Extended Forecast Discussion NWS Weather Prediction Center College Park MD 420 PM EDT Wed Oct 13 2021 Valid 12Z Sat Oct 16 2021 - 12Z Wed Oct 20 2021 ...Overview... Most guidance advertises a fairly progressive upper pattern over the lower 48 ahead of a mean trough that amplifies over the eastern Pacific next week. For the weekend, expect warm temperatures and rain in the East before upper troughing pushes a cold front across the region, while ridging leads to dry conditions over much of the rest of the country. Then, there is increasing uncertainty for weather details in the West to the Plains for the first part of the workweek, depending on what becomes of a shortwave trough forecast to approach the West Coast by early Sunday. The leading part of the Pacific trough should reach far enough eastward to have some influence on the West Coast by next Tuesday or Wednesday. ...Guidance Evaluation/Preferences... Model guidance shows reasonably good agreement with the large-scale upper-level pattern Saturday, and despite some minor timing and position differences with a surface front pushing through the East and an associated wave of low pressure, a multi-model deterministic blend was sufficient for the early part of the medium range period. Then for the early part of the week, bigger differences begin to arise as shortwave energy and troughing push into the West Coast, which has been a forecast challenge for at least the last couple of days. The trend in some of the deterministic models and some ensemble members has been to support deeper and slower troughing in the West stemming from the shortwave, pushing the ridge eastward. This trend is by no means universal among guidance, as the 00Z UKMET and 00Z GFS show a faster northern stream shortwave moving across the north-central U.S., with the 00Z GFS forming a small closed low over the Midwest by Tuesday, much like its previous runs. But, the 00Z and previous 12Z ECMWF have a deeper shortwave trough, the 06Z GFS broke with its continuity to show a similar solution, and the CMC has had deeper troughing all along, though likely is too aggressive and slow with it. There is also a considerable proportion of ensemble members particularly from the EC that show the latter type of solution. Once again the 00Z deterministic CMC had one of the deepest troughs, while the 00Z CMC ensemble members tend to have a stronger ridge, another reason for lower confidence. In terms of the newer 12Z runs, the 12Z GFS is pretty similar to its 06Z run but with a bit faster track, while the 12Z ECMWF is faster than the 00Z. Perhaps guidance is attempting to reach some sort of middle ground consensus, but will continue to monitor. Though confidence is somewhat low, today's updated forecast attempted to lean toward a solution with more troughing across the West, but not as aggressively as some of the deterministic guidance. This was accomplished using a fairly even blend of the 06Z GFS, 06Z GEFS mean, 00Z ECMWF, and 00Z EC ensemble mean for the latter half of the medium range period. Thus the main changes to the previous forecast were to slow down the cold front passing through the northwestern to north-central U.S. Monday through Wednesday given the slower trough movement, and to add some light precipitation to the north-central Rockies eastward, which would be supported if the shortwave does track as such. The aforementioned blend also worked for additional height falls in the Pacific coming near to the West Coast by Wednesday, which does show some timing differences among guidance. ...Weather/Hazards Highlights... Portions of the Northeast will likely see moderate to heavy rainfall on Saturday as strengthening eastern Canada low pressure pushes a front across the region. Expect rain to be much lighter to the south. Some lingering rain may fall to the lee of the eastern Great Lakes and over favored windward terrain during Sunday. Otherwise much of the East will see a drier trend aside from the Florida Peninsula where developing easterly low level flow may gradually increase rainfall toward midweek. Precipitation over the Pacific Northwest will extend into the weekend as an upper trough and developing surface system approach, with heaviest totals likely to be over the Olympics and far northern Cascades. Uncertainty with the ultimate evolution of upper trough energy lowers confidence in the precise southward extent of precipitation along the West Coast as well as where and how much may fall over inland portions of the West and into the Plains. Recent trends with that upper trough have led to less rain developing toward midweek over the southern High Plains, but with low confidence. Finally, the West Coast should see another increase of moisture by Tuesday-Wednesday ahead of a more amplified East Pacific upper trough. Areas near the East Coast will start Saturday with lows 10-25F above normal but the cold front crossing the region will quickly bring a cooling trend, with Saturday highs only 10-15F above normal. Moderately below normal highs to the west of the Appalachians on Saturday will continue eastward Sunday and then modify toward normal. There may also be a few colder pockets in the Rockies on Saturday while warm flow ahead of a frontal system crossing the northern tier may produce some warmer readings of 10-15F above average over parts of Montana on Sunday and perhaps into the Upper Midwest/Upper Great Lakes by Monday-Tuesday. Elsewhere most temperature anomalies should be in the single digits on either side of normal. 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 Hazards: - Heavy rain across portions of the Great Lakes, the Northeast, and the Pacific Northwest, Sat, Oct 16. - Flooding possible across portions of the Southern Plains. - Flooding occurring or imminent across portions of the Southeast. 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