Extended Forecast Discussion NWS Weather Prediction Center College Park MD 142 AM EDT Fri Mar 15 2019 Valid 12Z Mon Mar 18 2019 - 12Z Fri Mar 22 2019 ...Overview... A blocking mid/upper-level ridge initially centered from the Pacific Northwest to western Canada early next week is expected to very slowly shift eastward through the forecast period. With the ridge axis across western North America early next week, a mean trough will be in place across the Great Lakes/Northeast, with numerous small scale shortwaves traversing the larger trough. Strong westerly flow across the North Central Pacific will split as it encounters the ridge, with some shortwave energy directed north into the higher latitudes of Canada, and some separating into the southern stream. By the latter portion of next week as the western ridge axis shifts east, this may allow for stronger height falls from the North Pacific to approach the Pacific Northwest. ...Guidance Assessment/Preferences... In general a blend of deterministic models (ECMWF/GFS/CMC/UKMET) served as a good forecast starting point during days 3-5 (Mon-Wed). Overall, spread was relatively low during this time frame. Models differed on the amplitude of a northern stream trough digging into the central U.S. Tue-Wed, but no clear outliers were evident and a consensus approach was preferred. Models were also initially very similar with handling of an amplified southern stream shortwave/upper low approaching California by Wed. Differences in amplitude emerge as the system approaches the coast, with the GFS weaker and faster than the consensus of the ECMWF/UKMET/CMC which show a deeper system and slower progression. For this reason, the GFS from 12Z Thu (which was closer to the model/ensemble consensus) was used in the blend over the later runs. By days 6-7 (Thu-Fri), spread gradually increases as the southern stream system moves east into the Four Corners region and stronger height falls approach the Pacific Northwest. Weighting of the ECENS and NAEFS ensemble means was boosted during this time period to account for the increased spread ...Weather Highlights and Hazards... A lingering frontal boundary south of the Florida Peninsula (with a wave of low pressure developing along it) will support widespread and potentially heavy rains for the southern half of the Florida Peninsula Mon-Tue. The southern stream trough/upper low will spread rain and mountain snow into the Sierra Nevada, the Great Basin, and the central/southern Rockies by the middle to latter portion of next week. Precipitation should increase in coverage across the Pacific Northwest by late the week as the upper-level trough approaches. Temperatures will initially be 5 to 10 deg F below average early next week across the eastern third of the U.S. under the influence of the Great Lakes/Northeast upper trough, but temperatures across these areas will gradually moderate to near seasonal averages by the mid-latter portion of the week as the trough lifts out. Much of the West will start next week 5 to 10 deg above average, and the warm temperatures will gradually shift east through the week along with the upper ridge. Temperatures across the Midwest could reach 15 deg above average by late next week. 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