Extended Forecast Discussion NWS Weather Prediction Center College Park MD 246 AM 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... ...Overview... Expect the large scale pattern to become more blocky with time, as a western U.S. ridge drifts toward the Rockies and an Upper Great Lakes/Ontario upper low closes off within a deepening eastern trough. The western ridge will produce heat threats over portions of the Southwest and California late this week, with below normal temperatures prevailing across the Midwest to the East. The majority of rainfall during the period will be across the South and East. Weak energy near the Gulf Coast will interact with a persistent wavy surface boundary to focus heavy rainfall over parts of the Southeast, followed by interaction with a northern stream cold front into the Northeast. An increasingly large part of the country should see dry/quiet weather by Sunday and beyond. ...Guidance/Predictability Assessment... Models continue to show good agreement on the large scale pattern evolution during the period, but some differences in the timing of smaller waves, especially around the deep low north of the Great Lakes. This affects timing of the cold front through the East early period, and interactions with weak energy and surface low as it lifts up/off the East Coast. By early next week, uncertainty increases as to whether the low lifts more northward into eastern Canada or pushes through the Northeast. The GFS has been consistent showing it lifting north, but the new 00z runs of the CMC and ECMWF (which were available after forecast generation time) pull it eastward towards the Northeast. Out West, some larger scale differences in timing and amplitude emerge as the next trough moves inland Monday- Tuesday, but new 00z model runs seem to be trending slower with this feature. The WPC forecast for tonight used a blend of the deterministic models amidst good agreement for the first half of the period, incorporating up to 50 percent of the ensemble means in the blend by Day 7 to smooth out differences with the Eastern low and western trough. ...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 (covering Friday-Friday night). 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. After Saturday though, 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. 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. 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