Preliminary Forecasts
Issued by NWS
Issued by NWS
495 FXUS02 KWNH 210701 PREEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Tue May 21 2024 Valid 12Z Fri May 24 2024 - 12Z Tue May 28 2024 ...Heavy Rainfall/Flooding Threats through the Mid-South and vicinity through the weekend... ...South Texas to Central Gulf Coast and South Florida Heat Risk... ...Overview... Multiple and generally progressive upper shortwaves/closed lows and surface low pressure/frontal systems will traverse the West to the Plains and Midwest during the medium range period. The first will be moving through the Northern Plains by Friday, eventually lifting into south-central Canada next weekend. Meanwhile, the next upper low will drop into the Northwest next weekend, moving in an eastward direction into early next week. A building ridge behind this over the Western U.S. early next week will allow this system to dig more into the Midwest than previous systems. Multiple surface lows/fronts will track with these systems across the CONUS with some recurring threat for heavy rainfall mainly across the mid-South/Tennessee Valley to parts of the Mid-Atlantic as fronts tend to stall in that region. Strong upper ridging will also build 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... The latest models and ensembles continue to show very good agreement on the overall pattern across the CONUS during the medium range period, but with plenty of uncertainty in the details. There was good enough agreement the first half of the period to use a blend of the deterministic models as a starting point for WPCs forecast. After the weekend, there is some growing spread in the exact track and speed of the second upper low through the Northern Plains/Midwest although the ensemble means are quite a bit weaker with the system. Given there seems to be good agreement for a deeper surface low (including in the various ML/AI models), maintained more deterministic models in the blend than usual late period, ending up with a 60/40 split deterministic/ensemble means. Also some uncertainty early next week on various shortwaves rotating through a deep upper low parked off British Columbia which may skirt the Pacific Northwest, but very good consensus for an amplifying upper ridge over the interior West and a strong upper high reaching into south Texas and the central Gulf Coast. ...Weather/Hazards Highlights... A slow moving/stalled front across the Mid-South/Tennessee Valley into the southern Mid-Atlantic will be the main focus for convection late week. A marginal risk is in place across this region for the Day 4/Friday Excessive Rainfall Outlook. A southern stream shortwave into the Southern Plains next weekend should ignite more heavy rainfall potential into the Mid-Mississippi Valley where a marginal risk is highlighted for the Day 5 ERO. This looks to continue and possibly get more widespread for Sunday into the Tennessee Valley. 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 a hazardous heat threat to expand from South Texas through the Central Gulf Coast and also South Florida 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. 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. 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 $$