Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 AM 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 to start the week... ...Heavy precipitation threat over California and another event possible over the East mid-late week... ...Weather Pattern Overview and Guidance/Predictability Assessment... An active weather pattern next week will initially feature a mean trough aloft near the West Coast with individual features digging into and then ejecting from this trough. The leading system will likely bring a broad precipitation area of various types across the eastern half of the country early in the week. The next system should produce areas of enhanced precipitation over parts of the Great Basin/Southwest early in the week. Guidance is still trying to come into focus for what becomes of this system by the time it crosses the eastern U.S. by Wed-Thu but there is a potential for meaningful precipitation over some areas. Meanwhile during this time guidance appears to be in the process of converging toward some retrogression of large scale flow over the eastern half of the Pacific. This would lead to a farther west axis of the next amplifying trough and a period of strong southwesterly upper flow that would produce an episode of heavy precipitation focused over portions of California. Looking at guidance from the 12Z/18Z cycles, agreement has improved compared to earlier runs by way of various trends. For the first system which crosses the East early in the week GFS/GEFS mean runs have trended more toward recent ECMWF runs--maintaining better definition of the ejecting upper shortwave and reducing the influence of northern stream flow. The 18Z GFS was still a bit faster/weaker than other operational models and the new 00Z run appears to have continued the trend toward consensus. Then solutions have displayed considerable spread and in some cases variability for the next system ejecting from the West. Overall trend has been toward the more strongly developed GFS/GEFS mean though perhaps waiting until slightly farther east to deepen the system. Operational ECMWF runs have been oscillating over the past couple days with 12Z runs closer to the GFS in concept and 00Z runs very weak/diffuse. Recent trends have also been in favor of recent GFS/GEFS mean runs for the mid-late week upper trough amplifying near the West Coast. Earlier ECMWF/ECMWF mean runs had been broader and more progressive. Latest CMC runs have generally settled into the improving consensus pattern as well. Latest comparisons favored an updated forecast consisting of an operational model composite to reflect the most common ideas guidance during the first half of the period followed by a model/mean blend to balance the typically increasing uncertainty for specifics and potential merit of some operational model details in principle. ...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 is ongoing uncertainty over the precise axis and intensity of heaviest rainfall but interaction of moist Gulf inflow with a leading front should promote some moderate to heavy rainfall. 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 flow 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. 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