Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Sat Sep 09 2023 Valid 12Z Tue Sep 12 2023 - 12Z Sat Sep 16 2023 ...Overview... Latest models and ensembles show varying degrees of upper troughing over eastern North America during the period. Within this mean trough, the most prominent feature will be an upper low forecast to close off over the Great Lakes by midweek and then lift northeastward as diffuse shortwave energy entering the western U.S./Canada eventually feeds into the trough. The Great Lakes low and surface system should produce some moderate to heavy rainfall from the Great Lakes/Mid-Atlantic into the Northeast. By the end of next week into the weekend, the track and timing of Hurricane Lee will require monitoring for any potential effects on New England and the Canadian Maritimes. Over the West, shortwave energy will push through the region early in the period before upper ridging ultimately settles in by late next week into the weekend. The combination of western shortwave energy and a persistent Southern Plains front will favor multiple episodes of rain and thunderstorms over the southern Plains through the period, and extending as far west as Arizona Tuesday-Wednesday. Cooler than average highs will prevail over a majority of the central/eastern U.S. while moderately above normal highs should become established over the far northern Plains and Pacific Northwest by late next week/weekend. ...Guidance Evaluation/Predictability Assessment... The forecast period begins with two main upper-level features of interest indicated amongst the guidance: an upper trough/developing cutoff low over the Upper Midwest/Great Lakes shifting eastward and more subtle shortwave energy approaching the Pacific Northwest from upstream over the Pacific. There is decent clustering around the evolution of the developing cutoff low moving into the Northeast mid-period by all guidance except the GFS, which remains an outlier. Both the most recent 00Z/06Z runs depict a weaker and more progressive open wave, and continuously variable solutions in previous runs decreases confidence in the GFS. Naturally there is more disagreement both amongst the latest runs and previous runs on the timing/phasing of the shortwave approaching the West considering the scale, but the fact it is at least captured in some respect by all of the guidance early in the period, including the ensemble means, helps bolster some confidence that the guidance has a reasonable handle on its evolution. This wave along with additional upper-level energy eventually helps to develop weak mean troughing over the central U.S. following the departure of the upper low over the Midwest in most of the guidance with the exception of the 12Z CMC, which has a slower wave that dives further southward into the Southwest. This should lead to several days of energetic northwest flow over the Plains. Mean ridging looks to evolve over the West in tandem with the troughing towards the central/eastern U.S. The last, and potentially biggest challenge of the forecast is the track of Hurricane Lee and how it may or may not come close enough to the East Coast to interact with the weather pattern over the CONUS. The GFS has remained faster and further northwest with the track, with the system eventually being absorbed into the upper trough/low lifting through the Northeast. The ECMWF and CMC are slower and further south, allowing this trough to depart northeastward into Canada as the storm remains further south over the western Atlantic, and any subsequent interaction with troughing over the Northeast occurring beyond the current forecast period. For now, have remained with the slower solutions, with any additional updates and more specific track information available from the National Hurricane Center. The updated WPC forecast begins with a composite blend of the deterministic guidance given good agreement in the early period. The means are gradually increased in the mid- to late period as forecast uncertainty increases and the contribution from the GFS is removed given the noted outlier nature of the solution with regards to the upper trough/cutoff low and handling of Hurricane Lee. The latest update from the 12Z GFS continued these noted trends. ...Weather/Hazards Highlights... The Days 4-5 Excessive Rainfall Outlooks (EROs) that cover Tuesday through Wednesday night reflect heavy rainfall/flash flood potential over two general areas, one over central/northern portions of the East and the other over/west of the southern Plains. During the Day 4 time frame, the system beginning to evolve over the Great Lakes may generate some locally enhanced rainfall from the Great Lakes/Appalachians into the Mid-Atlantic and interior Northeast. Guidance is not fully agreeable on the exact forecast scenario QPF-wise but this area will have wet antecedent conditions from recent and near-future rainfall. By Day 5 expect a more pronounced potential for heavier rainfall over portions of the Northeast, where the slower non-GFS majority cluster for the system heading into the region merits a Slight Risk. From the southern Plains westward, shortwave energy will interact with anomalous moisture to yield areas of locally heavy rain and thunderstorms. A front persisting over the southern Plains will provide another rainfall focus. The corresponding Marginal Risk area on Day 4 extends from parts of Texas into the Four Corners region, while the Day 5 Marginal Risk reflects an eastward shift in activity into Colorado/New Mexico but a drier trend over Arizona. A Slight Risk has been introduced for portions of the southern Plains on day 5 where confidence has increased in the threat for heavy rainfall, particularly overnight Wednesday into early Thursday morning. While there remains some uncertainty in the specifics such as the timing of upper-level energy, strength of the low-level jet, the location of a lingering surface boundary, and overall available moisture starting to trend downward, the guidance is relatively unanimous in depicting localized totals of several inches over the region, with areal average totals and high ensemble probabilities for over 1". The overall pattern has been very hot and dry over the region this summer, but multiple days with rainfall chances through mid-week should also help to lead to wetter antecedent conditions. Some moderate to heavy rainfall may continue into eastern New England through late next week. 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 thunderstorm chances look to shift south and eastward towards the Lower Mississippi Valley/Southeast and Gulf Coast late week, while chances over the Rockies should trend lighter as upper-level ridging sets in. Below normal high temperatures over the central U.S. and Midwest/Great Lakes as of Tuesday should gradually spread farther eastward, though with a moderating trend by Friday-Saturday. Best potential for highs 10-15F below normal will be during Tuesday-Thursday over the central/south-central Plains as well as the Great Lakes/Ohio Valley as post-frontal high pressure settles in and clouds/precipitation also keep temperatures down. Slightly above normal temperatures may hold on over far southern Texas a little beyond midweek, while the Pacific Northwest eastward to the far northern Plains may trend somewhat above normal by Thursday-Saturday. Putnam/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