Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Mon Jun 24 2024 Valid 12Z Thu Jun 27 2024 - 12Z Mon Jul 01 2024 ...Hazardous heat threats to expand across parts of the South late week into next weekend... ...Overview... An upper ridge initially centered over the southern Plains will gradually expand eastward with time and intensify bringing an increased threat for hazardous heat to portions of the Southern U.S.. To the north, a series of shortwaves/troughs will progress from West to East across the northern tier into early next week. Showers and thunderstorms will accompany the associated frontal boundaries, with the best chance for heavy to excessive rainfall across parts of the Midwest Thursday into Friday. ...Guidance/Predictability Assessment... The latest suite of guidance continues to offer very good agreement on the large scale, and typical uncertainty in the details that could have impacts on sensible weather/QPF. Agreement on the timing and amplitude of a trough exiting the East on Thursday has trended better in recent days, with model agreement on an upper ridge building back in over the Southeast by next weekend. The next shortwave should reach the Northwest by Thursday, with some continued uncertainty as it reaches the northern Plains and Great Lakes regarding interaction with additional energy from south- central Canada. This system should progress across the Northeast early next week, with some question on timing of the shortwave/surface front through the East at that time. A third trough enters the West late weekend and also offers plenty of uncertainty in the timing/details. The CMC was a little slower than the better consensus of guidance and was not included in the late period forecast blend. The WPC forecast for tonight was based on a blend of the GFS, ECMWF, CMC, and UKMET for the first half of the period amidst sufficient model agreement. Incorporated more of the ensemble means the second half (with the GFS and ECMWF) to help temper some of the guidance differences by that time. Overall, this maintained good agreement with the previous WPC forecast as well. ...Weather/Hazards Highlights... A low pressure system into the Northern Plains on Thursday will bring a threat for heavy rainfall across the north-central Plains into the Upper Midwest. There remains a lot of uncertainty in the QPF details/amounts, but did maintain a broad marginal risk on the Day 4/Thursday Excessive Rainfall Outlook across the region given ample moisture and instability associated with this system. Compared to previous model runs, the signal a more targeted area of heavier rain has decreased, but did opt to maintain a slight risk centered across northern Iowa given very wet antecedent conditions and sensitivity to additional rainfall. By Friday, the system will shift east, and there is some increased heavy rainfall signal from eastern Iowa into southern Wisconsin overlapping with areas which have seen significant rainfall recently. A slight risk was placed in this region on the Day 5 ERO, with a broad marginal risk surrounding across parts of the middle to upper Mississippi Valley and lower Great Lakes. Showers and thunderstorms associated with frontal boundaries may also linger across the Southeast/Florida through the period, but dry conditions/high FFGs continue to preclude any excessive rainfall risk areas. Anomalous moisture with widespread precipitable water values over the 95th percentile will persist over the southwestern U.S. into Thursday and slowly lessen by the weekend. This should support monsoonal- type showers and thunderstorms with heavy rain. A marginal risks remains across the Four Corners states and vicinity for the threat of flash flooding on Thursday, with burn scars and slot canyons particularly vulnerable. Though perhaps not everywhere in the quite broad Marginal Risk areas will see heavy rain, slow- moving convection is a concern for these areas and warrants the risk depiction in the ERO. Coverage of storms should decrease by Friday but did maintain a small marginal risk on the Day 5 ERO for southeast Arizona to southwest New Mexico. Rainfall coverage may increase once again especially in New Mexico over the weekend. The focus for hazardous heat will initially be across parts of the southern Plains on Thursday, where a long duration heat wave will be ongoing and heat indices near 110F for some is leading to widespread major to extreme HeatRisk. Heat currently across the Southeast should briefly moderate on Thursday with troughing moving through, but the upper ridge will build back into the region and northward into the Mississippi and Ohio Valleys late week, bringing several days of above normal temperatures (both daytime highs and overnight lows) from the south-central Plains eastward. Temperatures may be a few degrees above average in the Desert Southwest, equating to highs 105-115F. Meanwhile the main area of cooler temperatures will be across the northern tier, where upper shortwaves and surface fronts provide some relief. Santorelli 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