Extended Forecast Discussion NWS Weather Prediction Center College Park MD 252 PM EDT Fri May 24 2024 Valid 12Z Mon May 27 2024 - 12Z Fri May 31 2024 ...Heavy Rain and flooding possible for the Northeast on Monday... ...Hazardous heat will continue through early next week for South Texas, the Central Gulf Coast, and South Florida... ...Overview... Multiple shortwaves will pivot around a deep upper low positioned north of the Upper Midwest over southern Canada, which will drive multiple surface fronts south across the central and eastern U.S. next week. Precipitation chances will be highest in the Great Lakes and Northeast in the vicinity of the upper low and near the Gulf Coast where one of the frontal boundaries will stall. By mid-week, another deep upper low is forecast to push into the Pacific Northwest and southern British Columbia and bring a frontal system across the Northwest and Intermountain West. This frontal system will emerge into the Plains later in the week, producing widespread shower and thunderstorm chances. A ridge will build over the Midwest during the second half of the week as the leading upper low/trough shifts east. Hazardous heat is forecast to continue for portions of South Texas, the central Gulf Coast, and South Florida through early next week, but the Gulf Coast may see some relief mid-week with a cold frontal passage. ...Guidance/Predictability Assessment... Model guidance shows good agreement on the overall large scale upper level pattern through the medium range period. As usual, there are some differences in the details, which will likely affect the sensible weather forecast. The highest uncertainty in the forecast concerns interactions between the Great Lakes/Northeast upper low and pieces of shortwave energy rotating around and through the upper trough. The strength, timing, and location of these features will have an affect on one another and the surface features moving across the central and eastern U.S.. Models have come into better agreement on the timing and strength of the upper low moving into the Northwest mid-next week, but solutions diverge later in the week as the low moves into the Upper Midwest/southern Canada. The WPC blend for the afternoon forecast was comprised of a blend of the deterministic GFS/ECMWF/CMC/UKMET for the first two days (Monday-Tuesday). For Wednesday, the UKMET strayed away from the other model guidance, so the previous WPC forecast blend was substituted in its place. For the rest of the period, ensemble means from the GEFS/ECE were added to the blend to smooth out differences between the deterministic GFS/ECMWF/CMC. ...Weather/Hazards Highlights... Scattered to numerous showers and thunderstorms are forecast across the Mid-Atlantic and Northeast on Monday, compliments of an amplifying upper trough and an incoming cold front. Model guidance continues to show heavy rain focusing over eastern Pennsylvania, northern New Jersey, and southern New York, so a Slight Risk of Excessive Rainfall was maintained for this area in the Day 4 (Monday) ERO. Elsewhere across the nation, moisture convergence along a frontal boundary and dryline across Texas and Oklahoma will likely generate scattered thunderstorms with the potential for locally heavy rainfall early next week. The threat of heavy rainfall will be highest in Texas Tuesday into Wednesday as an upper level shortwave moves over the southern Plains, so there is a Marginal Risk of Excessive Rainfall for this region in the Day 5 (Tuesday) ERO. Shower and thunderstorm chances will expand north across the central and northern Plains later in the week as a frontal system moves into the central U.S. from the northwest. Early summer heat and humidity will continue to impact the southern half of Texas, especially in the Rio Grande Valley, and portions of the Gulf Coast and South Florida through Memorial Day. Conditions will improve for the Gulf Coast and South Florida as a cold front sinks south mid-week, but heat, though less widespread, will likely hang on a little longer for some locations in South Texas as the front stalls then lifts back north as a warm front. The combination of daytime highs ranging from the mid 90s to 110 degrees and dewpoints of 70-80 degrees will produce dangerously high heat indices that could exceed 115 degrees in some places. Above average temperatures are also forecast across much of the western and central U.S., but below average temperatures will develop in the Pacific Northwest as the upper low and associated frontal system push inland mid-next week. Below average temperatures will also shift from the Upper Midwest to the Mid- Atlantic and Northeast behind a series of cold fronts next week. Dolan 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