Extended Forecast Discussion NWS Weather Prediction Center College Park MD 211 AM EST Sat Mar 07 2020 Valid 12Z Tue Mar 10 2020 - 12Z Sat Mar 14 2020 ...Heavy Precipitation threat early-mid next week for Southern California/Southwest... ...Guidance/Predictability Assessment... A relatively progressive northern stream and somewhat less progressive 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 trend toward a somewhat slower progression of this system during days 3-5. This trend gets larger 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. In general, 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, with more weight toward the ensembles by days 6-7 (Fri-Sat) given the uncertainty. A number of solutions showed a relatively amplified shortwave reaching the Pacific Northwest by next Fri-Sat. This feature is present across multiple runs/models, but run-to-run continuity is relatively poor with respect to the specific nature of the system (e.g. closed upper low or very progressive/positively-tilted shortwave). The more ensemble weighted blend described above for days 6-7 should resolve this feature as reasonably well as possible given the uncertainty. ...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. 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, showers and thunderstorms, some strong, are expected to become more widespread from the Southern Plains to the TN/OH Valleys then Appalachians ahead of a cold front. There remains much uncertainty with the main axis of any heaviest amounts. The Pacific Northwest should see an increase in rain/mountain snow by late next week as the aforementioned trough and associated frontal system affect the region. Given uncertainty in the timing and structure of this system, precipitation amounts were kept relatively conservative at this time, until models get a better handle on the synoptic evolution. Temperatures during the medium range are forecast to be near to above average across most of the CONUS, as the progressive northern stream keeps any cold air bottled up at the higher latitudes. High temperatures are forecast to be 5 to 15+ deg F above average across most of the central/eastern U.S. through most of next week. Slightly below average high temperatures are forecast across portions of the Southwest as the upper low moves inland. 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