Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Wed Aug 14 2024 Valid 12Z Sat Aug 17 2024 - 12Z Wed Aug 21 2024 ...Hazardous heat likely across the southern Plains and central Gulf Coast states through this weekend and early next week... ...Overview... A slow-moving upper pattern will be in place this weekend into early next week. Ridging will stretch northward from an upper high in the southern High Plains and promote hazardous heat from the southern Plains to the central Gulf Coast. An upper low is forecast to dig into the eastern Pacific off the West Coast, leading to a cooler and wetter pattern for the Northwest. Meanwhile, an upper trough will slow and deepen over the Great Lakes this weekend. This feature and a surface low pressure/frontal system will promote scattered to widespread thunderstorms with the potential for heavy rain across the eastern U.S. through much of the period. Tropical Cyclone Ernesto is forecast to track north over the western Atlantic and remain offshore of the U.S. East Coast. ...Guidance/Predictability Assessment... Model guidance continues to show good agreement on the large scale pattern through the first part of the medium range period with an expected amount of model spread/uncertainty that increases over time. By Sunday, the 12Z CMC and 18Z GFS jump farther north into Canada with the Great Lakes surface low compared to the broader agreement of other models and ensemble means keeping it farther south. The newer 00Z GFS looks more favorable, while the 00Z CMC is still on the northern side but not as far off as the 12Z run. A larger scale difference that emerges into early next week is the potential for a central Canada upper low to the north or northeast of the ridge axis. The 12Z ECMWF has this feature east of other guidance, serving to deepen the eastern trough for longer than other guidance. Other models like the GFS runs and the 12Z CMC show this upper low to the west where it instead suppresses the northern side of the ridge. The new 00Z GFS may take this suppression of the ridge a bit too far as it is quite strong with the low. Meanwhile farther west, models show reasonably good agreement with the eastern Pacific upper low through about Monday, with more spread after that with if/when the troughing comes inland or continues to hover offshore. This depends on low confidence upstream energy coming from Alaska, so a middle ground model/ensemble blend seems best there. The trend over the past couple of days for the eastern trough to amplify and stall does allow for what is currently Tropical Storm Ernesto to edge a bit closer to the U.S. East Coast, but on the whole models still keep it east of the U.S. over the western Atlantic. However, Bermuda and the Canadian Maritimes should keep a close eye on the forecast as some guidance does have Ernesto passing over or nearby those areas as a notable low (whether it is tropical or not by then). The WPC forecast used a multi-model deterministic blend favoring the 12Z ECMWF and 12/18Z GFS early in the period. Since each deterministic run had some aforementioned issues, the proportion of deterministic runs was gradually reduced in favor of the GEFS and EC ensemble means to over half by days 6 and 7 to reduce the individual model differences in the overall agreeable pattern. ...Weather/Hazards Highlights... The upper trough in the Great Lakes building south and east with time, along with a surface low pressure/frontal system, will provide forcing for rain and thunderstorms across the eastern U.S. this weekend. Instability and above normal moisture will be in place and could lead to locally heavy rain. Marginal Risks of excessive rainfall are in place Day 4/Saturday from the central/northern Appalachians and Lower Great Lakes into the northern Mid-Atlantic and portions of the Northeast, and Day 5/Sunday a bit east. While widespread heavy rain is not particularly likely, embedded heavy totals could cause flooding concerns especially considering wet antecedent conditions in much of the region. Rain may persist across the Eastern Seaboard into early next week. Monsoonal convection will return this weekend with moist southerly flow west of the upper ridge axis. A Marginal Risk is in place for Day 4/Saturday for much of Arizona and Utah into surrounding areas. Moisture and instability looks to make it even farther north by Sunday, reaching the northern Rockies, so the Marginal Risk for Sunday stretches farther north and east. Convection may continue across these areas into early next week but gradually reduce in scope. Farther north, the upper low setting up in the eastern Pacific will promote rounds of modest rain across the Northwest. Rounds of storms are also possible across the central U.S. as impulses "ride the ridge" in northwest flow. There is particularly low confidence in where these storms form on any given day/night, as model guidance ranges across the Plains, Mississippi Valley, and Tennessee Valley to Southeast. Expect changes to the rainfall forecast in future cycles, and there is certainly not enough agreement for any excessive rainfall risk at this time. Meanwhile, a front slowly moving through Florida and above normal precipitable water values could lead to locally heavy rain, especially into Saturday. The greatest concern for possible flooding issues would be if heavy rain rates affect the Miami/vicinity urban area. Upper ridging over the south-central U.S. will maintain above normal temperatures by 5-15 degrees from the central and southern High Plains east into the Lower Mississippi Valley/central Gulf Coast. Highs well into the 100s in much of Texas could set daily records. From eastern Texas into the Lower Mississippi Valley, high dewpoints will combine with the hot temperatures to produce heat indices likely above 110 degrees. Low temperatures in the upper 70s to low 80s will exacerbate heat stress and could set records for warm minimum temperatures. Experimental HeatRisk continues to show Major to Extreme impacts from this lengthy heat wave. North of this region, most areas should see near normal highs and near to slightly above average lows. The Pacific Northwest into northern California can expect temperatures a bit below normal in response to the low aloft. Tate 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