Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 807 PM EDT Wed Oct 11 2023 Valid 12Z Mon Oct 16 2023 - 12Z Fri Oct 20 2023 ...Possibly heavy precipitation lingers over Southcentral Alaska into early next week... ...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 Sunday, a surface low south of the Aleutians/Alaska Peninsula will have weakened considerably compared to its short range strength, but may drift north and enhance precipitation particularly for the Kenai Peninsula to Prince William Sound. Another low tracking north across the northeastern Pacific Monday-Tuesday could reach the Gulf. 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 across Southcentral to Southeast Alaska, with winds possibly increasing as well. ...Guidance Evaluation and Preferences... Model guidance is in reasonable agreement with the pattern described above. But there are some notable differences by Monday-Tuesday with the northeastern Pacific surface low strength and track. GFS runs have been stronger and farther northwest with the low than the non-NCEP guidance. GFS runs seem like they may be too aggressive with its depth, given the relatively nearby proximity of Bolaven to the southwest, and the GFS runs pivoting it west towards Kodiak Island or so while maintaining a strong feature also seems overdone. Preferred a blend of the ECMWF/CMC/UKMET and the ensemble means with this feature instead. Bolaven is forecast by the Joint Typhoon Warning Center to be fully extratropical by the Day 4/12Z Sunday period, with a latitude/longitude position of around 41N 172E, which is a bit faster than the GFS runs (especially the slow 06Z) and a bit slower than the ECMWF and CMC. There continues to be some notable west-east spread in its track. But models regardless have the strong and large post-tropical cyclone tracking east and then pivoting northward closer to and into the Gulf by Tuesday-Thursday, while overwhelming/absorbing the remaining energies and surface lows to dominate the pattern. For Bolaven, preferred the positioning of the GEFS and EC ensemble means as a good middle ground of the guidance. The 06Z GFS ended up close to the means, as did the 12Z CMC. The 12Z GFS appeared too far north (and dangerously close to Alaska by Wednesday-Thursday) while the ECMWF was south of consensus and ended up farther east too. This spread in the low positions does cause considerable differences with the sensible weather like precipitation and winds, so will continue to monitor. Overall the WPC forecast blend favored the deterministic models early on and transitioned to an ensemble mean-heavy blend by Days 7-8 amid the increasing variability among models. But did manually enhance the central pressures and winds as needed. ...Weather and Hazards Highlights... A low or two across the Gulf of Alaska could lead to rounds of precipitation for Southcentral Alaska, with the heaviest amounts looking most likely near the Kenai Peninsula to Prince William Sound, with easterly flow favoring 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 mainland as well. Then as Bolaven moves northeast toward the southern part of the state, the Aleutians may see brisk northerly winds and then the Gulf could see increasing easterly winds for early to mid-next week. Moist inflow will also lead to increasing precipitation chances on its northern and eastern sides, yielding a wet pattern for Southcentral to Southeast Alaska by Wednesday-Thursday and likely lingering beyond the forecast period as well. With ridging for the eastern part of the state and mean southeasterly flow on its west side, expect a steady warming trend during the the period to yield 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, with the possible exception of the North Slope. Tate Hazards: - Heavy rain across portions of mainland Alaska, Sat, Oct 14 and Wed, Oct 18. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Wed, Oct 18. 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