Extended Forecast Discussion NWS Weather Prediction Center College Park MD 308 PM EDT Tue Sep 3 2024 Valid 12Z Fri Sep 06 2024 - 12Z Tue Sep 10 2024 ...Hazardous heat likely late week across parts of the Southwest and California with heavy rainfall threats across the Lower Mississippi Valley to the Southeast during the weekend... ...Overview... An amplified upper-level pattern with an upper low slowly moving through the Northeast will support a period of enhanced rainfall over portions of New England this weekend, with a surge of fall- like dry and cool air mass behind it across much of the eastern half of the country. A lingering front will keep decent rain chances from central Gulf Coast eastward to Florida, with heavy rain most likely Friday and Saturday along central to eastern Gulf Coast. Meanwhile, a ridge shifting slowly east across the western U.S. will increase the threat of excessive heat over portions of the Southwest and California late this week. Tropical moisture moving into the southwestern Gulf could bring heavy rain into southern Texas by early next week. ...Guidance/Predictability Assessment... Models continue to indicate the likelihood for a deep upper low to form over the Great Lakes region by this weekend as a ridge axis shifts slowly eastward across the western U.S., allowing the next trough from the Gulf of Alaska to approach the Pacific Northwest early next week. There has been a tendency for the models to slow down the eastward progress of the upper low out of the Great Lakes this weekend in response to a trend for a deeper and more closed- off upper low circulation. Along the Gulf Coast, there has been a trend to slow down the motion of a frontal wave late this week into the weekend, allowing heavier rainfall to fall along the coast but this did not affect the timing and location of the heavy rain just of the Southeast coast all that much. Elsewhere, models have not shown too drastic of a change lately. The WPC medium-range products were derived based on 40% from the 00Z EC/EC mean, 40% from the 06Z GFS/GEFS, and 20% from the CMC/CMC mean, and leaning toward the consensus of the ensemble means for Days 6 and 7. ...Weather/Hazards Highlights... The main heavy rainfall focus during the period will be across the Southeast on Friday as shortwave energy interacts with a lingering surface front along/near the Gulf coast. Precipitation coverage and totals remain somewhat uncertain, but the overall dynamics and anomalous moisture support a heavy to excessive rainfall threat as highlighted by a broad marginal risk on the Day 4 Excessive Rainfall Outlook. A Slight Risk area has been introduced along the central to eastern Gulf Coast due to a general increase of model QPFs with a slowing frontal wave. Models continue to suggest a southward trend with the overall threat and so compared to yesterdays Day 5 ERO, trimmed the northern extent of the marginal a bit for the new update. There may be a need for an eventual upgrade to a slight risk, particularly for the central to eastern FL panhandle where the stronger instability should be, but decided to hold off on that with this cycle given continued uncertainty in placement. The front should linger across northern FL for a few days, with some pockets of heavier rainfall possible so did maintain a narrow marginal across this area for the Day 5/Saturday ERO as well. Farther north, some heavier rainfall may accompany the cold front as it passes through the Northeast, aided by wrap back moisture from a surface low along the coast. Covered this with a marginal risk on the Day 5 ERO for now, with a possibility for an embedded Slight Risk. From Sunday into early next week, much of the country should dry out, though shortwave energy within and around western U.S. upper ridging may produce isolated to scattered rainfall with mostly light amounts. Meanwhile, tropical moisture moving into the southwestern Gulf could bring an increasing threat of heavy rain into southern Texas by early next week. Above normal temperatures with hazardous heat over the West should be most pronounced on Friday, with HeatRisk showing widespread major threats across the Southwest and central valleys of California and moderate to locally major into the Pacific Northwest. California highs may reach up to 10-15F above normal while the Pacific Northwest could see readings up to 15-20F or so above normal. Some daily records are possible. Heat should persist to some degree into the weekend over the Northwest and slowly expand into the northern High Plains. In contrast, a much cooler airmass will filter eastward from the northern Plains and Great Lakes into the East this weekend and early next week, with daytime highs 10-20 degrees below normal. Kong/Santorelli 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