Extended Forecast Discussion NWS Weather Prediction Center College Park MD 311 PM 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.... ...Increasing confidence in a Winter Storm and Heavy Rainfall to impact California next week... ...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 en route 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. An Aleutian low is forecast to dislodge from the region late this weekend and descend down toward the West Coast of the CONUS early-to-mid next week. ...Guidance/Predictability Assessment... A general model blend consisting of the 00z EC/UK/CMC and 06z GFS were utilized through day 4. The pattern becomes more uncertain over the East on day 5, so the 00z ECE and 06z GEFS were introduced to mitigate some of the inconsistencies found in the deterministic guidance. An even split between the EC/GFS and ensemble mean suite was used on day 6 to capture some of the potential shortwave energy riding along the northern periphery of a mean upper trough propagating through the mid-section of the country. There's reasonable agreement between the different guidance elsewhere. The same models are used on day 7 but weighting is shifted to the ensemble means due to greater differences regarding the East Pac system approaching the West Coast. Timing differences are most notable between the 00z EC and 06z GFS but run to run inconsistencies are also glaring as well as potential for further deepening of the upper-low as noted in GEFS/CMCE/ECE ensemble spread. ...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. A deep Aleutian low may descend into the East Pac, tap an anomalous plume of subtropical moisture and generate another Atmospheric river event across California beginning next Tuesday night. The CPC outlook highlights the threat for this on Wednesday. 24hr 1" precipitation exceedance probabilities between 20-40% for much of the California coast and inland areas with some ensemble members suggesting potential for additional deepening of the upper trough off the West Coast next Tuesday are enough to support increased messaging for this threat. Kebede/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