Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sun Jan 26 2020 Valid 12Z Wed Jan 29 2020 - 12Z Sun Feb 02 2020 ...Overview and Guidance/Predictability Assessment... During the latter half of the week model/ensemble guidance continues to advertise a transition to a more amplified mean pattern with a strengthening Pacific ridge building over the western U.S. and a downstream trough taking shape over the Plains and east-central states. Then during the weekend there is good agreement that upstream flow should finally begin to push northeastern Pacific mean troughing eastward, with the trough axis most likely reaching far western Canada and near the Pacific Northwest coast by late Sun. As this occurs expect the western U.S. ridge aloft to flatten while the eastern U.S. mean trough continues eastward. While there is good agreement and continuity with the overall pattern evolution, forecast specifics continue to have lower than average confidence. Not only do difficulties persist in resolving individual shortwaves ahead of the building Pacific/western U.S. ridge but details of how flow around the top of the ridge splits--plus interaction among shortwaves--continue to be very uncertain with such evolution tending to have below average predictability beyond the short range time frame. Early in the period the guidance has yet to settle on a particular solution for the system expected to be over the Lower Mississippi Valley/Gulf Coast region as of early Wed. Multi-day continuity had shown a pronounced weaker/suppressed trend only to be followed in the past day by at least a partial trend back to a more phased shortwave aloft and farther north surface system as has been suggested by recent CMC runs. The updated forecast reflects the best consensus of 00Z-06Z guidance. Trailing evolution provides increasing complexity with time. There is decent clustering/continuity for amplifying western U.S. energy to form a fairly sharp southern Rockies/northwest Mexico upper trough with a closed or nearly closed low as of early Thu. However after that time spread rapidly increases for energy that drops into the central U.S., affecting ejection of the Rockies/Mexico trough. Uncertainty then compounds as a fairly wide envelope exists for how energy flowing around the western ridge drops into the east-central U.S. mean trough after early Fri. Playing a role in the Fri-Sun eastern U.S. evolution as well as forecast specifics over and near the Northwest, there are meaningful differences in strength/position for the western ridge during that time. Given typical guidance error 5-7 days out in time, an intermediate solution between stronger GFS/GEFS mean runs and somewhat flatter ECMWF/ECMWF mean provides a reasonable starting point. Meanwhile the 00Z CMC shows the lowest heights over the West due to a farther west upper ridge axis and the CMC mean does not support that idea. The 12Z CMC compares better to other guidance. The intermediate preference reflects sensitivity to details in the southern half of the upper trough approaching the Pacific Northwest, with various degrees of potential separation possibly developing between 130-150W longitude during the weekend--another aspect of flow that has low predictability especially 6-7 days out in time. As for surface evolution over/near the eastern U.S., surface low plots still show very little clustering during Sat-Sun but with a little more density from the eastern Gulf through the western Atlantic. Especially around Sat the 00Z ECMWF becomes flatter than most other solutions with eastern U.S. flow aloft so its well offshore surface system appears questionable--though the new 12Z run persists with such a scenario. The 06Z GFS matched up well with the 00Z ECMWF mean for timing/track close to WPC continuity so those three were emphasized for the weekend mass field forecast. The 06Z GEFS mean has a similar idea but 12 hours later. Thus far the GFS has been fairly consistent for timing/latitude of the surface low but have varied between near-coastal tracks (06Z and 12Z runs) and more inland ones. ...Weather/Hazard Highlights... The forecast pattern evolution will maintain a focus for periods of heavy rain and mountain snow over the Pacific Northwest, especially over favored terrain along the Cascades and near the coast. Best confidence for highest five-day totals is over Washington while subtle differences in strength of the upper ridge building over the western U.S. and overriding shortwaves will affect the extent to which significant activity reaches into northwestern Oregon. Expect some of this moisture to produce heavy snow in the extreme northern Rockies as well, but again with some uncertainty over southward extent. Shortwave energy dropping through the West around midweek will produce mostly light/moderate precipitation over the central/southern Rockies. Farther east expect the weakening system over the Lower Mississippi Valley/Gulf Coast on Wed to produce mostly light-moderate rainfall over parts of the southern tier early in the period. One or more of the shortwaves amplifying downstream from the building western U.S. ridge aloft may produce some precipitation (mostly rain) over the southern Plains around Thu, followed by expansion of precipitation over parts of the South and East from Fri into the weekend as surface low pressure evolves ahead of upper troughing. Model/ensemble spread and inconsistency with specifics at the surface and aloft--not to mention uncertainty in thermal profiles--keep confidence low in determining coverage and amounts of precipitation as well as precipitation type. From the probabilistic perspective, best potential (but still moderately low in absolute terms) for some snow extends from the Appalachians through parts of the Mid-Atlantic and Northeast. For the period as a whole, expect warmest temperature anomalies across the northern tier with highs averaging 10-20F above normal and morning lows 15-25F above normal. The western U.S. will see a pronounced warming trend as ridging builds overhead, leading to increased coverage of plus 10-20F anomalies. By next weekend a trend toward westerly flow aloft/downslope flow at low levels should promote increasing warmth over an expanding portion of the High Plains. The central High Plains may see readings as high as 10-25F above normal next Sun. Locations from the central-southern Rockies through the extreme southern tier may see a day or so of near to slightly below average temperatures. 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