Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM 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... ...Heavy Snow Threat from Central Rockies northeastward... ...Guidance/Predictability Assessment... Latest guidance continues to show that ridging aloft between Alaska and Hawai'i will support mean troughing over the eastern Pacific into the western U.S.. Meanwhile cyclonic flow over Canada will extend its influence into the northern tier of the lower 48 at times and an upper ridge initially centered over the southern Gulf of Mexico/northwestern Caribbean will expand from Wed into Fri and then become oriented more east-west next weekend. 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 for a time 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. 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 next weekend. Overall the guidance has been less stable/agreeable than desired with specifics of low pressure tracking northeast from the central High Plains after early Thu. 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. Based on data through the 12Z/18Z cycles the GFS was the deepest/sharpest with the Canadian trough, leading to a deeper and farther northwest surface low. The GEFS mean leaned in its direction in principle. Based on continuity and full guidance spread, the preferred solution was to trend an operational model blend into day 4 Thu toward a blend of recent ECMWF runs as well as 12Z ECMWF mean/CMC and 18Z GEFS mean. New 00Z guidance is split as far as how much to support some aspects of the GFS. The 00Z UKMET adjusted fairly close albeit with less extreme surface low depth, while the more suppressed ECMWF is only a fraction to the left relative to its prior run. Regardless of these specifics, there has been a more pronounced model/ensemble mean trend toward faster progression over the past couple days. 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 past 24 hours of ensemble mean runs have been been reasonably stable so they provide a good template for the overall pattern. A lagged average of ECMWF runs compared favorably to the means, as did some valid times of the 12Z CMC. The new 00Z CMC drops its upper low well westward of other guidance by next Sat-Sun. The blend mentioned above works well for depicting forecast preferences over this area. ...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 specific about exact 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 to the east a bit to the north, now over 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, lingering moisture may continue to support rainfall over parts of the South 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. 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