Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1155 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. 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 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 compact/robust surface low off the Eastern Seaboard Thu night-Fri. Trend in the ensemble guidance has been toward a bit slower/deeper and closer to the coast solution, forming a middle ground between the deeper 00Z ECMWF and 06Z FV3-GFS and weaker/quicker 00Z/06Z GFS and 00Z Canadian/UKMET. This blend sufficed for the west/central states as well given some overall minor detail differences. 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. Timing differences become more significant Sat-Mon in the West as subsequent Pacific shortwaves move inland. In general, the 00Z GFS continued to be quicker than the consensus of other guidance and was excluded from the forecast. 06Z GFS and 06Z FV3-GFS were slower like the ECMWF and its ensemble mean, which have been much more consistent with the cross-continental flow over the past several runs compared to the GEFS. Used a blend of those slower solutions with continuity for a starting point which only resulted in a few minor changes to the fronts, but perhaps more changes to the PoPs given timing uncertainties of return flow from the Gulf this weekend. ...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 expand 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 the Pacific Northwest and 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. Above average temperatures are expected to develop by late this week across the Upper Midwest and then gradually spread eastward across the Great Lakes to the Eastern Seaboard through the weekend as surface high pressure moves offshore. 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. Cooler than average temperatures in the Southwest will moderate a bit late this weekend ahead of the Pacific system just offshore next Monday. Fracasso/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