Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Tue May 21 2024 Valid 12Z Fri May 24 2024 - 12Z Tue May 28 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 Risk... ...Overview... Multiple upper level lows/shortwaves will parade across the continental United Stated during the medium range period. The first closed low will swing across the Upper Midwest on Friday and deepen north of the Great Lakes over the weekend. The next low will drop into the Pacific Northwest over the weekend and dig east towards the Midwest early next week while a ridge builds over the West. At the surface, multiple frontal systems will accompany the upper level lows, creating an unsettled pattern for much of the central and eastern United States. A couple of additional southern stream shortwaves will move from the southern/central Plains to the Mid- Atlantic ahead of the upper lows, providing support for multiple rounds of heavy rainfall/flooding for portions of the Mid- Mississippi, Tennessee, and Ohio Valleys. A strong upper ridge will remain in place through the period across Mexico, the Gulf, South Texas, and Florida, which will result in dangerously high heat index values each day and anomalously warm nighttime lows. ...Guidance/Predictability Assessment... Models remain in good agreement on the broad upper level pattern, but there are some differences in how the models handle interactions between upper lows/shortwaves, especially during the second half of the period. The 00Z guidance suite was showing a trend towards a faster moving southern stream shortwave ahead of the second closed low, which would limit the interaction between the two features and may result in a weaker upper level low. A general blend of the deterministic GFS/ECMWF/CMC/UKMET seemed fine for the first half of the forecast period, but ensemble means from the GEFS and ECENS were added to the blend for the second half of the period to mitigate some of the model differences. Even with the ensemble means added to the blend, deterministic models still made up 50-70% of the blend in order to retain some of the smaller scale details. ...Weather/Hazards Highlights... A slow moving front across the Middle/Lower Mississippi and Tennessee Valleys and southern Mid-Atlantic will become a focus for convection late this week. A Marginal Risk of Excessive Rainfall (level 1/4) is in place across this region for the Day 4/Friday Excessive Rainfall Outlook. A separate low pressure system will also be moving towards the Upper Midwest on Friday, and there will likely be enough moisture and instability to support isolated areas of heavy rainfall in this region. For this reason, a second Marginal Risk area was introduces for portions of the Upper Midwest on Friday. A southern stream shortwave will likely ignite more heavy rainfall potential over the weekend in the Mid-Mississippi and Tennessee Valleys, and a Marginal Risk is in place for this region in the Day 5/Saturday ERO. Heavy rain looks to continue and possibly become more widespread and expand further in the Tennessee and Ohio Valleys on Sunday. Northern stream energy will keep conditions showery and generally unsettled across the northern tier into the Great Lakes during the period. A more amplified pattern early next week should support a strong cold front into the East with showers and thunderstorms ahead of it across the East and into parts of the South as well. Expect hazardous heat to expand from South Texas through the Central Gulf Coast and South Florida during the period with highs persistently running 10-15 degrees above normal with max heat index values possibly reaching 115 degrees. 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. 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. Above normal temperatures will build again across the West early next week underneath of amplified upper ridging. 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