Extended Forecast Discussion NWS Weather Prediction Center College Park MD 248 PM EDT Sun Jul 30 2023 Valid 12Z Wed Aug 02 2023 - 12Z Sun Aug 06 2023 ...Hot temperatures and high heat indices from the south-central Plains and Mississippi Valley to the Southeast this week... ...Heavy Rain/flash flood threat to focus across the north-central Rockies states and vicinity midweek... ...Overview... An amplified and slow moving upper ridge will support above normal temperatures from the south-central Plains through the Mid-Mississippi Valley to the Southeast for much of this week along with high heat indices. Deepened monsoonal moisture lifting on the western periphery of the ridge this period will favor a slow shift of a main convective rainfall focus from the Great Basin to the north-central Rockies/High Plains. Meanwhile, shortwaves rounding the top of the ridge in the northwest flow will spark strong to severe thunderstorms, and locally heavy rain, midweek across the Midwest to the Appalachians/Mid-Atlantic and up to the Northeast late week. This is also along and in advance of an organized surface low/frontal system that should moderate temperatures in the wake of its passage. ...Guidance Evaluation/Predictability Assessment... Models and ensembles still have a good handle on the overall large-scale pattern but plenty of uncertainty in the details and timing of systems rounding the top side of the upper ridge. 06z GFS was notably faster with an initial shortwave skirting the Upper Great Lakes midweek, also faster with surface cold front progression into the East later in the week too. Majority of the rest of the guidance suggests a little slower progression, which is the direction the WPC forecast trended as well. This continues to offer plenty of uncertainty in the resulting QPF forecast from the Midwest into the East later this week. Otherwise, good agreement on the gradual expansion of the upper ridge across the Southern U.S. as well as an upper low meandering off the Northwest coast. The WPC medium range product suite used a general deterministic model blend for the first half of the period, which does help mitigate the timing differences with the Great Lakes shortwave. Incorporated more ensemble mean guidance late period to tone down some of the noise with any one individual solution. This maintained pretty good consistency with the previous WPC forecast, though some adjustments (especially to QPF) were needed based on current guidance trends. ...Weather/Hazards Highlights... The greatest heat anomalies should settle this week across the south-central Plains to the lower Mississippi Valley, where above normal temperatures by 5 to 10+ degrees are likely as actual temperatures near or exceed 100F. The southern Plains to lower Mississippi Valley and Southeast may experience heat indices soaring above 105F, even above 110-115F in some areas. Record hot highs and warm minimum temperatures are widely possible in these regions next week. Heat should settle southward with time as heights lower across the central U.S. and into the East. Areas from the Great Basin to the north-central Rockies/High Plains will see diurnally and terrain driven convection as fueled by deep monsoonal moisture and embedded shortwave energies on/around the periphery of the main upper ridge. QPF, moisture, and instability fields seem to support slight risks of excessive rainfall continuing into Wednesday and Thursday across portions of Idaho/Utah/Wyoming and Colorado where flash flood guidance values are lower and burn scars. For day 5/Thursday, new guidance trends necessitated a northward shift in the slight risk to across northern Wyoming and far southern Montana. Still a lot of uncertainty in the details so further refinement of these risk areas is likely in the coming days. Meanwhile, the north-central U.S. can also expect scattered storms as shortwaves/upper troughing work into the region. A heavy rain/flash flooding threat will likely be tied to the potential for organized convective systems in the northwesterly flow. Marginal Risks have been included on both Days 4 and 5 to cover this potential from the Midwest to the Appalachians/Mid-Atlantic, with eventual more concentrated Slight Risk areas possible. This activity will likely work across the Northeast later week in a pattern to monitor. Activity will also be aided as an organized low pressure system subsequently tracks eastward across Canada, and a cold front trails back across the lower 48 and stalls back into the central Plains. This all occurs well to the north of a lead cold front pushed unseasonably far south early in the period to offer a more defined than normal summertime convective focus over Florida. There could be some lingering localized high rain rates given stalling moisture pooling/instability focus as the front slowly weakens over time. Santorelli/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, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: 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