Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM 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... Moderate differences that develop for the timing and shape of the upper trough reaching the Plains as of Friday continue to cause difficulties for the forecast of a strengthening frontal wave likely to track northeast from the southern Plains or Midwest. Through the 12Z/18Z cycles guidance was arranged similarly to previous days--with the faster GFS/UKMET shortwave leading to a farther north developing surface wave relative to the ECMWF and slowest CMC. The new 00Z GFS/UKMET maintain their position while the CMC has finally caught up to the 12Z ECMWF. Finer details of the new CMC aloft actually lead to a surface wave faster/north versus the 12Z ECMWF. Persistence of these clusters led to favoring a blend among these ideas to maintain flexibility depending on future trends, which are now leaning farther away from the slow side given the faster adjustment in the 00Z ECMWF. Farther upstream, CMC runs are now oscillating between 12Z runs that fit the model/mean consensus for the upper trough entering western North America by Sunday and continuing eastward thereafter versus 00Z runs that drop a trough/low into the West early next week. 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. There may be a weak southern stream feature that tracks into the Southwest though. Otherwise, for day 7 Tuesday recent runs of the GFS/ECMWF have both trended noticeably farther south with the upper low contained within the trough reaching southern Canada and the Upper Midwest/northern Plains. For now a reasonable starting point seems to be a model/mean blend that yields more of a closed low than the open wave seen in the means but with a closed low track a little north of the 18Z/00Z GFS or 12Z ECMWF (00Z ECMWF adjusted north). Also of note, recent GEFS means have been a little east of other means with the East Pacific trough and a bit weaker with the downstream ridge/trough. Thus the ensemble mean part of the blend tilted somewhat more toward the ECMWF mean. ...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 Tropical Storm Pamela (expected to become a hurricane soon per National Hurricane Center Forecast). The latest 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. Light to moderate precipitation over the northern Pacific Northwest on Friday should become heavier and extend southward by Saturday as an upper trough and developing surface system approach. Precipitation will become lighter late in the weekend and possibly trend toward dry conditions early next week as an upper ridge builds over the West. Temperatures over the West will begin to moderate slightly on Friday with 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 will 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 will be 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 and its passage will bring near to slightly below normal highs for Sunday-Tuesday. 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