Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Thu Oct 31 2019 Valid 12Z Sun Nov 03 2019 - 12Z Thu Nov 07 2019 ...Overview and Guidance/Uncertainty Assessment... Expect mean ridging aloft over the eastern Pacific and Alaska to support persistence of a mean trough over the lower 48 states. On any particular day troughing will vary in precise amplitude and shape in response to individual shortwaves progressing within the mean flow. As has been the case over recent runs, guidance shows decent clustering in principle into about early Tue. Then model/ensemble spread and variability increase due to differences in how ridging over and near Alaska evolves and this influence on shortwave energy reaching northwestern North America early in the week. A blend emphasizing the latest operational guidance through the 12Z/18Z cycles provided a reasonable starting point for the updated forecast during the first half of the period when guidance is fairly agreeable (Sun into early Tue) aside from detail differences that tend to have fairly low predictability 3-5 days out in time. After a leading shortwave/cold front departs from the East Coast early Sun the primary feature of interest will be the next amplifying shortwave that should bring low pressure across the northern tier/Upper Great Lakes region. The trailing front will drop south/southeast across the central U.S. while the western part settles along the eastern slopes of the Rockies. From later Tue into Thu the central U.S. front should continue south and east. Based on the current array of guidance there is greater confidence in progression of the northern part of the front, with upstream issues leading to the uncertainty in progression/southward extent of the remainder of the front. As for the upstream evolution there is decent agreement in principle that another surge of cold air dropping through western Canada should reach the northern U.S. around Tue-Tue night. However by Wed-Thu there is rapid divergence in solutions for evolution of the supporting dynamics aloft in response to the shape of the Pacific/Alaska ridge--leading to some dramatic differences in southward/westward extent of cold air. At the very least the 12Z ECMWF appeared less likely than most other solutions as its western trough amplification (and resulting cold push through most of the West) exceeded nearly all of the ECMWF ensemble members. Reviewing remaining guidance, 12Z ECMWF ensembles as a whole have trended toward somewhat lower Northwest U.S. heights aloft than continuity or latest GEFS/CMC means while latest GFS runs show a hint of the ECMWF mean scenario. Teleconnections relative to strong positive height anomalies over/near Alaska offer potential for lower heights over this region as well, either within or beyond the forecast period. On the other hand the 12Z CMC ensembles adjusted to the GEFS scenario after previously showing more potential for lower heights over the Northwest. Based on the sum of these considerations the forecast ultimately trended toward a blend of the 18Z GEFS/12Z ECMWF means and minority input from the 12Z/18Z GFS and old 00Z/30 ECMWF. ...Weather Highlights/Threats... The northern tier/Great Lakes system and the cold cyclonic flow in its wake will bring periods of snow and rain to the Great Lakes region. Lighter precipitation will extend across the northern Plains. Portions of the northern Rockies/High Plains should see a couple periods of snow on the cold side of fronts dropping south from Canada. Meanwhile there is still considerable debate in the guidance over the coverage and intensity of rainfall over the eastern U.S. ahead of the front that should reach a Great Lakes to southern Plains orientation by Tue. Southern stream energy aloft may support more rain than forecast by the GFS/GEFS but ECMWF/ECMWF ensemble runs could be overdone. Depending on uncertain details at the surface and aloft, rainfall could begin to increase over or near the southern Plains around or shortly after next Thu. Easterly low level flow and one or more fronts will likely bring periods of mostly light rain to the Florida Peninsula. From Sun into Mon southern Plains into eastern U.S. high pressure will bring a brief period of below normal temperatures, with decent coverage of minus 10-15F anomalies on Sun. After this high departs the focus for coldest air will extend from the northern High Plains southeast into the central Plains and east into the Upper Midwest/Great Lakes. The northern High Plains region has the best potential for multiple days with highs 15-25F below normal while other areas should see one or more days at least 10-15F below normal. Western areas should remain modestly above normal with plus 5-10F anomalies for highs, while the East will see readings vary with system progression Tue-Thu. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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