Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sun Sep 10 2023 Valid 12Z Wed Sep 13 2023 - 12Z Sun Sep 17 2023 ...Overview... Today's guidance is fairly consistent with the large scale pattern evolution, featuring mean troughing over the eastern half of the lower 48 through the period and a fairly strong upper ridge building over the West by Friday-Sunday. Within the eastern trough, a Great Lakes upper low will lift northeastward while multiple shortwaves initially over the western U.S. and Canada should feed into the eastern trough while the western ridge strengthens. The Great Lakes upper low and associated surface system should produce some moderate to heavy rainfall over the Northeast around midweek. By next weekend, the track and timing of Hurricane Lee will require monitoring for any potential effects on New England and the Canadian Maritimes. Specifics will be dependent on finer details within the eastern North America mean trough, with typical predictability declining a week out in time. During mid-late week the shortwave energy emerging from the West will help to generate areas of rain and thunderstorms over the Rockies, and potential for heavier totals over the southern Plains where a wavy front may add focus. Cooler than average highs prevailing over a majority of the central/eastern U.S. mid-late week should moderate some by the weekend while above normal highs should become established over the Pacific Northwest into northern High Plains late week through the weekend. ...Guidance Evaluation/Predictability Assessment... Guidance continues to show some timing/detail differences for the upper low tracking northeast from the Great Lakes and associated surface system. GFS runs continue to be on the fast side, with the GEFS mean a tad slower, and most other solutions somewhat slower yet. However the slower majority has nudged slightly faster over the past day. Preference continues to be with the majority cluster but perhaps with confidence not quite as high given those trends. Behind this system, there is reasonable agreement that a combination of shortwaves over southern Canada and the western U.S. should ultimately feed into the long-term mean trough over the eastern half of the country. The scale of each feature limits predictability in specifics by 6-7 days out in time, favoring a conservative blend approach for now. Operational runs offer potential for an embedded upper low somewhere between the Upper Midwest/Great Lakes and Hudson Bay/James Bay by next Sunday. By next weekend, the specifics of the two eastern U.S. trough features and northward timing of Hurricane Lee as it tracks west of Bermuda will determine what effects Lee may have on New England and the Canadian Maritimes. Recent GFS runs have consistently been on the fast extreme of the spread while the much slower 12Z ECMWF is on the western fringe of the envelope. A blend of the 12Z CMC/ECens/CMCens and 18Z GEFS provided the best editable starting point to depict a reasonable extrapolation of the official National Hurricane Center track of Lee. Some guidance indicates that Lee may transition to an extratropical system by the time its track reaches east of New England. Minus the new 00Z CMC that strays from other guidance in keeping more shortwave energy lingering over the West, there is good consensus among models/means for the strong ridge building over the West by Friday-Sunday. Forecast considerations for the initial Northeast system favored an early blend of the 12Z ECMWF/UKMET/CMC and 18Z GEFS mean. Then as detail uncertainties increased the forecast increased weight of the 12Z ECens/CMCens along with the 18Z GEFS while steadily reducing ECMWF input, so that by the end of the period the blend primarily consisted of the means and a small 12Z CMC component. ...Weather/Hazards Highlights... The Days 4-5 Excessive Rainfall Outlooks (EROs) that cover Wednesday through Thursday night reflect heavy rainfall/flash flood potential over two general areas, one over the Northeast (Day 4) and another over portions of the Rockies and southern Plains (both days). The system affecting the Northeast on Wednesday continues to show some timing uncertainty but there is a persistent signal for the vigorous dynamics and moisture input to produce areas of heavy rainfall across a region that will have wet soil conditions due to recent/forecast rainfall leading up to the outlook time frame. This scenario favors maintenance of the Slight Risk area with a minor eastward adjustment to account for guidance trends. Meanwhile the Marginal Risk area was extended a little southward along the Mid-Atlantic coast in light of signals for some activity along the trailing cold front during the day Wednesday. Shortwave energy emerging from the West will promote at least a couple days of convection over the Rockies, with the best signals currently appearing over Colorado and New Mexico on Day 4 and Wyoming/Colorado by Day 5. Meanwhile leading energy interacting with a wavy front should focus an area of potentially heavy convection over the southern Plains Wednesday-Thursday. Expected rainfall totals and convective organization support maintaining a Slight Risk area over the south-central Plains on Day 4 and continuing southeastward into Day 5. The surrounding Marginal Risk area also encompasses the Rockies activity which has a somewhat more diffuse signal in the guidance. After the Wednesday system departs from New England, rainfall and other sensible weather details for New England by next weekend will depend on the track and timing of Hurricane Lee. Continue to monitor National Hurricane Center products for the latest information on Lee over the coming days. Rain and thunderstorms over the Rockies should gradually trend lighter with more southward suppression late week into the weekend as upper ridging builds over the West, while some showers/storms may extend across the South. Expect below normal high temperatures over a majority of the central and eastern U.S., with some moderation likely by the weekend. Coolest readings relative to average should be during Wednesday-Thursday when parts of the central/south-central Plains may be up to 15-20F below normal due to clouds/rainfall and the Great Lakes/Ohio Valley may be 10-15F below normal under the upper low/trough that eventually lifts away. Continued hot weather with highs up to 5-10F above normal will be possible over southern Texas mid-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. 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