Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1009 AM EST Fri Feb 15 2019 Valid 12Z Mon Feb 18 2019 - 12Z Fri Feb 22 2019 ...Multi-day Heavy Rain Threat for the Southeast and Southern/Central Appalachians... ...Overview and Guidance/Predictability Assessment and Preferences... The models and ensembles continue to show a pattern where waves/upper troughs progress over the top of the northeast Pacific ridge and reinforce the persistent western US trough. Deep layer southwest flow over the east reinforces the southeast ridge, with the majority of models/ensembles showing a strong/persistent anticyclone off the southeast coast with heights near 2 standard deviations above normal. This pattern favors wet conditions in the southeast US and as discussed below, potential remains high for waves of heavy rain in portions of the southeast US. The 00z ECMWF/Canadian/00z ECMWF Ensemble Mean/06 UTC GFS/GEFS were equally weighted days 3-6 for Mon-Thu. Models show 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. Towards the end of next Thu (day 6) through Fri 22 Feb (day 7), guidance continues to show relatively good agreement with the big picture, but differences develop regarding how quickly a spot or closed low in the southwest US ejects to the east/northeast. The forecasts switched to give less weight to the slightly more progressive GFS and more weight to the 00z ECMWF Ensemble Mean and 06z GEFS Mean. ...Weather Highlights/Hazards... Multiple waves traversing the southeast front combine with the enhanced precipitable water and lift to bring several rounds of widespread and likely heavy rain. Model/ensemble guidance show the strongest signal for heavy rains over the Southeast/Southern Appalachians/eastern Gulf coast. There is a threat of multi-inch rainfall totals in the southeast next week as the blocking ridging channels surges of moisture and lift along the same axis. The 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 snow/ice for the Great Lakes, portions of the Ohio Valley, northern Appalachians, and New York/New England. As the main upper trough axis moves inland, snows will focus across the Southwest/southern Rockies terrain. Heavy snows are possible in the ranges of AZ, including the Mogollon Rim. Rain/mountain snows 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 25 deg F below average will initially be concentrated across the northern/central High Plains Sun, in the southwest to central Plains Mon, southwest Tue and Wed. Otherwise the western US temperatures to the plains states remain below normal. Record cold maximum temperature are quite possible over portions of the Southwest/southern California through the period. Farther east near the upper ridge, record high temperatures will be possible along the Gulf Coast into Florida. Petersen/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