Extended Forecast Discussion NWS Weather Prediction Center College Park MD 150 PM EST Sat Mar 07 2020 Valid 12Z Tue Mar 10 2020 - 12Z Sat Mar 14 2020 ...Heavy Precipitation Threat next week for Southern California/Southwest... ...Guidance/Predictability Assessment... An interesting and active flow regime is on tap for the lower 48 and vicinity next week as a relatively progressive northern stream and less progressive/separated southern stream are forecast across the CONUS during the medium range. At least through the first half of next week, models/ensembles showed relatively good consensus, but that degrades quite a bit toward the end of the forecast period. A couple northern stream shortwaves will accompany a front/low pressure system across the Great Lakes and Northeast Tue-Wed. Farther west, across the east Pacific, a deep cutoff upper-level low is forecast by all guidance to approach Southern California by Tue-Thu, eventually reaching the central U.S. Models/ensembles over the past 24 hours have shown a significant trend toward a slower progression of this system. This trend becomes quite robust by days 6-7 when the latest guidance is quite a bit slower than previous runs. This slower progression makes for less phasing/interaction with northern stream energy than was shown in previous runs, as southern system now completely misses the northern stream energy in terms of timing. Thus, latest solutions show a continued separate southern stream wave emerging into the Plains by Fri. Ensemble spread by this time remains rather large, however. Given the impact on timing with potential for northern stream interactions, forecast confidence across the central U.S. by Fri-Sat is low, but generally less progressive recent GFS runs are now with latest 06 UTC and now 12 UTC guidance more in line with the ECMWF and ensemble means. In general though, a blend of the ECMWF along with ECENS/GEFS ensemble means depicted what seemed to be a reasonable consensus solution for this system through the period. An amplified shortwave trough/low is slated to reach the Pacific Northwest next Fri-Sat. This feature is present across multiple runs/models and recent run-to-run continuity has improved despite lingering uncertainty with respect to guiding upstream ridge specifics. Accordingly, a WPC guidance composite has trended continuity in favor of a deeper and wetter system. ...Weather/Hazard Highlights... A round of snow is expected for portions of northern New England Tue/Tue night as a wave of low pressure traverses the region. Portions of southern California and the Southwest will see a period of heavy precipitation potential through midweek as the upper-level low moves across inland. Rain is likely for most areas, but snows are possible at the highest elevations of Southern California, and more likely farther north across portions of the Sierra and downstream into higher elevations of the southern Great Basin/Rockies. The heaviest rains are forecast across the Transverse and Peninsular Ranges in Southern California, where 2-4 inches of rain are possible (with locally higher amounts). Heavy rains are also forecast cross the Mogollon Rim. Farther east, moist southerly flow should focus showers and thunderstorms along a stationary front from the Mid-Mississippi Valley to the Ohio Valley Wed-Thu. As the Pacific shortwave energy (former upper low) reaches the central U.S. by next Fri/Sat, showers and thunderstorms, some strong, are expected to become more widespread from the Southern Plains northeastward to the Midwest ahead the main system and overrunning a northern stream frontal drape. This solution has trended slower and more amplified than continuity. There remains much uncertainty with the main axis of any heaviest amounts and there seems enough cold air in upslope flow to the north of the drape to support some snow/ice on the northwest periphery of the expanding precipitation shield. Meanwhile, the Northwest should see an increase in rain/mountain snow heading into next weekend as the aforementioned trough and associated frontal system affect the region with increasing vigor. Ryan/Schichtel 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