Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Thu Mar 19 2020 Valid 12Z Sun Mar 22 2020 - 12Z Thu Mar 26 2020 ...Heavy Rain Threat across portions of the South... ...Overview... Latest guidance is consistent for the depiction of the overall pattern evolution during the period along with the embedded features that have had varying degrees of uncertainty. There is good continuity in principle along the West Coast as a vigorous trough/upper low descends from the Northeast Pacific/Gulf of Alaska to the West Coast during the weekend and first half of next week. This amplifying trough will open up/eject an upper low initially offshore California, with fast westerly-southwesterly mean flow shearing this low's energy across the country after early Mon. Yet another shortwave rounding the strong East-Central Pacific mean ridge aloft should help to push the deep trough over the West Coast inland by next Thu. This evolution will spread a broad area of rain and mountain snow across the West. Farther east the spread persists for lower-predictability shortwave details, both within the southern Canada/northern tier U.S. stream and the flow emerging from the West. This keeps confidence lower than desired for individual surface systems and associated weather impacts. There is a general signal for one system to evolve along or slightly offshore the East Coast around Mon-Tue with one or more upstream surface waves farther west. ...Guidance/Predictability Assessment... The updated forecast emphasized the 12Z GFS/ECMWF/UKMET with only modest 18Z GEFS/12Z ECMWF input for days 3-4 Sun-Mon. Then the forecast transitioned close to a nearly even model/mean blend for day 5 Tue and then 70 percent to the means by days 6-7 Wed-Thu, especially due to differences for individual features becoming more pronounced between the central U.S. into the western Atlantic. Models are still having a lot of difficulty resolving the shortwave specifics between the Northwest and Northern Plains at the start of the period Sun. On the positive side the 12Z ECMWF and now the 00Z CMC have trended more open their energy reaching the Northwest. The GFS remains the broadest and most progressive with this energy while latest 00Z UKMET runs are generally a compromise closer to the 12Z ECMWF. Precise details of this shortwave energy as well as separate southern stream energy--plus their possible interaction--will be crucial in determining the evolution of low pressure near the East Coast. The high uncertainty of this evolution favored keeping the surface low track near the middle of the significant spread. Current solutions range between the 00Z UKMET that shows a coastal track with accompanying significant precipitation and more offshore 00Z GFS/CMC that bring very little moisture to New England. Some moderate timing differences persist for the upper low initially off California and then opening up as it reaches the coast on Mon, with the latest GFS/CMC runs a bit faster than GEFS mean/UKMET runs and 12Z ECMWF. There has been sufficient variability over recent days to favor an average of these solutions, with the slightly slower 12Z GFS favored over the 18Z run in the blend. Operational models are starting to cluster toward the idea of this energy supporting surface low pressure that reaches the Plains around Tue and then continues across the eastern half of the country before emerging into the western Atlantic by next Thu. Ensemble means either lose this wave or track it into the Great Lakes instead of the Atlantic. Strength of the operational model surface lows allowed for some reflection of the model scenario even though the blend trended more toward the means on a percentage basis. The ensemble means have been the most consistent guidance for the trough amplifying over the West Coast and beginning to push inland next Thu. 18Z/00Z GFS runs are a bit on the faster side of the spread late in the period, again favoring the 12Z GFS in the blend, while the GEFS mean and 12Z ECMWF/ECMWF mean are fairly close relative to typical guidance error that far out in time. ...Weather/Hazard Highlights... The forecast is still on track for a broad area of rain and mountain snow to overspread the West from about late Sun or Sun night onward. Highest totals for the period should be over the Sierra Nevada while favored terrain along much of the West Coast and Cascades as well as the Great Basin and northern-central Rockies will likely see significant precipitation as well. Precipitation should taper off from west to east around midweek and than another area of moisture should reach the Pacific Northwest around next Thu. Potential still exists for some locally heavy rainfall across portions of the South as a front lingers near the Gulf Coast into early next week. Specifics will take more time to resolve though. Precipitation coverage and intensity farther north will depend on uncertain details of expected surface low development near the East Coast on Mon-Tue. Current solutions for track are far enough apart to make a difference between only light precipitation over New England or a meaningful snowfall between the central Appalachians and New England. Latest winter weather probabilities show at least 30 percent potential for a quarter inch or more liquid equivalent in the form of snow over parts of New England. Then a system emerging over the Plains Tue should spread another area of precipitation across the eastern half of the country. The majority of precipitation with this system should be in the form of rain, except for possibly some snow at northernmost latitudes. Modestly below normal temperatures over the eastern U.S. Sun-Mon will be followed by increasing coverage of above normal readings to the east of the Rockies, while the amplifying West Coast trough and the eventual start of its eastward progress will bring a cooling trend to much of the West. By next Wed-Thu expect a fairly broad area of plus 10-20F anomalies over the eastern half of the country and generally minus 5-15F anomalies for highs (with more moderate temperatures for morning lows) over the West. 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