Extended Forecast Discussion NWS Weather Prediction Center College Park MD 224 AM 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 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 and 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 ongoing differences that arise for upper pattern details over southern Canada and the northeastern U.S. ...Guidance Evaluation/Predictability Assessment... There remains uncertainty in the evolution of the closed low over the West as it moves into the north-central U.S. this weekend, with plenty of detail variability with bigger implications for rainfall amounts/QPF. Issues arise due to an initial southern Canada trough/trailing ridge, where some of the guidance yesterday suggested a secondary closed low to form out of this. The 18z/Sep 18 GFS was the greatest outlier suggesting the two lows attempt to phase, but the latest 00z run this morning (available after forecast generation time) pulled back on this. There is a little bit better consensus now that the Western low should remain mostly in tact as it moves east, weakening and possibly opening into more of a wave later in the period. Still, there remains some spread in exact placement of this low as it meanders across the Upper Midwest region Sunday-Tuesday and any possible interaction with the East Coast low. This southern Canada flow uncertainty also contributes to dramatic divergence of some guidance for the surface low along the East Coast and whether it eventually gets pulled inland or stays near the coast. Latest GFS runs remain on the faster side, though the new 00z run doesn't pull it as far west as previous runs. CMC runs are also fast/farther inland, while the ECMWF and UKMET (with support from the ensemble means) suggest a much slower progression north along the coast. Current preference remains in line with the ECMWF and ensemble means which maintains continuity for now awaiting any dramatic shifts in guidance clustering. The National Hurricane Center is monitoring 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. However fairly minor differences do have a more significant influence on exactly how much moisture reaches the region late in the period. The WPC blend for tonight leaned towards the better clustered ECMWF/CMC/UKMET the first half of the period with the Western upper low, trending quickly towards the ensemble means and ECMWF the second half of the period as spread increases across both the north-central U.S. and the East Coast. Overall, this maintained good continuity with the previous WPC forecast. ...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. On Day 5, introduced a slight risk from southern Minnesota to northern Missouri where QPF consensus was greatest for potentially hazardous/excessive rains, with a broad marginal surrounding and back into the northern Plains. 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 drawn for the Days 4 and 5 EROs. Extension of this farther inland and an upgrade to a slight with time are possible, but highly dependent on how close to the coast the low eventually tracks and how far inland the QPF extends (which some of the guidance is suggesting). 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. Santorelli/Rausch 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