Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Sat Oct 06 2018 Valid 12Z Tue Oct 09 2018 - 12Z Sat Oct 13 2018 ...Central U.S. Excessive Rainfall Threat Extends Into Midweek... ...Separate Areas of Tropical Moisture May Reach into the Gulf Coast-Southeast and Southwest-Southern Rockies... ...Active Pattern Continues over the West Through at Least Midweek... ...Pattern Overview and Guidance/Predictability Assessment... During days 3-5 Tue-Thu most guidance agrees that the pattern will commence its advertised transition toward a more positively tilted western U.S. trough while anomalously strong ridges over the eastern Pacific and western Atlantic gradually weaken. Models/ensembles show a similar shape to the flow but important differences exist for important details. The majority cluster suggests that energy aloft dropping into the Northwest as of the start of the period Tue will likely be deeper, more closed, and reach farther southward than depicted in recent GFS runs. This feature will promote ejection of the leading trough initially over the southern Rockies. Around Wed-Thu there is a decent signal for some interaction among the two features, but poor agreement on exactly how this occurs lowers confidence in specifics of low pressure expected to track into the upper MS Valley and upper Great Lakes. Already by days 4-5 there is enough detail spread to support including some ensemble mean guidance. Also of note there has been a notable slower trend for this system compared to the past 12-24 hours of guidance. The greatest uncertainty in the large scale forecast by days 6-7 Fri-Sat regards how much progression occurs for eastern Pacific/western U.S. features. The 18Z and new 00Z GFS runs become quite fast compared to other guidance including recent GEFS means. On the other hand the 12Z ECMWF digs an upper low to the California coast and then brings it into the Southwest. Ensembles capture both extremes and then some. Teleconnections relative to a core of negative height anomalies over the Aleutians by the D+8 time frame hints a the potential for at least a modest degree of split flow near 120W longitude. This seems to recommend tilting the forecast a little more toward the 12Z ECMWF/ECMWF mean relative to the GEFS mean in principle but downplaying the more extreme specifics of the 12Z ECMWF, while removing the least-supported GFS from the forecast. Regarding Hurricane Sergio, the faster jump by yesterday's 00Z ECMWF has yielded improved clustering among the GFS/ECMWF/UKMET over the past day. The CMC as well as multiple ensemble members still suggest meaningful timing uncertainty though. Preferences that lean away from the GFS evolution aloft by the latter half of the period would lead to a much less pronounced surface feature reaching the Plains Fri-Sat. There is increasing confidence that a disturbance over the extreme western Caribbean will become better organized over the coming days as it tracks northward into the Gulf of Mexico. However model/ensemble spread remains quite broad--both over the Gulf and farther north where the character of mid-latitude flow comes into play as well--so confidence in a particular track and timing is much lower than that for the system's existence. Especially after early Wed the centroid of ensemble members has shown a somewhat faster/eastward shift compare to 24 hours ago. The manual forecast nudges continuity a bit faster as a result but only toward the slower side of the operational model envelope (12Z ECMWF). Based on the above considerations the updated forecast started with an operational model blend for day 3 Tue followed by gradually increasing ensemble mean weight and elimination of the 18Z GFS after day 5 Thu. Days 6-7 Fri-Sat emphasized the 18Z GEFS/12Z ECMWF means with a small lingering 12Z ECMWF component. ...Weather Highlights/Threats... Heavy rainfall over the Plains on Tue along a wavy front will begin to move eastward from Wed onward, as developing low pressure tracking into the upper MS Valley/Great Lakes pushes the frontal boundary toward/into the East. Some activity may still be fairly intense to the east of the Plains but amounts should be lower given faster progression. Locations along the eastern half of the Gulf Coast and into the Southeast should monitor forecasts for the possible tropical feature that could bring significant moisture into this area. Depending on the relative timing of this system and cold front reaching the East, potential exists for some of the tropical moisture/heavy rainfall to extend well northward. A separate moisture feed into the Southeast/southern Appalachians ahead of the tropical feature may also yield areas of enhanced rainfall. Farther west in the cold sector, snow will be possible over higher elevations of the northern-central Rockies and into the northern Plains, while rain prevails at lower elevations. Northern parts of the West should trend gradually drier as upper troughing becomes more positively tilted. On the other hand the arrival of Sergio's moisture should bring increasing rainfall into Arizona and the southern Rockies late this week. Some of this moisture may extend into the Plains but with decreasing confidence for specifics given divergence of solutions for larger scale flow. Precipitation could linger over parts of the Southwest/southern Rockies depending on how much upper troughing persists near this area late in the period. Expect continued below to much below normal temperatures from the northern Plains/Rockies into the Southwest. One or more days may feature highs 20-30F below normal over and near the northern High Plains. Some of this chilly air will gradually push southeastward across the central U.S. Much above normal temperatures will prevail over most of the East during the first half of the period with min temps in particular (some plus 20F or greater anomalies) possibly reaching daily record values. The cold front crossing the East later in the week will finally bring temperatures closer to normal. 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