Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM 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/Southeast through the week into next weekend... ...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 into the Southeast. An upper low is forecast to dig into the eastern Pacific and West Coast late week into the weekend, leading to a cooler and wetter pattern for the Northwest while the southern stream ridge builds and retrogrades a bit in response. A shortwave trough and a surface low pressure system are forecast to track across the north-central to eastern U.S. and promote scattered to widespread thunderstorms with potential for heavy rain and flooding, gradually shifting east with time. Meanwhile, the tropical low pressure system that NHC has now designated Potential Tropical Cyclone Five will continue to be monitored, but fortunately most models and latest NHC guidance continue to take the low system well east of the lower 48 into the western Atlantic. ...Guidance/Predictability Assessment... Guidance remains generally agreeable regarding the large scale pattern, including troughing diving south in the eastern Pacific late week and forming a closed low that meanders just offshore of the Pacific Northwest through the weekend/early next week. There is good agreement for an upper high to become centered over the southern High Plains with ridging building north, though the eastern extent will depend on the relatively more uncertain troughing to its east. This (initially shortwave) trough supporting a surface low will track through the north-central U.S. Thursday- Friday and is likely to slow atop the Great Lakes/east-central U.S. this weekend into early next week as the pattern amplifies overall. The depth of the trough is in question even into late week, which impacts the surface low placement and QPF placement. The 12Z ECMWF was on the shallower side with the trough, resulting in faster movement of the QPF east than other guidance, limiting QPF and ERO confidence. The newer 00Z ECMWF is a bit slower/farther west than its previous run, but there is still some spread in rainfall placement. Model guidance generally shows the trough deepening somewhat and slowing, maintaining an axis over the east- central U.S. into early next week. This is behind an initial trough over the East Coast/western Atlantic late this week, which looks to steer Potential T.C. Five east of the lower 48. The WPC forecast used a blend of mainly deterministic models early in the period, using the 12Z ECMWF and CMC and 18Z GFS along with a bit of the EC ensemble mean. As the period progressed, lessened the proportion of deterministic guidance in favor of the ensemble means to reduce the individual model differences. ...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. Model differences in placement of heavy rain precluded any upgrade in the ERO, but given the potential for high rain rates in an unstable environment, a Slight Risk in future cycles is certainly a possibility. 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. 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