Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 651 PM EST Tue Dec 17 2024 Valid 12Z Sat Dec 21 2024 - 12Z Wed Dec 25 2024 ...Stormy pattern through this weekend into next week with multiple episodes of southern coast into Panhandle precipitation and hazardous maritime conditions over the North Pacific/Gulf of Alaska... ...Overview... Most guidance agrees on a persistent mean ridge aloft from western Canada into portions of the mainland through the period, with an initial storm track from the eastern Pacific into the Alaska Peninsula/Gulf of Alaska through Monday and then increasing dominance of a strong North Pacific/Aleutians storm into midweek. These systems will produce focused precipitation along the southern coast while the North Pacific and then the Bering Sea into Aleutians will see periods of strong winds. The forecast pattern will likely support above normal temperatures over most areas aside from parts of the North Slope, as well as perhaps some far western areas toward midweek as Arctic into Bering Sea upper troughing deepens to the west of the mean ridge. ...Guidance Evaluation and Preferences... Aside from the typical stray model solution here and there plus low-predictability smaller scale differences, today's dynamical and machine learning (ML) models plus the ensemble means displayed remarkably good agreement for the pattern and individual systems through the entire days 4-8 period (Saturday-Wednesday). Thus the favored guidance blend was able to use various weights of operational solutions to yield systems with better definition than the means while toning down uncertain specifics of individual model runs. The first half of the forecast placed the greatest emphasis on the 12Z ECMWF/UKMET with less input from the 12Z CMC, especially on Sunday. As of 12Z Saturday, the dominant dynamical/ML guidance clustering shows fairly strong low pressure a little south of Kodiak Island and tracking toward the Alaska Peninsula. The 12Z GFS is an eastern extreme, with the new 18Z run adjusting favorably back to the 12Z ECens mean track. Into Sunday, most models/means track the low into the southeastern Bering Sea. This is a refinement from continuity, and seems to reflect the occasional tendency for guidance to take a little extra time to bring defined low pressure westward into the Bering Sea in this type of pattern. The 18Z GFS continues to look a lot better than the 12Z run by this time, trending stronger toward the ECMWF/UKMET. The CMC held the surface low south of the Alaska Peninsula, thus its minimal weight around Sunday. The next vigorous system should reach the Gulf of Alaska by Monday. While the overall position of latest GFS runs looks reasonable, the model is currently running a little weak versus other dynamical models, the ECens mean, and the average of recent ML models. This system should steadily weaken near the southern coast thereafter. The theme of unusually good guidance agreement in principle continues after early Monday through next Wednesday when the overwhelming focus is on a deep North Pacific/Aleutians storm that may reach close to the Alaska Peninsula by midweek. GEFS/ECens means have been remarkably stable with the overall depth (mid 950s-mid 960s mb, with the 12Z GEFS deepest at 953 mb on Tuesday) over the past couple days, with the average of ML models in this range as well. ML models also support the dynamical model/mean storm track, reaching 46-50N as of early Tuesday and between 50N and the Alaska Peninsula by early Wednesday. Some individual dynamical model runs have brought the central pressure as low as the 920s-930s mb at some point early-mid week, but a deterministic forecast minimizing in the upper 940s mb (as denoted by a closed 952 mb isobar early Tuesday) seems reasonable at this time. By Tuesday-Wednesday the models suggest that potential frontal wave development will add some uncertainty to the specifics of the overall system. The late Monday through Wednesday part of the forecast started with a blend of half 12Z/00Z ECMWF with the rest distributed among the 06Z/12Z GFS and 12Z CMC. ...Weather/Hazards Highlights... From Saturday into Monday, two strong northeastern Pacific systems will bring enhanced precipitation to the southern coast along with areas of strong winds to the east of each low's track. Expect the first low to follow a path across the Alaska Peninsula into the southeastern Bering Sea Saturday-Sunday, while the trailing one that reaches the Gulf of Alaska by Monday will push the zone of strongest winds farther eastward. The flow around each storm will bring the potential to strengthen barrier jets and gap winds over some areas. While precipitation along the southern coast and Panhandle trends lighter as the Monday storm steadily weakens, focus will turn to the potentially deep North Pacific/Aleutians storm that may reach near the Alaska Peninsula by midweek. This system should produce moderate amounts of precipitation across the Aleutians with somewhat greater enhancement eventually possible into the southern coast. Expect a broad area of strong winds as well, including on the north side early in the week followed by northerly flow in the storm's wake from the Bering Sea into North Pacific as well as in leading southerly/southeasterly flow heading into midweek. Currently the Days 3-7 Hazards Outlook depicts no confident areas of heavy precipitation or high winds reaching hazardous thresholds, but either parameter may still be hazardous on a localized scale and will continue to monitor for updates in coming days. The pattern will favor above normal temperatures over most areas, with anomalies for min temperatures tending to be higher than those for highs. Parts of the North Slope may remain somewhat below normal during the period, along with a few pockets in the eastern Interior. Some far western areas may trend a little below normal by next Tuesday-Wednesday as upper troughing deepens just west of the mainland. Rausch Additional 3-7 Day Hazard Information can be found on the WPC medium range hazards outlook chart at: The WPC Hazards Outline depicts these threats. 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