Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Mon Sep 18 2023 Valid 12Z Thu Sep 21 2023 - 12Z Mon Sep 25 2023 ...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 intially 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 intially 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... Anamolous moisture associate with the upper low in the West late this week will allow for possibly heavy rainfall from the northern Rockies into the Plains. There remains a lot of spread in the details and placement though, so only a broad marginal risk remains for now on the day 4/Thursday excessive rainfall outlook. The inherited Day 4 Excessive Rainfall Outlook featured 2 areas highlighted by Marginal Risks. The southern one spans from northeast Texas to northern Missouri where moisture anomalies are greatest along a slow moving/stationary boundary. With the latest trend in the guidance minor expansions were made westward along the Texas and Oklahoma border and eastward along the Missouri and Arkansas border. The northern risk area spans from the eastern Dakotas to the Minnesota/Wisconsin border. No changes were made to this area at this time. The amplifying upper low over the West by midweek should lead to prolonged precipitation across the northern/central Intermountain West and Northern Rockies. As the low deepens the rainfall is expected to increase keeping the threat for excessive rainfall and local flash flooding elevated. The Day 5 ERO has a large Marginal Risk spanning from eastern Nevada northward to Montana and east to northern Minnesota. Higher elevations in the northern Rockies are likely to see snow that the extent to which depends on the position and depth of the upper trough/developing low. Increasing potential for more widespread heavier rain develops next weekend across the northern/central Plains into parts of the Mississippi Valley as the West upper low begins to push into the region. Minor adjustments were made to the western bounds over central Idaho and central Nevada and brought further northeast into northern Minnesota. Meanwhile, a stalled frontal boundary will maintain a fairly wet pattern over the Florida Peninsula through much of the week, though confidence in where that would focus is low enough to not raise an excessive rain outlook area in Days 4 or 5 at this time. The possible surface low off the Coast by Friday could tap tropical moisture, increasing the potential for heavier rains along the Atlantic coast from Florida to the Mid-Atlantic. Amounts and extent inland of the heavier rains remains extremely uncertain at this point. An area of max temperatures generally 10-15F above normal will shift east with the broad ridge axis from the Upper Midwest on Wednesday through the Great Lakes Thursday-Saturday moderating some with time. The upper low over the West looks to bring a cooling trend across much of the West, with high temps around 15F below normal forecast from Montana through California through at least Saturday. Modest above normal temperatures may return to parts of Texas this week, while much of the Eastern U.S. should be fairly near normal. Campbell/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