Extended Forecast Discussion NWS Weather Prediction Center College Park MD 327 PM 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 East Coast late this week and weekend... ...Overview... An upper level closed low dropping through the West Thursday and 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 next weekend. Most guidance suggests the upper low will slow down for a time across the central U.S. as upper ridging builds into the Western Atlantic and parts of southern 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 as Mid-Atlantic as it moves slowly up the coast. From the weekend onward there is increasing uncertainty in the track of this surface system and the coverage of associated rainfall, corresponding to significant differences that arise for upper pattern details over southern Canada and the northeastern U.S. ...Guidance Evaluation/Predictability Assessment... There's decent agreement on the upper pattern forecast across the CONUS from a multi-day mean perspective, but some of the day-to-day details vary significantly and have big implications for rainfall amounts/QPF. With the upper low initially over the West, a general model blend sufficed for the early part of the period. The models have been signaling potential for the upper low to weaken/open around the High Plains as of Saturday, while detail issues for an initial southern Canada trough and trailing ridge that builds in become an issue by mid-late period. The 00Z CMC and new 12Z GFS appear to have lower probability solutions for a southern Canada upper low, with the 00Z GFS/ECMWF and new 12Z CMC (plus an extrapolation of the slightly slower 12Z UKMET through early Day 6 Sunday) offering the best clustering for the upper low that should maintain definition into next Monday. This generally fits the trough position from the ensemble means whose varied members lead to a more open mean depiction. Southern Canada flow uncertainty contributes to the dramatic divergence of some guidance for the East Coast system. Latest CMC runs and the 06Z/12Z GFS track the surface low well inland while ECMWF runs and the 12Z UKMET maintain a more suppressed wave. Thus far the ensemble means have supported the more suppressed/coastal scenario. Current preference is in line with the ECMWF/00Z GFS/ensemble mean solution that would maintain continuity while awaiting any dramatic shifts in guidance clustering. The National Hurricane Center is monitoring this system for potential sub-tropical development, but regardless of characteristics, 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. However fairly minor differences do have a more significant influence on exactly how much moisture reaches the region. ...Weather/Hazards Highlights... Anomalous moisture associated with the upper low in the West late this week will allow for a potential of 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 through Saturday. The set up is favorable for heavy to excessive rainfall, given ample moisture and instability. Based on the pattern and latest guidance signals, there appear to be two areas favorable enough for a Slight Risk area within the broader Marginal Risk areas depicted in the Days 4-5 Excessive Rainfall Outlooks that cover the Thursday-Friday night period. On Day 4, there is a proposed Slight Risk area centered over the central Dakotas in the vicinity of a stalled surface front. For Day 5, there is some clustering among model/ensemble guidance for an axis of heavy rainfall potential from parts of southern Montana/northern Wyoming into western South Dakota--just north of the forecast upper low track--where a Slight Risk area has been proposed. A stalled frontal boundary will continue a fairly wet pattern over the Florida Peninsula into late week, though confidence in placement and amounts of rainfall are sufficiently low to include even a Marginal Risk area 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 inland extent of the heavier rains continue to be extremely uncertain at this point, but a Marginal Risk area seems to be a reasonable starting point in the ERO along the Southeast Coast for Day 5 (Friday-Friday night). Slight adjustment to the area in this afternoon's update keeps the outlook in the middle of the guidance spread that starts to broaden at that time. 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. Some activity may continue to be locally heavy. Expect additional rainfall over parts of the Mid-Atlantic and Northeast, but uncertainty for amounts/coverage continues to increase with time. 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 Texas Thursday and Friday. By Monday, much of the country should be near normal or within a few degrees of 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 Hazards: - Heavy rain across portions of the Central Plains, Southern Plains, Lower Mississippi Valley, Middle Mississippi Valley, Mid-Atlantic, and the Northeast, Sat, Sep 23. - Heavy rain across portions of the Central Plains, Southeast, Mid-Atlantic, Upper Mississippi Valley, and the Northern Plains, Fri-Sat, Sep 22-Sep 23. - Flooding likely across portions of northeast Arizona. 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