Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EDT Mon Mar 18 2019 Valid 12Z Thu Mar 21 2019 - 12Z Mon Mar 25 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. and Canada will very slowly shift eastward over the next week. With the strongest positive height anomalies remaining centered across north central Canada, 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 depictions are now relatively similar with respect to the upper low/trough crossing the Four Corners Thu-Fri and exiting into the Plains Fri night-Sat. Farther east, solutions differ more significantly with respect to small scale northern stream shortwave energy traversing the larger mean trough, and the potential for development of a surface low off the Eastern Seaboard Thu night-Fri. The majority of solutions keep any wave of low pressure relatively weak off the U.S. East Coast, delaying significant intensification until the feature nears the Canadian Maritimes where it potentially interacts with additional shortwave energy. Finally, models show general consensus that an another shortwave/frontal system should reach the West Coast Fri-Sat, with relatively minor timing and amplitude differences through Sat. Based on these factors and an assessment of moderate confidence, a blend of the ECMWF/GFS/UKMET was used a basis for the forecast during days 3-5. Timing differences become more significant as the second Pacific shortwave moves inland by Sun, with the GFS persisting in a much quicker progression relative to the slower ECMWF/CMC and ensemble means. The same holds true regarding yet another potential system approaching the West Coast. The GFS continues to be quicker than the consensus of other guidance in bringing another amplified trough/upper low to the West coast by Sun night-Mon. Given these differences, the GFS was excluded from the forecast during days 6-7, with the forecast based on a blend of the ECMWF and the ECENS/GEFS ensemble means. ...Weather Highlights and Hazards... Rain and high elevation snows are expected across the southern/central Rockies Thu-Fri as the leading Pacific system passes the region. Increasing moisture return ahead of the system will increase coverage of showers and some thunderstorms across the southern/central High Plains by Friday. The next system will bring another round of rain/snow to Pacific Northwest to the Rockies Fri-Sat, with further increased potential for rain and thunderstorms, some of which could be heavy, from the southern/central Plains to the Mississippi Valley through the weekend. As the upper ridge moves slowly east, above average temperatures are expected to develop by late this week across the Midwest, gradually spreading east across the Great Lakes to the Eastern Seaboard through the weekend. Maximum temperatures ranging from 5 to 15 deg F above average are forecast, with minimum temperatures approaching 20 deg above average for some areas across the Midwest and Mississippi Valley. 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