Extended Forecast Discussion NWS Weather Prediction Center College Park MD 153 AM EDT Tue Mar 19 2019 Valid 12Z Fri Mar 22 2019 - 12Z Tue Mar 26 2019 ...Overview and Guidance Assessment/Preferences... Split flow pattern is expected to persist across North America during the extended period. Ensemble guidance shows good consensus that a strong and blocking upper-level ridge axis initially across the High Plains of the U.S. into Alberta/Saskatchewan will meander in place over the next week. This will continue to split incoming energy from the Pacific into northern stream components directed northward toward Alaska, and southern stream components that traverse the southern/central U.S., undercutting the ridge. Downstream of the ridge, the Northeast will be subject to continued mean troughing, reinforced by a series of energetic shortwaves. Some guidance suggests that perhaps this northern stream amplified ridge/trough pattern may begin to weaken by early next week, but confidence in that is relatively low at this time given increased spread among the ensemble guidance as to the intensity and duration of the upper ridge. Model guidance has made a dramatic shift over the past 24 hours with respect to a low pressure system along/off the Eastern Seaboard. At this time last night, the consensus among the majority of guidance was for a track farther east, with significant deepening delayed until the system reached the Canadian Maritimes. Each consecutive run of the guidance, this morning and this evening has generally shifted the track west, with the ECMWF and GFS now showing a significant surface low just east of Cape Cod Fri morning (and the CMC/UKMET much farther southeast). Clustering of ensemble members around this idea is quite good among ECENS and GEFS members, with a secondary cluster farther southeast/offshore comprised primarily of Canadian ensemble members. Clustering around these solutions remains generally good into Fri night-Sat as the surface low moves northeastward, with some modest increase in spread. Farther west, a more stable consensus continues with respect to a shortwave/upper low moving from the Four Corners region to the Plains Fri-Sat. Solutions were generally well-clustered with respect to the timing and intensity of this feature during days 3-4. By day 5 (Sun), as the shortwave moves into the Midwest, solutions differ as to how quickly it will weaken, but there is general consensus that the feature should gradually shear out as it encounters a col between the northern/southern stream flow and the upper ridge. Farther west, additional Pacific shortwave energy is forecast to reach the West Coast Sat, and then split, with a more significant southern component also moving east across the Four Corners region Sun. Models differ on their handling of this splitting as well, with the ECMWF persistent in showing the strongest southern component. The complex nature of this process lends to low predictability and confidence in this aspect of the forecast. Based on these considerations, the forecast during days 3-5 was based primarily on a blend of the ECMWF and GFS, with some influence from their respective ensemble means as well. By days 6-7 (Mon-Tue) the second Pacific shortwave should reach the Southern/Central Plains in some form (solutions are similar in timing even if they differ on the amplitude), likely promoting development of a surface low. Ensemble members show some degree of clustering even through day 7 with respect to this system, and the ECMWF was generally well-centered in the spread. The GFS, on the other hand, seems to suffer its prevalent bias and accelerate the system eastward far more quickly than consensus. Differences also emerge in the northern stream by early next week, with different degrees of troughing across New England. Given some degree of persistent ridging upstream, would likely lean toward a somewhat more amplified trough for New England as shown by the ECMWF and 18Z/00Z GFS runs. The GFS has come more in line with consensus with another system off the West Coast early next week, now showing a closed upper low linger offshore rather than an open wave rushing inland as it showed last night. Based on these factors, the forecast during days 6-7 was based primarily on a blend of the ECMWF and the ECENS/GEFS ensemble means. ...Weather Highlights and Hazards... The low pressure system off the Northeast coast early Fri is expected to produce widespread precipitation for New England. Rain will be likely close to the coast, with snow possible for the interior. Marginal thermal profiles will make the precipitation type across New England highly sensitive to the eventual track of the surface low. Showers and thunderstorms should increase in coverage across portions of the Southern and Central Plains Fri-Sat ahead of the leading Pacific system. By Sun-Mon, as the second Pacific shortwave reaches the central U.S., rain and thunderstorms should spread east into the Mississippi/Ohio Valleys, Gulf Coast, and portions of the Southeast. Confidence is low as to the degree of moisture return and the strength of the surface low, but at least locally heavy rain appears possible. Across the West, the passage of these systems will bring a couple rounds of elevation-based rain/snow from the West Coast the Great Basin and Rockies. Precipitation may increase once again for northern California early next week ahead of the next Pacific low pressure system. Meanwhile, the Midwest will see the greatest potential for above average temperatures through the medium range. High temperatures of 5-15 deg F above average are expected Fri-Sat, with above average temperatures expanding eastward by Sun-Mon. Ryan Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are 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 https://www.wpc.ncep.noaa.gov/heat_index.shtml