Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 PM EDT Sun Mar 15 2020 Valid 12Z Wed Mar 18 2020 - 12Z Sun Mar 22 2020 ...Heavy Rain Threat from southern Plains into Ohio/Tennessee Valleys continues into late this week... ...Heavy Snow Threat from central Rockies northeastward Thu/Fri... ...Overview... Latest guidance continues to show that ridging aloft between Alaska and Hawai'i will favor mean troughing over the eastern Pacific into the western U.S. through the medium range period. Meanwhile, cyclonic flow over Canada will bring increasingly colder air into the Plains through the end of the week. A strong intrusion of cold air during mid-week will likely interact with energy ejecting from the western U.S. upper trough to result in a significant low pressure system tracking through the north-central U.S. late this week. Calmer and colder weather should then spread into the eastern two-thirds of the country by this coming weekend as precipitation associated with the next Pacific cyclone could reach California. ...Guidance/Predictability Assessment... Within the agreeable mean pattern there will be two dominant features. The first will be an upper low that should be elongating north-south as it reaches near 120W as of early Wed. The northern part of this feature may linger over the West while the southern part ultimately tracks across the Four Corners states and beyond, with Plains low pressure tracking northeastward as the dynamics progress onward--with some interaction from southern Canada troughing likely to have an influence on system evolution as well. This system will generate significant rain/mountain snow over the Southwest into the Colorado/Wyoming Rockies and then a band of potentially heavy snow to the northeast. At the same time the warm sector farther south will contain areas of heavy rainfall and possibly multiple days with severe weather threats prior to frontal passage this weekend. The second feature of interest, likely a closed low for a majority of the period, will dig southward over the eastern Pacific and should approach California by next weekend with some eventual assistance from yet more trough energy dropping into the Northeast Pacific. This evolution should increase precipitation over parts of California by this coming weekend. Overall the guidance continues to be less stable than desired regarding the timing of the low pressure forecast to track northeast across the central High Plains around Thu into early Fri. The primary factor has been the character of southern Canada troughing that may extend into the northern tier, with differences leading to timing issues for the cold front approaching the system from the northwest along with specifics of the system itself. The ECMWF has be pushing the system progressively faster eastward with each new run. The GFS has been relatively unstable regarding the track and intensity of this system. The CMC is the fastest among the guidance. The respective ensemble means are more agreeable with each other. Regardless of these specifics, there continues to be a trend toward faster progression over the past couple days. The WPC morning grids are based on a consensus of the 00Z ECMWF, 06Z GFS, and a smaller percentage from the 00Z CMC. More of their ensemble means are incorporated for Days 5-7 to deal with the uncertainty. Eastern Pacific details through the period have been fairly ambiguous so far when accounting for the full range of model and ensemble solutions. The most common theme has been for a trough/upper low to drop southward and then approach California next weekend as upstream energy reaches the northeastern Pacific. The WPC blend yields a fairly board eastern Pacific trough slowly edging closer toward California and into Baja this coming weekend. ...Weather/Hazard Highlights... The system progressing through parts of the West and then continuing onward will bring a broad area of significant weather to the lower 48. Highest rainfall/mountain snow totals will likely be over favored terrain in central Arizona and the Colorado/Wyoming Rockies. A band of potentially heavy snow will be possible from the north-central High Plains into parts of the Upper Great Lakes. However, uncertainty with system depth and track make it difficult to be more specific about the snow axis and amounts at this time. Some snow may extend into northern New England as well. The gradient between the surface low and high pressure building in behind it may produce a period of strong winds. Low level flow from the Gulf will provide some input for areas of heavy rainfall in the warm sector. The southern Plains area continues to be a part of the threat area while adjustments in system evolution have nudged the axis of heaviest rain potential a bit to the northeast into the Ohio Valley. Activity along a leading wavy front may also produce an episode of enhanced rainfall over some of these areas, with the combination of the two events leading to some flooding concerns. Also check latest outlooks/discussions from the Storm Prediction Center as there will be potential for strong to severe convection on one or more days with details to become more clear in upcoming days. While northern areas should clear out after system passage, the trailing cold front will appear to become stationary off the Gulf Coast. This will likely keep a good chance of rain through the Deep South and along the Gulf Coast into the weekend. The best consensus of guidance still shows an increase of moisture into California next weekend ahead of the next system dropping into the eastern Pacific. Specifics remain fairly uncertain but the overall pattern may ultimately support meaningful precipitation totals. California and the Southwest will tend to see below normal temperatures for most of the period with coolest highs of 10-20F below normal likely to be on Wed-Thu. Plains cyclogenesis by Thu and continued progression of the low thereafter will bring a surge of warm air into much of the central and eastern U.S. during the latter half of the week. Expect decent coverage of plus 15-30F anomalies for morning lows and plus 10-20F anomalies for daytime highs. Behind the system, chilly air with some highs 10-20F below normal will drop south and east from Montana and the Northern Plains mid-late week, with some moderation as the air reaches the East. Kong/Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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