Extended Forecast Discussion NWS Weather Prediction Center College Park MD 251 PM EST Wed Jan 06 2021 Valid 12Z Sat Jan 09 2021 - 12Z Wed Jan 13 2021 ...Overview... Latest guidance continues to show establishment of an amplified mean ridge aloft over western North America and downstream mean trough extending from eastern Canada through the southern Plains/northern Mexico. There is reasonable consensus that the core of the ridge over the Pacific should strengthen by next Tue-Wed, raising heights over the West at that time. With decent confidence expect Pacific shortwaves/fronts to support periods of precipitation focused over coastal Washington and Oregon with a tendency for heavier activity early next week. However within the downstream mean trough, individual model runs show a seemingly infinite array of possibilities for individual shortwaves/possible upper lows. Thus guidance clustering rapidly diverges after being more agreeable for Gulf Coast rainfall and associated northern Mexico/Gulf low pressure from the weekend into Mon, with confidence decreasing accordingly next week. This system will support a period of cool daytime temperatures across the southern tier while the large scale pattern should promote persistent warmth over the northern Plains and vicinity. ...Guidance/Predictability Assessment... Given the below average predictability of some of the fairly small features that could play a significant role in the forecast, the updated blend employed an operational model composite early followed by a transition toward relatively higher emphasis on the 00Z ECMWF mean and WPC continuity. Into about day 4 Sun there is decent clustering for the shortwave that drops into the southern Plains and northern Mexico. Then what becomes of this feature and Gulf low pressure depends on the evolution of various features within what could be at least two separate streams--one covering the northeastern quadrant of the lower 48 and another bringing energy south from Canada into the Plains and vicinity. Over the past couple cycles there have been increasing hints of an upper low dropping into the Plains (GFS being the last to trend this way, in its 12Z run) but there is still disagreement over which piece of energy creates this feature. Plus the new 12Z ECMWF has decided to divert this upper low energy eastward across southern Canada instead. Meanwhile Great Lakes/Northeast upper flow may also play a significant role in the eventual track of the wave crossing the Gulf along with the westward extent of precipitation during the first half of next week. By early next Tue the one aspect of the forecast maintaining some consistency from yesterday is a GEFS/ECMWF ensemble signal (a little better defined in the 00Z ECMWF mean versus the 06Z GEFS) for weak low pressure to be just off the East Coast. Then some combination of this wave and/or development associated with northern stream flow reaches south of the Canadian Maritimes by early Wed. The updated blend yielded a slightly slower trend versus continuity from the Gulf into the Atlantic but maintained a similar evolution in principle. Also this provided intermediate timing between the faster GFS/GEFS and slower 12Z CMC/00Z ECMWF. The main forecast consideration along the West Coast would be to downplay whichever solutions are strongest with individual shortwaves heading into the mean ridge. Notable examples include the new 12Z UKMET around Sun and early Tue as well as the 00Z ECMWF near the end of the period (approaching British Columbia). ...Weather/Threats Highlights... The Pacific Northwest will continue to see periods of precipitation with few breaks during the period. Low confidence in details of shortwaves carried by fast flow aloft will temper confidence for precise intensity and southward extent at any particular time. Still the overall guidance signal is for heaviest rain/high elevation snow to be during the first part of next week and especially over Washington. The system emerging from the southern Rockies on Sat may access enough Gulf moisture to produce some locally enhanced rainfall along and just offshore from the Texas coast, while recent southward trends suggest that areas farther east along the Gulf Coast will see only light to moderate rainfall. It would not take too great of a trend reversal to change that though. Some snow will be possible from the Colorado/New Mexico Rockies into parts of the southern Plains. Confidence remains low for precipitation coverage and type over the East during the early/middle part of next week. The fairly broad coverage of light amounts in the manual forecast reflects the high uncertainty and an ECMWF mean tilt while awaiting better clustering. Expect locations from Montana into Minnesota to be in the core of warmest temperature anomalies through the period, with some coverage of plus 10F or greater anomalies each day. Morning lows should generally be farther above normal than the daytime highs with plus 20F or greater anomalies for lows most likely on Sat and next Wed. Strengthening upper ridge should lead to more widespread plus 5-10F or so anomalies over the West Coast states into Nevada heading into midweek. Within a cool pattern for the southern tier, the best potential for highs more than 10F below normal will extend between the southern Plains and Lower Mississippi Valley from the weekend into the start of next week. Readings across the South should moderate gradually by midweek. Rausch/Fracasso Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Mon-Tue, Jan 11-Jan 12. - Heavy rain across portions of the Southern Plains, Sun, Jan 10. 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