Extended Forecast Discussion NWS Weather Prediction Center College Park MD 214 AM EDT Thu May 23 2024 Valid 12Z Sun May 26 2024 - 12Z Thu May 30 2024 ...Heavy Rainfall/Flooding Threats from parts of the Midwest to the Northeast Sunday-Monday.. ...South Texas to the Central Gulf Coast and South Florida Heat Threat into early next week... ...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 allowing the downstream trough to deepen and slow as it reaches 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, ridging over Mexico will help maintain a hazardous heat threat from south Texas to the Gulf Coast and southern Florida through about Monday before a cold front sweeps through the South. ...Guidance/Predictability Assessment... Models and ensembles continue to show reasonably good agreement on the overall large scale upper level pattern through much of the medium range period, but there are some small-mid scale system differences and run to run variances that remain more uncertain. Early in the period, a northern stream shortwave into the northern Plains and a leading southern stream shortwave should interact with a compact closed low and amplified troughing over the Ohio Valley by Monday. After Monday, there is a lot of uncertainty still on the evolution and speed of this trough which is dependent on additional closed low energy dropping in from south-central Canada. Eventually, this should create renewed troughing over the East that could be slow to move out of the region. Shortwave energy from a closed low just off the British Columbia coast should enter the Northwest around the middle of next week, but there is also a lot of uncertainty on the speed and amplitude of this feature. Through the 12z/18z May 22 model runs, the ECMWF was the most amplified with this, but the new 00z GFS did come in to match. But given the late period timing, a lot can still change. The WPC forecast tonight was based on a blend of the GFS, ECMWF, and CMC through Tuesday when model agreement was still decent. After this, trended more towards the GFS with the ensemble means to help mitigate the smaller scale differences while still maintaining some system definition. 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 remains in place across much of the Ohio Valley and into the Tennessee Valley on Sunday/Day 4, shifting into the Northeast on Monday/Day 5. Especially Sunday, the guidance continues to show elevated QPF totals, but still with a lot of spread in exactly where. It is possible that a the guidance signal becomes more defined, an embedded slight risk may be needed somewhere. The heaviest rainfall should exit the East Coast after Monday with the cold front, but renewed troughing across the region will allow for showers across the Great Lakes and into the Appalachians/Northeast beyond Monday. Elsewhere, rain chances should increase from the Northwest to northern Plains while additional showers expand across the Southern Plains as energy ejects out from the Southwest late period. Expect hazardous heat to continue into the medium range period from South Texas through the Central Gulf Coast and South Florida with daytime highs as much as 10-15 degrees above normal with max heat index values possibly reaching 115 degrees, especially for 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 as well as southern Florida. Temperatures should trend closer to normal after Monday across much of the South following a strong cold front passage, but southern Florida should remain warm into mid-next week. The forecast pattern will favor below average highs over the Northwest to northern Plains for Sunday still but above normal temperatures will build again across the West early next week underneath of amplified upper ridging that shifts into the Central U.S. Above average temperatures along the eastern seaboard will moderate back to normal next week with some chance for below normal highs around mid-next week. 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