Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EST Fri Jan 22 2021 Valid 12Z Mon Jan 25 2021 - 12Z Fri Jan 29 2021 ...Significant precipitation possible over parts of the Southwest and East early next week... ...Heavy precipitation threat over California and another event possible over the East mid-late next week... ...Weather Pattern Overview and Guidance/Predictability Assessment... Model guidance this morning is pretty much on track regarding timing of the next significant shortwave forecast to traverse the Desert Southwest and the southern Great Basin early next week. Meanwhile over the central and eastern U.S., recent runs from GFS have shown more significant northward shift in the track of a low pressure center across the Ohio Valley. The CMC also indicated a similar shift. The ECMWF has slowed down this cyclone across the Ohio Valley before downstream cyclogenesis takes over off the northern Mid-Atlantic coast. Thus, model consensus resulted in further northward shift in the QPF axis toward the Ohio Valley and across the Mid-Atlantic. Off the West Coast, models have come into much better agreement regarding the next upper trough digging down from the Gulf of Alaska as the ECMWF and EC mean have shifted much farther south toward what the GFS and GEFS have been advertising earlier. There is increasing confidence that a heavy precipitation event will impact California from Tuesday onward in association with this system. Meanwhile, models continue to indicate an increasing likelihood of cyclogenesis by mid- to late next week in the vicinity of the southeastern U.S. under an increasingly amplified upper-level pattern across the U.S. Model uncertainty remains very high regarding this system. Nevertheless, an expanding area of enhanced rainfall is forecast to develop on Wednesday across the Deep South as it merges with another disturbance moving across the central Plains to result in a swath of wintry precipitation across the interior eastern U.S. to possibly the Mid-Atlantic, with a round of meaningful rainfall across the Southeast during the latter part of next week. The WPC medium-range forecast package was based on consensus of the 06Z GFS/GEFS, the 00Z ECMWF/EC mean, with a smaller contribution from the 00Z CMC and the 00Z NAEFS. More of the ensemble means were used on Days 6 and 7 to handle the higher forecast uncertainty. ...Weather/Threats Highlights... The first system of interest, emerging from the southern Plains by the start of Monday, should track into the Mid-Atlantic by early Tuesday. There has been an ongoing northward shift in the rainfall axis. Currently the most likely area for highest totals extends from parts of the Ohio/Tennessee Valleys into the central Mid-Atlantic. Sufficiently high rainfall rates could lead to flooding issues in some areas, while below normal observed precipitation over the past month could be at least a modest mitigating factor. Cold air in the northern part of the moisture shield may allow for meaningful snow from portions of the central Plains/Midwest into the northern Mid-Atlantic. The second system will initially bring rain and higher elevation snow to locations across the southern Great Basin and the Southwest/southern California. Expect highest totals in this event to be over favored terrain in central Arizona and to a slightly lesser extent over southern Nevada/Utah as well as the coastal ranges of extreme southern California. As the upper energy continues eastward there is a slowly improving signal in the guidance for precipitation to spread across portions of the eastern U.S. around Wednesday-Thursday, though precise coverage and amounts are still rather uncertain. Expect the rain-snow line with this system to be somewhat farther south than with the first system. Meanwhile guidance displays improving agreement that an upper trough digging just off the West Coast around midweek will bring a period of strong moisture influx that would support an episode of heavy rain and mountain snow to favored windward terrain. Several inches liquid could be possible at some locations depending on the system's strength and timing which will get better calibrated over the coming days. Expect temperatures to be well above normal over the southeastern quadrant of the lower 48 early in the week in the warm sector ahead of the first eastern system. Highs should reach up to 10-20F above normal with some morning lows up to 20-25F or so above normal. System progression will lead to more moderate readings over the East for the rest of the period, with a tendency for modestly above normal readings over the South and somewhat below normal over the Mid-Atlantic/Northeast. Meanwhile the area encompassing California and the Great Basin/Southwest will be persistently below normal through the period, especially for highs with some minus 10-20F anomalies most days. Less extreme cool anomalies will extend into the Plains early in the week followed by a warming trend. Parts of the northern-central Plains and Upper Midwest could see readings exceed 10F above normal by late week. 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 Hazards: - Heavy precipitation across portions of the Central Appalachians, the Middle Mississippi Valley, the Mid-Atlantic, the Southern Appalachians, the Upper Mississippi Valley, the Great Lakes, and the Ohio Valley, Mon, Jan 25. - Heavy rain across portions of California, Tue-Wed, Jan 26-Jan 27. - Heavy rain across portions of California, Wed, Jan 27. - Heavy rain across portions of the Central Appalachians, the Tennessee Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, and the Ohio Valley, Mon, Jan 25. - Heavy snow across portions of California, the Central Great Basin, and the Pacific Northwest, Tue-Thu, Jan 26-Jan 28. - Heavy snow across portions of the Central Great Basin, California, and the Southwest, Mon, Jan 25 and Wed-Thu, Jan 27-Jan 28. - Heavy snow across portions of the Central Great Basin and the Southwest, Mon, Jan 25. - Heavy snow across portions of the Southwest, Mon-Tue, Jan 25-Jan 26. - High winds across portions of the Aleutians, Tue-Wed, Jan 26-Jan 27. 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