Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Mon May 20 2024 Valid 12Z Thu May 23 2024 - 12Z Mon May 27 2024 ...Heavy Rainfall/Flooding Threats from the Southern Plains through the Mid-South and vicinity midweek through the weekend... ...South Texas to Central Gulf Coast and South Florida Heat Risk... ...Overview... Multiple and generally progressive upper shortwaves/possible closed lows and surface low pressure/frontal systems will traverse the West to the Plains and Midwest/South during the medium range period. By Thursday, a potent shortwave and deep surface low will be exiting the Upper Great Lakes while its attendant cold front will push across the East and partway into the South beyond midweek, with rain and storms along and ahead of it and cooler temperatures behind. There is some recurring threat for heavy rainfall along the western side of this front from parts of Texas into the Mid- South/Tennessee Valley to parts of the Mid- Atlantic as the wavy boundary stalls and lifts back north as a warm front. Additional energy/closed lows look to traverse the northern and southern tiers of the nation late week and next weekend, but with more uncertainty as strong upper ridging builds into South Texas and the central Gulf Coast to spread a continued threat for hazardous heat as indicated in the WPC and CPC Hazards Outlooks. ...Guidance/Predictability Assessment... Multi-model and ensemble forecast spread and cycle to cycle run continuity remains decent Thursday to Saturday in a pattern with near average overall uncertainty at mid-larger scales. Smaller scale system and weather focus is not ideal, but a composite of the latest GFS/ECMWF/UKMET/Canadian models along with the National Blend of Models (NBM) and WPC continuity seems to provide a reasonable forecast basis as consistent with individual system predictability. Forecast spread growth and run to run continuity suffers at longer time frames, so prefer a composite of best clustered guidance of the GFS/ECMWF and GEFS/ECMWF ensemble means. ...Weather/Hazards Highlights... Main focus for late week convection will be from parts of the southern Plains into the Lower Ohio Valley as the western part of a frontal boundary stalls/lifts north ahead of the next upper low into the region. The Day 4/Thursday Excessive Rainfall Outlook continues to depict a fairly large slight risk area from northeast Texas through Arkansas and into western Tennessee where ample moisture/instability should be present within a very recently wet region. Heavy rainfall potential across this region is also possible at the end of the short range period/Wednesday. By Friday, some heavy rain/flood potential will be possible farther north and east across the Tennessee Valley to the southern Appalachians where a Marginal risk is in place for the Day 5 ERO. The Upper Low over the Northwest/northern Rockies on Thursday may also bring some heavier rain to parts of the northern High Plains, but with an overall lack of instability and anomalous moisture, combined with dryer antecedent conditions, not anticipating a flash flood risk at this time, despite some 1-2 inch QPF totals for central-eastern Montana. Showers and storms should progress with the accompanying cold front late week into the weekend through the Midwest. An additional heavy rainfall threat may develop farther south across the Middle Mississippi Valley/Mid-South region late this weekend or early next week depending on still very uncertain upper level dynamics and stream energies. Expect a hazardous heat threat to expand from South Texas through the Central Gulf Coast and also South Florida see multiple days of hazardous heat during the period with highs persistently running 10-15F above normal with max heat index values possibly reaching at least 110F. Highs near 100 degrees could stretch farther north across the southern High Plains later in the period as an upper high begins to build more into the region. Some daily records for highs/warm lows will be possible. Above normal highs will also track east into the Northeast/Mid-Atlantic on Thursday, but should moderate thereafter as the cold front pushes through the region. The forecast pattern will favor below average highs over the Northwest to northern Plains for most of next week but relatively normal temperatures everywhere else. 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