Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Tue Aug 13 2024 Valid 12Z Fri Aug 16 2024 - 12Z Tue Aug 20 2024 ...Excessive heat likely across the southern Plains and central Gulf Coast states through the weekend into early next week... ...Overview... An upper high/ridge meandering near the southern High Plains late this week into early next week will maintain the potential for high heat indices that could be hazardous across the southern Plains to Gulf Coast/Lower Mississippi Valley and toward the Southeast. An upper low is forecast to dig into the eastern Pacific off the West Coast this weekend, leading to a cooler and wetter pattern for the Northwest while the ridge builds and retrogrades a bit in response. Meanwhile, an upper trough pushing across the Midwest Friday will slow and deepen over the weekend. This feature and a surface low pressure/frontal system will promote scattered to widespread thunderstorms with potential for heavy rain across the east-central to eastern U.S. through the period. Meanwhile, Tropical Storm Ernesto is currently forecast to intensify and track offshore of the U.S. East Coast. ...Guidance/Predictability Assessment... Model guidance is rather agreeable on the large scale throughout the medium range period, as the eastern Pacific to West Coast trough, west-central to central U.S. ridge, and east-central to eastern U.S. trough pattern sets up and amplifies. Exact upper low/trough and surface low positions vary somewhat, as is typical. The Great Lakes surface low shows some spread in position by early Sunday, with the 12Z CMC taking it quickly north and now the 00Z GFS acting similarly, dependent on shortwave and trough structure details. With the eastern Pacific upper low, the 12Z GFS was farther west than other guidance, but the 18Z and 00Z GFS runs show better alignment. The 12Z ECMWF was on the faster side by late in the period in bringing the low/trough east, but EC-based AI/machine learning models were even faster, so the operational EC may be a good middle ground. Additionally, smaller details like shortwave energies rounding the mean flow vary as well, and these differences affect QPF placement. So while confidence in the overall pattern is pretty high, sensible weather details like QPF have lower confidence. UKMET runs continue to be outliers in producing very heavy QPF across the central U.S. into Friday, while ECMWF runs have been consistently faster with eastern U.S. QPF compared to other guidance, but fortunately the 00Z ECMWF is favorably slower. 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, the Canadian Maritimes should keep a close eye on the forecast as 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 18Z GFS and 12Z ECMWF early in the period. Gradually reduced the proportion of deterministic runs in favor of the GEFS and EC ensemble means as the period progressed, reaching half models/half means by Days 6-7 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 system, will provide forcing for rain and thunderstorms across the east-central U.S. Friday and gradually moving east into the weekend and early next week. Above normal moisture and instability in place could lead to heavy rain with potential flooding concerns. A Marginal Risk is in place Day 4/Friday from the Great Lakes region south into the Ohio and Tennessee Valleys, and across the northern Mid-Atlantic to portions of the Northeast for Day 5/Saturday. There is generally less signal for heavy rain causing widespread flooding compared to the short range period, but especially as rain gets into the East where there are wet antecedent conditions, ERO upgrades in future cycles are not out of the question. Rain looks to persist across the Eastern Seaboard into early next week. 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 from the Central Plains, Middle or Lower Mississippi Valley, Tennessee Valley, and/or Southeast. So expect changes to the rainfall forecast in future cycles, and there is certainly not enough agreement on any excessive rainfall risk. Monsoonal convection could creep back into Arizona Friday but begin in earnest by Saturday particularly across Arizona and Utah, focusing westward a bit compared to earlier in the week as the upper ridge retrogrades. A Marginal Risk has been delineated in the ERO for Day 5/Saturday for localized flooding potential. Some storms may make it north and east by Sunday-Monday as moisture advects around the ridge. Farther north, the upper low setting up in the eastern Pacific will promote rounds of modest rain across the Northwest. Additionally, a front slowly moving through Florida and above normal precipitable water values could lead to heavy rain especially Friday. The greatest concern for possible flooding issues would be if heavy rain rates affect the Miami/vicinity urban area, but will hold off on any Marginal Risk area since there is not agreement for exceptionally heavy rain. Upper ridging over the south-central U.S. will maintain temperatures around 10F above (already hot) normal values across the southern Plains into the Lower Mississippi Valley. Combined with the high dewpoints in most areas, heat indices above 110F are expected, and experimental HeatRisk shows Major to Extreme impacts. Low temperatures in the upper 70s to low 80s will exacerbate heat stress and could set records for warm minimum temperatures. North of this region though, most areas should see near to slightly below normal highs, with the most notable below average temperatures in the Northwest late week-weekend in response to the low aloft. Low temperatures should be near to a bit above average in most areas. 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