Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sun Nov 27 2022 Valid 12Z Wed Nov 30 2022 - 12Z Sun Dec 04 2022 ...Wet/snowy conditions expanding across the West with below normal temperatures... ...Heavy Rain possibly reaching into parts of the East by Wednesday... ...Overview... Most guidance continues to show a mean trough aloft becoming established near the West Coast and a ridge initially centered over western Cuba settling over the Gulf of Mexico, with west-southwesterly flow tending to prevail between these two features. This pattern should lead to cool daytime temperatures to the west of the Rockies along with increasing coverage of rain and snow over the West, with some of this activity potentially heavy. Day-to-day details of individual features within the West Coast upper trough still vary among the guidance so it will take time to resolve forecast specifics. Farther east, a deepening storm system tracking northeastward away from Lake Superior on Wednesday will sweep a strong cold front through the East, accompanied by locally heavy rainfall as well as a broad area of brisk to strong winds. Energy ejecting from the West Coast trough may bring a wavy frontal system into the central and eastern U.S. late this week through the weekend. Some precipitation may develop along this front by the weekend. ...Guidance and Predictability Assessment... Clustering has been steadily improving for the depth and track of the storm expected to be close to the northeastern shore of Lake Superior by the start of the period early Wednesday. Most guidance has been converging toward the average of the recent spread but the 12Z ECMWF was somewhat on the deeper side. Latest GFS runs have been a bit on the slower side with progression of the trailing front across the East, while latest runs have backed away somewhat from the idea of the pronounced frontal wave that had previously been suggested near/north of Maine by early Thursday. Into early Thursday there is decent agreement, and for at least the past 12 hours or so continuity as well, regarding the leading system forecast to track southeastward toward the Pacific Northwest Wednesday-Thursday. Then after early Thursday solutions begin to diverge for upstream shortwave energy (latest UKMET runs tending to be the fastest and most westward-elongated). This developing issue initially complicates the forecast near the West Coast, influencing exactly how the leading upper trough may incorporate a separate southern stream feature initially farther out in the Pacific. This evolution could bring a fairly compact but vigorous wave to the vicinity of northern California late in the week. Current guidance shows a fairly narrow corridor for the potential track of this wave but a lot of spread for timing and depth. Aforementioned issues with the upstream energy ultimately lead to a widening variety of possibilities for how the leading energy may eject eastward and reflect at the surface to the east of the Rockies, and of course for how the West Coast trough will look during the weekend. The new 00Z CMC seems overdone with the amount of energy cutting through the east-central Pacific ridge by next Sunday, leading to flatter flow along the West Coast, but otherwise it is hard to discount any individual solutions since most fit within the overall mean pattern even with different shortwave details. At least there is still decent continuity for the wavy front reaching the north-central Plains/Upper Midwest around Friday, but specifics for this front diverge thereafter. Based on the 12Z/18Z array of guidance, the updated forecast started with an operational model composite early and then steadily increased 18Z GEFS/12Z ECMWF mean input to half total weight by late in the period. This provided fairly good continuity into late week while detail changes in most guidance led to some adjustments by Saturday near the West Coast. ...Weather/Hazard Highlights... A system nearing the Pacific Northwest should continue spreading moisture into the region and the far northern Rockies during Wednesday with some locally moderate to heavy rain and higher elevation snow possible. Gradual amplification of upper troughing should expand the precipitation shield southeastward through the rest of the week, possibly reaching into central or southern California and the central Rockies. For a period of time late in the week, the front associated with the initial system could draw in some lower latitude moisture/waviness, which would enhance rain/snow totals over some areas--most likely in the vicinity of northern California. Confidence in the specifics of this potential evolution is still not great though. Another system dropping southeastward toward the West Coast may bring another round of precipitation around the end of the week or the weekend. Snow levels in the Northwest could decline to fairly low elevations over some areas. Strong low pressure tracking northeastward over Canada will push a cold front through the eastern U.S. during Wednesday-Wednesday night. Areas of moderate to heavy rain may accompany this front given the vigorous dynamics aloft and strong southerly flow of moisture ahead of the front. This storm system and its front may produce strong winds over some areas of the northeast quadrant of the lower 48. Expect a brief period of lake effect snow after system passage. A wavy front reaching the north-central Plains/Upper Midwest by Friday should not have too much precipitation with it as it continues across the northern-central U.S. into the first part of the weekend. More organized rainfall could then develop near the front as it stalls over the east-central U.S., depending on how much moisture the west-southwesterly flow aloft and possibly some low level Gulf inflow contribute. Much of the West and far northern Plains should see below normal temperatures for most of next week. This is especially the case for daytime highs, with coldest anomalies of 10-25F below normal possible over/near Montana mid-late week. Latest forecast trends have been toward slightly less extreme cold over the Northwest late in the week, so any daily records for cold lows/daytime highs may be fairly isolated. Meanwhile, the eastern U.S. will see a brief period of above normal temperatures (up to plus 10-15F anomalies for highs and slightly warmer for morning lows) on Wednesday ahead of the front crossing the region. Then the cold air behind the front will drop readings to moderately below normal over the central and then eastern U.S. Wednesday-Thursday, followed by a rebound to above normal readings from the central-southern Plains into parts of the Southeast. Rausch 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