Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 658 PM EST Thu Dec 14 2023 Valid 12Z Mon Dec 18 2023 - 12Z Fri Dec 22 2023 ...Windy conditions across the Aleutians... ...Snowy/wet pattern over the southeastern coast and Panhandle Tuesday onward... ...Overview... Today's guidance agrees well on a general pattern evolution featuring an initial upper low/trough along the western coast of the mainland lifting out and ultimately being replaced by incoming energy that supports a western Aleutians into Bering Sea storm and one or more leading waves. The overall area of low pressure reaching near the southern coast by late in the week should weaken as the upper system becomes more diffuse. With a lot of ambiguity in the details, there is a signal that trailing North Pacific low pressure may affect the Aleutians by the end of next week. The two systems expected to affect the Aleutians may bring periods of windy weather to that region while the surface waves reaching the Gulf of Alaska should focus precipitation over the southern coast and Panhandle for multiple days from Tuesday onward. ...Guidance Evaluation/Preferences... Latest operational model runs have good support from the means for the general northward/northwestward lifting away of initial west coast upper troughing/low and upstream dynamics gradually weakening by the time it reaches the western coast after midweek. A consensus blend looks good over and near the mainland aloft, while the typically more sensitive surface details show some locally meaningful differences. By early Wednesday the operational runs hold the parent low over the Bering Sea a little farther west than the ensemble means, while latest GFS/ECMWF/UKMET runs have come into better agreement for a leading Tuesday-Wednesday frontal wave that may develop near the Alaska Peninsula and track into the Gulf of Alaska. The 00Z ECMWF had tracked this wave northward along the western coast of the mainland instead. Given the compact nature of this wave, the ensemble means are not defining this wave yet by early Tuesday but the wave implied by the means thereafter offers added support for the GFS cluster. Also there is a decent signal in principle for another wave to follow a similar track about 24 hours later. The overall combination of waves over the Gulf and Alaska Peninsula (the latter some combination of lingering Bering Sea/North Pacific waviness) as of early Thursday should steadily weaken into Friday as the upper trough near the western coast becomes more diffuse. Looking farther west toward the end of the week, the ensemble means have recently been fairly steady in depicting a North Pacific/Aleutians system by day 8 next Friday. With the 12Z CMCens mean trending a bit south from the 00Z run, all three means show a track close to 50N latitude with similar timing. However operational model runs thus far have been a lot more varied with the storm's character, including whether one consolidated system or two separate lows along with timing/strength differences. Current preference is to lean toward the consistent ensemble means, with operational model enhancement coming from the 06Z GFS and 00Z ECMWF which were closer to the means than the more recent runs (and still depicted the consensus southwesterly flow aloft over the mainland on Friday). Based on the guidance comparisons, the Monday into Wednesday portion of the forecast started with primary emphasis on the 12Z GFS/ECMWF along with some UKMET input and the least weight on the CMC. Even into Thursday, model agreement was sufficiently good to require only the 20 percent total 12Z GEFS/00Z ECens input needed to make up for the UKMET run ending. Then considerations for the potential Friday North Pacific/Aleutians system led to a rapid increase of ensemble mean weight to 50 percent with GFS/ECMWF runs switched to the 06Z/00Z runs respectively. ...Weather/Hazards Highlights... Expect the most active weather during next week to extend from the Aleutians eastward along the southern coast into the Panhandle. Low pressure and a leading front affecting the Bering Sea and Aleutians should produce one or more periods of strong winds during the early-mid week time frame and then a storm expected to track just south of the Aleutians could bring a period of strong easterly winds and organized rainfall depending on the storm's track and intensity. The leading Bering Sea/Aleutians front should develop a wave that ultimately reaches the Gulf of Alaska by midweek, bringing precipitation from the Alaska Peninsula to the Panhandle. At least one more trailing wave could maintain the precipitation focus over the southeastern coast/Panhandle into late week. Current forecasts indicate that totals for snow and coastal/southern Panhandle rain should not reach hazardous criteria, but could still be significant given the potential for multi-day accumulations. Some light/scattered snow will be possible over portions of the central and northern mainland. Also of note and as indicted on the 3-7 Day Hazards Outlook, before the extended forecast period starts with a brief lull on Monday there should be a period of heavy snow extending into Sunday along and just inland from the southern coast--from just east of the Kenai Peninsula to near the far northwestern Panhandle. A majority of the state will tend to see above normal temperatures during the period, though some areas from the southwest through eastern interior as well as the Panhandle could see moderately below normal readings on one or more days (more likely for daily highs versus lows). 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