Extended Forecast Discussion NWS Weather Prediction Center College Park MD 317 PM EDT Sat May 17 2025 Valid 12Z Tue May 20 2025 - 12Z Sat May 24 2025 ...Severe weather and flash flooding forecast for parts of the Mississippi and Ohio Valleys Tuesday... ...Overview... A slow moving upper low over the central U.S. Tuesday shifts east and strengthens over the Great Lakes Wednesday and the Northeast on Friday before lingering over Atlantic Canada next weekend. Meanwhile, subtropical ridging over the Southeast becomes more suppressed from the eastern low/troughing by midweek. Finally, general ridging over the west will lead to warmer than average conditions across California and the Great Basin/Desert Southwest midweek, expanding across the Four Corners and southern High Plains late in the week. ...Guidance/Predictability Assessment... 12Z global guidance has improved agreement with the upper low closing off Tuesday evening over the central Plains, a new and deeper low development over the Great Lakes on Wednesday, a more southerly track of the low over the eastern Great Lakes Thursday and the departure of the low from the Northeast Friday into Saturday. The GFS remains the most progressive solution as it opens the low on Thursday while the EC/CMC keep it closed. One of the more notable shifts with 12Z guidance is QPF on Wednesday focused farther south over the Mid-Atlantic and central Appalachians compared to prior runs. Upstream, consensus on broad ridging over the Southwest midweek that shifts east to the southern Plains by Friday. The 00Z EC was the least with the next trough into the PacNW on Friday, but the 12Z consensus is now for a trough or low to reach Washington on Friday. The WPC forecast was based on a blend of mainly deterministic models for Days 3-5 with ECENS/GEFS ensemble mean inclusion for Days 6-7 as spread increased. ...Weather/Hazards Highlights... Ample moisture and instability over the Miss/OH valleys Tuesday ahead of a deep low over the east-central Plains along with strong dynamical lift aloft will lead to widespread rain and thunderstorms and mesoscale convective systems into Tuesday night. This covers a broad area including most of the TN/OH Valleys and back across the Midwest through southern MN where a broad Day 4 Slight Risk ERO remains with only minor edits to the shape. The severe weather threat area is focused over the southern portions of this precip area in the warm sector over the TN Valley per the Storm Prediction Center Day 4 Convective Outlook. Rain and thunderstorm potential shifts east over the Appalachians Wednesday as the low tracks east. Locally heavy rainfall is possible particularly for the Mid- Atlantic region on Wednesday where a Day 5 Marginal ERO remains. There is better agreement on the Day 5 precip area today, but the overall magnitude seems to be the greatest reason to not upgrade to a Slight Risk at time. Rain chances will make their way into the Northeast Thursday-Friday as the surface low pivots northward. Elsewhere, some weak troughing aloft and frontal systems tracking through the Northwest next week could produce rounds of modest precipitation there. Most precipitation should be rain aside from the highest peaks. Some rain may eject into the northern High Plains by Thursday, with possibly another round of storms emerging in the central U.S. by Friday, but with quite a bit of uncertainty. Heat lingers over south Texas and the Florida Panhandle through Tuesday, but the suppressed ridge should bring some temperature relief by Wednesday. The trough/low aloft will promote below normal temperatures across the northern tier with 10 to 15 degree below normal temperatures working east mid to late week. Upper ridging over southwestern U.S. will raise temperatures to above normal there by Wednesday, expanding east across the Four Corners states by Thursday and into the southern Plains late in the week. Highs will be well into the 100s in the Desert Southwest with temperatures nearing 100 again in parts of Texas eventually. Jackson 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