Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1059 AM EDT Wed Oct 10 2018 Valid 12Z Sat Oct 13 2018 - 12Z Wed Oct 17 2018 ...Sergio moisture/rains continue northeastward through the southern Plains... ...15 UTC Update... No significant changes were made to the medium range forecast. Overall, model spread was below average due to a relatively stable flow regime across North America and persistent blocking upper ridge off the North America west coast. Remnants of Sergio still expected to interact with a southward moving polar front across the central U.S., leading to potentially heavy rainfall for portions of the southern plains. As upper troughing amplifies across the north central U.S. early next week, a fast-moving clipper low pressure system is expected to cross the northern plains/Upper Midwest, bringing a reinforcing round of chilly air. Given decent model agreement with respect to these features, a blend of the latest model guidance including the ECMWF/GFS served as a reasonable basis for this forecast update through much of the forecast period. Ryan Previous Discussion (issued at ...Pattern Overview and Guidance/Predictability Assessment... Latest guidance agrees well with each other and continuity for the overall evolution of the mean pattern, featuring the eventual establishment of mean troughing over the eastern half of North America as an eastern Pacific ridge gradually builds into the northwestern U.S./western Canada while a modest mean low aloft meanders over the vicinity of the southwest U.S./extreme northwest Mexico. By day 7 Wed the majority of guidance shows the western U.S. pattern taking on the shape of a Rex block configuration. While consensus/continuity suggest above average confidence for the large scale pattern forecast, there are some significant uncertainties embedded within. Already during the first half of the period there is a lot of model/ensemble spread and run-to-run variability for the evolution of energy within the amplifying upper trough that drops into the northern/western U.S. 12Z UKMET and CMC/CMC ensembles were most aggressive in the amount of energy that is ultimately deposited over the West--while leaving a flatter shortwave to progress across the eastern half of the lower 48. On the other hand the 12Z/18Z GFS runs kept almost all energy within the northern stream leading to a more amplified eastern U.S. shortwave, while the past couple ECMWF runs have been somewhat closer to the GFS. Enough GEFS members concentrated less in the northern stream for the mean to support an intermediate solution rather than the operational GFS runs. Teleconnections relative to the core of positive height anomalies expected off the Alaska Panhandle early in the period favor some southwestward elongation of shortwave energy reaching the West but likely to a lesser extent than the UKMET/CMC. The uncertainty in shortwave energy distribution ultimately leads to increasing spread for the character of low pressure tracking across the Great Lakes-Northeast and/or adjacent Canada, timing of the trailing cold front, and the leading Sergio moisture streaming across the Plains and beyond. Guidance is slowly coming into better agreement for the compact upper low that should start the period off the southern California coast. The GFS and some other models picked up on this feature earlier than the ECMWF (which had been more open and progressive until the 12Z run), though sporadic GFS runs like the 12Z version have been a bit on the slow side of the envelope. What ultimately happens with this low will depend on how much mid-latitude energy amplifies into the West, with the GFS/ECMWF and their means currently providing a majority scenario that keeps a modest upper low over/near the Southwest for much of the period. Continuity has held up well so far for the next clipper in the series, most likely brushing the northern states during the latter half of the period. A model/ensemble blend would provide a reasonable starting point for this feature. The updated forecast incorporated various elements of the 12Z ECMWF/18Z GFS/12Z CMC in order of higher to lower weight early in the period with a modest 18Z GEFS/12Z ECMWF component. CMC weight was low enough not to have significant influence on the western U.S. forecast. Better comparison to the means mid-late period allowed for somewhat greater retention of the 12Z ECMWF versus 18Z GFS for that part of the forecast, while the overall solution increased ensemble mean influence due to good agreement for the large scale pattern but embedded detail uncertainty. ...Weather Highlights/Threats... During the weekend there is reasonable consensus in principle that Sergio's moisture will produce a band of locally heavy rainfall over the southern Plains--aided by interaction with an approaching cold front--but some ongoing spread for latitude of highest accumulations. The middle of the spread would place such an axis from northern Texas/southwest Oklahoma into northwest Arkansas and extreme southern Missouri. Some of this moisture may continue northeastward along the front into the Ohio Valley/Great Lakes/Northeast by early next week. The clipper system anchoring the aforementioned cold front will bring some light precipitation to locations near the eastern half of the Canadian border, and the next clipper in the series should spread another area of moisture across the northern tier states during the first half of next week. The northern-central Rockies should see one or more periods of mostly snow, while parts of the Southwest into southern Plains may see some rain due to the persistent upper low near the Southwest and southwesterly flow downstream. A front stalling over the Florida peninsula may help to focus rainfall in its vicinity during the weekend. Expect the Rockies/Plains to be the focus for the most extreme temperature anomalies during the period, with highs of 20-30F below normal progressing from the northern Rockies/High Plains on Sat into the southern High Plains by early next week. Min temperatures should be somewhat less extreme. In modified form some of this cool air will extend eastward, leading to near or below normal highs over all but the Southeast. Min temperatures over the East will tend to be near or somewhat above normal though. Meanwhile the Pacific Northwest should see a warm period (up to plus 10F or so anomalies) as upper ridging builds in, while southern California and the Southwest will likely remain somewhat below normal due to the slow moving upper low in the vicinity. 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