Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Fri Sep 08 2023 Valid 12Z Mon Sep 11 2023 - 12Z Fri Sep 15 2023 ...Hazardous heat from the south-central U.S to the Southwest to moderate next week... ...Overview... The guidance for next week expects mean troughing aloft over the eastern half of the country to become somewhat deeper than in the short range period, while a flat ridge over the West early-mid week could strengthen by Thursday or Friday as northeastern Pacific shortwave energy progressing into/through the northwestern U.S./western Canada eventually starts feeding into the downstream mean trough. In association with the deepening eastern trough, Great Lakes into New England/southeastern Canada low pressure and frontal system will promote an organized area of rainfall with highest amounts tending to be over the Northeast. Meanwhile a couple fronts over the southern Rockies and Plains will provide some focus for what could be a fairly wet period over that region. Southern tier heat should get suppressed after early in the week while cool daytime temperatures will likely spread eastward from the central U.S. over the course of next week. ...Guidance Evaluation/Predictability Assessment... Broadly speaking, there is decent agreement with the forecast evolution during the period. A positively tilted shortwave dropping into the Upper Midwest early next week should close off an upper low around midweek, with this upper low opening up and ejecting northeastward with the approach of Northeast Pacific energy crossing the northern tier U.S. and southern Canada. Some differences exist for exactly when the upper low closes off and how deep it is. The past couple UKMET runs are prominent extremes on the deep side, leading to a surface low that strays well northward of other guidance by next Wednesday. Among the means, the ECens has been closest to the operational consensus at the surface and aloft thus far while the GEFS/CMCens have been lagging a bit--being weaker aloft and flatter at the surface. The new 00Z GEFS mean seems to be trending in the right direction. Some operational model runs suggest that upstream energy reaching southern Canada and the northern tier of the U.S. could be somewhat deeper than the means by the end of the period, with an intermediate blend providing a reasonable starting point given typically lower predictability in specifics at that time frame. Also by next Friday, most guidance shows Hurricane Lee tracking northward between the East Coast and Bermuda, near 70W longitude. Any possible effects on New England and the Canadian Maritimes are likely to be just after the current medium range forecast period. Based on the 12Z/18Z guidance available for the latest update, the starting blend used the ECMWF/GFS/CMC/UKMET in order from more to less weight (GFS split between 12Z/18Z runs given details that were more or less favorable depending on the region) early in the period and then incorporated some 18Z GEFS/00Z ECens means by days 6-7 Thursday-Friday given decreasing confidence in some details at that time. ...Weather/Hazards Highlights... A primary region of focus in the Days 4-5 Excessive Rainfall Outlooks (EROs) that cover daytime Monday through Tuesday night will be over and eventually west of the southern Plains where multiple episodes of convection are possible. Contributing ingredients include shortwave impulses progressing west to east around upper ridging centered over Mexico and a couple surface fronts sagging southward over the southern Plains. A period of easterly upslope flow will be possible over the High Plains as well. Forecast rainfall amounts and guidance scatter within a background of moderately positive moisture anomalies and some instability, offset somewhat by relatively dry antecedent conditions, generally support a Marginal Risk area each day. The Day 5 area drops a bit southward versus Day 4 as well as extending farther west (into Arizona versus just into the High Plains). Refinements in future model runs may ultimately reveal areas of more pronounced flash flood potential. Meanwhile, a Marginal Risk area has been maintained over parts of New England for Day 4 as a wavy front and its associated moisture slowly move through. The developing upper/surface system reaching the Great Lakes could produce one or more pockets of moderate to heavy rainfall by the Day 5 time frame but current guidance signals appear too localized/diffuse for a risk area at this time. Rain and thunderstorms over the southern Plains and Four Corners states may continue into the latter half of the week, with a lighter and more scattered trend over Arizona and Utah. The system reaching the Great Lakes by midweek should produce some rainfall over parts of the Midwest early in the week and then spread an organized area of rainfall from the Great Lakes into the Northeast/Mid-Atlantic, with highest totals currently expected over the northern Mid-Atlantic/Appalachians into New England. Hazardous late summer heat will linger across the far southern tier of the U.S. and the Southwest into Monday. High dewpoints and humidity especially near the western Gulf Coast will lead to dangerous heat indices as high as 115F with actual air temperatures up to 10-15F above normal. Some record high minimum and maximum temperatures will be possible. Expect a cooler trend after Monday as supporting upper level ridging gets suppressed. Below normal high temperatures will prevail over the rest of the central U.S., spreading eastward with time. Some highs may be as cool as 10-15F below normal, especially over the central Plains early in the week. Temperatures in the Northwest may trend moderately above normal by late week under stronger ridging aloft. 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