Extended Forecast Discussion NWS Weather Prediction Center College Park MD 248 PM EDT Thu May 23 2024 Valid 12Z Sun May 26 2024 - 12Z Thu May 30 2024 ...Heavy Rainfall/Flooding possible from the Midwest to the Northeast Sunday and Monday... ...Hazardous heat will continue through Memorial Day weekend for South Texas, the Central Gulf Coast, and South Florida... ...Overview... An upper trough (which may be a combination of different energy streams) will traverse the CONUS through the medium range period. This will serve as the main driver for heavy rainfall potential early in the period from the Midwest/Mid-South into the East. After Monday, the overall pattern should turn more amplified as an upper ridge builds over the interior West, causing the downstream trough to deepen and slow as it moves over the East early next week. Upstream, a deep upper low anchored off the West Coast should eventually send energy into the Northwest and northern tier around the middle of next week. Meanwhile, a strong ridge to the south will maintain its influence over Mexico and will continue a hazardous heat threat from South Texas through the Gulf Coast and for South Florida through Monday. A cold front will drop south into the northern Gulf mid-next week and bring some much needed relief from the heat. ...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. Some of the highest uncertainty surrounds the timing and strength of a shortwave that will move over the northern Rockies and Central Plains on Sunday before interacting with a deep upper low over the Great Lakes region. The interactions between these two features will influence the timing and position of several successive frontal systems that will impact the eastern third of the nation next week. Shortwave energy should move into the Pacific Northwest and cross the northern tier of the nation mid-next week, but there is still a significant amount of uncertainty surrounding the speed and amplitude of this feature. Recent runs of the GFS and ECMWF have shown poor run-to-run consistency, so this will be a feature to keep an eye on in future forecast updates. The afternoon WPC forecast used a blend of the deterministic GFS/ECMWF/CMC/UKMET along with the previous WPC forecast blend through Tuesday, with the heaviest weight placed on the ECMWF and GFS. This method aims to find a middle ground solution given the differences in the latest model runs and lack of run-to-run consistency. After Tuesday, the WPC blend used a combination of the deterministic GFS/ECMWF/CMC and ensemble means from the GEFS/ECE to help smooth out model differences. This also maintained good continuity with the previous WPC forecast for Days 3-6. ...Weather/Hazards Highlights... A surface low and associated fronts will be the focus for strong to severe thunderstorms and heavy rainfall/flooding potential Sunday and Monday from parts of the Midwest to the East. A Marginal Risk of Excessive Rainfall (level 1/4) remains in place across much of the Ohio Valley and into the Tennessee Valley on Sunday (Day 4), shifting into the Northeast and Mid-Atlantic on Monday (Day 5). The guidance continues to show elevated QPF totals, but still with a lot of spread in exactly where. It is possible that the guidance signal will become more defined and an embedded Slight Risk (level 2/4) may be needed in the Midwest on Sunday. The heaviest rainfall should exit the East Coast after Monday with the cold front, but renewed troughing across the region will allow for shower chances across the Great Lakes into the Appalachians/Northeast through mid-next week. Elsewhere, precipitation chances should increase across the Northwest and northern/central Plains Tuesday through Thursday while additional showers and thunderstorms expand across the southern Plains. Expect hazardous heat to continue through Memorial Day weekend from South Texas through the Central Gulf Coast and South Florida. Daytime highs are forecast to be as much as 10-20 degrees above average with widespread heat index values above 100 degrees, and max heat index values may exceed 115 degrees in parts of South Texas. Highs near 100 degrees could stretch farther north across the southern High Plains on Sunday as well. The combination of dangerous heat indices and lack of overnight recovery/cooling will allow for a major to extreme heat risk across South Texas, with widespread major heat risk also extending into the Central Gulf Coast region and South Florida. Temperatures should trend closer to normal on Tuesday across much of the South following a cold front passage, but South Florida will likely remain warm through mid- next week. The forecast pattern will favor below average highs over the Northwest and northern Plains on Sunday, but above normal temperatures will build again across the West early next week underneath of an amplified upper ridge. Above average temperatures along the eastern seaboard will moderate back to near normal next week with some chance for below normal highs around mid-next week. Dolan/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