Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1157 AM EDT Sun Oct 13 2019 Valid 12Z Wed Oct 16 2019 - 12Z Sun Oct 20 2019 ...Pattern Overview and Guidance/Uncertainty Assessment... Today's guidance continues to show a relatively amplified but progressive pattern across the lower 48 during the extended period. A deepening upper trough should develop an embedded closed low as it tracks through the Great Lakes/Northeast Wed-Thu, while surface low pressure and frontal system cross the eastern U.S. ahead of the trough. As the initial surface low passes through the Great Lakes on Wed expect a new surface low to form across the Mid-Atlantic region. This southern low should quickly intensify as it moves northeastward along the New England coast Wed night-Thu. Farther west an intense Pacific upper-level jet and train of shortwave energy will reach the Pacific Northwest starting Wed night. This should quickly lower heights across the Northwest and eventually the northern Rockies as the systems move inland. By Fri expect the first significant shortwave in this series to exit from the Rockies into the central U.S. and push a leading cold front into the Plains. A decent majority of model/ensemble guidance suggests that a more significant trough will amplify into the Four Corners region or Rockies next weekend, moving into the Plains around or beyond the end of the forecast period. The most significant uncertainty in terms of guidance spread and associated sensible weather revolves around the upper trough expected to amplify into the western/west-central states next weekend. Over recent days nearly all operational models have been varying between a broader/open trough (such as the 00Z GFS, and to some degree the 00Z ECMWF that waits until reaching the northern Plains to close off an upper low) and slower/deeper/closed depiction (most prominently the 00Z CMC; 06Z GFS falls between the 00Z ECMWF and CMC) with major implications for precipitation coverage and intensity over portions of the Rockies and Plains. For a specific deterministic forecast the ensemble means have provided the most consistent overall forecast of a fairly amplified trough (with the 06Z GEFS trending a little slower/sharper from the 00Z run). Thus forecast preferences lean more toward the means by days 6-7 Sat-Sun. The 06Z GFS and 12Z/12 ECMWF runs provide the most representative/intermediate operational solutions for a possible embedded closed low by late Sun. For the most part guidance shows better agreement for days 3-5 Wed-Fri, favoring a model average in this time frame. This blend tends to give somewhat lower weight to GFS runs that in varying ways differ somewhat from the majority cluster for the deep storm that reaches near New England by Thu. The GFS has been trending gradually slower with the shortwave energy and leading surface front crossing the Northwest and central U.S., though it may still be a tad fast. Meanwhile most guidance shows an area of modestly lower surface pressures over the western/southwestern Gulf of Mexico. Sporadic operational runs have tried to bring a better-defined surface low toward the Gulf Coast but with general trends to back away from that scenario in later runs. Currently the 06Z GFS looks particularly questionable with its low reaching Louisiana by Sat, so the early-middle part of the forecast uses the 00Z run for GFS input. ...Weather Highlights/Threats... Expect widespread and at times locally heavy precipitation over the Pacific Northwest from midweek through next weekend as multiple shortwaves and surface fronts affect the region. Snow-levels should fall as lower heights aloft press inland. The heaviest precipitation is likely to fall across the coastal ranges as well as the Cascades, with multi-inch liquid equivalents possible. Less extreme precipitation should reach into extreme northwest California and favored terrain of the northern Rockies. The upper trough expected to amplify over the West next weekend will spread precip farther east/southeast into the Rockies. However confidence in exact coverage and intensity is low due to considerable uncertainty over how the upper trough will evolve. The central/east-central U.S. will likely see increasing coverage of rainfall by next weekend ahead of the amplifying western upper trough. High pressure moving from the eastern U.S. into the western Atlantic will promote a strengthening low level flow of moisture from the Gulf of Mexico and a front over the Gulf returning northward may provide a focus for heavier rain along parts of the Gulf Coast. The deep storm affecting the Northeast during the latter half of the week should bring heaviest precipitation to New England along with a period of fairly strong winds. Nearly all precip should be in the form of rain but it could be possible for highest elevations to see a little snow as the upper low passes through. Much of the central and eastern U.S. will see relatively cool conditions Wed into Thu with highs for many areas 5 to 15 deg F below average. These temperatures should warm by Fri and into next weekend. The warming trend should be more pronounced for min temperatures next weekend with the stronger flow of Gulf moisture. Meanwhile initially above average temperatures across much of the West (highs 5 to 10 deg F above normal) will be replaced by much cooler conditions as upper troughing moves inland. Highs of 5 to 10 deg F below normal are likely over the Northwest by Thu. By next weekend colder conditions should overspread much of the western U.S. as the main upper trough axis presses inland and amplifies, leading to highs 10 to 15 deg F below normal over some areas. Rausch/Ryan 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