Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Tue Oct 16 2018 Valid 12Z Fri Oct 19 2018 - 12Z Tue Oct 23 2018 ...Pattern Overview... A mean trough aloft will persist over eastern North America downstream from a Pacific Northwest/western Canada ridge. The gradual approach of a northeastern Pacific upper trough will begin to push the ridge axis a little eastward by the first part of next week. Under the eastern upper trough a series of Canadian fronts will swing through the central-eastern states to maintain below average temperatures and spark off some rain/snow showers especially in favored lake-effect areas. The strong upper ridge over Florida will slowly weaken by next week and get squashed into Cuba. On its west side will be a stalled front off the Texas coast and nearby surface trough which will keep a daily chance of rainfall in the forecast, over southern Texas in particular. Finally to the south of the Northwest ridge, a weak upper low near southern California will likely eject into the Great Basin early next week with the approach of more progressive Pacific flow, helping to wring out some light showers. ...Guidance/Predictability Assessment... Note that 00Z CMC/CMC ensemble runs were not available for the forecast. Remaining guidance in the 00Z-06Z cycles offer better than average agreement, and even by day 7 Tue operational run differences fall within typical error range. By late day 7 the 06Z GFS/00Z ECMWF trend a bit toward the deeper/sharper side of the spectrum with the shortwave nearing the East Coast but still compare acceptably to the overall pattern thus meriting inclusion for the forecast. ECMWF ensembles as a whole are still a little more eager to lower heights along West Coast late in the period but the operational 00Z ECMWF has made a favorable adjustment, toward slower arrival of shortwave energy than the 12Z/15 run. Due to an initial piece of Four Corners energy that the 00Z ECMWF pulls into the Northwest Fri-Mon in a strong low-confidence fashion (somewhat more sheared in the 00Z UKMET), the preferred days 3-5 Fri-Sun model blend included the past two ECMWF runs along with the 00Z/06Z GFS and 00Z UKMET. Days 6-7 Mon-Tue incorporated 40-50 percent total weight of the 06Z GEFS/00Z ECMWF mean to temper timing differences that arise among the GFS/ECMWF. Due to the aforementioned issue with the old 12Z ECMWF near the West Coast, that run was removed from the blend after day 5. The resulting forecast yielded modest and typical run-to-run detail/timing adjustments for individual features but good continuity overall. ...Weather Highlights/Threats... Expect above average temperatures over much of the western U.S. away from the influence of the southern upper low, generally 5-10F above average but with some locally higher anomalies over parts of the Northwest/extreme northern Rockies. Upper troughing farther east and cloud cover over Texas will keep temperatures below average over a broad area from the southern Plains into the Northeast. Those two areas should see the greatest negative departures from normal on one or more days. Frontal passages will provide some daily variation in temperatures. A few locations could see readings near record values for warmth over WA/OR and FL while a few daily records for cool highs may be possible over the Great Lakes/East Coast regions. Highest 5-day rainfall totals will be over portions of Texas and near the western Gulf Coast, in the vicinity of an inverted trough and to the northwest of a stationary boundary in the western Gulf. Some of this southern Plains moisture may extend northeastward along a cold front crossing the East during the weekend, though with some lingering uncertainty over precise rainfall amounts. The upper low in the Southwest will support rain and possibly high elevation snow showers mostly focused on/along the higher terrain while a stationary front across Florida will keep daily shower/storm chances in the forecast. Over the Great Lakes/Northeast lake-effect precipitation will start anew this weekend behind a cold front and then again late in the forecast with the passage of a weak system. Coastal WA/OR will see light-moderate rain develop and expand inland early next week as upper troughing approaches. Rausch/Fracasso 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