Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 653 PM EST Thu Dec 29 2022 Valid 12Z Mon Jan 02 2023 - 12Z Fri Jan 06 2023 ...Heavy precipitation/high wind threat with major New Year's weekend storm likely to linger into early next week for parts of the Upper Aleutians, Alaska Peninsula/Kodiak Island, and coastal Southcentral and Southeast Alaska... ...Overview and Guidance/Predictability Assessment... Guidance shows better than average agreement for the mean pattern during next Monday-Friday, with steady weakening of the initially strong deep-layer low over/near the Alaska Peninsula followed by an upper low emerging into the northwestern Pacific/Aleutians from the Sea of Okhotsk. Meanwhile upper ridging should generally extend north-northwest from western Canada. Within this agreeable overall pattern, there are embedded uncertainties over specifics of the general area of low pressure tracking into the Aleutians and vicinity Tuesday onward (perhaps multiple individual lows), the exact shape of the western Canada upper ridge whose influence could extend into parts of the mainland at times, and a northwestward-tracking Pacific system that may reach near or east of Kodiak Island by around next Wednesday. Differences for the initial Alaska Peninsula system are somewhat smaller in scale. At the moment the 12Z ECMWF/GFS/CMC provide the best cluster for depicting the midweek system that may track near Kodiak Island. There has been a decent signal for this feature in at least the past couple guidance runs, but a fair amount of spread for track/timing. Predictability appears moderate to low several days out given a combined dependence on how shortwave energy around the south/southeast side of initial troughing (anchored by the Alaska Peninsula low) develops and interaction with the lingering trough itself. 00Z/12Z ECMWF and CMC runs have been moderately stable while GFS runs have been inconsistent (00Z/12Z runs closest to the majority into Wednesday versus the slow 06Z run and fast 18Z run, though the 12Z GFS strays farther northwest than other runs by day 7 Thursday). The 12Z UKMET strays to the northeast side of the spread. Among the ensemble means, the past couple ECens runs closest to in principle to the favored operational model cluster (albeit slower/weaker) while the GEFS/CMCens are less defined. Over the Aleutians and vicinity, guidance agrees fairly well for the frontal system reaching the western Aleutians on Tuesday but then rapidly diverges for specifics of individual waves/fronts. From about midweek onward, individual ensemble members from the 00Z/12Z cycles depict just a broad area of potential low pressure waves from the North Pacific into the southern half of the Bering Sea with no meaningful clustering. Thus it is not surprising the operational models become quite diverse and inconsistent for specifics as well. In spite of this spread, the ensemble means have been more stable in depicting the general larger-scale area of low pressure--providing strong support for increasing ensemble mean weight in the forecast later in the period. Confidence is likewise low for exact details within/near the mean ridge anchored over and north/northwest of western Canada, with varying ideas for potential individual surrounding/embedded shortwaves. This recommends a blended/mean approach. A half 12Z ECMWF and half total 12Z GFS/CMC blend during Monday into early Wednesday provided the preferred scenario for the system tracking toward/east of Kodiak Island by Wednesday while it represented consensus for the initial low over/near the Alaska Peninsula and frontal system moving into the Aleutians. This blend quickly incorporated increasing weight of the 12Z GEFS/00Z ECMWF means to account for the increasing detail uncertainties after early Wednesday, with the means reaching 60 percent total weight by day 8 Friday. This yielded a forecast closest to the 12Z ECMWF/CMC for the feature possibly tracking between Kodiak Island and the Kenai Peninsula by late Wednesday/early Thursday, while capturing a combination of model/mean specifics over the Aleutians and vicinity. ...Weather/Hazards Highlights... Initially strong but steadily weakening low pressure near the Alaska Peninsula as of early Monday should extend some areas of locally heavy precipitation of varying types along the southern coast and Panhandle into Monday, with some gusty winds also persisting both to the west and east of the low center. Then a system tracking northwest toward or a little east of Kodiak Island around midweek (but with some uncertainty over exact timing/track) may bring another period of enhanced precipitation and winds, though this system looks to be more compact than its predecessor so the Panhandle should see a drier trend. Some moisture could linger along parts of the southern coast into late week. The rest of the mainland will likely experience mostly dry conditions next week. The Aleutians should initially see light precipitation on the back side of the Alaska Peninsula low. Then one or more waves/fronts reaching the Aleutians from the west should lead to additional areas of precipitation and varying winds, with low confidence for specifics mid-late week aside from the general pattern favoring unsettled conditions. Expect much of the state to see above normal temperatures next week, though perhaps with a gradual moderating trend as southerly mean flow aloft becomes a little weaker. Highs could be a little below normal near the western coast and Aleutians, under and west of the initial upper trough/low. Rausch Hazards: - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Sun-Mon, Jan 1-Jan 2. - Heavy precipitation across portions of mainland Alaska, Sun-Tue, Jan 1-Jan 3. - Freezing rain across portions of mainland Alaska, Sun-Mon, Jan 1-Jan 2. - High winds across portions of the Alaska Panhandle and mainland Alaska, Sun, Jan 1. 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