Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sat Jan 23 2021 Valid 12Z Tue Jan 26 2021 - 12Z Sat Jan 30 2021 ...Heavy precipitation threat over California and uncertain but potentially significant storm near the East Coast mid-late week... ...Weather Pattern Overview and Guidance/Predictability Assessment... The consensus of guidance displays some degree of retrogression for the pattern over the eastern Pacific and West Coast with time. With the ejection of energy from the Southwest to start the period on Tue, mean troughing along or slightly inland from the West Coast in the short range will shift to slightly offshore as upstream energy digs into a new trough near 130W longitude. Then this trough will push inland toward the end of the week (with an upper ridge building to its east) as yet another trough evolves yet farther upstream. The ejecting Southwest trough should generate a strong and windy storm over the western Atlantic by late week but there is significant uncertainty over this system's impact over the eastern U.S. leading up to that time, with important smaller-scale details aloft and exact timing yet to be resolved. Meanwhile the upper trough digging off the West Coast is increasingly likely to generate a period of strong moisture flow into California with heavy rain and mountain snow over some areas. By next weekend expect the progression of this trough and development of the upstream trough that should be somewhat broader to promote a drier trend over central/southern California while refocusing precipitation over the Pacific Northwest and/or northern California. Based on guidance available through the 12Z and 18Z cycles, the updated forecast incorporated the most common themes of the operational models for about the first half of the period and then transitioned to a model/ensemble mean blend--slightly more 18Z GFS than 12Z ECMWF for the operational model component. By days 6-7 Fri-Sat the ECMWF leaned a bit faster than the average of other guidance for both the upper trough poised to head into the West as well as the deepening system over the western Atlantic. Trends over recent days have been more in favor of the GFS/GEFS that have generally depicted slower timing for both features, especially the western trough. CMC input was phased out fairly early in the period as its southeastward track of a southern Canada upper low diverged from most other guidance. New 00Z models are adding to the uncertainty over the East after midweek, with both the GFS and CMC indicating sufficiently strong/westward closed upper low development to generate a significant snowstorm over some locations (in the GFS, essentially a broadening of significant totals relative to earlier runs)--in contrast to recent ECMWF/UKMET runs that are faster/weaker and more open aloft, thus producing only a modest event over the East. 00Z ECMWF/UKMET runs remain more open aloft. Spread for the trough near the West Coast is moderate for the time frame and slowly narrowing over successive model runs. The 00Z GFS did adjust a bit slower and the new CMC concurs while the UKMET came in a bit faster than the average through the end of its run. The slower solutions are certainly plausible in light of multi-day trends. What remains to be seen is the limit of such trends as the next broader trough develops upstream toward the end of the period. ...Weather/Threats Highlights... Over the West, parts of the Southwest/Four Corners states will see lingering precipitation to start the period as an upper shortwave crosses the region. Then guidance is gradually consolidating its signal for a heavy precipitation event over portions of California around mid-late week as an upper trough amplifies just offshore before moving inland. The most intense rain/mountain snow should initially focus over northern California and then drift southward with time. Favored terrain along the Coastal and Transverse Ranges as well as the Sierra Nevada will likely see the highest totals from this event and some locations could see at least 5-10 inches liquid for the five-day forecast period. A portion of this moisture should extend into parts of the Great Basin/Rockies and eventually the Southwest. If this trough reaches far enough eastward it could begin to generate precipitation over the central U.S. next weekend. Also next weekend, upstream Pacific flow may begin to focus precipitation more over the Pacific Northwest and northern California. The upper system emerging from the Southwest at the start of the period may produce a band of wintry weather from the central Plains through the Mid-Atlantic during Tuesday-Thursday with some moderate to heavy rain possible over the Southeast. Guidance continues to show important surface/upper level detail differences whose predictability tends to decrease a few days out in time. Thus confidence remains low in resolving the specifics of snowfall, especially over the Mid-Atlantic where the full guidance ranges between modest and heavy accumulations. There is better agreement on the potential for a period of brisk to strong winds along and offshore the East Coast late this week, between the storm that develops over the western Atlantic and trailing high pressure that builds into the East. Ahead of this system, a leading weaker feature should produce some lingering precipitation on Tuesday--snow most likely over the northern Mid-Atlantic and southwestern parts of the Northeast. Expect much above normal temperatures over the Southeast and vicinity on Tuesday, ahead of a frontal system expected to stall near the northern Florida border the next morning. Some morning lows on Tuesday may be 20-30F above normal and could approach/exceed daily record warm lows if they hold for the calendar day. Daytime highs should be 10-20F above normal. The East will trend toward moderately below normal readings after midweek as strong low pressure develops over the western Atlantic. The pattern near the West Coast will promote below normal highs through the period, especially over California and the Great Basin/Southwest. Coolest highs of 10-20F below normal will be most common on Tuesday as a leading shortwave begins to eject eastward, with some moderation to single-digit negative anomalies later in the week. The central High Plains may be similarly chilly to start the period. The central U.S. will trend warmer late week into the weekend, with an increasing area of lows 10-25F above normal and highs 10-20F above normal. 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, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml