Extended Forecast Discussion NWS Weather Prediction Center College Park MD 145 AM EST Fri Feb 15 2019 Valid 12Z Mon Feb 18 2019 - 12Z Fri Feb 22 2019 ...Multi-day Heavy Rain Threat for the Tennessee Valley and South-Central Appalachians/Mid-Atlantic and Heavy Snow/Ice Threats Overtop into the Ohio Valley, Central Appalachians, North-Central Mid-Atlantic and Southern New England... ...Wintry Western-Central U.S Weather focuses over Southwest/Southern Rockies... ...Overview... It remains the case that upper-level ridging across the North Pacific and near the Bahamas shows minimal signs in guidance of breaking down during the medium range, and thus will continue to influence the large scale flow across the CONUS. This reinforces a pattern with a cold/unsettling trough in the West/Southwest, arctic air intrusions into the n-central U.S. and wavy southwest flow over the eastern half of the CONUS through the next week. A rapid succession of Pacific shortwave energies will traverse the flow, amplifying the mean trough over the West before ejecting and interacting with a lingering polar front along the Gulf Coast to generate frequent frontal waves which affect the east-central and eastern U.S. ...Guidance/Predictability Assessment and Preferences... A composite of decently clustered guidance of the 12 UTC ECMWF/ECMWF ensembles and favorably trending of the 18 UTC GFS/GEFS seems reasonable as a basis for the forecast Mon-Wed. Models show 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 consensus was preferred. There is also relatively good consensus with amplified shortwave energy digging into the Southwest Mon-Wed. Strating Day 6/Thu and especially by day 7/next Fri, guidance continues to show relatively good agreement with the big picture, but larger differences begin to emerge on the specifics. Opted to derive this forecast period primarily from the 12 UTC ECMWF ensemble mean and to a lesser extent the 18 UTC GEFS mean to address growing uncertainty and maintain as much WPC continuity as feasible. The ECMWF ensemble mean offers more amplified/less progressive flow than the GEFS mean. ...Weather Highlights/Hazards... A polar front lingers over 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 on the northwest side of the upper high circulation over the Tennessee Valley and South-Central Appalachians/Mid-Atlantic. There is a threat of multi-inch rainfall totals next week as the blocking ridging channels moisture along the same axis. Ensemble situational awareness table highlighted this area with "MAX" values per the GEFS QPF relative to the reforecast database, signaling the rarity of the GEFS ensembles to be this wet this far out this time of year (Jan/Feb/Mar). Colder air settled in place on the northern periphery of these low pressure systems offers threats of heavy snow/ice for the Ohio Valley, central Appalachians and from the north-central Mid-Atlantic into southern New England. Meanwhile, conditions next week along the West Coast will be quieter than seen recently as the main upper trough axis moves inland. This will spread snows into Southwest/southern Rockies terrain. Some widespread rain/snow may return to the Pacific Northwest by midweek as a more significant shortwave approaches and dives south along the coast into the wintry Great Basin then Rockies/n-central U.S. 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 40 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. Record cold maximum temperature are quite possible over portions of the West including southern California through the period. Farther east near the upper ridge, record high temperatures will be possible along the Gulf Coast into Florida. Schichtel 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