Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Tue Sep 19 2023 Valid 12Z Fri Sep 22 2023 - 12Z Tue Sep 26 2023 ...Heavy rainfall potential across parts of the Plains/Upper Midwest as well as the East Coast late this week and weekend... ...Overview... A deep upper level closed low over the West on Friday should eventually shift east as the leading edge of amplifying/broadening Northeast Pacific troughing extends into the Pacific Northwest and western Canada by the weekend. Most guidance suggests the upper low will slow down for a time across the north-central U.S. as upper ridging builds into the Western Atlantic and southern/east-central Canada, creating a blocky pattern across the CONUS during the second half of the period. Elsewhere, a surface low forming off the Florida coast late this week will bring potential for heavy coastal rains to parts of the Southeast and Mid-Atlantic as it moves slowly up the coast. Models and ensembles develop more spread than usual for the overall central/eastern North America pattern, so there continues to be a lot of uncertainty for the ultimate path of the Plains upper low, as well as how eastern Canada/northeastern U.S. shortwave energy may evolve and its possible influence on the East Coast system/associated rainfall. ...Guidance Evaluation/Predictability Assessment... Given the dramatic divergence in some aspects of guidance, the updated forecast based on 00Z/06Z runs stayed closest to the 06Z GEFS/00Z ECens-CMCens means in principle with some incorporation of operational guidance where feasible--especially early in the period when components of the 00Z GFS/ECMWF/UKMET made up 60 percent of the overall weight. Means were two-thirds of the blend by late in the period. This provided the best combination of continuity in the overall forecast evolution while incorporating some modest trends. For the upper low tracking from the West into the north-central U.S. this weekend, there is a decent majority cluster up through the point of reaching the northern High Plains on Day 4 Saturday. There is still a small minority (represented by the 00Z ECMWF, and in eastward/delayed fashion by the new 12Z CMC) suggesting potential for initial shortwave energy extending south/southwest from Hudson Bay to pull off an upper low just north of the Plains or Great Lakes and become dominant relative to the Plains low. Another minority scenario, most recently depicted in the 06Z GFS and new 12Z UKMET, would be for the Plains upper low to open up and lift northward around the western side of the upper ridge trailing the aforementioned Canada shortwave. Adding to the forecast difficulty, some solutions are showing the potential for the eastern Canada shortwave energy to amplify into the Northeast--possibly whisking away the East Coast system into the Atlantic (06Z/12Z GFS, 12Z ECMWF) or even retrograding underneath the Canadian ridge and reaching a position close to the Upper Midwest/western Great Lakes low position shown by the means and the 00Z GFS. The average of guidance shifted a little northward with the upper low track, so the preferred blend reflected this adjustment albeit with fairly low confidence. Minus the UKMET, the 12Z model average is a little farther south again. As for other aspects of the East Coast system, there is a slight majority among new runs leaning toward somewhat faster northward progression than 00Z/12Z ECMWF and 00Z ECens runs and 06Z GFS, with a compromise providing a reasonable deterministic solution for now. The 00Z ECMWF tracked the system a little farther inland from the East Coast (adjusting back to the east in the new 12Z run), while the CMC is now the only holdout depicting a surface low track well northward of the Mid-Atlantic. Otherwise there is fairly overwhelming support for what has been the ensemble mean idea of the surface wave staying fairly suppressed, underneath strong eastern Canada high pressure. The National Hurricane Center continues to monitor this system for potential sub-tropical development, but regardless of classification, this surface low has the potential to bring significant rainfall to parts of the Southeast and Mid-Atlantic coast this coming weekend. Over the Pacific Northwest the guidance spread for the leading edge of Northeast Pacific upper troughing appears fairly typical, with shortwave differences having low predictability for the time frame in question. By late in the period 00Z/06Z GFS runs appeared to be on the fast side with the progression of leading height falls but the new 12Z run seems to compare better to other guidance. There is fairly good model agreement that a strong system will develop a ways offshore the Pacific Northwest coast around Sunday-Monday. The greater majority sides with the ensemble means for the latitude of this storm, while the 00Z ECMWF/12Z UKMET were farther south. Some differences along the coast would appear minor but have a more significant influence on exactly how much moisture reaches the region at times. ...Weather/Hazards Highlights... Anomalous moisture associated with the upper low in the West late this week will allow for the potential for heavy rainfall from the northern Rockies into the High Plains. There could also be some snow in the highest elevations depending on the depth of the upper low. As the low moves east into the Plains, rainfall should begin to 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. For the Day 4/Friday Excessive Rainfall Outlook, a slight risk remains across parts of the High Plains from southeast Montana/northeast Wyoming into western South Dakota, with a broad marginal risk surrounding this from the northern Rockies into the north-central Plains. Latest guidance shows more of a split between northern Plains moisture and another band with a southern Canada front, favoring removal of some locations near the Canadian Border from the Marginal Risk area. On Day 5, the Slight Risk area from southern Minnesota through northern Missouri was extended southward some for the afternoon update, in accordance with guidance trends for convective potential. Guidance agrees on fairly high values of instability over this area, adding to the potential for hazardous/excessive rains. A surrounding broad Marginal Risk now extends farther back into the High Plains per latest model trends. Some locations over the northern Plains could eventually see an upgrade in risk level if guidance becomes clustered better. Some of the convection over the Plains may also be strong to severe. Check the latest outlooks from the Storm Prediction Center for more information on severe threats. The surface low developing off the northeast Florida coast by Friday should tap tropical moisture, with increasing potential for heavy rainfall along at least the Atlantic coast from Florida to southern New England. Amounts and inland extent of the heavier rains continue to be extremely uncertain at this point, so just a marginal risk was maintained for the Days 4 and 5 EROs with only modest adjustments. Extension of this area farther inland and an upgrade to an embedded Slight Risk are eventually possible, but highly dependent on how close to the coast the low ultimately tracks and how far inland the QPF extends. Through the rest of the weekend into early next week, rainfall over the central U.S. should continue gradually eastward along with the upper low and surface system with some activity locally heavy. The Pacific Northwest should see gradually more rainfall with time, but to what extent is still a question mark that will take additional time to resolve. The axis of moisture may be fairly narrow, with minor changes in position being the difference between minimal totals or significantly more. 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 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. Rausch/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