Extended Forecast Discussion NWS Weather Prediction Center College Park MD 140 AM EST Tue Feb 12 2019 Valid 12Z Fri Feb 15 2019 - 12Z Tue Feb 19 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. This will leave the flow regime across the CONUS largely quasi-zonal. Teleconnections relative to the strongest 500 hPa positive height anomaly center across the North Pacific continue to favor a positively-tilted mean trough near the Pacific Northwest with weak positive height anomalies from the southern Plains east into the Southeast, and a broad region of modestly cyclonic upper-level flow across much of the CONUS. Shortwave energy ejecting from the northeastern Pacific every couple days will move cross the CONUS, undergoing gradual amplification in the broad cyclonic flow. Model guidance suggests that the dominant storm track should gradually shift southward over the next week, with a much more active southern stream taking shape by early next week. ...Guidance/Predictability Assessment and Preferences... Despite relatively good agreement among the guidance on the large scale flow regime, the smaller scales remain highly chaotic with low predictability. The system crossing the central/eastern U.S. on day 3 (Fri) has shown continued volatility in model solutions over the past several runs, with varying degrees of interaction between northern/southern stream shortwave energy. The trend over the past 12 hours has been overwhelmingly toward a more phased system with low pressure moving northeastward much more quickly, with a frontal wave developing across the lower Mississippi Valley in response to additional shortwave energy. Given the time range under consideration, one would expect the degree of run-to-run variability to be quickly decreasing; but models have demonstrated difficulty for days with their handling of this system, and would prefer to see another run of more stable solutions before forecast confidence increases. Farther west, additional Pacific shortwave energy is expected to cross the Southwest and approach the central U.S. by Sat. Given the intense frontal zone lingering near the Gulf Coast, this wave seems like to generate a frontal wave by day 5 (Sun) although model solutions vary widely as to the details of any frontal wave (or whether one occurs at all). The GFS continues to develop the strongest wave, with the ECMWF weaker. The WPC forecast during days 3-5 was based on a blend of the 12Z ECMWF/18Z GFS, along with their respective ensemble means, and a small component of forecast continuity to temper the dramatic adjustment slightly with the northern stream system days 3-4. Meanwhile, a more amplified upper trough appears set to move into the Southwest on Sun, crossing the Four Corners region on Mon and reaching the southern/central plains by Tue. While model/ensemble consensus is initially good with this system, differences energy by the time the shortwave reaches the central U.S., with differing levels of northern stream influence resulting in varying surface evolutions. One thing in common among the guidance that is different from the system late this week is that the southern stream feature appears more dominant. Given the persistent and intense baroclinic zone across the Gulf of Mexico/Gulf Coast by that time with the subtropical ridge building off the Southeast U.S. coast, this at least opens the door to the development of a significant frontal wave somewhere in the vicinity of the Gulf by next Tue. Confidence in the specifics of any such feature is quite low at this point, however. The WPC forecast during days 6-7 was based heavily on a blend of the ECENS/GEFS ensemble means, with minority components of their respective deterministic solutions. ...Weather Highlights/Hazards... Areas from central/northern California to western Oregon and Washington will see continued rounds of precipitation (rain and snow varying with elevation) through Sat before at least somewhat of a lull by late in the weekend/early next week as the main upper trough shifts inland. Another relatively weak shortwave/frontal system arriving by Mon may produce another round of precipitation for the Northwest. The low pressure system expected to cross Ontario/Quebec on Fri may produce areas of wintry weather across the interior Northeast in a region of warm-advection precipitation. Farther south, a frontal wave crossing the Southeast Fri-Sat will produce widespread moderate rains and perhaps a few thunderstorms along the Gulf Coast. The next southern stream frontal wave crossing the Southeast Sun-Mon looks to produce a more significant area of precipitation from the lower Mississippi Valley to the southern Mid-Atlantic, with an area of heavy rainfall possible across the Tennessee Valley and the southern Appalachians/foothills. Model solutions vary as to the degree of cold air north of the system, but given the rather tight thermal gradient depicted in much of the model guidance across the region, it seems likely that at least some areas on the northern portion of the precipitation area may see some potential winter weather. 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 upper trough move into the West and heights fall across much of the western/central U.S. 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 (Tue). 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