Extended Forecast Discussion NWS Weather Prediction Center College Park MD 248 PM 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 into the Southeast through the week to next weekend... ...Mid-Mississippi Valley Heavy Rain/Runoff threat by midweek... ...Overview... A summertime upper ridging will be in place across the south- central U.S. for much of the coming week, leading to high heat indices that could be hazardous across the southern Plains to Gulf Coast 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 may retrograde a bit in response. Shortwave energy and a growing signal in support of organized surface low pressure system cyclogenesis 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 western Atlantic. ...Guidance/Predictability Assessment... 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. Guidance forecast spread and system uncertainty in this pattern recently has tended to ramp up generally after only 4 days from current. Accordingly, felt comfortable using a GFS/ECMWF/UKMET/Canadian model blend for max details Wednesday into early Thursday. Beyond then into longer time frames, prefer to graviate to a blend with an ensemble mean focus supplimented by model guidance on the less progressive/amplified side of the full envelope of solutions for mid-latitude flow. In this period that placed more emphasis on the 00 UTC Canadian ensemble mean and to a lesser extent the 00 UTC Canadian model and the 06 UTC GFS model. This seems to overall better align with latest 12 UTC guidance trends. However in contrast, 00/12 UTC ECMWF system and associated QPF progression across the east-central to eastern U.S. seems too fast considering ample upstream amplitude more locked in to develop across the West Coast. ...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 an energetic shortwave provides support for thunderstorms likely in the form of multiple mesoscale convective systems tracking west-east or northwest-southeast on Wednesday and Thursday. A Slight Risk Day 4/Wednesday ERO has been introduced For the Mid-Mississippi Valley given a steady/growing signal with warm frontal and upper system approach in a forecast time period prior to most guidance forecast spread recently. Broad Marginal Risks also 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 for Day 5/Thursday, but given the potential for high rain rates in an unstable environment, a Slight Risk in future cycles are certainly a possibility. Rain is also possible farther north into the northern Plains and portions of the 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 East 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/Schichtel 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