Extended Forecast Discussion NWS Weather Prediction Center College Park MD 243 PM EDT Sat Jul 29 2023 Valid 12Z Tue Aug 01 2023 - 12Z Sat Aug 05 2023 ...Hot temperatures and high heat indices are likely across the south-central Plains and Mississippi Valley into the Southeast next week... ...Heavy Rain/flash flood threat possible mid next week across parts of the north-central Rockies... ...Overview... The upper high that has been anchoring over the southwest/south-central U.S. for weeks now is finally forecast to drift eastward next week, finally bringing temperatures in the Desert Southwest closer to normal (though still hot). Ridging stretching north from the upper high will also promote above normal temperatures across the central Plains to Mid-Mississippi Valley with the Southeast also likely to warm above normal, with high heat indices. A "ring of fire" type of pattern is likely in the precipitation forecast, as monsoonal moisture leads to diurnally driven convection in the Four Corners states and vicinity, shortwaves push through northwest flow and spark strong to severe thunderstorms in the north-central U.S., and a well-defined cold front pressing unusually far south into the mid-section of the country with a risk of heavy rainfall over the northern Rockies to the central High Plains midweek, and perhaps across the Northeast late next week. ...Guidance Evaluation/Predictability Assessment... The latest guidance continues to have a good handle on the overall pattern which features various systems/shortwaves rounding the top of the stubborn southern U.S. ridge. Some timing/details differences remain, most notably with the shortwave into the Upper Great Lakes later next week. The 00z ECMWF was the slowest of the guidance with this and the associated cold front through the Midwest, but the new 12z run (available after forecast generation time) did come in faster. Some differences with an upper low meandering off the Northwest coast by late week/next weekend in the 00z CMC so it was dropped off from the blend after day 5. The WPC medium range product suite was primarily derived from a composite blend of the deterministic models days 3 and 4, with more emphasis towards the ECMWF and GFS (with ensemble means) later in the period, along with the generally compatible 13 UTC National Blend of Models. This maintains excellent consistency with the previous WPC forecast as well. ...Weather/Hazards Highlights... A lead cold front should push unseasonably far south to offer a more defined than normal summertime convective focus over the Southeast. A Marginal Risk has been maintained in the updated Day 4/5 EROs for portions of Georgia and Florida, with extension into coastal South Carolina, as there could be high rain rates possibly causing excessive rainfall given stalling moisture pooling/instability focus and upper support. The combination of an additional frontal passage as well as the lingering/wavy southern front is then likely to favor periodic activity through mid-later next week. Meanwhile, the north-central U.S. can also expect scattered storms as shortwaves track through the region. A heavy rain/flash flooding threat will likely be tied to the potential for organized convective systems in the northwesterly flow. Day to day positions of heavy rain and MCSs are uncertain at this point, but the pattern is there. Marginal Risks have been included on both Days 4 and 5 to cover this potential, with eventual more concentrated Slight Risk areas possible. The Four Corners states and into Wyoming are likely to see diurnally driven convection each day as monsoonal moisture comes in. As the upper high shifts eastward with time, this may increase moist southerly flow and allow for additional shortwave energy to work into the region. There was enough support in the QPF, moisture, and instability fields to include a slight risk on day 5/Wednesday across portions of central Wyoming and north-central Colorado where flash flood guidance values are lower. Around the middle to latter part of next week, an organized low pressure system will track eastward across Canada with a cold front trailing back into the central Plains. Suspect an area of rainfall with possibly locally heavy rain may focus from north-central Rockies to the central High Plains where the monsoonal moisture appears to interact with the trailing front and with another frontal wave over the central Plains. There was enough support in the QPF, moisture, and instability fields for the addition of a slight risk area on the day 5/Wednesday ERO across central Wyoming into north-central Colorado consistent with lower flash flood guidance values. Later on, lead flow should also fuel an expanding area of precipitation from the Ohio Valley through the Appalachians/Mid-Atlantic and the soggy Northeast. The hot upper high that has been centered over the southwest/south-central U.S. will re-position eastward into early-mid next week, allowing for high temperatures in the Desert Southwest to finally ease to near normal. Highs should still be over 100F in many areas but at least there should be minor relief compared to the past few weeks. Temperatures by begin to creep back up later next week into the weekend though as the ridge tries to expand back westward. The most persistent/greatest anomalies should move from the central Plains to the mid-Mississippi Valley, where above normal temperatures by 5 to 15 degrees are likely as actual temperatures near or exceed 100F. The southern Plains to lower Mississippi Valley and Southeast may see slightly lower anomalies of 5-10F above normal, but the heat combined with high dewpoints will lead to 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. There is hope for some relief to slowly work into this heat dome from the north through later next week with possible cold frontal approach. 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