Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 658 PM EST Tue Dec 19 2023 Valid 12Z Sat Dec 23 2023 - 12Z Wed Dec 27 2023 ...A deep surface low tracking across the North Pacific/Gulf will spread heavy precipitation to Southcentral and Southeast Alaska along with a maritime wind threat this weekend into early next week... ...Overview... Over the weekend, initially separate upper lows (one near the Bering Strait and one just south of the Alaska Peninsula early Saturday) are forecast to combine, perhaps incorporating additional energy from the west as well. This will lead to an overall mean trough/low over the eastern Bering/western Alaska into at least the first half of next week. Downstream ridging over western Canada may extend into the mainland especially through the weekend. This evolution should produce a strong surface low pressure system tracking just south of the Alaska Peninsula into Kodiak Island this weekend, leading to areas of strong winds across the North Pacific along with a threat of heavy precipitation along the southern coast of the mainland into Southeast Alaska. As another surface low tracks through the northeast Pacific and pivots, rounds of precipitation should persist across these areas into next week. ...Guidance Evaluation/Preferences... Model guidance shows reasonably good agreement on the large scale with the pattern described above, with typical differences in the individual details. There are some differences in the timing and evolution of the southern upper low's phasing with the northern one, and how far east into the Gulf the southern stream energy may track before doing so. The UKMET was on the eastern side with this energy and resulting surface low on Sunday, but other than that most guidance seemed reasonable. Then there is some spread regarding another shortwave coming eastward through the Pacific along the southern side of the upper trough and then pivoting north a bit, which supports another surface low. The UKMET appeared somewhat fast/northeast by Monday while the ECMWF was on the slower side of the guidance envelope. GFS runs and the CMC looked closer to the ensemble means timing-wise though with some west-east spread. By Tuesday-Wednesday, the axis of the main upper trough/low also becomes questionable in terms of how far east/inland it pushes and suppresses the ridge to the east. The CMC was on the aggressively pushing eastward side of the solutions, with the upper low centered over the Mainland rather than the Bering, unlike the ensemble means. A multi-model deterministic blend worked for the early part of the forecast period, with the blend serving to average out some individual model differences. The ensemble means showed good agreement and thus increased their proportion in the blend with increasing lead time to around half by days 7-8. ...Weather/Hazards Highlights... A deep surface low is likely to cause a broad area of brisk to strong winds across the North Pacific as it tracks from south of the Aleutians toward Kodiak Island and perhaps into the Gulf of Alaska by Sunday-Monday. This storm track will direct a weak atmospheric river into the region and focus heavy precipitation along the southern coast from the Kenai Peninsula eastward across the southeast Mainland into the Panhandle during the weekend. There may be a brief break in the heaviest precipitation intensity sometime early next week, but the pattern should be favorable for additional precipitation especially by around Tuesday as another surface low could track through the northeast Pacific. The specifics are uncertain and subject to change, given the differences in the details of the low track among guidance. Elsewhere, the Aleutians and vicinity will see unsettled conditions for most of the period with frequent light precipitation, with some possible snow farther north into the southwestern Mainland especially this weekend. Lighter snow may be possible for western and then northern parts of the Mainland, with some continued uncertainty over whether much of the moisture reaching the south will reach farther northward. The mean trough across the western part of the state will promote below normal temperatures for the Aleutians, Alaska Peninsula, and western Mainland this weekend and continuing to cool into the first half of next week. Conversely, mean southwesterly flow and upper ridging poking into the East should lead to above normal temperatures on average for the eastern two-thirds of the state, especially by early next week. Anomalies for lows should generally be greater relative to those for highs. However, some areas of near normal to lightly below normal highs may be possible for parts of the central Interior, particularly closer to midweek. 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