Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Wed Oct 30 2019 Valid 12Z Sat Nov 02 2019 - 12Z Wed Nov 06 2019 ...Overview and Guidance/Uncertainty Assessment... Most guidance maintains good agreement and continuity with the expectation of a persistent and fairly amplified east-central U.S. mean trough downstream from a northeastern Pacific ridge. There has been some typical spread/variability with details, which favors maintaining a model/ensemble mean approach to represent the most common ideas of guidance while downplaying less confident specifics. Within the forecast mean pattern expect a leading shortwave over the Mississippi Valley early day 3 Sat to lift northeastward during the weekend and push a leading cold front through the East. Upstream shortwave energy over southern mainland Alaska and the Gulf of Alaska on Sat will quickly drop southeast and support a northern tier surface low and trailing front late weekend into early next week. The primary consideration for this feature is that recent GFS runs (including the new 00Z run) carry their surface wave slower and farther south than the majority cluster that consists of the ECMWF/UKMET and the ensemble means. The CMC is on the weaker side of the spectrum but the low track in the 00Z run is close to consensus as well. An operational model blend to start the period and then by day 5 Mon adding some 18Z GEFS/12Z ECMWF mean input and reducing the 12Z/18Z GFS a bit reflects consensus early and a non-GFS solution for the early week northern tier system. Later in the period the GEFS/ECMWF means have been very agreeable and stable at least for their mass fields. Thus prefer to trend the more distant part of the forecast 50-60 percent in their direction. The 12Z ECMWF compared better to the means than the 18Z GFS in particular, though the new 00Z GFS has trended favorably into early Wed. As a result the late-period operational component used more 12Z ECMWF than 12Z GFS while phasing out the 18Z run. This solution brings the next upper trough into the East by next Wed with the leading front reaching the East Coast by then. The greater model/ensemble discrepancy involves moisture return ahead of the late-period eastern front. Localized ECMWF precip amounts and ECMWF mean coverage could be a little aggressive but consensus suggests that GFS runs could be too fast with southern stream energy that could play a role in developing at least some areas of rain. As suggested by variability in recent operational model runs, there is still a fair degree of uncertainty over wave details along the mean frontal boundary likely extending from southwest Canada southeastward into the Rockies/High Plains. This is connected to various potential solutions for what happens with shortwave energy expected to be over Alaska/northwestern Canada early in the week. Thus far GEFS/ECMWF ensembles are narrowing their spread toward keeping the northeast Pacific ridge rather strong and not allowing the shortwave energy to elongate westward like the 18Z GFS and to some degree 12Z GFS. On the other hand CMC ensembles continue to show lower heights over the northwestern states--seemingly a low probability scenario at this time given the robust consensus of the GEFS/ECMWF means and operational GFS/ECMWF/CMC runs. Finally, GFS/ECMWF runs and their means have come into good agreement for the weak upper low offshore California. ...Weather Highlights/Threats... From the weekend into the start of next week the majority of precipitation will be with areas of mostly light-moderate snow and rain between the northern Rockies/High Plains and Great Lakes. Cool cyclonic flow around a leading system tracking away from the Upper Great Lakes will promote a period of lake effect activity while the next wave tracking into the northern tier and trailing front will provide another precipitation focus. From Mon into Wed the passage of the northern wave will generate more precipitation over the Great Lakes with lower confidence in precip coverage to the west. Rainfall coverage may increase somewhat over the eastern half of the country as the northern tier system's front heads toward the East Coast, but to what extent remains uncertain and dependent on evolution/timing of upper dynamics. One other localized area of rainfall during the period will be over the Florida Peninsula in association with a stalling front and easterly low level flow. The eastern periphery of the Pacific ridge aloft will provide modestly warm temperatures to most of the West during the period, with most anomalies for highs around plus 10F or less. One chilly airmass will progress across the central/eastern states during the weekend with most locations seeing at least one day with highs 10-20F below normal. Well below normal anomalies should be less extensive with the next push of cool air Mon-Wed, with areas from the northern High Plains into the Upper Midwest/Great Lakes most likely to see highs 10-20F below normal. 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