Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1015 AM EST Thu Jan 24 2019 Valid 12Z Sun Jan 27 2019 - 12Z Thu Jan 31 2019 15 UTC Update... Model consensus has improved over the past day, especially during the latter portion of the medium range period. A highly amplified large scale flow regime will persist with ridging off the West Coast and a broad trough/closed upper low from Hudson Bay south into the Great Lakes/Midwest. Shortwave energy originating in the arctic will traverse the eastern side of the ridge and move into a broad region of cyclonic upper flow across the eastern half of the CONUS, reinforcing the upper-level trough. While models continue to show some timing/amplitude differences with respect to these individual shortwaves, consensus has generally improved over recent model runs. The region from the central plains to the Great Lakes and Northeast will be the dominant storm track during the period, with a couple low pressure systems traversing. Model solutions showed reasonably good clustering with each of these systems. By days 6-7 (Wed-Thu) the main question becomes to what degree north Pacific shortwave energy will be able to flatten the ridge, and whether more blocked flow begins to evolve across the arctic, with a number of solutions suggesting an anomalous closed ridge developing near the Bering Strait (which would reinforce the existing amplified flow pattern across the CONUS. Given these considerations, a multi-model blend of the latest deterministic guidance served as a forecast starting point for days 3-5 (Sun-Tue), with an increase in weighting of ensemble means (ECENS and GEFS) during days 6-7. This yielded a forecast very close to continuity. Ryan Previous Discussion (issued at 0659 UTC) ...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 first system of note will affect the Great Lakes/New England early in the period with some leading snow over New England and a period of lake effect snow behind the low. The next system dropping southeast from western Canada will bring a band of potentially significant snow to locations just north of the low track and then in leading warm advection over New England. Currently the highest snowfall probabilities extend from the northern Plains through the Midwest and lower Great Lakes into New England. This is still subject to some change due to track variability in the guidance thus far. How much moisture this system produces is another uncertainty, with most operational models generating significantly more QPF than the ensemble means. The means have trended wetter over the past day while the highest solutions may be overdone for being in cold air--favoring an intermediate forecast perhaps a little more than halfway toward an operational model average as a starting point. 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 little snow with frontal passage. After the system departs expect another period of lake effect snow. Elsewhere, the details remain quite uncertain but one or more low latitude frontal waves may produce locally heavy rainfall over the southern Florida Peninsula in the late weekend/early week time frame. An area of locally enhanced snow will drop southward along the Rockies Sun-Mon behind a strong cold front. The most prominent focus for coldest air remains over the Upper Mississippi Valley/Midwest/Great Lakes. Some temperatures may be locally 10-25F below normal Sun-Mon but over a much more limited area than the cold outbreak expected to occur behind the storm tracking through the Midwest early next week--enhanced by a compact Arctic upper low whose track could track close to or over Lake Superior. By the middle of next week expect broad coverage of readings 10F or greater below normal over the eastern half of the country with the Upper Mississippi Valley and vicinity potentially seeing temperatures exceed 30F below normal for at least one day. The High Plains will be quite warm on Sun before a strong cold front brings temperatures below normal over the Rockies/Plains early next week. The West Coast states will remain above normal through the period with localized plus 10F or greater anomalies most likely on Sun. Rausch 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