Extended Forecast Discussion NWS Weather Prediction Center College Park MD 151 AM EST Wed Feb 13 2019 Valid 12Z Sat Feb 16 2019 - 12Z Wed Feb 20 2019 ...Overview... Models/ensembles continue to show strong consensus that a Rex block pattern across the central/north Pacific, extending from near Hawaii to Alaska, should persist through the next week. The strongest positive height anomaly center across the North Pacific is forecast to shift a bit south over the next several days, with heights falling at least somewhat across Alaska. The downstream result of this slight modulation in pattern across the Pacific will allow the mean upper trough which has been persistent near the Pacific Northwest to shift inland, to a position across the Rockies by the middle of next week. Numerous smaller scale shortwaves will continue to traverse this flow across the CONUS, originating in the higher latitudes of the North Pacific. Model guidance continues to suggest that the dominant storm track across the CONUS should shift southward over the next week, with a much more active southern stream taking shape. Additionally, the subtropical ridge will intensity off the Southeast U.S. coast early next week, which will aid in the transport of Gulf of Mexico moisture northward ahead of a potentially significant low pressure system for the eastern third of the CONUS next Tue-Wed. ...Guidance/Predictability Assessment and Preferences... Frequent smaller-scale shortwaves crossing the southern tier and the large western trough axis shifting inland will be the most significant features this forecast. An active southern stream will be quickly taking shape by day 3 (Sat) with a frontal wave likely crossing the Southeast U.S. The past few model cycles have revealed a much clearer picture and model/ensemble solutions are now decently clustered for this feature as it quickly moves northeast into the Atlantic by late Sat. Next shortwave-induced frontal wave should get going late Fri across the southern plains/lower Mississippi Valley and quickly lift northeast toward the Tennessee Valley/southern Appalachians Sat night/Sun. Spread is a bit higher on this feature, with some solutions trying to take the low farther north into the Ohio Valley, but spread has also reduced significantly over the past 24 hours with respect to this system. As the large upper trough shifts east into the Southwest Sun-Mon, models show good consensus on the large scale aspects of this feature. Given these considerations, the WPC forecast during days 3-5 was based on a multi-model blend of deterministic guidance including the 12Z ECMWF/UKMET and 18Z GFS. Even in the latter portion of the extended forecast period, model consensus has improved over the past few runs. The speed with which an amplified leading shortwave ahead of the mean trough moves east into the central U.S. Tue-Wed continues to show some variability in solutions, but the trend has been toward a slower ejection eastward, and this idea seems reasonable given the amplifying large scale flow and evolving trough/ridge pattern across the CONUS. Some consensus is evident among deterministic/ensemble solutions that an area of low pressure should develop along a strong frontal boundary draped along the Gulf Coast Tue as the upper wave reaches the central U.S., with the wave lifting northeastward toward the Appalachians Tue night-Wed, and perhaps a secondary coastal low taking shape near the Mid-Atlantic coast. Some modest clustering is evident among ensemble lows even on day 7, so forecast confidence is at least a bit higher than it has been in recent days. Finally, farther west, shortwave energy will continue to skirt the Pacific Northwest even into the middle of next week - with some consensus among the guidance that one weakening wave should arrive on Tue with another stronger one by Wed. Kept 40 percent of the GFS/ECMWF deterministic solutions in the blend through days 6-7, with the slight majority of the forecast based on their respective ensemble means. ...Weather Highlights/Hazards... The active southern stream storm track setting up will mean a period of generally wet conditions for areas from the southern plains to the Eastern Seaboard. Low pressure traversing the Southeast/southern Mid-Atlantic on Sat could produce an area of relatively light snows north of the system across the Mid-Atlantic, with light to moderate rains for portions of the Southeast. The next frontal wave on Sun may bring another round of light to moderate rain/snow to portions of the Mid-Atlantic and Southeast. The more significant low pressure system expected to develop along the western Gulf Coast early next week will have access to much deeper moisture courtesy of the amplifying trough to the west and subtropical ridge to the east, and this system looks to be a major precipitation producer. Isentropic lift across a lingering polar front ahead of the developing low pressure system is expected to produce widespread moderate to heavy rain from portions of the lower Mississippi Valley to the Southeast and Tennessee Valleys late Mon into Tue/Wed. Colder air to the north of the system may result in an area of wintry precipitation from portions of the mid-Mississippi Valley to the Mid-Atlantic. Current model guidance suggests some potential for a significant winter weather event across portions of the Mid-Atlantic on Tue, with snow, sleet, and freezing rain all possible. Please refer to the Winter Weather Outlook probabilities for days 4-7 for further details on the winter weather threats in the medium range. Meanwhile, while a significant shortwave Sat-Sun will ensure that rain/mountain snow continue along the West Coast, this region will see some respite by Mon-Tue as the main trough axis shifts east and heights rise a bit. Arrival of another shortwave by late Tue into Wed may bring another round of precipitation to the Northwest. Well below average temperatures will persist and become more widespread across the western/central U.S. through the medium range. The core of the cold air late this week will initially be from the northern plains to the Upper Midwest, with temperatures 15 to 25 deg F below average. By Sun-Mon as the western trough pushes inland expect high temperatures of 15 to 30 deg below average across much of the western and central U.S., continuing through the end of the forecast period (Wed). 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