Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Mon Aug 12 2024 Valid 12Z Thu Aug 15 2024 - 12Z Mon Aug 19 2024 ...Excessive heat likely across the southern Plains and central Gulf Coast states through the weekend into early next week... ...Heavy rain potential across the Midwest to portions of the Ohio Valley and Appalachians Thursday to Friday... ...Overview... Summertime upper ridging across the southern tier will lead to 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 late in the period, leading to a cooler and wetter pattern for the Northwest while the southern stream ridge builds and retrogrades a bit in response. A more amplified trough and associated low pressure system will promote scattered to widespread thunderstorms with potential for heavy rain and flooding across the Midwest to portions of the Ohio Valley and Appalachians Thursday to Friday. Meanwhile, Potential Tropical Cyclone Five is currently forecast to intensify and track well off the East Coast of the U.S. ...Guidance/Predictability Assessment... Guidance remains generally agreeable with one another regarding the large scale pattern evolution across the mainland U.S. through early next week. The shortwave trough that is forecast to move across the northern and central Plains later this week has been a tricky feature to handle. In the past couple of days, models have generally trended toward a more amplified solution, leading to a sharper upper trough and a stronger surface low pressure system to move across the Great Lakes by around Friday. The impact of the more amplified pattern is a better-defined forcing to support heavy rain potential over the Midwest on Thursday ahead of the system. However, the ECMWF QPFs remain noticeably farther north than the GFS/GEFS QPFs. This discrepancy continues downstream into the eastern U.S. by late in the forecast period. Meanwhile, the corresponding upper ridge over the southern Rockies to the southern Plains shows a tendency to be stronger as well. The amplified pattern slows the eastward progression of the trough over the eastern U.S. by early next week. The slow-moving and more amplified trough did help to steer Potential T.C. Five closer to the East Coast with the 12Z GFS run but not with the 12Z ECMWF. There is also a tendency for the closed low off the Pacific Northwest to be broader/more amplified and a bit further off the coast. The WPC forecast was derived from a blend of the deterministic and ensemble means from the 00Z ECMWF, 06Z GFS, and 00Z CMC, transitioning toward a larger proportion from the ensemble means by Day 7. ...Weather/Hazards Highlights... Upper troughing and surface cyclogenesis will provide forcing for rain and thunderstorms across the Midwest Thursday in an unstable and moist environment. There may be multiple mesoscale convective systems producing heavy rain rates across the region possibly causing flooding concerns. A Marginal Risk remains in place for the Day 4 ERO for portions of the Upper/Middle Mississippi Valley and the western Ohio Valley. A Slight Risk has been introduced inside the Marginal given better-defined forcing with a more amplified pattern. Thunderstorm chances move east on Friday, reaching the Upper Great Lakes region to Ohio and Tennessee Valleys and into the central/southern Appalachians. A broad Marginal Risk is in place for those areas for the Day 5 ERO to cover the flooding threat with embedded heavy rains. Convection should push slowly east toward the Eastern Seaboard over the weekend. The monsoon may take a relative break into Thursday, but expect monsoonal convection to get renewed Friday and especially into the weekend particularly across Arizona and Utah, focusing westward a bit compared to earlier in the week as the upper ridge retrogrades. Some storms may make it north and east by Sunday 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, above normal moisture pooling near a front may enhance storm chances over Florida later this week, but there is likely not enough flooding risk to delineate any area in the ERO, since Florida is not sensitive to flooding unless there are exceptionally heavy rain rates/totals. Light to modest showers are forecast in the Northeast into Thursday ahead of more widespread rain approaching into the weekend. Upper ridging over the south-central U.S. will maintain temperatures up to 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 from the western to north-central to eastern U.S., 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. Kong/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