Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 PM 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... Guidance today continues to show good agreement on the large scale with the forecast evolution next week. 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. UKMET runs continue to be deeper with the upper low, leading a much farther north surface low by next Wednesday. The GFS is also somewhat deeper with the shortwave (new 12z run much faster too) later next week, which seems to have influence on the late period speed of Hurricane Lee, currently forecast to track northward between the East Coast and Bermuda next week. The GFS is much faster to pull Lee north becoming absorbed into the exiting Northeast shortwave, as early as next Friday. The ensemble means and the ECMWF/CMC are slower with Lee, suggesting that any possible impacts on New England and the Canadian Maritimes are likely to hold off until just past the end of the period. This will need to be monitored closely. Refer to forecasts from the National Hurricane Center for the latest information on the strength and track of Lee. The updated WPC forecast for today used a starting blend of the operational deterministic models through day 4. Removed the UKMET after day 4/Tuesday per issues above, and trended away from the GFS in the later periods due to its faster track of Lee. The blend after mid week trended mostly towards the agreeable ensemble means which helps mitigate timing differences with the next shortwave into the Northwest later next week (ECMWF faster than the other guidance). This approach maintains good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... A primary region of focus in the Days 4-5 Excessive Rainfall Outlooks (EROs) that cover Monday and Tuesday 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, but for now there continues to be enough disagreement in the placement of higher amounts to preclude a slight risk. 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 may linger across far southern Florida, Texas, and the Southwest into Monday. High dewpoints and humidity especially near the western Gulf Coast will lead to heat indices as high as 110-115F 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. 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