Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1057 AM EDT Tue Oct 09 2018 Valid 12Z Fri Oct 12 2018 - 12Z Tue Oct 16 2018 ...Michael and associated heavy rainfall move away from the East Coast Fri... ...Sergio moisture/rains to reach Arizona-Southern Rockies and extend northeastward to the southern plains... ...15 UTC Update... No major changes were made to the inherited forecast from overnight. A blend of the GFS and UKMET was closest to the official NHC forecast tracks for both Hurricane Michael and Hurricane Sergio. Please refer to products issued by the NHC for the latest information on these tropical systems. Elsewhere, a multi-model blend served a good starting point during days 3-5 (Fri-Sun) for this forecast update (including the GFS/UKMET/ECMWF), with a trend toward heavier weighting of ensemble means (ECENS/GEFS) during days 6-7 (Mon-Tue). The forecast continues to show a frontal wave crossing the mid/upper-Mississippi Valley and Great Lakes Sun-Mon. The latest model guidance showed a slightly slower trend with respect to this feature, and this is reflected in the updated WPC forecast. As upper trough amplification occurs across the central U.S. early next week, a fast-moving clipper-like system should cross the northern plains and Midwest. Forecast confidence is actually slightly lower with respect to the Sun-Mon low pressure wave (compared to the clipper) due to the potential incorporation of mid/upper-level energy from Sergio, introducing a bit of increased spread with respect to both the location and intensity of this feature. Ryan Previous Discussion (issued at 0647 UTC) ...Pattern Overview and Guidance/Predictability Assessment... Today's guidance continues to show a large scale evolution toward a slow/amplified eastern Pacific to West Coast ridge and corresponding downstream trough, but with some important trends. At the very start of the forecast early day 3 Fri consensus is stronger with shortwave energy reaching western Canada and thereafter the upstream amplifying ridge is somewhat stronger/slower than continuity from 24 hours ago. As a result by the weekend the central/western U.S. trough aloft shows considerably more southwestward elongation. Teleconnections relative to the core of positive height anomalies off the Alaska Panhandle in D+5 multi-day mean charts support the idea of some trough elongation over the West. The changes for handling of the upper troughing lead to a more diffuse appearance of the clipper system by the time it reaches near the upper Great Lakes during the weekend. Meanwhile there is still the tendency for energy to collect/persist over or near the Southwest during the period--consisting of energy over or offshore southern California and then possibly additional energy from amplifying northern stream energy over the West. Consult latest National Hurricane Center products for the latest information regarding Michael and Sergio. Currently expect Michael to reach a position just off the East Coast by early Fri and then continue tracking northeastward. Timing differences are still significant but less extreme than in previous days. Sergio has generally been better behaved in terms of guidance consensus for the surface reflection, likely tracking from Baja California into the extreme southern Rockies/High Plains from early Fri into early Sat. However there had been considerable uncertainty over how Sergio's energy aloft would interact with energy to the west as well as northern stream flow. This aspect of the forecast has come into much better agreement. Majority clustering now shows Sergio's energy/moisture progressing steadily northeastward across southeast AZ, NM, and central Plains/Mid MS Valley. Early in the period a blend of primarily operational models provided a good representation of consensus trends while resolving differences over some areas. The 12Z run was preferred for the GFS component as the 18Z version pulled the initial CA upper low farther west than most solutions. However note that new 00Z model runs seem to be following the lead of the 18Z GFS. Some 12Z ECMWF/CMC input was reasonable since their more amplified western trough reflects recent trends. The 18Z GEFS/12Z ECMWF means had greater weight in the forecast later in the period given lower predictability differences among operational solutions at that time. ...Weather Highlights/Threats... East Coast rainfall associated with Michael should be tapering off on Fri. Meanwhile Sergio's moisture is most likely to stream across southeast AZ and then continue northeastward across the extreme southern Rockies, central/south-central Plains, middle MS Valley and to a lesser extent (and with greater uncertainty of specifics) Ohio Valley/Great Lakes. Best potential for locally moderate-heavy rainfall should be over the Plains portion of the precip shield given potential interaction with a surface trough and an approaching front. Some aspects of the pattern are still trending in the guidance but general forecast confidence is definitely higher than it was yesterday. The upper trough dropping into the West will spread an area of precipitation--a decent percentage in the form of snow--southeastward along the Rockies from about Fri night onward. By the latter half of the period low level upslope flow south of strong high pressure building southward over the Rockies may serve to enhance rainfall of varying intensity over parts of the southern Plains. Northern Tier/Great Lakes locations will see some precipitation with the low pressure/frontal system affection the region early-mid period. Light precipitation may accompany another clipper system around next Tue. The Florida peninsula should see periods of rain as a front stalls over the area. Recent trends aloft have led to a colder forecast temperatures over the west-central U.S./Plains, with strong high pressure building down from western Canada supporting one or more days with highs at least 20F below normal from the weekend into early next week. The northern half of the East will tend to see below normal high temperatures with Mon likely being closest to normal, while a majority of the eastern states will see near to above normal mins. Locations near the West Coast should see modestly above normal readings. 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