Extended Forecast Discussion NWS Weather Prediction Center College Park MD 354 PM EST Mon Jan 27 2020 Valid 12Z Thu Jan 30 2020 - 12Z Mon Feb 03 2020 ...Overview and Guidance/Predictability Assessment... A strong Pacific ridge aloft building into the western U.S. from Thu into Sat will encourage a larger scale mean trough to evolve over the Plains/Mississippi Valley. Then during the weekend into early next week an upper trough approaching the Pacific Northwest will move into the western states while the downstream trough crosses the eastern U.S. and continues into the western Atlantic. The mean trough crossing the eastern half of the country will consist of multiple shortwaves, leading to continued low confidence in evolution of surface low pressure that could affect parts of the eastern U.S. and western Atlantic. The eventual arrival of upper troughing into the West will finally break the pattern that has favored periods of significant precipitation over the Pacific Northwest. Operational models and ensembles continue to depict a very complicated evolution involving a number of shortwaves (each having moderate to low predictability) that drop into the developing central-eastern U.S. upper trough after streaming around the northern periphery of the building western ridge. These shortwaves include one over the southern Rockies/Plains and northern Mexico on Thu, additional energy dropping into the central U.S. Thu-Fri, and yet more energy reaching into the central/eastern U.S. from Canada Fri-Sat. From Fri into Sat the full range of guidance continues to depict a broad area within which one or more surface lows may exist, with greatest density from the eastern Gulf of Mexico to just off the East Coast. East-west ensemble spread valid 12Z Sat appears to have narrowed somewhat compared to past days but expands thereafter. GFS/ECMWF runs through 06Z have converged somewhat, with the GFS trending a bit farther offshore with low pressure and the 00Z ECMWF trending westward after the prior two runs had appeared excessively far eastward due to being quite flat with eastern troughing aloft. Timing issues persist with the ECMWF mean (probably the most consistent piece of guidance overall thus far) a bit in the faster portion of the full spread versus the 12-18 hours slower GEFS mean that has been on the other side of the spectrum. New 12Z GFS/UKMET runs side with the faster progression while the 12Z CMC is somewhat slower but catches up to the 00Z ECMWF mean by early Sun. The 12Z ECMWF has changed details aloft yet again, leading to faster timing of Atlantic low pressure but maintaining an offshore theme. In spite of the various ways of getting there, better agreement exists in principle for deep low pressure to reach near the Canadian Maritimes by Sun or Mon as the core of the upper trough approaches. Due to the high uncertainty the preference was to maintain an intermediate approach with some account for recent trends, mainly through the 00Z/06Z GFS and 00Z ECMWF. Upstream the GFS/GEFS versus ECMWF/ECMWF ensemble differences have narrowed somewhat for strength of the western ridge aloft, with the stronger GFS/weaker ECMWF appearing to gravitate toward a middle ground. By days 6-7 Sun-Mon there is better than average agreement among the 00Z/06Z operational models for the upper trough moving into the West. However there is still uncertainty due to the possibility that some energy in the southern part of the trough may separate between 130-150W longitude during the weekend--leading to questions over how amplified the western trough will be by next Mon. GFS runs had been flat/progressive with the trough before latest 00Z through 12Z runs conformed to recent ECMWF/ECMWF runs and 00Z CMC. On the other hand the latest GEFS means and 12Z CMC separate more trough energy upstream to yield a flatter western trough. Teleconnections relative to a core of positive height anomalies which D+8 means have been showing around 40-50N/140-150W support fairly amplified western U.S. troughing. Therefore late-period preferences side with the 00Z/06Z operational models and 00Z ECMWF mean. ...Weather/Hazard Highlights... From late week into the weekend the Pacific Northwest--and especially favored terrain along the Washington Cascades and coastal areas of Washington--will continue to see a favorable pattern for periods of heavy precipitation as moist flow rounding the strong western U.S. ridge aloft impacts the region. There is still some uncertainty over exact latitude of best focus though guidance spread has narrowed somewhat compared to yesterday. During this time frame expect snow levels to be fairly high. Lesser totals of snow should extend into the extreme northern Rockies. An upper trough moving into the West by early next week will finally push the overall precipitation shield southeastward and with less concentration, favoring a trend toward a broader area of light-moderate rain/high elevation snow over the West with snow levels decreasing. From late this week into the weekend expect an area of precipitation to spread across portions of the southern tier and eastern U.S. as low pressure develops over the Gulf of Mexico and most likely just off the East Coast. Specifics involving the low-predictability individual shortwaves expected to influence surface development still make it difficult to resolve coverage, timing, and intensity of precipitation. Recent trends suggest heaviest activity should remain offshore. Precipitation type will also be very dependent on low-confidence details. Exact values have fluctuated with run-to-run adjustments in guidance but winter weather probabilities have been fairly consistent in suggesting at least some potential for snow from the Appalachians into the Mid-Atlantic and Northeast. An overall warm pattern incorporating Pacific-origin flow and the generally open nature of eastern U.S. troughing aloft (aside from one or more small-scale embedded features not well-resolved in the medium range period) support the fact that these probabilities for significant snow have so far remained in the lower half of the full spectrum. Meanwhile shortwave energy crossing northern areas may produce one or more periods of mostly light snow from the Upper Midwest eastward. The upper system crossing the southern Rockies/Plains on Thu should produce a localized area of highs 5-15F below normal. Clouds/rainfall may also lead to near or slightly below normal highs across the southern tier/Florida late week into the weekend. Expect above normal temperatures over the rest of the contiguous U.S. through the weekend. Warmest readings versus normal (plus 10-25F anomalies) will focus over the northern tier, as well as over the West with upper ridging building over that region. Temperatures may approach or reach daily record values for highs/warm lows over some western locations by Sat-Sun. Expect warmth to expand over the Plains during the weekend with Sun likely to be the warmest day as highs reach 15-30F above normal. By next Mon the upper trough moving into the West will bring a pronounced cooling trend to that region while the emphasis for above normal readings will extend from the southern half of the Plains through much of the East. 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 Hazards: - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Thu-Sun, Jan 30-Feb 2. - Flooding occurring or imminent across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, the Ohio Valley, and the Northern Plains. 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