Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 PM EDT Tue Aug 13 2024 Valid 12Z Fri Aug 16 2024 - 12Z Tue Aug 20 2024 ...Hazardous heat likely across the southern Plains and central Gulf Coast states through this weekend and early next week... ...Overview... An upper high/ridge will remain nearly stationary over the southern High Plains late this week into early next week and will maintain high, potentially hazardous, heat indices across the southern Plains to the Gulf Coast/Lower Mississippi Valley and 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. Meanwhile, an upper trough pushing across the Midwest Friday 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 east-central to eastern U.S. through the period. Tropical Cyclone Ernesto is forecast to track north over the western Atlantic and remain offshore of the East Coast. ...Guidance/Predictability Assessment... Model guidance continues to show good agreement on the large scale pattern through the medium range period with an expected amount of model spread/uncertainty. The 06Z GFS continues to be a little faster than other guidance with the progression of upper level lows, while the 00Z ECMWF falls more in the middle of the guidance spread. The 00Z CMC is in good agreement with the GFS and ECMWF through the first half of the period, but upstream differences in the Gulf of Alaska resulted in it falling away from the general model consensus in the second half of the period. For these reasons, more weight was put on the ECMWF and less was put on the CMC in the WPC forecast blend. 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 06Z GFS and 00Z ECMWF early in the period. The proportion of deterministic runs was gradually reduced in favor of the GEFS and EC ensemble means 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 system, will provide forcing for rain and thunderstorms across the east-central U.S. Friday and into the East this weekend and early next week. Above normal moisture and instability will be in place and could lead to heavy rain with potential flooding concerns. A Marginal Risk of Excessive Rainfall is in place on 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 wet antecedent conditions in portions of the East could contribute to a higher flood risk. Rain looks to persist across the Eastern Seaboard into early next week. 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. 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. Expect changes to the rainfall forecast in future cycles, and there is certainly not enough agreement for any excessive rainfall risk. 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 locally heavy rain, especially Friday. 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 temperatures around 10 degrees 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 110 degrees 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, most areas should see near to slightly below normal highs, with the most notable below average temperatures in the Northwest late week into the weekend in response to the low aloft. Low temperatures should be near to a bit above average in most areas. Dolan/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