Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 724 PM EDT Thu Oct 12 2023 Valid 12Z Mon Oct 16 2023 - 12Z Fri Oct 20 2023 ...Watching what is currently Typhoon Bolaven for impacts to southern Alaska next week... ...Overview... The storm track over the Alaska forecast area will favor low pressure systems over the northern Pacific and Gulf of Alaska into next week. As the forecast period begins Monday, a surface low south of the Aleutians/Alaska Peninsula will have weakened considerably compared to its short range strength, but may linger long enough to continue enhanced precipitation particularly for the Kenai Peninsula to Prince William Sound. Another low tracking north across the northeastern Pacific Monday-Tuesday will approach the Southeast. Aloft, troughing is forecast to extend across western Alaska and broaden south of the state supporting these surface lows, while ridging pokes into the eastern mainland from western Canada. By Tuesday and beyond, these energies are likely to become absorbed/dominated by the extratropical reflection of current Typhoon Bolaven. Bolaven looks to track quickly eastward through the Pacific early in the week and then slow a bit south of Alaska, but drift north by midweek next week, increasing precipitation chances particularly for Southeast Alaska, with winds increasing as well along the southern coast. ...Guidance Evaluation and Preferences... Two forecast challenges are noted early in the period which complicate guidance preferences due to variable differences with respect to each region. The first is the evolution of a low pressure system in the northeastern Pacific and the second the approach of the extratropical remnants of Bolaven. For the northeastern Pacific system, the 12Z GFS is initially slower than the other deterministic guidance which were reasonably well clustered on Monday (Oct 16), though by Tuesday (Oct 17) a slower northward progression in the 12Z ECMWF lines up better with the GFS compared to the CMC and UKMET which progress faster to the northeast. The slower solutions are favored with a potentially slightly stronger and longer lasting low moving towards Southeast Alaska early in the period compared to the previous forecast. To the west, the 12Z GFS/ECMWF/UKMET were reasonably well clustered compared to the 12Z CMC which was slower with the progression of the remnants of Bolaven. This combination of factors results in a blend weighted more towards the 12Z ECMWF/GFS with lower contributions from the 12Z UKMET and CMC. The UKMET then accelerates this system northeastward, with the other guidance remaining reasonably well clustered through the middle of the period as the progression of the 12Z CMC catches up with the GFS and ECMWF in the Gulf of Alaska. The deterministic solutions diverge rather rapidly towards the end of the period, with the remnants of Bolaven lingering in the GFS while a secondary shortwave follows the weakening remnants in the 12Z ECMWF, leading to a pattern that is out of phase. There are also notable differences to the west with a system upstream over the Bering Sea. While the deterministic guidance is in agreement on the approach of this next system, the ECMWF has trended towards a very positively tilted trough as the GFS and CMC have remained negatively tilted, with the means not surprisingly favoring a more neutral tilt. Given the noted divergence in the deterministic solutions, the updated WPC forecast favored an increasing contribution from the GEFS and ECens means late period. It should be noted that longer-term trends do favor a positively tilted trough, so this will need to be monitored. ...Weather and Hazards Highlights... Precipitation chances may continue for the southern coast early in the period as a low lingers in the Gulf of Alaska, with the heaviest amounts looking most likely near the Kenai Peninsula to Prince William Sound as easterly flow favors upslope areas/windward terrain there. One or more shortwaves embedded within the mean flow aloft could produce periods of generally light snow farther north over the Central and Eastern Interior. As another low over the northeastern Pacific and the extratropical remnants of Bolaven move toward the southern part of the state, the Aleutians and Alaska Peninsula may see brisk northerly gap winds with gusty southerly winds for Southeast Alaska. Moist Pacific inflow will also lead to increasing precipitation chances to the northeast of the approaching systems, yielding a wet pattern for Southeast Alaska as early as Tuesday. Precipitation chances should spread further northward along the southeastern coast towards Prince William Sound by Wednesday, and then likely linger for the Southeast beyond the forecast period. With ridging for the eastern part of the state and mean southeasterly flow on its west side, expect above normal temperatures over much of the state by early next week and lasting through much of the week. Highs look to be above freezing in most lower elevations for southern portions of the state with highs around freezing in the Interior. Highs look to trend downward slightly mid- to late next week. Putnam/Tate Hazards: - Heavy rain across portions of mainland Alaska, Wed, Oct 18. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Tue-Thu, Oct 17-Oct 19. 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