Extended Forecast Discussion NWS Weather Prediction Center College Park MD 319 AM EDT Mon Sep 18 2023 Valid 12Z Thu Sep 21 2023 - 12Z Mon Sep 25 2023 ...Heavy rainfall potential across parts of the Plains as well as the Southeast Coast late week and next weekend... ...Overview... An upper level closed low dropping through the West Thursday and Friday should eventually shift east as additional troughing moves into the Pacific Northwest next weekend. Guidance suggests the upper low will linger across the Plains for at least a few days as upper ridging builds into the Western Atlantic creating a blocky pattern across the CONUS the second half of the period. Elsewhere, a surface low forming off the Florida coast late this week brings potential for heavy coastal rains to parts of the Southeast as Mid-Atlantic as it moves slowly up the coast. ...Guidance Evaluation/Predictability Assessment... Overall, there's good agreement on the upper pattern across the CONUS, but a lot of spread in the details, even early in the period which have big implications for rainfall amounts/QPF. With the upper low initially over the West, a general model blend sufficed for Thursday and Friday, but there is divergence in the guidance as the low shifts into the Plains next weekend regarding placement and timing. The 12z/Sep 17 ECMWF seems most displaced with the low as it brings it well into the Lower Mississippi Valley, while the GFS and CMC (and ensemble means) suggest something deeper and farther north across the central Plains. The CMC is also much faster lifting the surface low up the Southeast coast and the ensemble consensus would suggest something slower and closer to the GFS/ECMWF. The UKMET is also an outlier early in the period as it tried to initially form the surface low well west into the eastern Gulf of Mexico. The NHC is monitoring this system for potential sub-tropical development, but regardless of characteristics, has the potential to bring significant rainfall to parts of the Southeast and Mid-Atlantic coast next weekend. ...Weather/Hazards Highlights... Anomalous moisture associate with the upper low in the West late this week will allow for possibly heavy rainfall from the northern Rockies into the High Plains and depending on depth of the upper low, there could be some snow in the highest elevations too. As the low moves east into the Plains, rainfall should begin to expand in coverage across the central and northern Plains through Saturday. The set up is favorable for heavy to excessive rainfall, given ample moisture and instability, but there remains too much uncertainty in the guidance for anything higher than a broad marginal risk right now on the Days 4 and 5 Excessive Rainfall Outlooks. A stalled frontal boundary will continue a fairly wet pattern over the Florida Peninsula into late week, though still low confidence in placement and amounts of rainfall to include even a marginal risk on the EROs. The surface low off the coast by Friday could tap tropical moisture, increasing the potential for heavy rainfall along the Atlantic coast from Florida to the Mid-Atlantic. Amounts and extent inland of the heavier rains remains extremely uncertain at this point, but did introduce a marginal risk in the ERO along the Southeast Coast for Friday. Temperatures as much as 10-20 degrees below normal associated with the deep Western upper low from California to Montana 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 Texas Thursday and Friday. By Monday, much of the country should be near normal or within a few degrees of 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