Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Sat Apr 06 2019 Valid 12Z Tue Apr 09 2019 - 12Z Sat Apr 13 2019 ...Confidence increasing that a significant late-season storm will impact portions of the Northern Plains and Midwest with heavy rain and snow mid to late next week... ...Overview and Guidance Assessment/Preferences... A moderately progressive northern stream early in the medium range will become more amplified over time, with split flow becoming increasingly prominent through the extended period. One southern stream shortwave is expected to cross the Southeast U.S. Tue-Wed, with another crossing from the West Coast to the Four Corners/Rockies during the same time frame. The second of these waves is forecast to quickly amplify as it approaches the central U.S. late Tue into Wed, with support increasing for a significant late-season cyclone across the central U.S. by Wed-Fri. Additional shortwave energy is expected to drop southward along the West Coast Wed-Sat, eventually consolidating into an anomalous upper trough (500 hPa height anomalies of -2 to -3 standard deviations) across the southwestern states. A multi-model deterministic blend was used as a forecast starting point during days 3-5 (Tue-Thu), including the 06Z GFS and 00Z ECMWF/UKMET/CMC. Guidance has shown a trend toward more separation of streams across the eastern U.S. Tue-Wed, keeping a somewhat less amplified and more distinct southern stream system that crosses the Southeast and moves east into the Atlantic. This trend was reflected across a range of model/ensemble solutions and is well-represented by a general model blend. Farther west, models handle the shortwave crossing the Western U.S. Tue-Thu similarly, with only minor differences in timing and/or amplitude. Models have shown significant difficulty over the past couple days handling this system as the surface low deepens rapidly across the Central Plains Wed and then lifts northeastward. Solutions have varied between keeping a more open upper wave and quickly moving the surface low eastward, and a more closed upper low with a deeper surface low that progresses east much more slowly. There was a large reduction in spread during the last ensemble cycle, with solutions overwhelmingly trending toward the idea of a deeper/slower system. Interestingly, the UKMET has been the most consistent deterministic solution, showing a solution similar to the current consensus for at least the past three runs. Given these considerations, and solutions which were better centered within the ensemble consensus, the 00Z ECMWF/UKMET solutions were weighted a bit more relative to the GFS/CMC, especially by Wed-Thu. Spread still increases with respect to the evolution of this system during days 6-7 (Fri-Sat), and its potential interaction with the northern stream as it moves into Canada. Given significant continued run-to-run variability from Fri onward, opted to lean much more heavily on ECENS/GEFS ensemble means during that time frame. This preference holds along the West Coast as well, where models have struggled to deal with the specifics of a series of shortwaves dropping southward across the West Coast/Great Basin. There is some consensus, however, that energy should eventually consolidate into a broad and anomalous upper-level trough across the Southwest by Fri-Sat. ...Weather Highlights/Hazards... Confidence has increased that a deep late-season storm system should track from the Central Plains to the Midwest/Great Lakes during the Wed-Fri time period. The upper-level shortwave will initially bring rains and potentially heavy mountain snows to much of the Rockies Tue into Wed. After the system emerges into the Plains, models show improved consensus that significant band of dynamically forced precipitation is likely to the north of the low track, from portions of eastern Wyoming and the Dakotas east to the Upper Midwest. A variety of precipitation types are likely, but sufficient cold air should be in place to support a band of moderate to heavy snow. Some areas near the precipitation transition zone will likely see the potential for both heavy rain and snow. Additionally, the deep nature of the low (central pressures likely in the mid 980-mb range), will likely result in the potential for strong and gusty winds across a large area of the central U.S. Farther south, limited moisture return and potential for a capping inversion will limit precipitation coverage ahead of the system across the Southern Plains, although given the strong and dynamic nature of the low pressure system, any isolated to scattered convection that develops could be strong. As the low moves into Canada by Fri-Sat and the cold front sweeps into the eastern U.S., widespread rains are likely from the Ohio Valley to the Mid-Atlantic and Northeast. Farther west, the series of shortwaves diving south along the West Coast Wed-Sat will support continued scattered and periodic rounds of rain and mountain snow for much of the West. Warm temperatures are expected ahead of the deep low pressure system Tue-Wed from the Plains to the lower/mid-Mississippi Valley, where high temperatures of 5 to 15 deg F above average are likely. In the wake of this low pressure system, a colder polar air mass will overspread areas from the northern Rockies to the Upper Midwest, with high temperatures as cold as 15 to 25 deg below average expected. A broad area of cool temperatures is expected across the interior western U.S. by late next week as upper troughing amplifies overhead. 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