Extended Forecast Discussion NWS Weather Prediction Center College Park MD 242 AM EDT Tue Mar 14 2023 Valid 12Z Fri Mar 17 2023 - 12Z Tue Mar 21 2023 ...Deep storm heavy snow/wind threat for the Upper Midwest/Great Lakes to interior Northeast Fri into Sat... ...Cold Canadian high pressure plummets into the nation behind the low, especially into the central U.S.... ...Overview... Southern stream energy entraining into a powerhouse northern stream upper trough/closed low will support development of a deep Great Lakes surface low by Friday. The storm offers a heavy snow/wind threat out from the Upper Midwest through the Great Lakes, southern Ontario/Quebec and interior Northeast Friday into Saturday. This occurs as trailing and wavy cold frontal passage sweeping to the south of the low focuses showers/thunderstorms, especially over the Southeast. Elsewhere, less certain Pacific system energies/precipitation reach the West Coast then Intermountain West/Rockies this weekend enroute to fast progression across the U.S. southern tier to the Southeast U.S. then western Atlantic by early next week where there may be some coastal low potential. ...Guidance/Predictability Assessment... The 18 UTC GFS and 12 UTC ECMWF/UKMET/Canadian solutions are well supported by ensembles Friday/Saturday and a blend along with the 01 UTC National Blend of Models offers a good forecast basis with detail consistent with a pattern with seemingly above average predictability. Forecast spread increases Sunday into early next week, mainly with variance with Pacific systems working into the West Coast and downstream within an emerging/zonal southern stream flow. The 12 UTC ECMWF was most in line with a model composite. A blend of the ECMWF with compatible GEFS/ECMWF ensemble means was used into these longer time frames. WPC product continuity is well maintained and newer 12 UTC guidance is supportive. ...Weather/Hazards Highlights... Deep storm/low track to the Great Lakes Friday and downstream system transitions across the Northeast and vicinity into Saturday will favor continued wrap-around snows as per the WPC Winter Weather Outlook (WWO). The highest chances for impactful snowfall will shift out from the Upper Midwest through the Upper Great Lakes and southern Canada/northern tier of the Northeast Friday into Saturday. Blustery winds with the low may produce areas of blowing snow and lake effect activity in it's wake. A broad area of high pressure will set in bringing an end to precipitation chances for most of the eastern 2/3rds of the country. Temperatures will tend to be cooler than average by 10-25+ degrees, especially into the central U.S. with cold frontal passage as a wintry Canadian airmass robustly digs in. Gulf moisture/instability return to the Southeast along/ahead of a trailing cold front will fuel showers and thunderstorm driven rains Friday. The WPC experimental Day 4 Excessive Rainfall Outlook (ERO) has a "Marginal Risk". Locally heavy rains/rain rates may focus over southern Alabama/Georgia and the Florida Panhandle, limited by a progressive frontal nature. Otherwise, moderate rainfall chances will overspread the East Coast Friday into Saturday. Elsewhere, it remains the case that there looks to be a brief reprieve from the recent string of anomalously strong and wet storms over the West after Thursday and into Saturday before a series of uncertain upper-level waves/systems increases precipitation chances. More moderate lower-elevation coastal/valley rain and higher elevation snow will be possible, particularly along a southeasterly track from the West Coast across the Great Basin and into the Central/Southern Rockies Sunday-Monday. High temperatures look to remain around 5-10 degrees below average in general, with a pocket of colder temperatures 15-20 degrees below average lingering over the Rockies. Progressive system translations across the U.S. southern tier offers increased uncertainties Sunday-next Tuesday. Lead cold airmass intrusion through Texas may lead to the potential for some light upslope snow/ice over the Southern High Plains. The best opportunity for moderate rainfall seems downstream across the Gulf Coast/Southeast with frontal wave developments. Schichtel 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