Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 AM EDT Wed Sep 20 2023 Valid 12Z Sat Sep 23 2023 - 12Z Wed Sep 27 2023 ...Heavy rainfall potential across parts of the Plains/Upper Midwest as well as the East Coast late this week and weekend... ...Overview... The leading edge of amplifying/broadening Northeast Pacific troughing extends into the Pacific Northwest and western Canada by the weekend. An omega block remains downstream from central North America eastward into the western Atlantic. A frontal wave/upper level shortwave around the southeast side of the Omega block and has the potential for heavy coastal rains to parts of the Mid-Atlantic and coastal New England as it moves up/offshore of the coast. ...Guidance Evaluation/Predictability Assessment... The guidance, in general, agrees on the deep cyclone offshore the West Coast migrates eastward and brings a cold front through portions of the West. Otherwise, the 12-00z Canadian runs became an outlier near the US/Canadian border, retrograding a closed low out of New England into southeast Canada where all the other guidance had ridging. The 12-00z Canadian were not judged useful beyond Sunday across the Great Lakes and East. For the potentially convective, possibly occluded, low moving up the coast, used to the typical overlap of the 12z ECMWF/GEFS members (fast side of the ECMWF and slow side of the GEFS) and tried to guide the low parallel to height lines due to its potentially convective nature. This kept good continuity early on and ended up fairly close to the 00z ECMWF solution. However, this led to faster progression mid period onward which led to changes. ...Weather/Hazards Highlights... As the low moves east into the Plains and then southward into the Ohio Valley, rainfall should expand in coverage across the central and northern Plains into the Mississippi Valley through Saturday. The set up is favorable for heavy to excessive rainfall, given ample moisture and instability. On Day 4/Saturday into early Sunday, the excessive rainfall risk areas maintained reasonable continuity. The new day 5/Sunday into early Monday outlook, a Slight Risk was placed where the best overlap in heavy rainfall was seen in the guidance, which should be near where the better moisture transport and instability resides. The surface low moving offshore the Carolinas should tap tropical moisture, with increasing potential for heavy rainfall along at least the Atlantic coast from the Mid-Atlantic coast into southern New England. Amounts and inland extent of the heavier rains continue to be fairly uncertain at this point, so just a marginal risk was maintained for the Days 4/Saturday into early Sunday ERO with only modest adjustments. Activity should be more convective near the coast with more frontogenetic/comma head rains extending somewhat inland Through the rest of the weekend into early next week, rainfall over the central U.S. should shift gradually east and southeast along with the upper low and surface system with some activity locally heavy, with lesser amounts expected by mid next week. The Pacific Northwest should see gradually more rainfall with time. The axis of moisture may be fairly narrow, with minor changes in position being the difference between minimal totals or significantly more. Below normal temperatures out West should moderate by Sunday as the low shifts east. Elsewhere, temperatures could be moderately above normal across the Upper Midwest/Great Lakes and parts of the southern Plains Friday into Saturday. By early next week though, much of the country should be near normal or within a few degrees of normal. The one exception may be along the West Coast where incoming rainfall may keep daytime highs at least 5-10 degrees below normal. Roth/Rausch 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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw