Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 658 PM EST Mon Nov 11 2024 Valid 12Z Fri Nov 15 2024 - 12Z Tue Nov 19 2024 ...Gusty winds and increased snow for Southwest Alaska followed by heavy precipitation over the Southcentral coast and Panhandle late this week... ...Overview... Guidance shows a transition from progressive North Pacific/southern Bering Sea flow toward a more amplified regime by next Tuesday, supported by a large scale trough/upper low emerging from eastern Asia and ridge building into the Aleutians/Bering Sea. This evolution should yield an eastern Pacific trough. The combination of initial northern mainland/incoming Bering Sea energy plus flow around an Arctic upper low potentially settling near the northern coast will also establish mean troughing over the mainland with a pronounced colder trend going into next week. Low pressure nearing the southwestern coast of the mainland by Friday and transitioning to the Gulf thereafter will bring an episode of wind and enhanced precipitation to southern areas, and then North Pacific/Aleutians low pressure may reach a position just south of the Gulf by next Tuesday. ...Guidance Evaluation/Preferences... For leading Bering Sea low pressure reaching near the southwestern coast by early Friday, details leading into that time are still a bit uncertain but there is decent clustering for a track into the eastern Bering Sea/southwestern coast. The 12Z CMC is a bit on the northern fringe of the spread, with some other solutions having recently trended away from that side of the envelope. There is good agreement that low pressure should transition into the Gulf of Alaska for the weekend. There is considerable spread for details of North Pacific/Aleutians low pressure during the weekend into early next week. Among latest guidance, the 12Z ECMWF/CMC/UKMET represent a stronger/slower cluster with the ECMWF deepest. The old 00Z ECMWF was even more pronounced with its depth and slow timing. On the other hand, the 06Z/12Z GFS trended faster than its already somewhat more progressive 00Z run. ECMWF mean runs are a little faster than the ECMWF while the GEFS/CMCens favor timing closer to the GFS. While the dynamical models/means agree fairly well on a latitude around 50N or the Aleutians, some machine learning (ML) runs also offer the possibility of a farther south track or a very weak evolution that would be of much less consequence for the Aleutians. ML runs are not supporting a depth below the 990s mb until possibly early next week as it reaches south/southeast of the Alaska Peninsula. By next Tuesday as the upper pattern becomes more amplified and larger in scale, there is actually some improvement with respect to upper trough and surface low position. Finer details in the 12Z GFS/12Z CMC bring some degree of low pressure farther north into the Gulf than most other guidance. Overall preference is for intermediate timing (within the bounds of the ECens mean and 00Z GFS while near the Aleutians) and moderate depth, which keeps the forecast close to continuity in principle. For the Arctic upper low originating from the Canadian Archipelago, the 12Z CMC offers the most extreme evolution with a loop as far west as the northwestern coast of the mainland and then into the Interior. Remaining dynamical/ML guidance suggests this upper low is more likely to settle along or north of the northern coast, though with differing ideas for how much flow may try to dig to its south. Today's forecast started with primary emphasis on the 12Z GFS/ECMWF Friday into early Saturday with a small 12Z UKMET contribution. Then the forecast switched GFS input to the 00Z run while splitting the ECMWF component partially with the 00Z ECens mean to yield the desired intermediate solution for the weekend Aleutians system. With the ECens/CMCens means maintaining better definition of this system by Monday than the GEFS mean, 40 percent total ensemble mean input late period used those two means. Detail/position differences favored splitting GFS input among the 00Z/06Z runs by Day 8 Tuesday while keeping modest ECMWF weight. ...Weather/Hazards Highlights... The system likely reaching near the southwestern coast of the mainland by the start of the period early Friday will likely increase winds and raise the potential for some coastal flooding for the Bristol Bay region late this week, with increased snowfall moving inland. Breezy conditions are also likely across much of the Interior above the influence of the surface temperature inversion as the storm system moves inland and weakens. The Days 3-7 Hazards Outlook depicts an area of high winds over a portion of the far southwestern mainland between Norton Sound and Bristol Bay for Thursday (Day 3, late in the short range time frame). Winds may not quite reach objective hazardous criteria but the hazard area also incorporates the potential for coastal effects. Expect flow ahead of a triple point low transition into the Gulf to focus a plume of Pacific moisture into Southcentral and the Panhandle. This will likely produce heavier precipitation in these regions, especially along the coastal terrain. The Days 3-7 Hazards chart reflects this threat valid during the Friday-Saturday period. The Alaska Peninsula and vicinity may see a period of brisk to strong winds on the back side of the surface low during the weekend. The next system should track farther south, potentially bringing a period of enhanced wind and rain to the Aleutians, but system details are still very uncertain at this time. While this storm's moisture should stay south of the Alaska Peninsula, it may eventually bring increasing moisture to the Panhandle and vicinity around next Tuesday-Wednesday as it reaches a position just south of the Gulf of Alaska. Also by next Tuesday, southern tier areas may see stronger winds due to the tightening gradient between this low and high pressure that builds into the northern mainland. During the late part of this week, expect a mix of below/above normal temperatures with a tendency for more above normal min temperatures as low pressure brushes the southern mainland. A cold front dropping south through the mainland as an Arctic upper low settles near/north of the northern coast, with high pressure building into the northern mainland by next Tuesday, will bring a pronounced colder trend from the weekend into early next week. Coldest anomalies should be over Interior/southwestern areas while the North Slope and parts of the far southeast could remain a little above normal. Rausch Additional 3-7 Day Hazard Information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php 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