Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM 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. There is reasonable agreement for the upper trough crossing the eastern states Saturday-Monday and then a modest trailing ridge. However models and ensembles have become more divergent over the past day for what ultimately becomes of a shortwave trough forecast to approach the West Coast by early Sunday. This issue also affects a weak southern stream shortwave tracking into the Southwest. Thus there is increasing uncertainty for some weather details from the West into the Plains from late Sunday onward. 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... The models persist with some detail differences for a strengthening wave expected to be near the eastern Great Lakes or farther north over Canada at the start of the period early Saturday. A blended approach is reasonable given the fairly subtle detail differences aloft (low predictability multiple days out in time) and modest timing differences for the trailing front that sweeps across the East Coast. As for the shortwave nearing the West Coast early Sunday, the 12Z ECMWF and a meaningful number of its ensemble members have joined recent CMC runs in pulling off energy from the southern part of the feature to form a closed or nearly closed low that crosses the West. This scenario contrasts with the prior majority consensus, still consisting of the GFS/UKMET and recent GEFS/CMC means, which keeps the trough energy in the northern stream as it progresses across the northern U.S./southern Canada. Interestingly, while the ECMWF had support from some of its ensembles, none of the 12Z CMC ensemble members reflected the CMC/ECMWF scenario. It is still a question mark how much energy can separate and track over the West given the developing mean ridge over the region in response to the amplifying Pacific trough. At the very least if that does occur, the resulting feature should be fairly weak/progressive. The weak southern stream shortwave that may track into the Southwest would maintain itself as a distinct feature if the aforementioned trough remained in the northern stream, or else be incorporated within/ahead of the flow from any feature that pulls off from the West Coast trough. For the purposes of preparing a deterministic forecast based on data available through the 12Z/18Z cycles, preference sided with maintaining the ideas of continuity until the evidence for a significant change becomes more compelling. Farther upstream the 12Z ECMWF strayed a bit on the faster side to bring leading Pacific height falls into the Northwest by Tuesday-Wednesday. Again this was within the ensemble spread but the majority of models/ensembles favored a slower and more amplified depiction of the trough. Forecast preferences led to starting with a 12Z/18Z operational model blend to start the period on Saturday. With time the blend steadily transitioned toward replacing the 12Z ECMWF with its prior 00Z run while incorporating some GEFS/ECMWF/CMC means along with holding onto the GFS. GFS input transitioned to the 12Z run for about the latter half of the period due to the 18Z run questionably holding back the upper low near James Bay and merging it with the system coming across the northern tier. ...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. Another question mark exists over the southern High Plains where some rain could develop toward midweek if southern stream shortwave energy remains separate from the trough reaching the West Coast during the weekend. 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. Elsewhere most temperature anomalies should be in the single digits on either side of normal. There may 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 over parts of Montana on Sunday and perhaps into the Upper Midwest/Upper Great Lakes by Monday-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