Extended Forecast Discussion NWS Weather Prediction Center College Park MD 238 PM 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 generally hold over the Central U.S., anchored by an upper high centered over the southern High Plains to promote hazardous heat from the southern Plains to the central Gulf Coast. An upper low is forecast to meander 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 Northeast 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 period with some fairly typical model spread/uncertainty in the details. Most of the pattern uncertainty surrounds an upper shortwave (or low) into Central/Southeast Canada that may erode the top portion of the Central U.S. ridge. The GFS is the most bullish on this, suggesting a deep closed low over Central Canada next week and shortwave troughing through the northern Plains/Upper Midwest. The ECMWF and CMC (with support from the means) hold onto more ridging across that area. Across the East, most of the guidance has trended towards more amplified troughing over the East, but a lot of uncertainty about if/how Ernesto may get intertwined as it moves northward through the western Atlantic. Consensus is for Ernesto to stay well enough East, but there is some guidance which brings the system close to or through the Canadian Maritimes. WPC progs used a general blend of the operational guidance for the first half of the period. Trended towards just over half of the ensemble means by the end of the period to account for increasing spread in the guidance. The CMC was not included late period due to it being slightly faster/east with the Eastern U.S. trough. ...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 and Day 5/Sunday from the central/northern Appalachians and Lower Great Lakes into the northern Mid-Atlantic and portions of the Northeast. 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. 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. Santorelli/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