Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 651 PM EST Sun Dec 01 2024 Valid 12Z Thu Dec 05 2024 - 12Z Mon Dec 09 2024 ...Wet/snowy pattern for the southern coast and Panhandle continuing late this week into early next week... ...Overview... With some day-to-day differences in details, expect mean ridging aloft to persist from the Panhandle and western Canada northwest through Mainland Alaska. Meanwhile an upper low just south of the eastern Aleutians and Alaska Peninsula late this week should then open up with the approach of a mid-latitude Pacific system, followed by a larger scale upper low drifting toward the western Aleutians by next Monday. This pattern will tend to support surface low tracks from the Pacific toward the Alaska Peninsula (with potential leeway into the Bering Sea or Gulf of Alaska) late week into the weekend and then a westward shift by Monday. The southern coast and Panhandle will likely see multiple episodes of enhanced precipitation while above to well above normal temperatures become established across much of the state. ...Guidance Evaluation/Preferences... Reflecting what has been an agreeable large scale pattern containing lower-predictability details at the surface and aloft from the short term into late this week into Saturday, there are some model refinements still in progress along with lingering spread for Thursday-Saturday systems. As of early Thursday when today's forecast period starts, latest guidance has become somewhat better defined with the lingering reflection of a short term surface low (crossing the Alaska Peninsula) reaching near Nunivak Island. This wave should dissipate shortly thereafter. Continuity has held up fairly well for the next system in the series, a fairly compact but vigorous one reaching just south of the Alaska Peninsula by early Friday. Through that time, latest ECMWF runs have had the most coherent evolution of this system and the 12Z UKMET has come close to joining it. The 12Z GFS is somewhat disjointed with the details but the overall pattern is acceptable enough to include as a minority component of a blend. CMCens/ECens means and machine learning (ML) models comprise the consensus cluster as well, while the GEFS mean is a bit west. More spread develops for this system's evolution/track by Saturday. Most of the latest ML runs want to pull the system eastward into/near the Gulf of Alaska before shearing it out. The 12Z ECMWF is closest to this idea. On the other hand, the 12Z CMC is a northern extreme as of Friday and then lifts the dynamics into the mainland with minimal support from other guidance. Latest guidance continues to depict a strong system tracking well to the south of the Aleutians Friday into early Saturday, followed by some increasing spread regarding northeastward progression through Sunday. Some of this spread relates to interaction with the dynamics and frontal system approaching from the western Pacific/Aleutians. The most common theme in the dynamical/ML models and ensemble means is for continued progression into Sunday (reaching just south of the Alaska Peninsula as of 12Z), as opposed to the slow solution seen in the 12Z CMC/00Z ECMWF due to merging of the lead system with upstream dynamics. Dynamical models and ensemble means that depict the progressive scenario suggest this system should lose its identity by Monday as the western Aleutians system becomes dominant. Some ML models hold onto more definition into Monday while differing with details upstream. The AIFS ensemble mean is close to the dynamical means though. 12Z/18Z GFS details look somewhat questionable but again the overall pattern compares acceptably enough to consensus for minority weight. Guidance comparisons led to starting the Thursday-Friday part of the forecast with half 12Z ECMWF weight and the rest split between the 12Z GFS and UKMET. Day 6 Saturday started to add some of the ensemble means while days 7-8 Sunday-Monday increased the means (12Z GEFS/CMCens and 00Z ECens) to 45-60 percent respectively. By Monday an even split of the 12Z GFS/ECMWF comprised the remaining 40 percent operational component. ...Weather/Hazards Highlights... With the large scale pattern supporting moist southerly flow aloft for most of the period, areas along and near the southern coast from the Alaska Peninsula to the Panhandle should see continued episodes of enhanced precipitation from late this week into early next week. A surface low forecast to track close to the Alaska Peninsula/Kodiak Island by Friday-Saturday and its associated frontal system may provide some added focus to activity. Potential eastward progression as it weakens Saturday could provide a brief lull in precipitation over some areas for a part of the weekend. Another Pacific system forecast to track near the Alaska Peninsula by Sunday and continued southerly flow behind it should lead to a rebound in moisture later in the weekend and early next week. Coastal and nearby low elevations should tend to see rain, with mostly snow elsewhere. However milder conditions over the southern Panhandle will likely yield broader coverage of rain with snow confined to higher elevations. At least on the larger scale, most guidance signals are for the Sunday-Monday period to have somewhat greater precipitation totals compared to late this week. Low pressure systems could produce periods of brisk to strong winds as well. The Aleutians should see lighter and more scattered precipitation late this week, while conditions likely become more unsettled in terms of winds and moisture by Sunday-Monday. Away from the southern mainland moisture focus, portions of the western and north-central mainland may see periods of light snow while other locations should be fairly dry. Above to well above normal temperatures should already become established over most of the state as the forecast period starts early Thursday. North Slope locations may still be moderately below normal late this week before finally flipping to above normal by the weekend and some valley locations where cold air typically persists longer may also take a a little extra time to trend above normal. Expect the pronounced warm anomalies to persist through at least early next week, with min temperatures tending to exhibit greater departures from normal versus the highs. 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