Extended Forecast Discussion NWS Weather Prediction Center College Park MD 246 AM EDT Mon Sep 20 2021 Valid 12Z Thu Sep 23 2021 - 12Z Mon Sep 27 2021 ***Heavy rainfall expected for portions of the Northeast U.S. to close out the work week*** ...Weather Pattern Overview... The longwave trough that is currently crossing the central U.S. is forecast to evolve into a closed upper low over the Ohio Valley and Great Lakes region by early Thursday, and then lifting to the northeast by the end of the week as the cold front exits the East Coast. Another trough and surface cold front will be tracking from the Pacific Northwest to the Upper Midwest by Friday night and this may also evolve into an upper low, although not as strong as the preceding system based on current model guidance. In its wake will likely be a developing upper ridge centered over the Intermountain West for next weekend, along with a weak upper low trapped under the ridge near the California coast that may bring some moisture northward across portions of the Desert Southwest. ...Overview and Guidance Evaluation/Preferences... There has actually been an increase in model uncertainty with the evolving upper low over the Great Lakes and Ohio Valley for the end of the week compared to yesterday, and forecast confidence for this system is below average for a day 3 and 4 forecast, while near to above average for the remainder of the nation. The ECMWF and CMC are considerably slower and more southwest with the core of the upper low compared to the more progressive GFS and GEFS mean. The UKMET and also the ICON model are generally between these two sets of solutions, and after reviewing trends in the guidance and previous WPC continuity, the forecast was hedged more in the faster direction through Friday. However, if the slower CMC/ECMWF solutions come to pass, the passage of the cold front could be delayed and heavier rainfall will likely persist longer across the Northeast. Elsewhere across the continental U.S., another synoptic scale trough is likely to develop over the Great Lakes region in time for the weekend with perhaps a closed low forming over Michigan and southern Ontario, with the GFS slightly faster and holds onto the feature longer over the Northeast U.S. by next Monday, whereas the other guidance lifts the trough out faster. There are also some longitudinal differences with the upper low near the California Coast, with the CMC/ECMWF farther offshore and the GFS/GEFS mean near the coast. The WPC fronts/pressures forecast was primarily based on the 12Z/18Z GFS, UKMET, and 18Z GEFS mean for Thursday and Friday, particularly for the East Coast storm system with slightly more weighting on the 12Z UKMET given its middle ground solution with the closed low over the northern Ohio Valley. The ECMWF and its ensemble mean were introduced into the forecast blend for the second half of the forecast period after the departure of the East Coast system. ...Weather and Potential Hazards... Widespread moderate to locally heavy rain is expected to be ongoing Thursday across northern portions of the Mid-Atlantic and much of the Northeast U.S. as the cold front intercepts a deep plume of moisture ahead of it. The potential exists for widespread 1-2 inch rainfall totals, with isolated amounts up to 3 inches possible through Friday evening, particularly across the elevated terrain of upstate New York and New England. There may be some instances of flooding if higher rainfall rates repeatedly affect the same areas and for those regions with saturated soils. There may be some strong thunderstorms as well, but the overall severe potential appears limited for now. In terms of temperatures, it will feel fall-like behind the cold front across the Ohio Valley and the Appalachians on Thursday, and the cold front should make it to the East Coast by Friday along with improving humidity levels. It should also feel more pleasant across the Deep South and Southeast after the front passes through the region. The opposite will hold true across much of the western U.S. as a warming trend is likely to commence by next weekend in response to a building upper level ridge, with highs expected to be on the order of 10 to 20 degrees above normal across the western High Plains next Sunday and then expanding eastward to the Midwest by Monday. Meanwhile, temperatures should be close to seasonal averages for the eastern third of the nation for this same time period. Hamrick 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