Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 625 PM EST Fri Jan 31 2020 Valid 12Z Tue Feb 04 2020 - 12Z Sat Feb 08 2020 ...Overview... Today's guidance maintains better than average agreement for the large scale pattern evolution through the period (Tue-Sat). During the early-middle part of the forecast expect an Arctic upper low to descend close to the northwest corner of the mainland while initial Bering Sea low pressure/dynamics aloft weaken. Progressive North Pacific-Aleutians flow will carry along multiple waves/fronts. Then by Fri-Sat consensus brings a fairly strong area of low pressure into the Aleutians/Bering Sea while the associated upper trough promotes a rapidly strengthening downstream ridge that should extend from the northeastern Pacific into the mainland. This ridge will likely weaken/displace the upper low lingering near the northwest corner of the mainland through late week. Within the consensus pattern evolution there are persistent differences for various details. ...Guidance Evaluation... With the Arctic upper low and surrounding flow, guidance continues to show enough spread to have a significant effect on the northward extent of moisture over the mainland. Early in the forecast GFS/GEFS mean runs remain on the eastern side of the spread for the upper low while the CMC continues to define the western side. Eventually the GFS/GEFS mean runs pull enough flow from the western side of the feature to yield an upper low close to the consensus position. The 00Z ECMWF was a bit west of the guidance average while the 12Z run strays somewhat eastward after early Thu. Ultimately prefer a centered solution as close to continuity as possible, per a blend of the 00Z/12Z ECMWF runs and 12Z GFS/UKMET early and a model-mean blend (a little more 00Z ECMWF mean than 12Z GEFS mean) thereafter. Farther south the models are showing difficulty in resolving the proportion of initial Bering Sea energy that retrogrades versus progressing into the mainland with corresponding surface differences. The 12Z ECMWF came in with an evolution that reflected a variation of the 12Z GFS that holds back a western surface low into Thu while some other 12Z solutions and 00Z ECMWF are more consolidated/eastward with low pressure. The aforementioned blend provides the best option for a single deterministic forecast, with a hint of the western low. Within the progressive North Pacific stream the operational models offer a decent signal for separate waves reaching the Gulf of Alaska by around early Tue and early Wed, albeit with typical track/strength/timing differences. Not surprisingly these features appear somewhat weaker in the means. A weakening front should follow close behind the second wave. Guidance is now more agreeable that the system nearing the western Aleutians by early Wed will maintain definition into Thu before quickly weakening so the forecast through Thu has been adjusted accordingly to reflect a model blend. For the larger scale system expected to track into the Aleutians/Bering Sea by Fri-Sat, ensemble mean trends over the past 1-2 days (GEFS slower, ECMWF mean faster) have brought those solutions fairly close together. The main difference between the models and means is that most models are suggesting that a leading frontal wave should develop ahead of the parent low while the means are more consolidated. Prefer about an even weight of the models (00Z ECMWF and 12Z ECMWF/GFS) and ensemble means to resolve this detail difference. The 12Z ECMWF mean has come in with some support for this idea. ...Weather Highlights... Highest five-day precipitation totals should exist from just east of the Kenai Peninsula through the Panhandle. Some moisture will extend over the western mainland with northward extent sensitive to the ultimate evolution/position of the Arctic upper low, though with some tendency to focus near a front draped over the central/southwest mainland. Strong flow ahead of the system reaching the Aleutians and Bering Sea Fri-Sat may enhance amounts over the eastern Aleutians/Alaska Peninsula and push moisture into the western mainland. Below normal temperatures should prevail over northern areas with a slow southward push of colder readings over the western mainland Tue-Fri, followed by a northward retreat on Sat ahead of the aforementioned system. Above normal temperatures will be more prevalent over southern/eastern areas. Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Mon-Thu, Feb 3-Feb 6. - Much below normal temperatures across portions of mainland Alaska, Mon, Feb 3. - Much below normal temperatures across portions of mainland Alaska, Thu-Fri, Feb 6-Feb 7. - High significant wave heights for coastal portions of mainland Alaska and the Aleutians, Mon, Feb 3. WPC medium range Alaskan products including 500mb, surface fronts/pressures progs and sensible weather grids can also be found at: https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html