Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 PM EDT Mon Sep 11 2023 Valid 12Z Thu Sep 14 2023 - 12Z Mon Sep 18 2023 ...Hurricane Lee looms over the western Atlantic... ...Heavy rainfall threat for the south-central Rockies/Plains... ...Overview... It remains the case that the models and ensembles agree fairly well on the idea of a strong upper ridge building over the West late this week into the start of next week, while departing western U.S. shortwave energy and southern Canada energy combine to yield an amplifying upper trough over the eastern half of the country. Ahead of this developing trough, a leading upper low/trough will eject across southeastern Canada and the northeastern U.S. late this week. Precise upper flow details near the East Coast and specifics of ridging farther east over the Atlantic will be important in determining the exact track and timing of Hurricane Lee and corresponding effects on New England and especially the Canadian Maritimes that may prove significant. The initial shortwave energy over the West into the Plains along with easterly low level flow over parts of the southern Plains will help to generate areas of rain and thunderstorms that could be locally heavy over the south-central Rockies Plains. Cooler than average highs prevailing over a majority of the central/eastern U.S. late this week should moderate some by the weekend while above normal highs should become established over the Pacific Northwest into northern High Plains late this week into the start of next week. ...Guidance Evaluation/Predictability Assessment... The main weather story will likely turn to Hurricane Lee whose northward lifting in most guidance over the western Atlantic this week could lead to a track offshore New England to the Canadian Maritimes by the weekend. Relative to the full range of guidance, recent GFS runs have leaned somewhat on the western side of this forecast envelope, but the 12 UTC GFS has shifted some east. Recent ECMWF/ECens/UKMET/Canadian solutions seemed more preferable as they were better in line with the NHC track forecast and were decently clustered for much of the rest of the lower 48 into the weekend, but the caveat is that the 12 UTC ECMWF in particular has trended faster with Lee. Model and ensemble members still show increasing detail spread within the overall trough amplifying over the east-central U.S. later weekend/Monday. Possibilities range between an open trough and one or more embedded lows anywhere between the Midwest/Great Lakes and Canada. Poor clustering and run-to-run variability still favors a conservative approach more like the open trough of the GEFS/ECens ensemble means at these longer time frames. ...Weather/Hazards Highlights... During the Day 4 Excessive Rainfall Outlook period of Thursday-Thursday night there should be a broad area of rain and thunderstorm potential from the central half of the Rockies into the southern Plains, with one area of shortwave energy over the Great Basin and some weak impulses flowing into the Plains. Easterly low level flow directed over the southern High Plains into a potential surface trough may also support convection. A Marginal Risk area encompasses this overall region that could see locally heavy rainfall. There are two embedded Slight Risk areas that reflect the best combination of guidance signals/parameters. One is over the southwestern Oklahoma/northern Texas, reflecting some indication that enhanced convection could extend northwestward versus prior ERO issuance. Especially the western part of this Slight Risk area is expected to have meaningful rainfall before Day 4. Meanwhile a separate Slight Risk area centered mostly over Wyoming was introduced, with guidance starting to show a more pronounced signal for a slow moving Great Basin shortwave and anomalous moisture combining to support localized heavy rainfall potential. The Day 5 Excessive Rainfall Outlook that covers Friday-Friday night depicts a Marginal Risk area from the central High Plains into parts of Texas. Convection should continue into this time frame but most guidance shows some decrease in moisture anomalies and the best focus for rainfall becomes less defined. Some rain/thunderstorms should continue over parts of the southern Rockies/Plains and perhaps the South beyond early Saturday but likely with less coverage and lighter totals. Meanwhile a front settling over the Florida Peninsula may produce one or more days of rain over that region. Over New England, expect any lingering rain from the midweek system to depart on Thursday. Continue to monitor National Hurricane Center products regarding the track of Hurricane Lee and potential effects on New England and the Canadian Maritimes by late week and the weekend. Uncertainty with details for the upper trough evolving over the eastern half of the country during the weekend and early next week keeps confidence low for what the surface pattern may look like along with any areas of rainfall, though into Friday or so a front reaching the Upper Midwest could produce some areas of mostly light rain. Below normal high temperatures will prevail over most of the Rockies, central/south-central Plains, and much of the East late this week. Expect coolest readings up to 10-15F below normal over parts of the Rockies and south-central Plains where clouds/rainfall should be most persistent. The Great Lakes/Ohio Valley/Appalachians should see highs 5-10F or so below normal behind the cold front pushing off the East Coast on Thursday. Temperatures should moderate Saturday-Monday, reaching near normal by early next week. Continued hot weather with highs up to 5-10F above normal will be possible over southern Texas through late week before a cooler trend. The upper ridge building into the West will support highs 10F or more above normal over the Pacific Northwest from late week through the weekend, with a broader area of morning lows 5-15F above normal across the West. Some of this warmth may extend into the northern High Plains while the Pacific Northwest may start to see a cooler trend by next Monday as a Pacific front approaches. Schichtel/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