Extended Forecast Discussion NWS Weather Prediction Center College Park MD 226 AM EDT Fri May 24 2024 Valid 12Z Mon May 27 2024 - 12Z Fri May 31 2024 ***Heavy Rainfall for the Northeast on Monday, and hazardous heat will continue through early next week for Southern Texas and South Florida*** ...General Overview... An amplifying upper trough over the Great Lakes region early in the week will result in a pattern change across much of the northeastern quadrant of the U.S. through midweek. A relatively strong cold front will bring a quality airmass from the Northern Plains to the East Coast after recent warm and humid conditions. Another cold front is also forecast to cross the Pacific Northwest and then the Intermountain West through mid-late week, while an upper ridge tries to build northward across the Plains. The heatwave across the Gulf Coast region is expected to abate some by midweek, although it will still be quite warm and humid for the remainder of the week across southern Texas. ...Guidance/Predictability Assessment... Model guidance shows good agreement on the overall large scale upper level pattern through the medium range period, but there are some small/mid scale differences between models and runs that will affect how upper lows and shortwaves interact. Shortwave energy should move into the Pacific Northwest and cross the northern tier of the nation mid-next week, but there is still some uncertainty regarding the speed and amplitude of this feature. The WPC fronts/pressures forecast incorporated a blend of the deterministic GFS/ECMWF/CMC/UKMET along with some previous WPC continuity through Tuesday, with slightly more weighting applied to the ECMWF and GFS. For the Wednesday through Friday time period, the WPC blend used a combination of the deterministic GFS/ECMWF/CMC and ensemble means to help smooth out model differences, with ensemble means accounting for about 40% of the blend by the end of the week. ...Weather/Hazards Highlights... Scattered to numerous showers and thunderstorms are likely across the Mid-Atlantic and into the Northeast U.S. for the beginning of next week, compliments of an amplifying upper trough and an incoming cold front. The models are coming into better focus with an area of more concentrated convective rains from eastern Pennsylvania to southern New York during the new Day 4 period Monday afternoon into Monday night. Therefore, a small Slight Risk area is planned for this region. Elsewhere across the nation, moisture convergence along and south of a frontal boundary across Texas and Oklahoma, along with a dryline, will likely generate scattered storms with heavy rainfall on both Days 4 and 5 (Monday through Tuesday), but the models vary on the placement of the QPF maxima. A Marginal Risk area is planned on both days across portions of the southern Plains to account for this potential. Early summer heat and humidity will continue to make weather headlines across the southern half of Texas, especially in the Rio Grande Valley, through Memorial Day before slight improvement arrives. The combination of daytime highs from the upper 90s near the coast to 105-110 degrees well inland, along with dewpoints of 75-80 degrees closer to the coast, will combine to produce hazardous heat indices of 110 to perhaps 120 degrees for these areas. Farther to the north, a more refreshing change is coming from the Midwest to the Mid-Atlantic as a cold front advects a cooler and drier airmass into the region to close out the month of may, with highs in the 60s and 70s for many of these areas by Wednesday and Thursday. Hamrick 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