Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 141 PM EST Thu Jan 25 2018 Valid 12Z Mon Jan 29 2018 - 12Z Fri Feb 02 2018 The area extending from the southern mainland/northeast Pacific across the southern Bering Sea/Aleutians/northern Pacific appears to have the highest forecast uncertainty within the blocky pattern indicated by most guidance. There is reasonable agreement that an upper high over eastern Siberia should drift a little north and/or east with time while energy flowing around this high tracks over the southern mainland/Gulf of Alaska and then along/near the Aleutians. Meanwhile some energy within/ejecting from a mean trough becoming established over the central Pacific may interact with the energy flowing around the southern side of the upper high. Typically low predictability for specifics of flow around upper highs and potential stream interaction combine to lower forecast confidence over the aforementioned area. In addition, teleconnections relative to the upper high and central Pacific trough suggest that the latitude of the upper weakness currently forecast over and southwest of the southern mainland will be fairly sensitive to the relative strength of the two large scale features. The current model/ensemble mean majority (the 00z CMC being the main exception) shows the Pacific trough becoming the more influential feature with associated downstream ridge extending into the northeast Pacific. A more dominant upper high would push the southern mainland weakness aloft farther south. During days 4-5 Mon-Tue the primary features of interest will be deep northeast Pacific low pressure likely tracking just south of the Alaska Peninsula and a weak Arctic system bringing a front into the northern mainland. Stronger ensemble mean trends over the past day provide support for the operational consensus that is deeper than the means, while most models favor somewhat slower timing than the 00z/06z GFS. The 12z GFS has adjusted somewhat slower, favoring that run as part of the operational blend. The 00z CMC starts out east of consensus. Farther north a compromise looks reasonable for the Arctic system bringing a trailing cold front into the northern mainland. Thus the first two days of the forecast consisted of the 12z GFS/00z ECMWF/00z UKMET. By days 6-8 Wed-Fri there are some general themes in the guidance but also increasing uncertainty for specifics. The most agreeable surface features are Gulf of Alaska low pressure that briefly exists around midweek in response to energy flowing around the upper high, and strong high pressure extending from the Arctic across the North Slope and into northwestern Canada. In varying ways GFS/ECMWF runs and their ensemble means show some combination of retrograding southern mainland energy and flow within/around the central Pacific trough helping to guide an area of low pressure into or near the central Aleutians. There is a less pronounced signal for some waviness extending into the Gulf of Alaska with recent GFS/ECMWF runs inconsistent for such a feature. Prefer leaning more toward the ensemble means over this area until a better consensus emerges. Overall for days 6-8 incorporating 40-60 percent of the 06z GEFS/00z ECMWF means helped to emphasize the most agreeable aspects of the large scale pattern. Holding onto some 12z GFS/00z ECMWF input added a little detail where they were agreeable with the means in principle but individual weight was low enough to downplay less confident details. Rausch