Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Fri Jan 25 2019 Valid 12Z Mon Jan 28 2019 - 12Z Fri Feb 01 2019 ...Overview... Downstream from a mean ridge tending to be aligned just off the West Coast, very deep and amplified mean troughing will persist over the eastern half to two-thirds of North America. The exact shape of the overall trough will vary from day to day in response to embedded shortwaves. This pattern will maintain cold weather over many areas east of the Rockies, with the west-central U.S. receiving a glancing blow of chilly air for a time as well. The most notable change in the guidance over the past day appears over and north of the east-central Pacific. Latest solutions are showing a more pronounced upper high building over or west of the Bering Strait while troughing digs over the east-central Pacific. This evolution ultimately keeps the mean ridge in place closer to the West Coast. At least for the moment the guidance is showing better agreement with the most significant features but there are still stray operational solutions and some ensembles that temper confidence to some degree. ...Guidance Evaluation/Preferences... Within and around the core of the mean trough, the leading feature of interest will be an upper low forecast to be over Ontario at the start of the period early Sun. This upper low will support low pressure that should track across southeastern Canada with associated frontal system crossing the Great Lakes/Northeast. Over the past 24-36 hours the operational GFS/ECMWF have generally led the ensemble means for depicting this system's definition while recent trends have been somewhat faster. Behind this system a vigorous shortwave dropping southeast from western Canada will bring a rather strong clipper type system across the northern Plains and Midwest, then most likely the lower/eastern Great Lakes into New England and eastern Canada. A strong cold front will extend southwest from the surface low. Guidance has provided a signal for this system over multiple days but so far there has been a fair amount of waffling on the exact latitude of surface low track. Thus the preferred consensus/average approach could well be extended to a multi-run mean. What appears to be the most extreme aspect of the forecast is a deep upper low which a number of models (minus the 12Z CMC) bring down toward Lake Superior by day 6 Wed. This low has its origin over the Arctic beyond the Canadian Archipelago. Interestingly the ECWMF mean has been leading the GEFS mean for the southward extent of this upper low--with the 18Z GEFS mean finally matching the ECMWF mean position over Lake Superior early Wed--but GFS runs have generally been a little less erratic than the ECMWF with specifics of the upper low and surrounding flow. Thus not yet confident in the 12Z ECMWF that brings the upper low farther south than the aforementioned means. Depending on exact upper low depth, 500mb height anomalies may reach at least 3-4 standard deviations below normal along/south of the low's path. This feature may promote further strengthening of the Midwest/Great Lakes system through New England and eastern Canada. Deeper trends in the means for the upper low but some lingering track spread in the guidance favor a combination of model/mean ideas. Around the southern/eastern periphery of the overall trough there continues to be a more suppressed trend for any waviness across the Gulf of Mexico and into the western Atlantic. Model inconsistency and spread keep confidence below average for specifics that affect intensity of rainfall over the Florida Peninsula. Guidance as a whole is maintaining recent trends toward lower potential for any waves to have a significant effect on the East Coast. Along the West Coast the aforementioned adjustments with the upstream pattern lead to fairly good agreement in the models/means toward a more persistent ridge aloft with GFS/ECMWF runs generally a little stronger than their means. The 00Z CMC has strengthened its ridge compared to the 12Z run that was more rounded so there is support for operational model input to enhance the means late in the period. Based on the above assessment the updated forecast used mostly operational guidance during the first half of the period and then trended toward an even model/ensemble mean blend employing the 12Z-18Z GFS/12Z ECMWF and the 18Z GEFS/12Z ECMWF means. ...Weather Highlights/Hazards... The main storm system of interest during this forecast period will be a clipper type system dropping southeast from central Canada along with an arctic front. This will bring a band of light to moderate snow across the southern Great Lakes and extending across the northern Ohio Valley on Monday, and then reaching the Northeast U.S. and Mid-Atlantic region by late Tuesday. This is still subject to some change due to track variability in the operational guidance. Once the front trailing from the low reaches the Mississippi Valley and points east, some rain may develop in the warm sector and then possibly end as a period of snow with the frontal passage. After this system departs, more lake effect snow is likely downwind of the Great Lakes. Another thing that will be making weather headlines by the middle of the upcoming week will be an impressive round of frigid temperatures as a lobe of the polar vortex sinks southeastward across Canada and then reaches the northern Great Lakes. Temperatures are expected to plummet well below zero across the Upper Midwest, most of the Great Lakes region, and the northern Ohio Valley by Wednesday and Thursday. Anomalies on the order of 25 to 35 degrees below normal are a good possibility for these regions, and it is quite likely that this would warrant wind chill warnings for many of these areas. Although the airmass will modify some by the time it reaches the East Coast, it should still be quite cold for the Mid-Atlantic and Northeast U.S. on Thursday with highs well below freezing. D. Hamrick WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation and winter weather outlook probabilities 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