Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Thu Feb 23 2023 Valid 12Z Sun Feb 26 2023 - 12Z Thu Mar 02 2023 ...Significant storm system to track from the Southwest into the Northeast from late this week into early next week... ...Record cold possible over/near California while record warmth is possible in the Southeast... ...Overview... An amplified mean trough aloft will persist near the West Coast during the period. Within this feature, a leading upper low will initially eject from the Southwest and produce a strong storm system to the east of the Rockies early next week with rain/thunderstorms and possibly severe weather from the Plains into Mississippi Valley along with snow/ice from far northern areas into the Northeast. A trailing eastern Pacific upper trough dropping in early next week and then pushing gradually inland mid-late week will spread another area of rain and higher elevation snow across portions of the West. The West Coast mean trough will keep temperatures chilly over the western U.S. and an upper ridge over Cuba or northwestern Caribbean will promote above normal temperatures over many areas across the southern two-thirds of the central/eastern U.S. ...Guidance/Predictability Assessment... Guidance is steadily trending closer together for the handling of the upper low ejecting from the Southwest and associated storm system to the east of the Rockies. Consensus maintains a sharp and neutrally/negatively tilted upper trough reaching the Mississippi Valley by Monday with associated low pressure as of 12Z Monday in the mid 970s-low 980s mb range per the latest and recent operational model runs. Such a depth could challenge some February sea level pressure records at select locations depending on exact track. This low should weaken thereafter as the upper system shears out. Latest guidance has reinforced recent trends toward the ECMWF/ECMWF mean that were earliest to suggest that secondary low pressure off the East Coast would track along the southern side of the prior envelope, offshore the northern Mid-Atlantic. For the large scale upper trough reaching the West Coast around Tuesday and then pushing inland, GFS/GEFS runs are still tending to be a bit on the eastern side of the spread at some valid times but by late in the period the 06Z GFS actually held the trough back a bit relative to the GEFS mean. Thus by day 7 Thursday the GFS runs and 00Z ECMWF/CMC plus their ensemble means compared quite well. There is greater uncertainty with what becomes of leading energy that may eject from the trough and affect the surface pattern to the east of the Rockies mid-late week. By day 7 the CMC runs are straying to the fast side with its surface wave while the GFS is somewhat north of the means with its Great Lakes low pressure. A blended/ensemble mean approach looks best for resolving this aspect of the forecast. Model consensus supported a composite of operational guidance for about the first half of the period. Then the forecast transitioned to a blend of the 06Z GFS/00Z ECMWF-CMC (phasing out the CMC by day 7) and 00Z ECens/CMCens means. ...Weather/Hazard Highlights... The upper low initially ejecting out of the Southwest will produce some light to moderate rain/mountain snow over the Four Corners states on Sunday. Then this feature will produce a strong storm system over the Plains/Midwest into Great Lakes with a broad area of precipitation. The most favorable conditions for heavy rainfall should exist over the Central Plains and the Midwest, where copious amounts of moisture (up to plus 3-3.5 standard deviations in the ensemble means and 4+ sdevs in the operational models) and some instability may combine to produce potentially heavy short-term rainfall rates. WPC maintains a Marginal Risk area over this region in the Day 4 experimental Excessive Rainfall Outlook (12Z Sunday-12Z Monday). Most of the precipitation will likely fall in liquid form as the airmass will not be as chilly as the previous system. The Storm Prediction Center continues to monitor the potential for severe weather over portions of the Plains. There will be at least a potential transition among precipitation types in the Upper Midwest/Great Lakes, with possible issues for locations receiving snow from the previous system and rain from this one. Meanwhile probabilities for significant snow are somewhat higher over New England, with the most favored axis depending on the details of coastal/offshore low development around next Tuesday. Current model trends support an offshore low and with a track in the southern part of earlier guidance spread. This would hold colder air/snow over New England with enough suppression to keep most snow to the south of far northern New England. This system may produce some very strong and gusty winds through its existence, and in particular over the southern High Plains and from parts of the Midwest/Ohio-Tennessee Valley into Lower Great Lakes. Light to moderate mountain snow will spread across the Northwest this weekend as a weakening front approaches. However, a stronger system approaching the West Coast late weekend into next week will likely bring heavier precipitation to the West Coast region, with highest totals expected along the central West Coast and the Sierra Nevada. The Day 5 Excessive Rainfall Outlook (12Z Monday-12Z Tuesday) plans to introduce a Marginal Risk area along the central West Coast, mainly due to the combination of the approaching upper trough/front providing enhanced focus for locally heavy rain early in the outlook period. A broader area of precipitation should spread farther south and inland across the West. By next Thursday the upper trough may reach just far enough east to start drawing up Gulf moisture into the eastern half of the country, with rainfall beginning to expand in coverage and intensity at that time. Specifics will be resolved in coming days. Cold high pressure reaching New England behind a late-week front may lead to high temperatures of 10-20F below normal over the region into Sunday but should trend back toward near to only moderately below normal levels. Meanwhile the strong upper high over Cuba/northwestern Caribbean will maintain warmer temperatures over the South with some record highs possibly lingering into next week. A broad area of the southern two-thirds of the central/eastern U.S. should see one or more days of temperatures 10-20F above normal with some days featuring even warmer morning lows. The West will see a persistent cool pattern from the weekend through Thursday with central/southern areas most consistently seeing highs up to 10-20F below normal. Some daily records for cold temperatures are possible over/near parts of California. Rausch/Campbell 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