Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sat Jan 23 2021 Valid 12Z Tue Jan 26 2021 - 12Z Sat Jan 30 2021 ...Heavy precipitation appears likely for California beginning Tuesday night... ...Potential for a significant storm increasing near the East Coast late Wednesday to Thursday... ...Weather Pattern Overview and Guidance/Predictability Assessment... The medium-range period should commence with heavy precipitation moving into northern California late Tuesday ahead of the frontal system associated with the next Pacific cyclone. Models have lately been quite consistent with the timing of this heavy precipitation event as the bulk of the moisture is forecast to slide southeastward and engulf much of northern and central California on Wednesday and Thursday. Some of the heavy precipitation should also reach the Coastal Ranges of southern California late Wednesday and then linger into Friday. Another Pacific system could reach the West Coast by next weekend with noticeable model spread in the timing at present. Meanwhile, wintry or mixed precipitation over parts of the northern Mid-Atlantic should be on the wane Tuesday night as the associated low pressure wave moves rapidly off the East Coast. Over the Deep South, model consensus has been trending toward a faster progression of a developing low pressure wave near the central Gulf Coast midweek near a stationary front. At the same time, wintry precipitation associated with a shortwave moving from the northern/central Plains to the Midwest on Wednesday will likely merge with the developing low pressure wave pushing into the Southeast. Models this morning have come into better agreement on the potential for significant cyclogenesis Wednesday night into early Thursday near the Carolina coasts. A swath of wintry precipitation is expected to traverse the Ohio Valley and central Appalachians and parts of the Mid-Atlantic during Wednesday to early on Thursday. Model consensus has been showing a gradual trend for the precipitation to slide faster offshore with an increasingly amplified upper-level ridge across the central U.S. By next weekend, models are beginning to indicate the possibility of the next heavy rain event developing over the Deep South ahead of a developing low pressure system in the central High Plains. The WPC medium-range forecast package was based on the consensus of the 06Z GFS/GEFS, the 00Z ECMWF/EC mean, with a smaller contribution from the 00Z CMC mean. More of the ensemble means were used on Days 6 and 7 to handle the increasing model spread. ...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 in excellent agreement on a heavy precipitation event over portions of California beginning late Tuesday 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 should produce a band of wintry weather from the central Plains through the Mid-Atlantic during Tuesday-Thursday while some moderate to heavy rain is possible over the Southeast. Models this morning have come into better agreement on the potential for significant cyclogenesis Wednesday night into early Thursday near the Carolina coasts. But 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. Kong/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