Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 PM EDT Sat Jun 22 2024 Valid 12Z Tue Jun 25 2024 - 12Z Sat Jun 29 2024 ...Hazardous heat possible for southern/central portions of the Plains and Mississippi Valley to Southeast for parts of next week... ...Overview... An upper level ridge will remain anchored over southwestern to south-central parts of the nation through the week, leading to multiple days of above normal to possibly hazardous heat. To the north, one shortwave through the northern tier will amplify somewhat as it crosses the Midwest/East mid- to late week. The next system reaches the West Coast around Thursday should push a defined surface low pressure system into the northern Plains by late week. ...Guidance/Predictability Assessment... The latest suite of guidance continues to offer overall good agreement on the large scale, and typical uncertainty in the mesoscale features that could have impacts on sensible weather/QPF details. The first shortwave skirting the Northern Plains/Great Lakes Tuesday-Wednesday and amplifying in the Northeast Thursday shows better agreement in recent model guidance. Out West, models are also converging better on timing for the upper low/trough entering the Northwest late Wednesday and tracking east. The main remaining outlier was the faster 00Z GFS, but the 06Z and 12Z GFS runs seemed more reasonable. This agreement seems to hold until around Saturday when the 00Z CMC stalls and amplifies the trough a bit more. The WPC forecast utilized a blend of the 06Z GFS and 00Z ECMWF/CMC/UKMET early in the period, gradually ramping up the proportion of GEFS and EC ensemble means as the period progressed to help temper some of the smaller scale uncertainties. Overall, this approach maintained fairly good continuity with the previous WPC forecast. ...Weather/Hazards Highlights... A low pressure system moving through south-central to eastern Canada and its associated fronts will provide a focus for possibly impactful convective rainfall. On Tuesday-Wednesday, copious moisture and instability in a typical summertime convective pattern south and east of the cold front should support widespread showers and thunderstorms progressing eastward with the boundary from the Mississippi Valley/Midwest into the East. As a result, large Marginal Risks are highlighted on the Days 4 and 5 ERO to cover this favorable environment. It is still too early to pin down any potential areas of higher risk and the front should be fairly progressive as well, limiting the overall threat. Anomalous moisture and sufficient instability should persist over the Four Corners states, eventually expanding into the southern Rockies and parts of the High Plains much of the period and potentially lead to some locally heavy rainfall, which could cause flooding impacts particularly over burn scars. Marginal Risks are in the outlook for Day 4/Tuesday across parts of Arizona and New Mexico, expanded west a bit toward the most anomalous moisture and east a bit to cover the Sacramento Mountains that are very sensitive to additional rainfall. By Day 5/Wednesday, moisture looks to be transported even farther north ahead of approaching upper troughing, and thus the Marginal is expanded into portions of Utah and Colorado and into the south-central High Plains. Though perhaps not everywhere in the Marginal will see heavy rain, slow- moving convection is a concern for these areas and warrants a Marginal Risk. As the northwestern to north-central U.S. upper trough and the low pressure/frontal system tracks eastward with it, the moisture combining with these systems is likely to produce rain/thunderstorms across the north-central U.S. again late week. Elsewhere, parts of the Southeast/Florida may also see episodes of diurnally enhanced convection, but overall dry conditions/high FFGs precluded any excessive rainfall risk areas at this time. Into Tuesday, above normal temperatures will stretch from the West to the East, though the focus for hazardous heat will be across the south-central Plains/Mississippi Valley into the Southeast where daytime highs 10-15F above normal, heat/humidity could lead to heat indices near 110F for some, and widespread major to extreme HeatRisk are present Tuesday-Wednesday. Hazardous heat is also possible across the Southwest where temperatures 105-115F are likely. Temperatures should moderate back towards normal for the East and West states as upper troughing moves through both regions mid to late week, but upper ridging in between should bring a return to above normal temperatures from the south-central Plains into the Ohio and Tennessee Valleys Friday-Saturday. Tate/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