Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Sat May 21 2022 Valid 12Z Tue May 24 2022 - 12Z Sat May 28 2022 ...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... ...Weather Pattern Overview... A rather active 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 western High Plains that will slowly move eastward through the week with the front reaching the East Coast by Friday. Meanwhile, a second storm system is progged to reach the Pacific Northwest by late Thursday and then reach the Rockies and the northern Plains by early Saturday, although not currently expected to be as impactful as the earlier event across the Plains. ...Guidance Evaluation/Preferences... The 00Z model guidance suite is in decent synoptic scale agreement for the beginning to middle of next week, and there has been more consolidation and less model spread compared to this time yesterday. However, there is still uncertainty with regards to placement of heaviest QPF maxima as these will be the result of multiple MCS clusters. There are more model differences apparent with the trough entering the Pacific Northwest by Thursday, with the 00Z GFS indicating a faster solution and becoming more amplified by Friday and Saturday with a closed upper low developing. The WPC fronts/pressures forecast was primarily derived from a multi-deterministic model consensus through Wednesday, and then incorporated more of the ensemble means for the Thursday-Saturday time period while still maintaining some of the 18Z GFS and 12Z ECMWF. ...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 central/eastern 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 both Tuesday and Wednesday across the ArkLaTex region and extending across Arkansas and into western Tennessee, 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. 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 by Saturday as the next front enters this region. 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