Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Tue Oct 12 2021 Valid 12Z Fri Oct 15 2021 - 12Z Tue Oct 19 2021 ...Overview... Latest guidance continues to suggest that the amplified western trough/eastern ridge upper pattern in the short range will become flatter and more progressive heading into the weekend and the start of next week. This will support moderating and in some cases more variable temperatures over much of the lower 48 along with a drier trend over the central/eastern U.S. The two main precipitation areas will be over the East with a wavy front late this week and over the Pacific Northwest with a system supported by a vigorous shortwave trough arriving into western North America by Sunday. An upper trough amplifying over the eastern Pacific early next week should start to build a ridge over the western U.S. by next Tuesday, possibly leading to a gradual deceleration of that West Coast trough (eventually with a possible embedded low) as it reaches the northern Plains/Upper Midwest into southern Canada at that time. ...Guidance Evaluation/Preferences... Model guidance differences are not terribly large for the early part of the medium range period, as the upper trough that is becoming more progressive pushes a cold front with waves of low pressure along it across the east-central U.S. Friday into Saturday. While there remain some mainly latitudinal differences with the placement of those surface lows, this 00Z/06Z model cycle came into better agreement with a surface low position near the Lower Great Lakes for 12Z Saturday, with a slight northward trend in the EC runs. Newer 12Z guidance does continue to show some spread in the exact position of this low. But overall, was able to use a deterministic model blend early in the medium range period, as the model consensus appeared a reasonable compromise. Farther upstream, models generally agree with the potent shortwave energy moving into the Pacific Northwest around Sunday, but there are some differences in the amplitude of the associated upper trough, with the 00Z CMC having the deepest solution, which becomes slower and more out of phase by Monday/Tuesday as it drops a low into the West. Teleconnections relative to the negative height anomaly corresponding to the East Pacific trough late in the period favor the model/mean majority that has a ridge, not the 00Z CMC trough/low--though there may be a weak southern stream feature that tracks into the Southwest. Meanwhile the 00Z UKMET was shallower with the Pacific Northwest trough, but the 00Z/06Z GFS and 00Z ECMWF line up well with the timing and depth of the feature through early next week. As this energy tracks eastward into the north-central U.S. and south-central Canada, model differences remain with if/when/where a low closes off, its center point, and if/how it connects to the northern stream. Used a blend of the 06Z GFS and GEFS mean and slightly higher weighting on the 00Z ECMWF and EC ensemble mean for the latter part of the period, which maintained good continuity with the previous forecast. ...Weather/Hazards Highlights... The southern Plains/Midwest frontal wave expected to strengthen as it tracks northeastward into Canada from early Friday onward should produce an axis of moderate to heavy rainfall from near the Lower Ohio Valley into parts of the Great Lakes/Northeast. To some degree the rain may be enhanced by moisture originating from eastern Pacific Hurricane Pamela. The 00Z model runs still vary some for the details of the frontal wave but at least are getting closer together than they were previously. Expect lighter rainfall to extend southward along the trailing cold front. Most of the East will be dry after Saturday aside from lingering rain over the eastern Great Lakes/northern New England and the Florida Peninsula. Precipitation is forecast to increase over the northern Pacific Northwest beginning Friday and lasting into the weekend as an upper trough and developing surface system approach. Precipitation could trend toward dry conditions early next week as an upper ridge builds over the West. Chilly temperatures in the West should continue into Friday, with lows 10-20F below normal and highs 10-25F below normal primarily centered over and near the Rockies. Less extreme anomalies will extend into the Plains. Western temperatures will continue the warming trend during the weekend. The north-central Rockies may stay moderately below normal into early next week while the Northwest through northern Plains could see temperatures vary with system progression. Warmth over the East will continue Friday-Saturday with many areas seeing highs 10-15F or so above normal and morning lows 10-20F/locally 20-25F above normal. A broad area of record warm lows is possible from the Lower Mississippi Valley northeastward and there may also be a few record highs over the Mid-Atlantic and Southeast. The front crossing the region will erode the warmth from west to east as its passage should bring near to slightly below normal highs for Sunday-Tuesday. Tate/Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are 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