Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Sat Feb 09 2019 Valid 12Z Tue Feb 12 2019 - 12Z Sat Feb 16 2019 ...Overview... Latest model/ensemble guidance persists in showing a North Pacific through Alaska mean ridge that will be a dominant influence on downstream flow affecting the lower 48 states and vicinity. Multi-day means are remarkably agreeable and consistent in showing a strong core of associated positive height anomalies south of the Alaska Peninsula near 50N 160W. Teleconnections relative to this center support the deep positively tilted mean trough which guidance keeps over the Pacific Northwest, while above normal heights should prevail over the southern/eastern states--around the northern periphery of a subtropical ridge. Over recent days guidance has been somewhat inconsistent with the degree of positive height anomalies over the South/East, with some differences noted again today. Within this flow regime expect an active pattern with storm systems affecting the West Coast and then intensifying as they track from the central Plains northeastward--along a strong gradient between very cold air over the northern Plains and warmth over the South/East. ...Guidance/Predictability Assessment and Preferences... The most prominent forecast uncertainty continues to be with upper troughing forecast to be near the West Coast around days 3-4 Tue-Wed. In the past couple 12-hourly cycles the ECMWF and its mean have reverted back a bit toward more westward elongation for a time before ejecting inland. On the other hand the 00Z UKMET strays well ahead of other guidance, leaving the 00-06Z GFS/GEFS mean and CMC as the intermediate solutions. At the same time as guidance spread for this trough increases, there is been an increasing signal toward a low latitude wave approaching California, with development along a forming frontal boundary to the east of the strong storm affecting Hawai'i. The exact timing of the northern stream trough will be crucial for determining the track/strength of the low latitude wave. Overall preference at this time is for an intermediate solution no slower than a compromise between GFS/GEFS/CMC and ECMWF/ECMWF mean clusters, given spacing considerations relative to rapidly approaching upstream North Pacific shortwave energy. This solution remains close to continuity, with a low latitude wave that is weaker and farther south than depicted in the past couple ECWMF runs. As the western trough ejects eastward, guidance actually trends closer together for the overall trough axis but differs considerably over exact distribution of energy within the trough and depth of any embedded upper low. As a result there is wide array of possible surface low track/depth--as shown by a broad/poorly clustered spread of surface lows among the ensembles. Over the eastern half of the country by Fri-Sat the GFS/GEFS mean/CMC runs show greater upper trough amplitude versus other guidance or what would be favored by the teleconnection-favored pattern so preferences would be to lean away from that scenario late in the period. Note that in recent days some guidance had been more amplified aloft than current forecast for the leading system expected to affect the East early in the period. As for this leading system, a majority cluster approach provides a reasonable starting point for the forecast. Overall the next storm system approaching the Pacific Northwest by late week/weekend shows average or better agreement relative to typical days 6-7 forecasts. The 00-06Z GFS runs lean somewhat toward the southwestern part of the full guidance envelope for the upper system and surface low center so prefer to lean away from the GFS solution for this part of the forecast. As forecast considerations evolve over the course of the period, the starting blend used somewhat more total 00-06Z GFS/00Z CMC contribution relative to the past couple ECMWF runs days 3-5 Tue-Thu and then transitioned to more ECMWF/ECMWF mean weight by day 7 Sat. ...Weather Highlights/Hazards... Over the West expect one period of enhanced precipitation to spread across the region Tue-Thu with the combination of a Pacific Northwest system and possible lower latitude wave that could bring a surge of heavy rain and high elevation snow to California including southern parts of the state. Another area of moisture is likely to overspread parts of the West from Thu or Thu night into the weekend with the next system dropping southeastward toward the Pacific Northwest. As has been the case with the preceding system there is considerable uncertainty over the southward extent/intensity of precipitation over California. Persistently cold temperatures will maintain the threat for snow at very low elevations over the Northwest, while rain and higher elevation snow will prevail farther south. Over the Interior West and Rockies the majority of precipitation should be snow except over southern areas. Expect highest 5-day totals over western Oregon and northern-central California. The system tracking from the Midwest into Great Lakes/Northeast Tue-Wed will bring potentially moderate to heavy snowfall to areas north of the low track. Overrunning moisture north of the leading warm front may also bring wintry precipitation to portions of the Mid-Atlantic and Northeast, with the potential for heavier snow across portions of New England as another low center develops along the coast Tue night/early Wed. The storm system emerging from the West by Thu and then tracking northeastward should bring another widespread area of precipitation to the eastern half of the country late week into the weekend. Depending on similarity of track, winter weather impacts may cover some of the same areas affected by the preceding storm. Warm sector rainfall may be heavy over and somewhat northwestward of the Southeast. Within the broad area of below normal temperatures over the West and northern Plains, expect the most extreme anomalies over the northern Plains and to some degree the interior of the Northwest with some readings 15-25F below normal. Within the generally warm pattern over the South/East (albeit with some day-to-day variability due to system progression) there may be multiple days with temperatures of at least 10-20F above normal. Rausch WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation and winter weather outlook probabilities are found 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