Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Tue Mar 08 2022 Valid 12Z Fri Mar 11 2022 - 12Z Tue Mar 15 2022 ...Winter storm threats for parts of the Mid-South to the interior Northeast Friday-Saturday... ...Heavy rainfall likely across portions of the Southeast on Friday... ...Overview... A potent shortwave in the southern Rockies and a northern stream shortwave through the Midwest early Friday will combine to support strong cyclogenesis into the Eastern U.S. by Saturday. This classic spring system will feature snow and cold to the north/northwest of the surface low, and warm/wet conditions ahead of the cold front. Upper flow should then become quasi-zonal Sunday-Monday with a weak shortwave into the Northwest, though may amplify with time as it reaches the East on Tuesday. The next shortwave should reach the Pacific Northwest by next Tuesday as well. ...Guidance Evaluation/Predictability Assessment... The guidance seems to be trending towards better agreement with the central/eastern system Friday-Saturday, although some timing differences continue with the cold front into the East. The GFS has consistently been on the faster side of the envelope, although has trended slower with each of its past few runs and is now in reasonable agreement. The 12z/Mar 7 UKMET is also a hair faster and also quicker to occlude the system over the Northeast. Incoming 00z guidance tonight overall had the same general feeling, although the UKMET was slower and more in line. Given the slower trend seen in all the guidance the past few days, the WPC progs favored something closer to the slower EC/CMC for days 3-4, however, overall the pattern remains rather progressive despite rapid deepening of the surface low, and further refinements slower or faster are probable. At this point, these are details that will take until the short range period to fully resolve. So it's hard to throw any solution off the table at this point. After this system, deterministic models diverge more within a quasi-zonal pattern. The 18z/March 7 GFS is quicker with the first shortwave into the Pacific Northwest Sunday, but the ECMWF catches up to it by Monday across the central Plains and both are rather progressive downstream. The 12z/Mar 7 CMC is the real outlier here, with a more amplified shortwave and overall slower progression which eventually ends up with a closed low over the lower Mississippi Valley next Tuesday. The second shortwave into the Pacific Northwest early Tuesday shows pretty good agreement right now timing wise. After day 5, the WPC blend leans increasingly more towards the ensemble means (with some modest contributions from the GFS/ECMWF) to help mitigate the differences. ...Weather Highlights/Threats... Showers and thunderstorms should still be ongoing Friday across the Southeast as a frontal boundary remains parked over the region with ample moisture and instability present. This boundary should finally become absorbed/pushed east/lift northward with the strong cold front on Saturday though. Rainfall will come in waves over the Southeast, some of which could be heavy and flooding/flash flooding may be a threat especially where multiple rounds of convection train over the same areas and combined with heavy rainfall the previous day in the same region. Given this potential, the WPC day 4 (Friday) newly experimental excessive rainfall outlook highlights a slight risk area from the Florida Big Bend to southeast Georgia. Cold air pouring into the Central U.S. behind the developing storm system into Friday will support rain changing to snow behind the front Friday evening, with accumulations possible as far south as the lower Mississippi Valley. The rapidly deepening surface low into the Northeast should allow for possibly heavy snow across the lower Great Lakes/interior Northeast. Widespread, though quick moving, rain is likely ahead of the front in the East. The arctic airmass behind the front will bring daytime highs 20-30+ degrees below normal as far south as Texas on Friday. Overnight lows across the northern Plains and in higher elevations of the Rockies are likely to be near or below zero for many locations. Modifying some with time, the cold air will shift into the Midwest on Saturday and the Eastern U.S. on Sunday following the frontal passage (with above normal temps ahead of it). Even for parts of northern Florida and the Southeast, low temperatures Sunday morning could be near or below freezing with record lows possible in some locations. By next week, parts of the West and central U.S. may see moderately above normal temperatures as periodic upper ridging slides through. Santorelli 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