Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Mon Feb 14 2022 Valid 12Z Thu Feb 17 2022 - 12Z Mon Feb 21 2022 ...Mid-late week system to bring potential for heavy rain and severe weather from the south-central to eastern U.S. and wintry weather from the Midwest through Great Lakes and interior Northeast... ...Overview... Northern and southern stream upper-level troughs will be in the process of phasing in the central U.S. as the medium range period begins Thursday, which will shift eastward through the end of the week. At the surface, low pressure is forecast to strengthen as it tracks quickly northeast and the associated strong cold front will push ahead of the trough axis, drawing moisture northward from the Gulf of Mexico and producing notable precipitation across the eastern third of the U.S. on Thursday. Expect rain and thunderstorms in southern areas, including the possibility of flash flooding and severe weather, while snow and wintry weather are likely on the northern and western side of the precipitation shield across the Middle Mississippi Valley and Midwest, Great Lakes, and possibly the interior Northeast. There is still uncertainty in the precise axis of meaningful snow though. By Friday-Saturday expect a clipper system tracking near the Canadian border to bring a cold front into the northern tier and some snow into the Great Lakes, while a weakening ridge making its way into the West Coast will be overcome by additional troughing entering the West early next week. ...Guidance Evaluation/Predictability Assessment... The overall pattern shows fairly good agreement to start the period Thursday-Friday. Model guidance seems to have come to a "happy medium" midpoint solution with the timing and track of the trough and surface low/front and thus the precipitation shield--faster and initially weaker than what was forecast a few days ago but trending slightly stronger and slower over the past day or so. Good continuity remains that the surface low will intensify quickly as it tracks northeast Thursday into Friday. After a few days of the CMC being out of tolerance with other solutions, the 12Z run seemed more in line especially later in the period as well as with its upper pattern. The early part of the medium range WPC forecast was based on a composite of the 00Z/06Z GFS and 00Z ECMWF and UKMET runs, with only modest nudges from continuity. Upstream of the initial trough, considerable vorticity/energy drops into central Canada and shifts toward the Great Lakes for the end of the week, and recent models show better than average agreement with the associated surface low/clipper track and strength through early Saturday, until some differences arise with the exact amplitude of the resulting upper trough and thus variability with the trailing front into the East. A more uncertain feature is a compact shortwave rounding the ridge in the eastern Pacific that could enter the Pacific Northwest around Friday night and move eastward, but with differences in the timing and placement, which may not be resolved for a while given the small scale of this feature. The larger scale trough making its way toward the West early next week demonstrates notable differences as energy spills in from the low predictability high latitudes. GFS runs have been on the stronger and slower side with the trough, but the 00z and 12z runs are somewhat slow compared to consensus as well. The ECMWF runs have been fairly stable in the middle so far, while the CMC has been slightly faster. Thus for the latter part of the forecast period, the WPC forecast favored the ECMWF in terms of the deterministic models, and gradually phased in some of the more agreeable GEFS, CMCE and EC ensemble means to about half by the end of the period to temper individual model differences. ...Weather Highlights/Threats... On Thursday, showers and thunderstorms will be widespread from the Lower Mississippi Valley north and eastward across the Tennessee/Ohio Valleys, Appalachians, and Eastern Seaboard as a surface low/frontal system advects substantial moisture from the Gulf of Mexico northward. Strong storms are possible, with the Storm Prediction Center outlook showing some severe threat for the Mid-South. There could also be flash flooding concerns where heavy rainfall occurs in similar regions and spreading toward much of the Eastern Seaboard on Thursday and Thursday night. Current forecasts show the Southern Appalachians could see particularly high rainfall totals given terrain enhancement there. Continue to monitor forecasts as uncertain details of these potential threats become better refined. At the same time, expect a band of snow in the colder air on the northwestern periphery of the system. The best potential for meaningful accumulations on Thursday into Thursday night currently extends from the Middle Mississippi Valley into the Midwest/Great Lakes region and interior New England, while mixed precipitation/ice cannot be ruled out in the transition zone between the snow and rain. Totals/location of the snow axis will be very sensitive to precise strength and track of the system given that wintry weather could be within a fairly narrow band. Gusty winds are possible with the system as well, and at this point appear most likely near the Northeast coast on Thursday night/Friday morning in strong southerly flow ahead of the quickly strengthening surface low. Dry conditions should prevail over much of the lower 48 behind this system, with a few exceptions. The clipper system along the Canadian border may bring some snow into the Great Lakes region by Friday and Saturday. Meanwhile, some rain showers could persist in Florida with lingering moisture there, but rainfall amounts remain uncertain. Light precipitation could also make its way into the Northwest by Friday, with amounts increasing to locally moderate and spreading south and eastward across the West over the weekend as the upper trough comes in. Ahead of the cold front pushing across the eastern U.S. late in the workweek, warmer than average temperatures by 10-25F will be widespread across the eastern third of the U.S. on Thursday, and daily records could be set for some locations, especially for warm lows, while a few record high maxima could be challenged as well. On the backside of the front, temperatures should flip to near or below average, with some 10-20F below average readings possible over parts of the central/east-central U.S. on Thursday and then moderating closer to the East Coast Friday and Saturday. Coldest anomalies should be over/near Minnesota around Thursday with some places 20-30F below normal. An increasing portion of the West and eventually the northern/central Plains should warm up to above average once again by the end of the week into the weekend. Parts of the Great Basin and vicinity could see highs at least 10F above normal, while central parts of the Plains could even reach 20F above average with temperatures in the 60s by Sunday. Kebede/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 WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml