Extended Forecast Discussion NWS Weather Prediction Center College Park MD 502 PM 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 of mean troughing over the eastern Pacific/West Coast region with southwesterly or westerly mean flow downstream across most of the rest 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... A general model blend represented the latest preferences well and has support from the ensemble means east of the Rockies. However issues over the East Pacific/West Coast precluded use of the GFS/UKMET/Canadian to varying degrees and for different reasons through the period. 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. 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 (outside the 00Z UKMET) 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. Model solutions became wildly divergent for the upper low path, with the GFS farther east, the Canadian farther south, and the UKMET farther west. The ensemble means (trending slower) were mostly aligned near the 00Z ECMWF, which was seen as more stable and least different than continuity and offered the choice of least regret at this time. By the latter half of the period the different GFS evolution ultimately leads to much stronger/quicker height falls progressing through the West into the central U.S., even beyond nearly all 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 lead weakening upper low over southern California early next week. Based on guidance available through the 06Z cycle, the 00Z ECMWF with WPC continuity and the 06Z GEFS/00Z ECMWF ensemble mean 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. Initial part of this system will slide into the short range with today's WPC guidance (see the Winter Weather Desk discussion QPFHSD for more info). 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 but with confidence-limiting factors still in play. Northern areas will dry out after system passage and a fairly strong high pressure area for the latter half of March. 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 Mon with some moisture spreading farther inland. Slowed down/tempered the precipitation onset/amounts, respectively, given the slower ECMWF ensemble trend. 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. This could push temperatures into the low 80s as far north as parts of the mid-Atlantic region which would approach/tie daily records. Overnight lows and dew points in the upper 50s to low 60s Thursday night may very well be above normal daily high temperatures and more reminiscent of May or even June. Fracasso/Rausch Hazards: - Heavy precipitation across portions of the southwest Colorado into northern New Mexico, Thu, Mar 19 and Sat, Mar 21. - Heavy precipitation from portions of the central Rockies across the High Plains into the northern Plains, Thu, Mar 19. - Heavy rain from central Texas through portions of the lower to mid-Mississippi Valley, the Tennessee Valley, and into the Ohio Valley, as well as portions of the interior Northeast, Thu-Fri, Mar 19-Mar 20. - Heavy rain from portions of eastern Texas into the lower Mississippi Valley, Sat-Sun, Mar 21-Mar 22. - Heavy snow along the Sierra Nevada, Sun-Mon, Mar 22-Mar 23. - Flooding possible across portions of the Dakotas, the middle Mississippi Valley, and Oklahoma. - Flooding occurring or imminent across portions of the Southeast, the lower Mississippi Valley, and eastern South Dakota. - Flooding likely across portions of the northern Plains. - High winds from across portions of the the central Rockies, through much of the central to northern Plains, into the upper Midwest and upper Great Lakes, Thu, Mar 19. - High winds from across the Great Lakes and northern Ohio Valley through the Northeast, Fri-Sat, Mar 20-Mar 21. - Much below normal temperatures across much of the northern and central High Plains, through the northern Plains into the upper Midwest, Thu-Fri, Mar 19-Mar 20. 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