Extended Forecast Discussion NWS Weather Prediction Center College Park MD 301 PM EDT Sat May 21 2022 Valid 12Z Tue May 24 2022 - 12Z Sat May 28 2022 ...Multi-Day Heavy Rainfall and Strong Thunderstorm Threat from the Southern Plains and Lower Mississippi/Tennessee Valleys to the Ohio Valley and Great Lakes early-mid next week to work across the East later week... ...Weather Pattern Overview... A stormy weather pattern is expected to develop across the continental U.S. going into the middle of next week, as multiple waves of low pressure develop along a frontal boundary situated over the Plains and extending to the Great Lakes. This will be in response to a building upper level trough over the High Plains that will slowly move eastward through the week with the front reaching the East Coast by Friday. A second system is slated to dig through the Northwest to the north-central Intermountain West/Rockies and northern Plains Thursday-Saturday. ...Guidance Evaluation/Preferences... Models and ensembles continue to offer below normal forecast spread/uncertainty days 3-7, bolstering forecast confidence to above normal values. While local convective rainfall focus remains less settled, guidance offers consistent 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, WPC continuity and the 13 UTC National Blend of Models (NBM) days 3-5 (Tuesday-Thursday) before enough uncertainty with flow progression and convective interactions compounds to favor blend introduction of the 06 UTC GEFS mean and 00 UTC ECMWF ensemble mean to smooth the rough edges consistent with predictability. This composite solution now also seems overall in line with latest 12 UTC models and ensembles. ...Sensible Weather/Threat Highlights... 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 Texas to the Great Lakes with moist return flow into another slow moving and wavy frontal system. There is an increasing potential for some flash flooding with the slower moving storms within a very moist and unstable atmosphere. The latest WPC experimental Excessive Rainfall Outlook indicates a Slight Risk area for Tuesday into Thursday from the ArkLaTex region into the lower Mississippi/Ohio valleys. and the potential exists for 3 to 6+ inch rainfall totals over that 48 hour time period. Some severe storms are certainly within the realm of possibility for these same general areas where parameters of shear and instability are greatest, although model differences remain on the placement of MCS activity. Widespread convection will also spread across the eastern states later week with less certainty/details but local threat potential. The west-central U.S. trough should initially keep temperatures below normal across Rockies and extending across the central Plains through about Wednesday before much warmer weather returns by the end of the week. Some late season snow showers will also be possible across the higher terrain of central Colorado on Tuesday. Much of the southwestern U.S. should remain dry through the end of next week as upper ridging builds over the region and warmer temperatures expand across the region. Building heat across interior portions of California and the Desert Southwest could result in highs running 10-20 degrees above normal for some areas, with widespread 100+ degree high temperatures for the lower elevations. There will likely be some showers across the Pacific Northwest by Friday and then reaching the northern Rockies/Plains with some thunderstorms by Saturday as the next front enters this region. 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