Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sat Sep 28 2024 Valid 12Z Tue Oct 01 2024 - 12Z Sat Oct 05 2024 ...Overview... Most guidance shows progressive mean flow across the northern lower 48 during the period, pushing along a series of frontal systems. One should push through the East around midweek while another moves into the Northwest, with the latter front reaching the central and eventually eastern U.S. late in the week. Another front may reach the Northwest toward the end of the week. These fronts may produce mostly light to moderate rainfall over some areas. Separate upper ridges will prevail over the southern half of the West and over/surrounding Florida, with the former in particular producing well above normal temperatures. There is significant uncertainty over how a potential weakness between the two ridges may eventually draw increasing moisture northward into the Gulf Coast region, with some signal for embedded tropical development as well. ...Guidance/Predictability Assessment... For about the first half of the period, the most impactful discrepancy involves the precise track of western Atlantic low pressure and what interaction there could be between this wave and the cold front supported by an approaching northern tier U.S./southern Canada trough. The full array of 12Z/18Z guidance recommended a compromise approach between the 12Z ECMWF/CMC that were farthest west with the surface low (leading to more interaction and higher Northeast rainfall totals) and other solutions that were more offshore (leading to less interaction and lower rainfall). The average of 12z machine learning (ML) models leaned toward the more offshore scenario, and indeed the new 00Z dynamical guidance overall has made a notable shift offshore. The prominent uncertainties later in the week involve the details of northern stream flow as well as the evolution over the Gulf of Mexico and southern tier U.S. Ensemble spaghetti plots become quite messy from the Pacific eastward, though in a broad sense there has been somewhat better clustering among the GFS/ECMWF, their ensemble means, and most ML models/AIFS ensemble mean the past couple days. This favored cluster would have an upper trough evolving over the eastern Pacific by Friday and reaching the West Coast or inland by Saturday, while a downstream ridge progresses into western North America with cyclonic flow to the east. Within this cluster, ML models include the range between the faster ECMWF runs and slower GFS for the incoming Pacific trough by Saturday--arguing for a blend approach. Meanwhile, CMC/CMC mean runs have been amplifying the Pacific trough farther westward. The new 00Z CMC still exhibits this characteristic but it does seem to be trending partially toward the majority. The forecast over the Gulf of Mexico and southern tier U.S. is complicated due to the low predictability of details regarding both the degree of tropical development (and path) reaching into the Gulf of Mexico and upper level flow specifics over the southern tier. Recent GFS/CMC/CMCens runs have been the most pronounced with their combination of tropical development and faster northward timing of a system. ML models and a majority of GEFS/ECens members favor holding what weak low pressure there may be over the southwestern Gulf into Saturday. At the same time there is a general signal for a potential weakness aloft over the southern Plains/Lower Mississippi Valley region, which could support an increasing flow of moisture into portions of the Gulf Coast region even in the absence of a defined surface low. This goes along with the 12Z ML models that show relatively greater QPF focus along the central/east-central Gulf Coast region toward the end of the week even without much of a surface reflection--interestingly, a similar region highlighted by the 00Z GFS/CMC/ICON in more pronounced fashion due to their developed surface systems. Current preference maintains continuity from yesterday's WPC/NHC coordination reflecting a nearly stationary weak low in the southwestern Gulf by late week. The updated forecast incorporated a blend of 12Z/18Z operational runs for the early part of the period and then trended toward an even model/ensemble mean mix, including splitting GFS/ECMWF input between their two most recent runs to downplay low-confidence specifics and phasing out the CMC due to disagreements with the majority cluster. ...Weather/Hazards Highlights... Forecasts have been showing rainfall progressing from the Great Lakes/Appalachians/Mid-Atlantic into New England from Tuesday through midweek, with various ideas for coverage and amounts depending on the track of western Atlantic low pressure and how much interaction occurs with an approaching front. 00Z model runs are trending toward more separation of these two features, likely leading to lower rainfall totals. Thus the Excessive Rainfall Outlooks for Days 4-5/Tuesday-Wednesday depict no risk areas. Expect rainfall to increase across the Gulf Coast region during the latter half of next week, but with low confidence regarding the details at this time due in part to guidance spread regarding possible tropical development over the Gulf of Mexico. Monitor forecasts over the coming days for better resolution of the rainfall/tropical forecast across this region. A frontal system may bring light to locally moderate rainfall to the Pacific Northwest by Friday as well. Temperatures will remain quite warm to hot across the southern half of the West next week with fairly broad coverage of highs 10-15F above average. Record highs should be most numerous through Wednesday but there may still be a few lingering into late week. Portions of the Plains will also be above normal one or more days from Wednesday onward in the warm sector ahead of a front. The Northwest will tend to be closer to normal. Temperatures over the East will vary with frontal progression, with highs averaging out to near normal and morning lows averaging above normal. 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 (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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