther south (around Cape Hatteras) upper low by the weekend, but spins southeastward in the GFS runs while keeping the core of the upper low to the north. Meanwhile the 12Z CMC maintains most of its energy within the low rather than splitting it off. Hurricane Sam may also be a factor to consider as it moves northward through the Atlantic late in the week. Overall confidence is low with the upper low placement and evolution by the weekend. At this point the WPC forecast tried to take a middle ground approach, with upper low placement around Downeast Maine on day 6/Saturday as a compromise center point. Ensemble mean guidance was only somewhat helpful as the differences in the upper low position in the ensemble members led to more of an open trough than individual models. Overall preferred the 12Z ECens mean compared to the GEFS means as the former maintained a closed low at least through Friday. Meanwhile in the West, an upper trough with shortwaves moving through for midweek has general agreement along the guidance. Models show shortwave energy splitting with some moving into the north-central U.S./south-central Canada, and some forming a southern stream trough or potential closed low centered in the Desert Southwest around Thursday. Some differences in the exact track and magnitude of both these features remain. But guidance all has similar ideas in lifting the Southwest energy northeastward through the Four Corners states for the end of the week that a model blend worked well to depict the feature. The WPC forecast began with a 12/18Z deterministic model composite blend, eliminating the 12Z UKMET by Friday, and incorporating the GEFS and especially the EC mean guidance for the latter part of the period and increasing the weight of the means by the weekend to minimize individual model differences. ...Weather and Potential Hazards... Central parts of the Plains will see showers and thunderstorms around midweek as moisture pools ahead of a shortwave and frontal boundary, and some rain could spread to the Upper Midwest on Thursday and Friday. But the greater chance for multi-day accumulations of multiple inches of rain will be across the southern High Plains beginning Thursday and lasting into the weekend, as the upper low in the Southwest provides moist inflow from both the Pacific and the Gulf of Mexico. The Pacific Northwest can expect precipitation on Wednesday into Thursday, with the highest totals in the Olympics and Cascades, and some snow possible at the highest elevations. Scattered showers and thunderstorms are expected across southern Florida as a couple of fronts move across, while areas of light rain are possible across the Eastern Seaboard due to the upper low/trough, mostly likely over the Northeast. Warmer than average conditions are forecast underneath the upper ridge from the Dakotas into the Upper Midwest through the period. Temperatures 10-20F above average will mean highs in the 70s and 80s there. Meanwhile, much of the West should see below normal temperatures given the upper trough. The most anomalous temperatures of 10-20F below normal are forecast in the Great Basin on Wednesday, while shifting east and slightly moderating as the week progresses. Cool high temperatures could linger in the Four Corners states into the weekend. Near to slightly below average highs are forecast for the East. Tate 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