Extended Forecast Discussion NWS Weather Prediction Center College Park MD 251 PM 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... The period begins as the upper low that has been lingering over the Ohio/Tennessee Valley this weekend and was associated with the remnants of Helene shears out and moves eastward as an upper wave just offshore the East Coast mid-next week, with a lingering weakness trailing southwest towards the Gulf Coast. This continues south of amplifying upper ridging over New England as a northern stream upper trough begins to drop south over the Upper Midwest/Great Lakes region. A coastal low that develops with the upper wave moving off the coast has been trending more offshore in the latest runs of the guidance, especially in the last few runs of the GFS. The GFS is also a bit more aggressive with the strength of the low, including the 12Z run, and takes a more northerly path compared to the 00Z ECMWF. Ensembles indicate a general clustering just offshore as well. The more eastern solutions offshore lead to less onshore flow ahead of the upper-trough and expected approaching frontal system to the west and results in a significant decrease of QPF over the northeastern U.S., which is shared by the NBM. Differences in the low position/strength also lead to spatial disparity in QPF totals, with higher amounts into New England in the GFS and the Mid-Atlantic in the ECMWF. Opted to keep a bit of continuity in the forecast as well as blend in some ECens mean given QPF footprint differences to avoid a more drastic change from the prior forecast, though still indicate a downward trend. Uncertainty increases by late next week with the timing of additional northern stream shortwaves in progressive flow along the U.S./Canadian border, and with the handling an upper-trough over the northeastern Pacific dropping into western Canada and the northwestern U.S. The GFS has trended more amplified, digging deeper into the western U.S. and slower in timing through the 00Z run, before shifting faster and back a bit eastward in the 06Z and 12Z runs. Timing and depiction of a lower amplitude wave remaining more in line with the U.S./Canadian border is more consistent in the last few runs of the ECMWF, but still shows varying differences in the solutions. The 00Z ECens and GEFS means offer a compromise solution indicating a less-amplified upper-trough over the northwestern U.S. with the ECMWF the most similar. The more amplified GFS solution would not surprisingly lead to more expansive precipitation chances into the northwestern U.S. Both the 00Z CMC as well as the CMC ensemble mean were out of phase with the other guidance during this timeframe. The potential for tropical development also leads to another region of uncertainty in QPF along the Gulf Coast late next week and into next weekend. The latest runs of the guidance show significant differences in the timing and path of a potential tropical system. The CMC and GFS are more aggressive in the development of a system in the northwest Caribbean that eventually moves into the Gulf of Mexico late next week/early next weekend, with the CMC faster in bringing the system up towards the Gulf Coast. The National Hurricane Center now anticipates an area of low pressure lifting north out of the northwestern Caribbean and into the central Gulf by next Saturday. Regardless of the exact nature of any development, increasing moisture should lead to rainfall across the region, but guidance shows significant differences, with the GFS much heavier compared to the ECMWF. Opted for a blend of the NBM and ensemble means which keeps amounts on the less extensive side for now given the degree of uncertainty and how higher QPF amounts would likely be tied to tropical development, but this will need to be closely monitored for both heavy rainfall potential and any other additional impacts. The updated WPC forecast consists of a composite blend early in the period given good clustering of the deterministic guidance. An increasing contribution from the ECens and GEFS means is included for the latter half of the period given the greater differences with the progressive flow in the northern stream and the eventual upper-trough approaching southwest Canada and the northwestern U.S. The deterministic 00Z ECMWF is emphasized during this period given how it best falls within the envelope of the two mean solutions. The GFS contribution varies in the mid- to late period, with a preference for the 00Z run initially given similar upper wave timing along the U.S./Canadian border, and then a preference for the 06Z run at the end of the period due to a closer solution to the ECMWF/ensemble means with respect to the northeastern Pacific upper-trough coming into the northwestern U.S. ...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 guidance has trended toward more separation of these two features, 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 timing, coverage, and amounts due to its connection to 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, with some low precipitation chances spreading into the northern Rockies by Saturday. 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. 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 (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