Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 650 PM EST Fri Dec 15 2023 Valid 12Z Tue Dec 19 2023 - 12Z Sat Dec 23 2023 ...Windy conditions across the Aleutians... ...Snowy/wet pattern over the southeastern coast/Panhandle mid-late week... ...Overview... Guidance maintains reasonable agreement and continuity for the large scale pattern evolution during the period but disagrees with some important embedded details. Expect an initial upper low/trough to continue lifting away from the northern mainland while incoming Bering Sea/Aleutians dynamics and nearby impulses form another mean trough aligned near the western coast of the mainland into the start of next weekend. Multiple surface lows (some possibly generating locally enhanced winds) will likely track from the southern Bering Sea into the Gulf of Alaska, spreading precipitation near their path and supporting a more persistent stretch of precipitation along the southeastern coast/Panhandle. Farther west, models and ensembles continue to advertise the threat of a North Pacific/Aleutians system late next week. There is poor agreement on the details, especially near the Aleutians on Friday, while there is a better consensus toward some precipitation/wind effects along the southern coast by Saturday. ...Guidance Evaluation/Preferences... The Tuesday-Thursday part of the forecast primarily focuses on the small-scale (thus having low predictability) surface lows forecast to track between the southern Bering Sea and vicinity through the Gulf of Alaska. Recent runs have been suggesting a leading wave tracking from near the southwestern Alaska Peninsula into the Gulf during Tuesday-Wednesday in association with a progressive shortwave aloft. However latest runs have significantly diverged in strength and track, with the ECMWF quite strong, the 12Z GFS weak and south, and the 12Z CMC becoming fast/south. Latest ECens/CMCens runs favor a weaker version of the latest ECMWF runs while the UKMET falls in line by day 5 Wednesday and the 06Z GFS was closer to these than the 12Z run. Details become even more ambiguous for one or more trailing waves within the overall favored area of low pressure. A consensus blend yields a developing wave reaching the Gulf about 24 hours after the first one, closely followed by another wave reaching somewhere near the southwestern mainland. Then Gulf low pressure should weaken into Friday. The favored blend yielded decent continuity for surface details in light of the ongoing detail uncertainty. For the potential Friday-Saturday North Pacific/Aleutians storm, another day of guidance runs has not improved confidence in specifics. This system may take a while to resolve, as dynamic support comes from an upper low/trough expected to spin up on the northern periphery of the mid-latitude Pacific jet rather than the bodily movement/evolution of a large scale feature. The models and even to a lesser degree the ensemble means are particularly divergent around Friday. At that time the latest ECMWF runs and CMCens mean (and 12Z ECens mean trending close as well) develop a leading wave while the GEFS mean is well westward. The 06Z/12Z GFS and 12Z CMC were closest to the middle of the spread (where the 00Z ECens mean was). Aside from the farther south ECMWF, there is at least better latitude agreement for track--generally between the Aleutians and 50N. Minus the fast ECMWF, there seems to be some consolidation by Saturday as a majority cluster reaches just south of the Alaska Peninsula, while operational runs show potential for a very deep storm with a central pressure that could reach under 960 mb. Preference sides with an average of the ensemble means and closest operational solutions, namely the 12Z CMC and 12Z/06Z GFS. Guidance/continuity preferences for the above components of the forecast led to a blend consisting of the 12Z ECMWF, 06Z/12Z GFS, and 12Z UKMET for days 4-5 Tuesday-Wednesday followed by returning some 12Z CMC to the mix by day 6 Thursday and adding 30 percent total ensemble means (12Z GEFS/CMCens, 00Z ECens) by days 7-8 Friday-Saturday. The blend maintained 12Z/06Z GFS and 12Z ECMWF input through the end of the period, with the latter included due to being a valid blending component for the western coast mean trough but with a low enough weight not to have an undesired effect on the North Pacific forecast. This approach ended up close to the means for the west coast mean trough aloft, while awaiting better agreement for what could be a better defined embedded low. ...Weather/Hazards Highlights... Multiple potential low pressure waves/frontal systems from the Bering Sea and Aleutians into the Gulf of Alaska should produce one or more periods of strong winds during the early-mid week time frame, especially over the Aleutians, while waves reaching the Gulf should spread some precipitation over the far southwest mainland and lead to a more persistent snowy/wet pattern over the southeastern coast/Panhandle mid-late week. Forecasts are consistent in showing totals for snow and coastal/southern Panhandle rain that 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. The potential remains for North Pacific/Aleutians low pressure to have enhanced wind and/or precipitation influence on the Aleutians and into the Alaska Peninsula/southern coast by next Friday-Saturday. There is still considerable uncertainty over the exact track and timing of this system which could be fairly strong, so continue to monitor forecasts as the sensible weather effects get better resolved in the coming days. Most of the state should see above normal temperatures during the period. Best potential for some pockets of below normal readings will be for highs on some days over parts of the interior and Panhandle, as well as over far western areas especially toward the end of the week as upper troughing becomes established nearby. 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