Extended Forecast Discussion NWS Weather Prediction Center College Park MD 243 AM EDT Mon Oct 08 2018 Valid 12Z Thu Oct 11 2018 - 12Z Mon Oct 15 2018 ...Michael Expected to Continue Lifting Northeast into the Western Atlantic... ...Moisture from Sergio May Reach into Arizona/Southern Rockies... ...Pattern Overview and Guidance/Predictability Assessment... Guidance has maintained good continuity for the evolution of the mean pattern, showing a transition from a positively tilted southern Canada into western U.S. trough as of day 3 Thu toward a ridge building near the West Coast and downstream trough settling over east-central North America. Meanwhile most solutions show some degree of smaller-scale troughing likely to persist over the southwestern U.S. and extreme northwestern Mexico. Over the West, the greatest uncertainty will be with the specifics of initial western energy for which models/ensembles are showing an increasing signal of closing off for a time near or offshore southern California--plus interaction with Sergio which should be tracking northeastward ahead of the aforementioned energy. This interaction will be very sensitive to the exact timing of Sergio and how the western U.S. shortwave energy is evolving at a specific point in time (and what proportion feeds southwestward versus continuing along in the westerlies)--thus predictability at extended time frames appears well below average favoring an intermediate solution. The 12Z ECMWF mean provides further support for bringing moisture farther northward than the operational run even if not as far as the GFS/GEFS mean. At the same time the past couple parallel FV3 GFS runs have kept moisture more suppressed than operational runs. Meanwhile the forecast for Michael, which has reached tropical storm status at this time and should affect parts of the East primarily in the first half of the period, is also quite uncertain in light of continued very broad model/ensemble spread for track and especially timing. At the very least the overall pattern appears supportive for faster progression than what has been depicted in ECMWF/ECMWF mean runs thus far. Consult National Hurricane Center Products for the latest information on Michael. Minus the uncertainty for Michael, there is good consensus for eastern Canada low pressure to push a trailing cold front through the eastern U.S. late this week with Plains high pressure building in behind it. Farther upstream continuity has held up fairly well for a clipper system expected to track across or near the northern Plains/Great Lakes/New England Sat onward. Operational models show typically better detail than the means while the means provide a good centered timing relative to the faster 18Z GFS and slower 12Z ECMWF. Therefore a model/ensemble mean blend provides a good depiction of this system and its trailing front. As this front reaches the East Coast by early next Mon much of the lower 48 should be dominated by a broad area of high pressure centered over the Rockies/Plains. The first half of the forecast uses more 18Z GFS/12Z UKMET input versus other models to provide a better starting point for the official track of Michael and given decent agreement with consensus elsewhere--including the fact that the ECMWF mean tilts away from some aspects of its operational run over the Southwest. Preferences for the mid-late period clipper system leads to an evenly weighted blend of the 18Z GFS/12Z ECMWF and their means for the latter half of the period. ...Weather Highlights/Threats... Parts of the eastern U.S. will likely see areas of heavy rainfall with the combination of T.S. Michael and interaction with an approaching cold front. Guidance continues to vary widely for track and timing of Michael so confidence is fairly low with respect to precise location of heaviest rainfall and timing-dependent duration of this rainfall. While Michael begins to track away from the East Coast, Sergio may bring moisture into portions of Arizona and the southern Rockies/Plains. Highly sensitive details for evolution aloft lead to low confidence in exact coverage/intensity of rainfall here as well--especially the latitude of the axis of heavier rainfall. This lack of confidence extends to how much moisture may interact with a front that drops into the Plains during the weekend--with solutions spanning the whole spectrum from a considerable amount to very little. Elsewhere, the initial front over the East will likely decelerate/stall over the Florida peninsula leading to periods of rainfall in its vicinity. The clipper system brushing the extreme Northern Tier will be accompanied by an area of mostly light rain and trailing rain/snow from Sat onward. The Rockies and High Plains will generally be the focus for the greatest negative temperature anomalies during the period. The northern half of the Plains/north-central Rockies may see some highs 20-30F below normal late this week. A broad area of minus 10-20F anomalies will persist as reinforcing high pressure builds in later in the period. By the start of next week clouds/rainfall will likely promote coolest readings versus normal over parts of the southern Plains. Cold frontal passage over the East early in the period will bring a significant cooling trend to near/moderately below normal readings. The West Coast states will trend gradually warmer versus normal as upper ridging builds toward/over the region. Rausch WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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