Extended Forecast Discussion NWS Weather Prediction Center College Park MD 419 PM 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... Strong upper ridging is forecast to meander in place in an omega/Rex block configuration between Hudson Bay and the Upper Midwest/Corn Belt/Mid-MS Valley. On its southeast side, a frontal complex will move into the MS Valley with heavy rain and severe weather chances. Over the Southeast, an upper low and developing coastal low (potentially sub-tropical) will likely lift north/northeastward this weekend with a rainfall shield over much of the I-95 corridor from VA into New England. Across the North Pacific, a strong upper jet arcing across the Aleutians will promote digging troughing southeast of the Gulf of Alaska and west of the Pac NW this weekend before translating eastward/inland early next week. ...Guidance Evaluation/Predictability Assessment... Aside from the East Coast, the 00Z-06Z guidance suite was in good agreement overall with some exceptions noted in the 06Z GFS off the Pac NW (farther north with the deep low) but otherwise mostly within tolerance. Over Canada, some models finally relented on showing a closed low moving southwestward across the Great Lakes and now the strong majority favor a more progressive trough into the Northeast this weekend, which should help kick out the coastal system. Regarding the evolution of said system, guidance generally agrees on lifting the low northward or north-northeastward across the Outer Banks and maybe SE VA before turning northeastward and moving out to sea. Will maintain the non-tropical structure (occluded cyclone) but some models do indicate potential for at least sub-tropical characteristics. Regardless, impacts (heavy rain, wind, choppy waters, etc.) will be the same. Split the difference between the faster (generally GFS/GEFS) and slower (generally ECMWF-led) camps and favored a more coastal vs inland track over VA. Across the West/Central CONUS, a consensus blend sufficed as well with good agreement on the slow track of the upper low across the Corn Belt next Tue or so between the recent GFS/ECMWF. Predictability was near to better than average in the West but perhaps a bit lower than average along the coast where there may be a sharp gradient between heavier rain and much lighter/less impactful rain. ...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 and despite dry antecedent conditions. SPC noted a severe potential as well. A Slight Risk outline is noted from the Upper Midwest/Northern Plains southward toward the Ozarks. On Sunday into early Monday, as the front continues southward and eastward, heavy rainfall will be possible over portions of TX/OK/AR/LA where a Slight Risk is shown. 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 as the deep moisture is wrapped northward and up and around the surface low (reminiscent of a cool season nor'easter). Amounts and inland extent of the heavier rains continue to be fairly uncertain at this point, so a Marginal Risk was maintained for the Day 4/Saturday ERO with room for adjustments per future guidance. By Sunday, a quicker exodus may reduce the heavy rain risk over southeastern New England but kept a small Marginal area for now. 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 as the cold front approaches then moves inland. 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 in the central CONUS. Elsewhere, temperatures could be moderately above normal across the Upper Midwest/Great Lakes and parts of the southern Plains Friday into Saturday and even approach record highs over South Texas. By early next week though, much of the country should be near normal or within a few degrees of normal. The one exception will likely be along the West Coast (esp NorCal northward into the northern Great Basin) where incoming rainfall may keep daytime highs at least 5-10 degrees below normal. Fracasso/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 Hazards: - Heavy rain across portions of Northern California, and the Pacific Northwest, Mon, Sep 25. - Heavy rain across portions of the Plains, and the Mississippi Valley, Sat-Sun, Sep 23-Sep 24. - Heavy rain across portions of the northern Mid-Atlantic and the Northeast, Sat, Sep 23. - Severe weather across portions of the Central/Southern Plains, and the Middle/Lower Mississippi Valley, Sat, Sep 23. - Hazardous heat across portions of the Southern Plains, Sat-Sun, Sep 23-Sep 24. 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