Extended Forecast Discussion NWS Weather Prediction Center College Park MD 320 PM EDT Wed Mar 15 2023 Valid 12Z Sat Mar 18 2023 - 12Z Wed Mar 22 2023 ...Deep storm heavy snow/wind threat for the Great Lakes/Northeast lingers 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... A deep winter storm work from the Great Lakes/Northeast Saturday to the Canadian Maritimes Sunday. The storm offers a continued heavy snow/wind threat from the Great Lakes through southern Ontario/Quebec and the Northeast into Saturday. This occurs as trailing and wavy cold frontal passage sweeping well south of the low focuses showers/thunderstorms over the Southeast into the weekend, especially Florida. Elsewhere, Pacific system energies/precipitation will 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 early-mid next week where there is an increasing signal for coastal low development. A deep Aleutian low is forecast to dislodge this weekend and descend down toward the West Coast of the CONUS early-to-mid next week. This may set the stage for another moist storm with a main impact back yet again into California. ...Guidance/Predictability Assessment... A general model blend consisting of the 00z EC/CMC/UKMET and 06z GFS was used through day 4, mainly due to the models agreeing on the progression of upper troughing moving through the East. Added weighting was given to the EC and GFS since they captured upstream shortwave energy propagating through the Pacific Northwest during this period. The 00z ECE and 06z GEFS ensemble means were introduced to the gmb on day 5 to mitigate deep shortwaves depicted by the deterministic Euro and GFS over the Pac NW and Upper Midwest. GFS/GEFS trends have been slower with the E Pac system forecast to impact the West Coast next week, while the EC/ECE have waffled back and forth with the latest 00z run suggesting a slower track as well. A more ensemble-centric blend is favored by days 6 and 7 to account for a more stratified distribution of guidance regarding southern stream energy propagating through the Gulf Coast/Southeast. The latest 00z EC diverged from the consensus solution to a coastal low evolution along the Southeast coast with the EC trending much weaker and faster than its previous runs and the other guidance. ...Weather/Hazards Highlights... Deep storm/low track out from the Great Lakes through the Northeast Saturday will support lingering wrap-around snows, mainly over northern New England. Blustery winds with the low may produce areas of blowing snow and lake effect activity in it's wake. A broad area of cold high pressure will subsequently settle into the eastern 2/3rds of the country. Temperatures will tend to be cooler than average by 10-25+ degrees, especially for the central U.S. with cold frontal passage as a wintry Canadian airmass robustly digs in for the rest of the week. Gulf moisture return to the Southeast along/ahead of a trailing cold front will fuel showers and thunderstorms through the weekend, especially over Florida. Elsewhere, it still seems evident 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. Expect some moderate rainfall downstream across the Gulf Coast/Southeast with frontal wave developments. Possible Southeast U.S. coastal low genesis Tuesday offers some risk of further development and subsequent lift up the East Coast/vicinity, depending on the model. Models have been less than stellar from run to run, but the signal is growing in support of of a coastal system. The 12 UTC ECMWF offers an inland option while the Canadian coastal low lifts more northward up the coast than the GFS. System details are uncertain, but latest WPC progs show a Carolina coastal low by Wednesday morning in a position closest to the middle of the forecast envelope Canadian that has the most GEFS and especially ECMWF ensemble support. Well upstream, 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 have become 25-45+% 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 before slamming inland into midweek with potentially a heavy rain/snow/wind etc. impact signature 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