Extended Forecast Discussion NWS Weather Prediction Center College Park MD 119 AM EST Thu Feb 14 2019 Valid 12Z Sun Feb 17 2019 - 12Z Thu Feb 21 2019 ...Multi-day heavy rain event possible for portions of the Southeast, Tennessee Valley, and Southern Appalachians next week... ...Overview... Upper-level ridge across the North Pacific shows no signs in the guidance of breaking down during the medium range, and thus will continue to influence the large scale flow across the CONUS. This will reinforce a pattern with a trough in the West and ridging off the Southeast U.S. coast through the next week. A rapid succession of Pacific shortwave energy will traverse the flow, amplifying the mean trough over the West before ejecting east and interacting with a lingering polar front along the Gulf Coast to generate frequent frontal waves which will affect the Southeast and Mid-Atlantic regions during the forecast period. ...Guidance/Predictability Assessment and Preferences... Model consensus was sufficient during days 3-4 (Sun-Mon) to use a multi-model deterministic blend (12Z ECMWF/UKMET/CMC and 18Z GFS) as a basis for the forecast. Models showed some variability on the specifics of a frontal wave crossing the eastern U.S. during that time frame, with variations as to how quickly energy transfers from a low over the southern/central Appalachians to a second low off the Mid-Atlantic coast, but a multi-model consensus was preferred. Models showed relatively good consensus with amplified shortwave energy digging into the Southwest Sun-Mon as well. From day 5 (Tue) onward, models continue to show relatively good agreement when it comes to the big picture, but larger differences begin to emerge on the specifics. Confidence is relatively high that energetic shortwave energy should move from the Four Corners region Tue morning into the central U.S., weakening to some degree as it begins to encounter anticyclonic flow on the western periphery of the subtropical ridge. Differences among the guidance as to how quickly this wave lifts out and the degree of interaction with the northern stream result in varying surface evolutions. There is at least general consensus that another frontal wave should develop in the northwestern Gulf of Mexico Tue and then lift northeastward across the Southeast Tue night/Wed. Run-to-run variability among deterministic solutions but a moderate degree of ensemble clustering lent to gradually heavier use of ECENS/GEFS mean solutions. Meanwhile, as additional shortwave energy dives south along the West Coast and reinforces the mean trough, solutions vary widely as to the specific nature of the next wave, and its effect on the larger mean trough. Teleconnections relative to the strong North Pacific ridge support the idea of keeping the western U.S. mean trough from progressing east too quickly, and the ECENS mean has more consistently held this feature farther west relative to the GEFS mean by the middle of next week. Thus, the forecast during days 5-7 was based more heavily on the ensemble means, with more weight placed on the ECENS relative to the GEFS. ...Weather Highlights/Hazards... The polar front will linger across the Southeast/Gulf Coast through the forecast period, with multiple waves/surface lows traversing the front bringing several rounds of widespread and potentially heavy rain. Model/ensemble guidance show the strongest signal for heavy rains across portions of central/northern Mississippi, Alabama, and Georgia, as well as Tennessee and western portions of the Carolinas. Multi-inch rainfall totals are possible across these areas from Sun onward into the middle of next week. Colder air in place on the northern periphery of these low pressure systems may result in areas of wintry precipitation for portions of the central Appalachians and Mid-Atlantic. Meanwhile, conditions along the West Coast will be at least a bit quieter than seen recently, as the main upper trough axis moves inland. This will spread snows into the southern Rockies Sun-Mon. More widespread rain/snow may return to the Pacific Northwest by next Tue-Wed as a more significant shortwave approaches. Well-below average temperatures will persist across much of the western/central U.S. during the next week as the upper trough axis moves inland. Temperatures as cold as 15 to 30 deg F below average will initially be concentrated across the northern/central High Plains Sun, but will become more widespread from Mon onward, with much of the western and central U.S. expected to see temperatures ranging from 10 to as much as 30 deg below average. Ryan WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation and winter weather outlook probabilities are found 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