Extended Forecast Discussion NWS Weather Prediction Center College Park MD 246 PM EDT Wed May 22 2024 Valid 12Z Sat May 25 2024 - 12Z Wed May 29 2024 ...Heavy Rainfall/Flooding Threats for the Mid-Mississippi, Tennessee, and Ohio Valleys through the weekend... ...South Texas to the Central Gulf Coast and South Florida Heat Threat... ...Overview... It remains generally the case that northern stream upper trough/compact closed low and southern stream shortwave/low energies and instabilities working from the West to Central U.S. will be the main drivers for heavy rainfall potential across parts of the Middle Mississippi Valley through the Tennessee/Ohio Valleys this weekend. By Monday, the overall pattern across the CONUS should turn more amplified as an upper ridge builds over the interior West and a large closed low anchors off the West Coast. This allows the downstream troughing to deepen and slow as it reaching the East early next week. A strong upper ridge over Mexico will keep hazardous heat a threat from south Texas to the Gulf Coast and southern Florida through at least the weekend with dangerously high heat indices coupled with anomalously warm nighttime lows. ...Guidance/Predictability Assessment... Models and ensembles continue to show reasonably good agreement on the overall mid-larger scale upper level pattern through the medium range period, but small-mid scale system differences and cycle to cycle variances and especially heavier convective QPF foci remain more problematic. Accordingly, the WPC medium range product suite was primarily derived from best clustered guidance from the 06 UTC GFS/GEFS and 00 UTC ECMWF/ECMWF ensemble mean that seems to provide detail consistent with individual system predictability. This also maintains good WPC product continuity. Newer 12 UTC guidance continues to exhibit these types of warm season variances. ...Weather/Hazards Highlights... A deepening surface low and associated warm front will be the focus for strong to severe thunderstorms and heavy rainfall/flooding potential this weekend across parts of the Mid- Mississippi Valley to Ohio Valley. A marginal risk of excessive rainfall remains in place from eastern Kansas to the lower Ohio Valley for Saturday as a frontal boundary stalls and lifts north through the region. An organized surface low moving into the Midwest later in the weekend should help tap additional moisture and instability for heavy rainfall development farther east on Sunday into the Ohio and Tennessee Valley. Much of the guidance is showing some elevated QPF totals, but still with a lot of spread in exactly where. Opted to maintain continuity with a broad marginal risk across this region on the Day 5/Sunday ERO, albeit with some expectation a more welll defined guidance signal closer to the event will likely prompt issuance of a Slight Risk area. Elsewhere, showers will move from the Northern Rockies into the Plains associated with the northern stream system this weekend, and then a strong cold front into the East should bring increased rain chances across the East and South early next week. Expect hazardous heat to expand from South Texas through the Central Gulf Coast and South Florida during the period 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 this weekend 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 next week across the South following passage of a cold front. The forecast pattern will favor below average highs over the Northwest to northern Plains for this weekend but above normal temperatures will build again across the West early next week underneath of amplified upper ridging. Above average temperatures along the eastern seaboard will moderate back to normal next week. Santorelli/Schichtel 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