Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sun Aug 11 2024 Valid 12Z Wed Aug 14 2024 - 12Z Sun Aug 18 2024 ...Excessive heat likely across the southern Plains and central Gulf Coast/Southeast through the week into next weekend... ...Overview... Upper ridging will be in place across the south-central U.S. through much of the week, leading to high heat indices that could be hazardous across the southern Plains to Gulf Coast/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 may retrograde a bit in response. Shortwave energy and a surface low pressure system will promote scattered to widespread thunderstorms across the central U.S., shifting into the East late week. Meanwhile, a tropical system that is likely to develop per NHC will continue to be monitored, but fortunately most models and latest NHC guidance continue to take the low system east of the lower 48 into the Atlantic. ...Guidance/Predictability Assessment... Model guidance remains generally agreeable regarding the large scale pattern, including the southern stream ridge likely rebuilding farther west with time as an upper low with okay placement/timing agreement comes into the eastern Pacific and Northwest. But a possible shortwave moving through the northern tier and its associated surface low/frontal system is the trickiest model diagnostics feature. The 12Z ECMWF/CMC has the shortwave tracking across the north-central U.S. Thursday-Friday and into the Great Lakes by Saturday, similar to earlier model consensus. AI/machine learning models also show a shortwave with reasonably similar timing/placement. However, the 12/18Z GFS runs pretty much dropped this shortwave feature after showing it a day ago, maintaining ridging. Leaned away from these GFS runs to be more consistent with previous forecasts and the non-NCEP guidance. Fortunately the newer 00Z GFS run has brought the shortwave back. There is some uncertainty with how this shortwave interacts with troughing that is along the East Coast at the start of the period. The pattern across the East will affect the track of the tropical system that is likely to form. The bulk of the model and ensemble guidance continues to show the system turning northeastward enough to take it out to sea rather than into the U.S., other than the rogue 12/18Z GFS runs that were not preferred. Thus the WPC forecast blend leaned heavily on the 12Z ECMWF and CMC and EC ensemble mean, with some incorporation of the 12Z GEFS mean, due to aforementioned issues with the 12/18Z deterministic GFS runs. ...Weather/Hazards Highlights... Into midweek, monsoonal moisture will flow into the Rockies for some scattered storms. Overall coverage and rainfall amounts should be lower than in the short range period, but likely still sufficient for a Marginal Risk from eastern Arizona into New Mexico and Colorado for the Day 4/Wednesday ERO. A relative break in monsoonal convection is possible on Thursday, but by Friday and especially Saturday some renewed moisture could come into the Four Corners states, focusing westward a bit compared to earlier in the week as the upper ridge retrogrades. Meanwhile, Pacific and Gulf moisture will likely combine across the central U.S. while a shortwave provides support for thunderstorms likely in the form of multiple mesoscale convective systems tracking west-east or northwest-southeast on Wednesday and Thursday. Broad Marginal Risks remain in place in the Day 4-5 EROs for central parts of the Plains into the Mid-Mississippi Valley and east into parts of the Ohio Valley by Thursday for flooding threats with these storms. Aforementioned model differences with the supporting shortwave lessened confidence in exact location of heavy rains, but given the potential for high rain rates in an unstable environment, Slight Risks in future cycles are certainly a possibility, especially across the Middle Mississippi Valley-- though recent models are focusing the heaviest rain a bit north of a day ago. Rain is also possible farther north into the northern Plains and Upper Midwest, where a Marginal Risk extends for Day 4/Wednesday across areas that still have wet antecedent conditions remain from heavy rains earlier this summer. Thunderstorm chances should shift into the east- central U.S. on Friday and into the Eastern Seaboard over the weekend. Scattered storms are possible in the Southeast as a front works its way through Florida. Coverage of storms may be maximized on Thursday in the Florida Peninsula where above normal moisture pools, but held off from any risk area in the ERO since most of Florida is not sensitive to flooding unless there are exceptionally heavy rain rates/totals, and there is not confidence of that occurring. Light to modest showers are forecast in the Northeast Wednesday-Thursday ahead of more widespread rain approaching into the weekend. And in the Northwest, coverage and amounts of rain will increase late week into the weekend as the low/trough aloft provides support. The upper ridge axis over the south-central U.S. will maintain temperatures up to 10F above (already hot) normal values across the southern Plains into the central Gulf Coast. Combined with the high dewpoints in most areas, heat indices above 110F are possible, and experimental HeatRisk shows Major to Extreme impacts. Low temperatures in the upper 70s to near 80 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 a cooling trend 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