Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sat Jan 25 2020 Valid 12Z Tue Jan 28 2020 - 12Z Sat Feb 01 2020 ...Overview and Guidance/Predictability Assessment... Initially low-amplitude flow containing progressive shortwaves embedded within multiple streams will evolve toward a more amplified regime as an upper ridge builds over the West while a downstream mean trough becomes aligned from the central/east-central U.S. into northern Mexico. Forecast details within this fairly agreeable large-scale evolution continue to be problematic. There is gradually improving--but not yet ideal--consensus for shortwaves early in the period. Then there are significant differences and run-to-run changes for how the energy within strong Pacific flow entering the Pacific Northwest/western Canada splits and possibly interacts with leading shortwaves in one or more streams, leading to low confidence in southern/eastern U.S. specifics by Fri-Sat. There is a more consistent/agreeable signal for the significant precipitation forecast over the Northwest but amplitude and timing differences for individual shortwaves lead to some uncertainty for specifics here as well. Best consensus shows a modest system over the southern Plains as of early Tue weakening with a track through the Gulf of Mexico and points eastward as the dynamics aloft shear out. However CMC runs including the new 12Z version have been stronger and more phased aloft leading to a somewhat deeper and much farther north surface low. Compared to the full ensemble spread the CMC scenario at the surface is not quite an outlier but fairly extreme. The CMC scenario appears unlikely given the overwhelming cluster of weaker/suppressed solutions (and thus was not included for the manual forecast blend) but the typical difficulty of resolving small-scale shortwaves and their possible interaction, plus considerable changes/trending in the guidance over the past few days, keep the door open for more adjustments. To the west there has been decent continuity/clustering over the past day for a Pacific Northwest shortwave to amplify through the Interior West toward northwestern Mexico during Tue-Thu, with a continued possibility that an embedded upper low may form around the time the trough core reaches the U.S.-Mexico border. After early Thu the forecast becomes increasingly complicated as models and ensembles have displayed meaningful differences for energy that flows around the top of the building western ridge and then amplifies, and in turn how this energy interacts with shortwaves to the south and east (which have their own timing uncertainties). The most prominent trend in the latest guidance is for less aggressive amplification of northern stream flow late in the period. By day 7 Sat this leads to slower progression of a front dropping through the central U.S. and a tendency for a somewhat farther east track of low pressure that emerges from the Gulf of Mexico. Ensemble low plots as of early next Sat show little clustering with possible tracks covering the southeastern third of the CONUS and the Gulf/western Atlantic, albeit with slightly more density over the Atlantic. The 12Z ECMWF adds another possibility, with much more amplified Plains troughing by Fri--the shortwave ahead of the one for which guidance has trended less amplified. Prefer a compromise among latest guidance and continuity due to the high uncertainty. For the rapid succession of systems/fronts affecting the Northwest, the 00Z ECMWF was a weak extreme for the wave forecast to be just offshore as of early Tue while there is reasonable agreement for the front that approaches on Wed and a slower trend in consensus for the next front approaching Thu-Thu night. The 12Z CMC trends lower with heights aloft than other solutions and thus is on the fast side. Toward the end of the period differences develop with respect to the shape of the surface pattern over and offshore the Pacific Northwest, with a blend/continuity approach serving as the best option at this time. The updated forecast emphasized the 06Z GFS and 00Z ECMWF/UKMET during the first half of the period and then transitioned to a blend including the GFS/ECMWF, their respective means, and WPC continuity. ...Weather/Hazard Highlights... Confidence remains above average that the Pacific Northwest will see an extended period of active weather next week as multiple frontal systems impact the region. Each of these should produce episodes of potentially heavy rain/snow dependent on elevation. The upper ridge building into the West should lead to a rise in snow levels by late next week as well as a somewhat lighter trend for precipitation over far northwest California/southwest Oregon. Potential still exists for some five-day totals of at least 5-10 inches liquid over favored Cascades and coastal terrain of Washington and possibly into northwest Oregon. Some of this moisture will extend into the northern Rockies to produce snow that may be heavy especially around Tue-Tue night and then again late in the week. The system over the southern Plains on Tue and most likely tracking into the northern Gulf by Wed should be accompanied by showers and possibly thunderstorms. Expect mostly light to moderate rainfall as the best deep moisture likely remains to the south of the surface low track. The wave of low pressure expected to develop/emerge from the Gulf of Mexico later in the week should bring a more expansive area of precipitation to areas across the southern and eastern U.S.. High uncertainty over surface low track and details of dynamic support aloft make it difficult to resolve precipitation type and amounts at specific locations. Probabilistic winter weather guidance continues to suggest best potential (though low in absolute terms) for some snow over parts of the central-southern Appalachians and interior Mid-Atlantic Fri or Sat. While the upper pattern changes in character during the period, the Pacific origin of most flow aloft should continue to support near to above average temperatures over most of the lower 48 through the end of the week. Highest anomalies will continue to extend from Montana into the Upper Great Lakes with readings 10-25F above normal. Plus 20 anomalies will be somewhat more likely for morning lows. The West will also see increasing coverage of plus 10-20F anomalies by late week and next weekend with the upper ridge expanding overhead. The central-southern Rockies will be an exception to the warmth with a couple days or so of modestly below average highs. Rausch 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