Extended Forecast Discussion NWS Weather Prediction Center College Park MD 114 PM EDT Mon Sep 20 2021 Valid 12Z Thu Sep 23 2021 - 12Z Mon Sep 27 2021 ...Heavy rainfall expected to continue pushing through the Northeast U.S. Friday into Saturday... ...Weather Pattern Overview... The longwave trough that is currently crossing the Great Plains is forecast to evolve into a closed upper low over the Midwest on Wednesday before lifting north into Ontario as it interacts with a potent wave or low approaching from the Canadian Prairies. These interacting systems over/north of the Great Lakes causes the cold front to slow over the Northeast, potentially struggling to clear Maine this weekend. West of the Great Lakes trough/low is an amplifying ridge with an axis over the Intermountain West/northern Rockies by Saturday. An upper low gets trapped under the ridge near the southern California coast Friday well into the weekend that should direct Pacific moisture northeastward across portions of the Desert Southwest. ...Overview and Guidance Evaluation/Preferences... Substantial model uncertainty continues with the evolving upper low over the Great Lakes/Ontario Thursday into this weekend, particularly in how it interacts with the approaching wave from the Canadian Prairies Thursday night into Friday. Given the disparate solutions in depth/timing/whether or not the systems phase maintains a below average forecast confidence for much of the northeastern CONUS while near to above average for the remainder of the CONUS. The 06Z GFS continues to be the most progressive solution, with the most aggressive northward push from the Midwest Thursday night which results in nearly no phasing and a stronger resultant low over the Great Lakes with a much faster cold frontal progression through the Northeast. The 00Z UKMET/ECMWF/CMC are considerably slower in lifting the low from the Midwest Thursday night while the 00Z ECMWF is the farthest north with the approaching central Canadian trough/low. None of the guidance fully phases the two lows, but the strong trough from the ECMWF and deeper low from the CMC make for a slow cold frontal solution with the UKMET focusing a low Friday night just north of Lake Superior makes for a cold front with timing between the much faster GFS and slower ECMWF/CMC. Therefore, as a bridge to the previous WPC forecast that included some of the GFS (which was not as progressive as the 06Z solution), the preferred model blend for the fronts/pressures leans heavily on the 00Z UKMET through Day 5 before going exclusively with an even blend of the 06Z GEFS and 00Z CMCE/ECENS which are in acceptable agreement with the large scale wave pattern of the trough over the West Coast, ridge over the central CONUS, and troughing in the east. ...Weather and Potential Hazards... Widespread moderate to heavy-at-times rain looks to slow in its progression through the Northeast Friday, lingering over much of New England on Saturday, and over Down East Maine on Sunday as the cold front intercepts a deep plume of moisture. The potential remains for widespread 1-2 inch rainfall totals with isolated amounts up to 3 inches possible Thursday night through Friday night east from the Hudson Valley. There may be some instances of flooding if higher rainfall rates repeatedly affect the same areas and for the pockets of areas with recent heavy rain. 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 Southeast and Mid-Atlantic through Friday with dry conditions and temperatures 5 to 10 degrees below normal. The opposite will hold true across much of the western U.S. as a warming trend is likely to commence by Friday 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 northern High Plains Sunday, expanding southeastward to the southern Plains and Midwest for Monday. Meanwhile, temperatures should return to seasonal averages for the eastern third of the nation Sunday into Monday. Jackson 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