Extended Forecast Discussion NWS Weather Prediction Center College Park MD 243 AM EDT Thu Sep 21 2023 Valid 12Z Sun Sep 24 2023 - 12Z Thu Sep 28 2023 ...Heavy rainfall potential across parts of the Plains/Lower Mississippi Valley... ...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. On the East Coast, an upper low and developing coastal low (potentially sub-tropical) will be lifting north/northeastward this weekend with a rainfall shield mainly just affecting parts of the Mid-Atlantic to New England by the start of the period on Sunday. Across the North Pacific, a strong upper jet arcing across the Aleutians will promote digging of troughing southeast of the Gulf of Alaska and west of the Pacific NW this weekend before translating eastward/inland early next week. ...Guidance Evaluation/Predictability Assessment... Aside from the East Coast low, the latest guidance suite was in fairly good agreement, with some exceptions. The GFS remains on the slightly farther north side of the guidance with the upper low off the Pacific Northwest coast and also much more amplified with an initial shortwave across the West mid next week. The ECMWF and CMC seemed to have formed a better consensus with the ensemble means for the Western trough. Along the East Coast, the new 00z guidance (available after forecast generation time) seem to be converging on a consensus now, with the exception of the CMC which remains on the much faster side. Through the 12z/18z (Sep 20) runs though, the GFS and CMC were offering a more inland track over the Mid-Atlantic while the ECMWF and UKMET were noticeably offshore. Both the ECMWF and UKMET with the new 00z run has trended closer to the coast/just inland over the Mid-Atlantic before turning northeastward and racing out to sea. Many of the models also indicate potential for at least sub-tropical characteristics, which the NHC is monitoring. Regardless though, impacts (heavy rain, wind, waves, etc.) will be the same. The WPC forecast progs for tonight split the difference between the more onshore GFS and offshore ECMWF, and luckily is still very consistent with the latest 00z guidance that has come in since then. Across the board, the WPC forecast suite for tonight was primarily derived from a deterministic model blend for the first half of the period, mostly for the East Coast system as the West was in good agreement the first half. For days 6 and 7, trended towards the ensemble means to try and mitigate differences out West, while also maintaining some detail using the ECMWF. ...Weather/Hazards Highlights... The surface low near the Mid-Atlantic by the start of the period on Sunday should continue to tap tropical moisture, with heavy rainfall possible still across parts of the Mid-Atlantic and southern New England coast 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 somewhat uncertain at this point, so a Marginal Risk was maintained for the Day 5/Sunday ERO from coastal New Jersey to Cape Cod. There are some suggestions that by Sunday, much of the heavy rain could be off the coast and so even a marginal risk may be generous. Rainfall should focus along a frontal boundary dropping across the Plains/Mississippi Valley Sunday into Monday. The set up is favorable for heavy to excessive rainfall continuing into Sunday, given ample moisture and instability, across portions of the southern Plains/lower Mississippi Valley. The day 4 excessive rainfall outlook continues a slight risk across eastern Oklahoma/northeast Texas/western Arkansas. Rainfall will continue across the general area into Monday, but should be lessening in coverage/intensity as the front weakens. For now, only a marginal risk was drawn for the day 5 ERO from east Texas into the lower Mississippi Valley but there are indications for locally heavy to excessive amounts, but plenty of uncertainty still on where. Elsewhere, troughing along the West Coast will possibly heavy rainfall to mainly coastal locations and ranges, with extent farther inland still quite uncertain. A marginal risk was added to far northwest California into southwest Oregon on Day 5/Monday ERO where models seemed to have a better consensus for significant rains. Generally showery weather should continue into the Eastern U.S. during the middle of next week, with some local enhancement along favorable terrain. Troughing along the Southeast Coast may bring another period of wet weather by next Wednesday into Thursday to coastal locations. Below normal temperatures out West should moderate by Sunday as an initial upper low shifts east in the central CONUS. Elsewhere, temperatures could be moderately above normal across parts of the southern Plains Sunday into Monday. By Tuesday and beyond 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. Santorelli 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