Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 PM EDT Sun Sep 26 2021 Valid 12Z Wed Sep 29 2021 - 12Z Sun Oct 03 2021 ...Overview... The Wednesday-Friday part of the forecast will feature an amplified upper pattern as ridging in the east-central U.S. separates an East Coast trough (most likely with an embedded closed low over the Northeast) and mean troughing over the West. Initial western trough energy should split, with the southern part forming a Southwest upper low and the rest tracking into the northern tier U.S./southern Canada. Heading into/through the weekend the pattern should flatten somewhat as a Pacific shortwave reaching western North America continues on toward the mean ridge position and heights over a majority of the West generally rise. Late this week the Southwest upper low and moist inflow will lead to heavy rainfall potential over southern portions of the Rockies and Plains. ...Guidance Evaluation/Preferences... Within the East Coast upper trough, guidance is slowly consolidating toward the idea of the initial eastern Canada upper low dropping into New England by around late Wednesday/Thursday with a possible northeastward drift by Friday. New 12Z model runs reinforce this scenario. GEFS means generally remain on the northern side of the spread while the 06Z GFS displayed a west-of-consensus wobble around Friday. Depending on specifics of the upstream pattern, digging flow on the western side of the upper trough could elongate the feature by Friday-Saturday. The 00Z ECMWF and 12Z UKMET are the proponents of that idea but the new 12Z ECMWF has adjusted flatter. Meanwhile the eventual combined evolution of shortwave energy reaching the northern Plains/southern U.S. late this week along with the ridge downstream has become increasingly problematic over the past day, in terms of the range of potential surface patterns and rainfall distribution late this week through next weekend. Latest GFS runs have been cutting the shortwave energy into the downstream ridge while also closing off an upper high over eastern Canada, leading to a wet system affecting northern/central parts of the East and ejecting the East Coast upper trough. With some different specifics the 00Z CMC also brought a system into the East, while the 12Z run abandoned that scenario. Remaining models as well as the ensemble means (including at most only a faint hint of GFS ideas in the GEFS mean) hold onto a more pronounced east-central North America mean ridge aloft with surface high pressure continuing to dominate over the eastern U.S. For the purposes of a single deterministic forecast the preference is to favor the ensemble mean/ECMWF scenario over the eastern half of the country next weekend, which also maintains reasonable continuity. On average today's guidance is a tad weaker with the core of the upper low expected to form over the Southwest by Thursday. A moderate amount of spread develops for the upper low track by Friday, at that point fairly typical for a forecast that far out in time. However after Friday the guidance shows greater divergence and run-to-run variability than seen yesterday. Now the envelope ranges from lingering over the Arizona/New Mexico border (00Z ECMWF) to shearing well northeastward into the Plains (GFS/GEFS mean/CMC runs). The main item of note for shortwave energy reaching the Northwest around Friday is that the new 12Z CMC is quite extreme relative to other guidance as the shortwave nears the coast and then forms a rather deep upper low that ends up off the northern California coast by Sunday. Consensus suggests only a small part of this shortwave may get left behind somewhere over the eastern Pacific/western U.S. while the rest continues onward to push a cold front across the northern U.S./southern Canada. The days 3-5 Wednesday-Friday operational model blend split the GFS component among 00Z and 06Z runs since the 00Z run compared somewhat better to other guidance for the New England upper low. Preferences toward the ensemble means/ECMWF over the East after Friday led to a rapid transition of the blend to 50-70 percent total ensemble mean weight for days 6-7, continuing with more ECens than GEFS due to the former better maintaining the New England/Canadian Maritimes upper low. The forecast maintained some 00Z ECMWF input while decreasing GFS weight to a low enough level not to affect the eastern U.S. pattern. ...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. The greater chance for multi-day accumulations of multiple inches of rain will be across the southern High Plains beginning Thursday or Thursday night and lasting into the weekend, as the upper low in the Southwest provides moist inflow from the Pacific and the low-level pattern brings a feed of moisture from the Gulf of Mexico. Uncertainty over the eventual track and timing of the Southwest upper low somewhat tempers confidence for the specifics of rainfall duration and intensity. The Pacific Northwest can expect precipitation on Wednesday into Thursday, with the highest totals in the Olympics and northern Cascades, and some snow is possible at the highest elevations. Scattered showers and thunderstorms are likely across southern Florida as a couple of fronts move through. Areas of light rain are possible across the Eastern Seaboard, most likely over the Northeast, due to the upper low/trough. Over the remainder of the East, high pressure should keep most areas dry through most of next weekend aside from any rainfall accompanying a front that could reach the Great Lakes by Sunday. However there is a low-probability chance of more moisture extending into the Great Lakes/Ohio Valley and beyond. Mean ridging aloft and warm low level flow ahead of a couple fronts will support above normal temperatures from the Dakotas into the Upper Midwest through the period. Most days should feature at least some locations with plus 10-20F anomalies and daytime highs reaching into the 70s and 80s. On the other hand much of the West will see well below normal temperatures on Wednesday with coolest highs of 10-20F below normal likely to be centered over the Great Basin. By late week and the weekend expect the coolest anomalies for highs to settle into the southern Rockies/High Plains due to the clouds/precipitation over the region. Also next weekend the West Coast states and vicinity should see temperatures rise to moderately above normal levels as the western U.S. mean trough aloft weakens and possibly even gives way to some degree of upper ridging. Fairly modest temperature anomalies should prevail over the East with above normal readings over the South mid-late week and below normal highs in the Northeast for as long as the forecast upper low persists over the region. Rausch/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