Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Mon Mar 16 2020 Valid 12Z Thu Mar 19 2020 - 12Z Mon Mar 23 2020 ...Heavy Rain Threat from southern Plains into Ohio/Tennessee Valleys... ...Heavy Snow Threat from the central Rockies northeastward Thu-Thu night... ...Overview... Guidance still depicts a large scale pattern consisting mean troughing over the eastern Pacific/West Coast region with southwesterly or westerly mean flow downstream across most of the lower 48. During the latter half of this week a southern Canada upper trough extending into the northern tier states, along with an associated cold surge, will interact with western U.S. energy to produce a significant storm system most likely to track from Colorado northeastward. This system should produce a band of potentially heavy cold sector snow along with heavy rainfall from southern portions of the Plains northeastward. By the weekend and early next week the trailing front stalling over the Gulf may become a focus for additional southern tier rainfall. At the same time one or more systems dropping into the East Pacific mean trough should bring more moisture into the West, especially California. ...Guidance/Predictability Assessment... Recent GFS trends away from a deep/northward evolution for low pressure tracking northeastward from Colorado Thu onward have provided somewhat better clustering for this system among 12Z-18Z solutions. A general model blend with somewhat greater focus on recent ECMWF runs (most stable in the past three runs after a faster adjustment) would represent latest preferences well and has support from the ensemble means. However issues over the East Pacific/West Coast precluded use of the GFS in the overall starting forecast blend beyond day 3 Thu. Even with some improved agreement there is lingering uncertainty due to unresolved issues with the southern Canada trough that extends into the northern tier states. For example the new 00Z CMC is sharper/westward with this trough and faster than consensus with the energy emerging from the West--leading to its solution now being a northward extreme. There is fairly good continuity for the trailing front that crosses the Southeast and settles into the Gulf of Mexico. The passage of another day has not added much clarity to the evolution over the eastern Pacific/western U.S. during the period. At the start of the period early Thu there is some agreement on an upper low a ways west of Vancouver Island and a separate feature around 35-40N 140-145W. From Thu into Sat the upper low should drop southward while the southern feature ultimately progresses toward the extreme southwestern U.S. and northwestern Mexico. With time solutions become wildly divergent for the upper low path, between the GFS whose latest runs bring it to the Pacific Northwest and the new 00Z UKMET that has it well offshore southern California by the end of its run late Sat. Latest CMC runs have been been near (12Z run) or west/southwest (new 00Z run) of the ECMWF that has generally been the most consistent though a bit slower than a day or so ago. The ECMWF from recent cycles has reasonable support from the ensemble means in principle. By the latter half of the period the different GFS evolution ultimately leads to much stronger height falls progressing through the West into the central U.S., even beyond most ensemble members. In addition to poor comparison to other guidance currently, extended GFS forecasts valid over recent days--as strong upper ridging became established over the Northeast Pacific/Gulf of Alaska--verified very poorly and since then GFS runs have been inconsistent with the handling of features dropping down the east side of the upstream mean ridge. Late in the period there is decent agreement that another upper low should drop into the northeastern Pacific offshore the northwestern U.S. and begin to eject the upper low. Based on guidance available through the 18Z cycle, the past couple ECMWF runs/12Z CMC and 18Z GEFS/12Z ECMWF means provided the most agreeable depiction of East Pacific/western U.S. features through the period. ...Weather/Hazard Highlights... The storm system tracking northeast from Colorado will bring the potential for heavy snow over parts of the Colorado/Wyoming Rockies into the High Plains with a band of significant snowfall also extending northeastward--most likely through parts of Minnesota/Wisconsin into the U.P. of Michigan. This band may be fairly narrow which could present some forecast challenges given lingering uncertainty with the exact low track. Also expect strong winds to add to the hazardous conditions given the tight gradient between the surface low and trailing strong high pressure building into the Plains. Farther south expect an episode of heavy rain from parts of the Southern Plains region into the Tennessee/Ohio Valleys as moisture carried northward from the Gulf interacts with the cold front trailing from the surface low. Expect lighter amounts over other areas. The Storm Prediction Center is also monitoring potential for strong to severe convection. Northern areas will dry out after system passage. However the combination of southern stream shortwaves embedded in flow aloft and the front eventually stalling over the Gulf of Mexico and starting to lift back as a warm front may produce additional significant rainfall over the Southern Plains/Lower Mississippi Valley with lighter amounts farther east. Over the West, lingering energy aloft may produce scattered rain/mountain snow over some central/southern areas in the region late this week. Specifics of precipitation expected to reach the West Coast states continue to be fairly uncertain due to guidance spread with important aspects of flow aloft. The most likely scenario based on the best common aspects of the overall pattern continue to point toward highest totals over California Sun-Mon with some moisture spreading farther inland. The large scale pattern will continue to promote below normal highs over the southwestern corner of the lower 48 during the period. Thu-Fri should be the coolest days with some highs at least 10-15F below normal. Thereafter expect anomalies of minus 5-10F to be more common. Farther east expect a strong push of cold air southward along the eastern slopes of the Rockies through the Plains from Thu into the weekend. Some areas may see one or more days with highs 10-25F below normal. On the other hand the warm sector ahead of the storm tracking out of Colorado will contain much above normal temperatures late this week. There should be considerable coverage of morning lows 20-30F above normal and highs 15-25F above normal. 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