Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 653 PM EST Mon Dec 23 2024 Valid 12Z Fri Dec 27 2024 - 12Z Tue Dec 31 2024 ...Bitter cold likely across much of the Mainland, the coldest temperatures of the season for the western/central Interior... ...Gusty winds possible over parts of the Aleutians late week and possibly for Cook Inlet and surrounding areas next week... ...Overview... An upper trough stemming from an Arctic to Chukchi Sea upper low phasing with preexisting energy along the southern coast to northern Pacific will lead to much colder temperatures across much of the state late week into early next week as it slowly traverses east. Some precipitation is forecast ahead of this trough, with moderate to locally heavy amounts along the southern coast and light snow for the Interior. Upstream, an upper low will spin near the western Aleutians next weekend into early next week and support a couple of surface low pressure/frontal systems, bringing some precipitation and gusty winds to the Aleutians, Alaska Peninsula, and toward Southcentral Alaska. ...Guidance Evaluation and Preferences... Model guidance shows rather good agreement on the large scale pattern through the early part of the forecast period, including the first upper trough moving across the Mainland, ridging traversing the Aleutians, and a Kamchatka to western Aleutians upper low sending troughing east. Relatively larger model differences come into play early next week with the evolution of the ridge axis interacting with troughing--especially around Sunday, which seems to be a transition day over the Mainland as the northern part of the ridge gets combined with western Canada ridging, but energy undercutting the ridge complicates matters. The 12Z GFS ended up out of phase with the pattern compared to other models but probably cannot be ruled out given the complexity of the pattern. While there is general model agreement for vort maxes to round the southern and eastern side of the western Aleutians upper low into next week, the details of these energies and associated surface lows are rather uncertain. Leaned toward the ensemble means at this point to avoid favoring any particular model's solution that can be expected to change with the next model run. The WPC forecast used a blend of the deterministic models (12Z ECMWF, 06Z/12Z GFS, and 12Z CMC) early in the forecast period. The GEFS and EC ensemble means were incorporated by mid-period, with quickly increasing proportions to half means by Day 7 and more than half means Day 8 given the increasing model spread. ...Weather/Hazards Highlights... Surface lows in the Gulf of Alaska and vicinity, along with energy aloft and moist inflow ahead of the main trough, will bring some precipitation to the southern coast late week. Moderate to locally heavy totals are possible from Prince William Sound to Southeast Alaska, with multiple rounds of precipitation in some areas. Precipitation amounts should decrease into next weekend as the surface lows weaken. Farther north, some light snow may linger into late week, getting pushed eastward with time as the upper trough and cold front bring in much colder and drier air on the backside. Northerly winds with this upper trough could produce gap winds in favored areas of the eastern Aleutians/Alaska Peninsula Friday-Saturday; models suggest a magnitude of around 30 knots for these winds. Then with surface systems tracking across the Aleutians, light to moderate precipitation and strong winds are possible. Winds look to be highest across the western to central Aleutians Friday-Saturday coming from the south ahead of a strong front, potentially reaching hazardous levels. Then by around next Monday, though there is some spread, models are leaning toward a surface low approaching Kodiak Island or nearby and funneling possibly high winds into Cook Inlet and vicinity. This pattern will also spread increasing precipitation chances to the Alaska Peninsula and Southcentral Alaska Monday-Tuesday. Significantly colder temperatures are likely across the state during the extended period given the upper trough. The cold will affect western Alaska Friday, gradually push into central Alaska by Saturday, and across the rest of the Mainland next Sunday-Monday. Temperatures are forecast to be around 20-35 degrees below average, with actual temperatures dipping into the -30s in the northwestern Interior and North Slope and -10s and -20s as far south as the Lower Yukon and Kuskokwim over the weekend. Temperatures should slowly moderate first in western areas Monday and across most areas next Tuesday as the ridge aloft comes in, slower to warm up than the upper ridge alone would allow because of a persistent cold surface high. Into Southeast Alaska, temperatures look to gradually cool from above normal late this week to near normal or perhaps a few degrees below for highs. Tate 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