Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sat Mar 19 2022 Valid 12Z Tue Mar 22 2022 - 12Z Sat Mar 26 2022 ...Strong low pressure system will likely produce heavy rain/severe storms across the southeastern quadrant of the U.S. into Tuesday-Wednesday with some wintry weather on the northern side... ...Overview... The dominant focus of the medium range forecast will be an upper low expected to track from the southern/central Plains Tuesday into the Midwest Wednesday-Thursday, opening up as northern stream energy phases with it late in the week. This feature will support reasonably strong low pressure that should track from the Southern Plains Tuesday northeastward across the Mid-Mississippi Valley and Great Lakes. Heavy rain that could cause flash flooding is expected for the Lower Mississippi and Ohio/Tennessee Valleys with some chance farther east as well, while severe storms are forecast for the Gulf Coast states. Possibly heavy snow over the central Rockies should be ending by early Tuesday, but some precipitation in the northern fringe of the moisture shield could be in the form of snow in the northern half of the Plains, Midwest/Great Lakes, and into the interior Northeast particularly in higher elevations. Behind this system a strong upper ridge will build into the West, bringing mostly dry and warm weather to the region, and the anomalous warmth over the West could bring record highs to parts of California around Tuesday-Thursday. The pattern should deamplify somewhat by late next week. ...Guidance Evaluation/Predictability Assessment... Model guidance from the 12/18Z yesterday cycle remained consistent and agreeable with the period starting Tuesday with a central U.S. (and Canada) trough and embedded closed low, which should track northeastward through midweek. Smaller scale shortwaves within the trough/low are more variable, and these do cause some minor differences in the specifics of the main surface low's position and depth. Meanwhile, models also agree that the upper ridge building into the West should be strongest Tuesday-Wednesday. Overall, a multi-model consensus of deterministic guidance led by the 12Z ECMWF and 18Z GFS worked well for the WPC forecast with the blend serving to smooth out the smaller differences, and this maintained good continuity from the previous forecast. By around day 5/Thursday, more notable model differences arise both with the main trough/low feature and upstream with Pacific shortwave energy that could serve to dampen the northern part of the western North America ridge, which are interconnected. For the 12Z/18Z cycle, the ECMWF/CMC maintained a closed low for the main feature through Thursday with a broader more rounded trough, while especially the 12Z and somewhat the 18Z GFS are more open/phased with the northern stream. There have also been run to run and model differences with how much energy extends back southwest into the south-central U.S. and northern Mexico around Thu-Fri, with better consensus for some compared to about a day ago, but GFS runs (especially the 12Z) have shown more extension southwest than the overall consensus. The WPC forecast maintained a similar compromise approach to the previous forecast, adding the influence of the GEFS and EC ensemble means to temper the operational model differences. The newer 00Z guidance continues to show some of these differences persisting, with especially the CMC presenting a different solution with the upper trough/low, much more rounded and closed through Saturday compared to any other guidance from multiple cycles. ...Weather Highlights/Threats... Ample Gulf of Mexico moisture flowing into the surface/upper-level lows tracking northeastward over the central U.S. will lead to widespread rain and thunderstorms for the Mississippi Valley to the East Coast particularly on Tuesday and Wednesday ahead of the cold fronts. Storms could be severe across the Deep South Tue/Wed per the Storm Prediction Center. Furthermore, heavy rain is likely to cause areas of flash flooding particularly due to high rain rates, with a Slight Risk indicated Tuesday for portions of the Lower Mississippi, Tennessee, and Ohio Valleys in the experimental WPC medium range Excessive Rainfall Outlook. Some isolated flash flooding is also possible Wednesday for the southern/central Appalachians and Southeast as the front progresses eastward and spreads rainfall to the Eastern Seaboard. Additionally, some precipitation appears likely from the northern parts of the Plains through the Northeast, which could be in the form of snow. Currently the highest probabilities for notable snow are in the Upper Great Lakes region. In the West, mainly dry weather is expected, with possibly a little light/scattered precipitation activity that could increase slightly in coverage by the end of next week. The strong upper ridge building into the West will lead to a broad area of highs 10-20F above normal Tuesday-Saturday with slightly higher localized anomalies possible. The best potential to reach daily record highs will be over California during Tuesday-Thursday. Leading upper level troughing should support temperatures 10-20F below average across the central/southern High Plains Tuesday and Wednesday. The cooler air should moderate closer to normal by the latter part of next week as it progresses eastward. Ahead of the trough and cold fronts, generally warmer than normal conditions are likely across much of the eastern third of the U.S. for much of the week, transitioning to near normal temperatures by Friday and Saturday. Isolated record highs are possible over parts of Florida Tuesday-Wednesday. Tate 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 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml