Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Sat Jun 22 2024 Valid 12Z Tue Jun 25 2024 - 12Z Sat Jun 29 2024 ...Hazardous heat possible for parts of the South/Plains early to mid next week... ...Overview... An upper level ridge will remain anchored over much of the Southern half 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 spreading a defined surface system into the northern Plains by next weekend. ...Guidance/Predictability Assessment... The latest suite of guidance continues to offer overall good agreement on the large scale, but plenty of uncertainty in the details which could have larger impacts on sensible weather/QPF across the affected regions. A shortwave skirting the Northern Plains/Great Lakes Tuesday-Wednesday may amplify some into the Midwest and Northeast by Thursday, with the ECMWF offering a more amplified and slower evolution. The better consensus (including machine learning guidance) suggests something a bit flatter and faster. The new 00z ECMWF (available after forecast generation time) shows a trend in this direction as well. Out West, timing uncertainty arises early in the period, with the UKMET the slowest and the GFS remaining on the faster side. Ensemble means/ML models though seem to support a little faster progression though as well. The WPC forecast tonight used a general blend of the 12z/Jun 21 ECMWF and CMC with the 18z/Jun 21 GFS for the first half of the period, introducing more of the ensemble means later in the period (with some GFS and ECMWF) to help temper some of the smaller scale uncertainties. Overall, maintained fairly good continuity with the previous WPC forecast for days 3-6. ...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 across parts of Arizona and New Mexico, expanding into Colorado and the south-central High Plains on Day 5 where the best moisture transport is for slow- moving convection. 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. A front reaching the Northwest may produce some light rainfall Wednesday into Thursday. For the first half of the week, above normal temperatures will stretch from the West to the East, though the focus for hazardous heat will be across the central Plains states into the Southeast where daytime highs 10-15F above normal, and 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 modestly above normal temperatures from the Plains into the Midwest Friday-Saturday. 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