Extended Forecast Discussion NWS Weather Prediction Center College Park MD 247 PM EDT Fri May 20 2022 Valid 12Z Mon May 23 2022 - 12Z Fri May 27 2022 ...Heavy Rainfall/Runoff and Strong Thunderstorm Threat from the Southern Plains and Lower Mississippi/Tennessee Valleys to the Ohio Valley and Great Lakes early-mid next week... ...Weather Pattern Overview... A lead front is forecast to push across the South and East into Monday to end the protracted heat wave over the South and also usher in a cooler and more comfortable airmass over much of the eastern U.S. after a sweltering weekend. Meanwhile, a rather active weather pattern is expected to develop across the continental U.S. going into next week, as multiple waves of low pressure develop along a resurgent frontal boundary from the Plains to the Great Lakes. This will be in response to a building upper level trough over the High Plains and this main system will slowly move eastward through next week. This offers a widespread threat for heavy rainfall/convection and local runoff issues as moisture and instability focus ahead/over the well organized and wavy system. The experimental WPC Day 4/5 excessive rainfall outlook has a "slight" risk from the southern Plains to the lower Mississippi Valley, with activity spreading up through the Midwest and eventually across the East. Meanwhile, a warming pattern over the West by early-mid next week may be partially supplanted late week as Pacific upper troughing works a front inland with modest precipitation across the Northwest in an otherwise mainly dry week west of the Rockies. ...Guidance Evaluation/Preferences... Forecast spread has decreased significantly with the latest model runs whose mid-larger scale flow evolutions are now much better clustered with the ensemble means days 3-7. While local convective rainfall focus remains much less settled through medium range time scales, guidance offers fairly consistent and uniform regional areas of threat. The WPC medium range product suite was primarily derived from a composite blend of the 06 UTC GFS and 00 UTC ECMWF/UKMET/Canadian and the 13 UTC National Blend of Models (NBM) days 3-5 (Monday-Wednesday) before enough uncertainty is introduced to gravitate more toward the 06 UTC GEFS mean and 00 UTC ECMWF ensemble mean along with the best matched GFS/ECMWF and NBM for better local detail consistent with predictability. This general plan maintains good WPC product continuity and seems in line with latest overall 12 UTC models and ensembles despite local convective focus differences along with some lingering run to run system timing/amplitude differences. ...Sensible Weather/Threat Highlights... Temperatures behind the cold front exiting the East Coast next week should return to near seasonal levels after a potentially record setting early season heatwave. Low pressure forming along the trailing front as it meanders near the Southeast Coast may continue a moderate to heavy rainfall threat into Monday as well for the southern Appalachians/Mid-Atlantic and Southeast. The next system developing over the south-central U.S. into early-mid next week is expected to become the main weather headline of this forecast period. There will likely be widespread convection and heavy rainfall from the southern Plains to the Ohio Valley with moist return flow into another slow moving and wavy frontal system. There is the potential for flash flooding with the slower moving storms within a very moist and unstable atmosphere. The potential exists for additional heavy rain as the system translates eastward into mid-later next week. The western-central U.S. trough should keep temperatures below normal across Rockies and Plains through at least Tuesday. Much of the Western U.S. should remain mainly dry through the middle of next week as upper ridging builds over the region and warmer temperatures expand across the region, with the exception of some showers entering the Northwest into Friday. This may lead to an excessive heat episode over interior California and the Desert Southwest next week, with lead transient system warming across the Intermounmtain West. Schichtel/Hamrick 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices are at: 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