Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1142 AM EDT Sun Oct 21 2018 Valid 12Z Wed Oct 24 2018 - 12Z Sun Oct 28 2018 ...Guidance/Predictability Assessment and Weather Highlights/Threats... A strengthening low will exit New England toward the Canadian Maritimes midweek, with lingering early season cold air/snows wrapping back into northern New England. Model spread has vastly improved with this coastal low. The main weather focus then shifts to a phasing pair of streams across the middle of the nation. This process ultimately leads to a development of a surface wave which tracks eastward across the northern Gulf of Mexico before making a northward turn toward the Gulf Stream next weekend. This offers a threat of heavy precipition for the Gulf Coast then East Coast with coastal storm potential. Upstream, additional mid-upper level trough amplification may also enhance development of a Midwest low with some snow along with a reinforcing trailing frontal surge next weekend. Considering the pattern across the eastern Pacific, after another powerful cyclone lifts toward the Gulf of Alaska, the mid/upper-level flow turns more zonal. Eventually marked height rises take shape toward the western U.S. which establishes a higher amplitude pattern into the following weekend. Northwesterly flow on the eastern extent of this mentioned ridge could lead to an impressive clipper-like system over the Mississippi River Valley by Day 6/7, October 27/28. As the pattern amplifies over the mid-nation late week, the 00 UTC Canadian/UKMET and to lesser extent the 00 UTC ECMWF stand out as outliers compared to the ensemble envelope by later week in bringing warming western U.S. ridging too progressively eastward into the n-central U.S., leading to sharper downstream trough amplification. The 06 UTC GFS is more in line with multi-model ensembles both with holding more amplified ridging over the West and downstream east-central U.S. trough amplitude. Accordingly, WPC progs blended best clustered 00 UTC GEFS/ECMWF ensembles with the 06 UTC GFS days 3-5, quickly transitioning to an ensemble mean blend days 6/7 amid growing forecast spread, not committing yet to significant 06 UTC GFS east coast low genesis/qpf. Shifting focus underneath to expected Gulf of Mexico development, ensemble low plots still exhibit a great deal of spread with multiple waves possible. This and QPF is further complicated by any moisture and downstream ripples from the remnants of Hurricane Willa and Tropical Storm Vicente that are both forecast to landfall on the Pacific coast of Mexico midweek. Then overtop, the suggested negative tilt/amplitude of the supporting shortwave aloft indicates a coastal low may subsequently form downstream over the western Atlantic, but details are less certain. Run to run continuity is lacking in the models which suggests diminished confidence with any forecast details. Even in cases when particular model runs happen to look similar at particular time steps, development evoluitons have been different. That said, recent model runs have overall trended toward at least moderate surface low deepening and closer to the coast track, as do WPC progs. The wettest conditions this period should train along the Gulf Coast region given the eastward migrating wave of low pressure and tropical moisture infusion. Recent guidance continues to favor the heaviest rainfall just offshore with best instability. However, it would not take much to move some of these convective elements inland. The focus gradually moves northeastward into the coastal Carolinas getting by Friday, working up the coast next weekend to effect offshore waters and inland to the eastern Mid-Atlantic and New England. Cooled fall temps over the Eastern Seaboard do not seem suffiently cold to warrant an early season snow threat into next weekend overall. However, there is modest potential for higher elevations of the Appalachians and portions of the Midwest/Great Lakes and interior Northeast. For the Pacific Northwest, persistent onshore flow keeps conditions wet for much of the period. Local orographics dominate activity/elevation snows to focus from the Olympics/Cascades that moderates quickly inland to the northern Rockies. Schichtel 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