Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1140 AM EST Mon Nov 26 2018 Valid 12Z Thu Nov 29 2018 - 12Z Mon Dec 03 2018 ...Multi-faceted storm from the Plains eastward this weekend... ...Overview... A blocking upper high near the Davis Strait will weaken later this week as upstream ridging over the northeastern Pacific into eastern Alaska becomes the new dominant upper feature. This favors troughing over the central CONUS and an attempt at subtropical ridging to push into Florida. This upper trough will support a robust storm system late this week through the weekend lifting out of the southern Plains into the Great Lakes region. ...Model Guidance Evaluation... Start of the period on Thursday will feature a storm moving into California where the models clustered fairly well (00Z UKMET was slower than the rest but not outside the ensemble spread). This will move through the Four Corners region Friday and into the Plains by Saturday. By that point, the models/ensembles diverge in the amplitude and speed of the developing surface low -- GFS and GEFS were generally quicker/weaker than the ECMWF and its ensemble mean with the Canadian a bit in between. Preferred the ECMWF and its ensemble mean due to its better consistency over the past few runs as the GEFS has alternated between favoring a stronger/weaker storm in its 12Z/00Z runs, respectively, the past two days. This would take the surface low into the central/eastern Great Lakes on Sunday and out through Quebec next Monday. Back to the west, another system is forecast to move into Oregon on Saturday though the 00Z ECMWF was different than the rest in its handling of two incoming systems from the west and northwest. Blended solution should suffice there as there has been less consistency overall among the guidance. That system will move through the interior west on Sunday and down into the southern Plains next Monday as broad troughing along 100W is flanked by ridging up/downstream per the ensembles -- more amplified in the ECMWF ensemble mean (preferred) than the GEFS mean. ...Weather Highlights... First round of precipitation in the medium range period for the west is expected Thursday as the strong Pacific low moves ashore. This should bring precipitation into Southern California given a southeasterly track of the system with enhancement along western-facing Coastal Ranges. Several inches to over a foot of snow is possible for the Sierras and also in the mountains outside Los Angeles (e.g., San Gabriel, San Bernadino, San Jacinto mountains). This system will reorganize in the southern Plains Friday where the Storm Prediction Center has outlined a chance of severe weather for northeastern Texas eastward (ArkLaTex to the ArkLaMiss). Convection will likely continue eastward on Saturday as the front moves eastward. Warm front along the Gulf Coast will lift northward but perhaps only slowly east of the Appalachians until enough southerly flow will take it northward. Widespread modest rainfall is likely along/ahead of the front with a strong moisture influx. To the north/northwest of the surface low, enough cold air should be in place for some snow or mixed precip from NE/SD northeastward perhaps into northern New England where icing could be in the mix as well given the surge of warm air aloft but marginally cold air at the surface. Along the Pac NW coast on Saturday, the next system will bring in rain/snow to the west again, but likely farther north than the previous one. Temperatures will be above average (about +5 to +15F anomalies) ahead of the western system Thursday through Sunday, first in the southern Plains then into the MS/Ohio/TN Valleys then into the east. Cooler/colder air will filter in behind it as troughing settles into the central states. Temperatures may be 10-20F below average in the interior west by next week. Fracasso WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes 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 https://www.wpc.ncep.noaa.gov/heat_index.shtml