Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM 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 continues to depict a similar theme regarding the overall synoptic pattern evolution but with a general trend toward faster progression of individual waves across the mainland U.S. through the medium-range period. 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... Models generally indicate good agreement on a frontal wave emerging into the northern High Plains on Sunday. The precise details of upper-level trough associate with this frontal wave, as well as its possible interaction with the southern stream energy, will be crucial in determining the evolution of a low pressure system forecast near the East Coast next Monday into Tuesday. There has been a general trend toward a faster departure of the system into the Atlantic on Tuesday, thereby lowering the chance of wintry precipitation for the interior Northeast. The GFS has been on the fast side with this system. The slower 00Z ECMWF has switched to a faster solution in the 12Z run but southeastern New England is still susceptible to having some impacts from this storm on Tuesday. Thereafter, models continue to cluster toward the idea of 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. The slower ECMWF as well as the faster GFS both show a trend toward a faster eastward progression of this feature. The ensemble means have been the most consistent guidance for the trough amplifying over the West Coast and beginning to push inland next Thu. Given the relatively progressive nature of the synoptic pattern during the medium-range period together with considerations of recent model performances, preference is given to the more progressive solutions. The updated WPC forecast is based on an even blend of the 06Z GFS and the 00Z ECMWF with increasing weights given to their ensemble means of up to 70 percent 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. ...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/Kong 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