Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 759 PM EDT Tue Oct 10 2023 Valid 12Z Sat Oct 14 2023 - 12Z Wed Oct 18 2023 ...Heavy precipitation threat over Southcentral Alaska late this week into the weekend... ...Watching what is currently Typhoon Bolaven for possible 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 this weekend into next week. As the forecast period begins Saturday, a low south of the Aleutians/Alaska Peninsula will have weakened compared to its short range strength. But another low tracking west across the Gulf could spread some locally heavy precipitation into Southcentral Alaska in particular. 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. Into next week, 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 and then slow a bit south of Alaska, but drift north by midweek next week, increasing precipitation chances once again across Southcentral to Southeast Alaska. ...Guidance Evaluation and Preferences... Model guidance is in reasonable agreement with the pattern described above. Early in the forecast period Saturday, there was some spread with the potential for lingering energy/surface low pressure in the Bering Sea, as shown most strongly by the 12Z UKMET and the 00Z ECMWF. Due to a delay from the European Centre, the 12Z ECMWF was not available at the time of 500mb heights/fronts/pressures forecast generation. Once it came in, the 12Z ECMWF did back off on this idea. This low should have a general track east in the northeastern Pacific through early next week. Meanwhile, a surface low or two could rotate northwestward around this feature in the Gulf over the weekend, and then models vary greatly with the strength of another surface low within the cyclonic mean flow moving generally north through the eastern Pacific around Monday. ECMWF runs including the newer 12Z run were among the weakest with this low, while GFS runs were the strongest, and the CMC/UKMET were in between. With the large scale pattern seemingly handled okay, but with some differences in the details even early on, utilized a blend of the 12Z GFS, 00Z ECMWF (latest available at the time), 12Z CMC, and initially small proportions of the EC and GEFS ensemble means. By next week, the the primary system of interest will likely be the extratropical reflection of Bolaven as it tracks across the mid-latitude Pacific. The Joint Typhoon Warning Center latitude/longitude position for Day 5/12Z Sunday of around 40N 169E in its 15Z advisory was closest to the 12Z GFS, compared to the slower 06Z GFS, and the faster ECMWF/CMC/UKMET runs and the faster ensemble means. There has been some notable west-east spread in its track over the past few days, but is generally trending faster. As Bolaven moves closer to and into the Gulf, it should overwhelm and/or absorb the remaining energies and surface lows, and dominate the pattern. Model agreement is pretty good that the large low should move northeastward Monday-Wednesday, crossing 50N late Tuesday-early Wednesday around 150-155W longitude. The 12Z GFS was even farther northeast than the bulk of guidance though, so incorporated the 06Z GFS into Days 7-8 as the 06Z run was better aligned by that time. Still maintained some of the 12Z CMC and 00Z ECMWF in the model blend through the late period, while increasing the proportions of the GEFS and EC ensemble means to half by Day 7 and just over half Day 8. ...Weather and Hazards Highlights... As an initial low pressure system weakens and drifts away south of the Aleutians/Alaska Peninsula, this should minimize wind and wave threats across those and surrounding waters as the forecast period begins Saturday. Meanwhile waves of low pressure in the Gulf could lead to rounds of precipitation for Southcentral Alaska, with the heaviest amounts looking most likely near Prince William Sound, and lesser amounts into Southeast Alaska. The upper level and surface pattern evolution should transition toward favoring more easterly flow along the southern coast by late weekend into early next week. This will likely lead to precipitation becoming somewhat less concentrated and focused more over windward terrain from the Kenai Peninsula southwestward. 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 could also lead to increasing precipitation chances on its northern and eastern sides, so a wet pattern for Southcentral to Southeast Alaska once again. The greatest impacts from Bolaven may even be after the medium range period, into later next week, depending on how it evolves. So will continue to monitor. 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. 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-Sun, Oct 14-Oct 15. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Fri, Oct 13. - High winds across portions of the Aleutians, Fri, Oct 13. 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