Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1112 AM EDT Thu Oct 11 2018 Valid 12Z Sun Oct 14 2018 - 12Z Thu Oct 18 2018 ...15 UTC Update... Given continued mid/upper-level blocking off the west coast of North America, models spread remains slightly below average, with forecast confidence slightly above average through the medium range. A multi-model deterministic blend (ECMWF/UKMET/GFS) served as a reasonable starting point for the forecast during days 3-5 (Sun-Tue). Some timing differences were noted between the GFS and the ECMWF/UKMET with shortwave energy crossing the northern plains/Great Lakes during this time period, with the GFS placing more emphasis's on a leading shortwave resulting in a faster progression of the trough, while the ECMWF/UKMET placed more emphasis on a second shortwave resulting in a slower overall progression. Given the overall amplified state of the flow, opted to lean a bit more toward the ECMWF/UKMET idea during this time period. During days 6-7 (Wed-Thu) models/ensemble show good consensus that a broad upper trough axis will cross the eastern U.S. with another surface front ushering in a reinforcing round of polar air from the Midwest to the East Coast. Meanwhile, confidence is high that the blocking upper ridge off the West Coast will hold strong, with perhaps a slow eastward migration to right along the West Coast by the middle of next week. Increased weighting of ensemble means (ECENS/NAEFS) during days 6-7, but confidence was sufficient to continue a sizable minority of ECMWF/GFS deterministic guidance in the forecast through the period. The previous discussion from overnight follows below. Ryan Previous Discussion (issued at 0645 UTC) ...Pattern Overview and Guidance/Predictability Assessment... Guidance consensus shows good continuity in principle with the idea of a strong eastern Pacific ridge aloft building into the northwestern U.S. and western Canada, with shortwaves in the downstream mean trough supporting a series of surface waves/frontal systems. The axis of the overall mean trough should reach eastern North America late in the period as the upstream ridge moves inland. There is also a persistent signal toward a weakness/upper low over the southwestern U.S./extreme northwestern Mexico but some question over exactly how this feature evolves. Within the agreeable mean pattern there are still details for which individual models/ensembles are having difficulty resolving. The greatest overall uncertainty involves the distribution of energy within the initial trough extending from the northern tier states into the West as of early day 3 Sun. Solutions continue to vary significantly with what proportion of energy in the western U.S. part of the trough might peel off into the mean low near the Southwest versus continuing onward in the westerlies. GFS runs had been fairly consistent with keeping most of the energy in the progressive northern stream but the 18Z and new 00Z runs have adjusted to an in-between scenario that takes longer for the southern part of the shortwave to eject through the central-eastern states. ECMWF runs have been erratic, the CMC has been on the progressive side, and the UKMET has generally wanted to hold back more southern energy in some fashion. Thus confidence is really not much improved versus the past 1-2 days. Through the 12Z/18Z cycles the overall signal from the guidance was to hold back the shortwave somewhat compared to the prior 12-hourly runs, but guidance volatility suggests not going completely that way. As a result by day 4 Mon prefer to include somewhat more 18Z GEFS/12Z ECMWF mean input (40 percent) than typically desired that early in the period to provide a slight toning down of the adjustments made by a general blend of the 18Z GFS and 12Z ECMWF/UKMET/CMC. The result is a slower frontal progression over parts of the East by Tue but not quite to the extent of an operational model average. Meanwhile energy along and just south of the Canadian border remains another forecast problem, with differences ultimately affecting the surface pattern over central/northern parts of the eastern U.S. around early next week. Sporadic solutions (new 00Z UKMET/CMC, old 00Z/10 ECMWF) lead to fairly strong surface development affecting the Northeast while the currently preferred remaining model/ensemble mean consensus is much weaker with any meaningful surface development waiting until reaching the Canadian Maritimes. With typical detail differences/variability, guidance shows better than average clustering and continuity for upstream energy that should enter northwestern Canada on Sun and then drop into the Midwest/Great Lakes Tue-Wed with accompanying low pressure and frontal system. Over the past couple days there have been some intermittent signals from guidance that this system may interact with waviness just off the Mid-Atlantic/Northeast coast around midweek. Another system may approach the upper Great Lakes by day 7 Thu but with much greater spread for details and thus low confidence at this time. For the upper low over or near the Southwest, individual solution spread tempers confidence in specifics of which piece(s) of energy may contribute to the feature but at least there is better confidence in the general existence of an upper low/trough. Following an operational model blend on day 3 Sun and the aforementioned model/mean blend the following couple days, the 18Z GFS and 12Z ECMWF/CMC were sufficiently comparable to the ensemble means for the overall pattern that the forecast was able to maintain about an even weight of models and ensembles through day 7 Thu. ...Weather Highlights/Threats... The series of clipper systems will bring periods of mostly light rain and possibly cold sector snow to parts of the Great Lakes and Northeast. The cold front trailing from the leading system will cross the eastern states early-mid period with some accompanying rainfall, possibly aided in part by moisture originally associated with Sergio. Rainfall distribution/timing is still fairly uncertain however due to question marks over specifics of shortwave energy aloft/surface wave(s). Early in the period an area of snow should progress southward over the central Rockies with rain over the southern Rockies/Plains. Depending on specifics of the upper pattern, rain may persist over the southern Plains with some scattered rainfall potentially extending back through the southern Rockies/Southwest. Strong high pressure building south/southeast from the northern Rockies will bring multiple days of below normal temperatures to the central U.S., though northern areas will see steady moderation with time as the upper pattern shifts eastward. Expect Sun-Mon to feature the greatest negative anomalies (some highs 20-35F below normal) and best potential for daily records to be approached/broken. Currently expect record cool highs to be more numerous than record lows. To a lesser degree this cool air will extend eastward to keep all but the Southeast below normal for highs most of the period. The upper ridge building into the Northwest will promote highs up to 10F or so above 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