Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 646 PM EST Thu Nov 28 2024 Valid 12Z Mon Dec 02 2024 - 12Z Fri Dec 06 2024 ...Wet/snowy pattern for the southern coast and Panhandle next week... ...Overview... Most guidance shows a blocky large scale pattern consisting of a southeastern Bering Sea upper low as of early Monday wobbling back into the central Aleutians and then eastward to a position near or south of the eastern Aleutians/southwestern Alaska Peninsula by next Friday, while an initial col region over the mainland transitions toward more ridging that extends northwest from the Panhandle and western Canada. The main upper low and various other shortwave impulses will likely support multiple surface lows between the Aleutians/North Pacific and the Gulf of Alaska, but with below average confidence in specifics for most of them. However the overall pattern should support a prolonged period of wet/snowy weather along the southern coast and Panhandle. Expect most of the mainland to lie within the gradient between high pressure over the Arctic and surface lows to the south. ...Guidance Evaluation/Preferences... The most notable difference aloft arises during the first half of the week with the southern Bering Sea upper low. There is a considerable majority cluster consisting of the 12Z ECMWF/UKMET/CMC and latest ECens/CMCens means that depict greater retrogression into the central Aleutians versus recent GFS runs through 12Z which drop it more southward. Latest machine learning (ML) models also point toward the ECMWF cluster. This difference has implications at the surface as well. The ECMWF cluster reflects a surface low associated with the upper low (of varied strength, ECMWF on the weak side and CMC strongest) with a separate zone of other surface lows from the mid-latitude Pacific northeastward. In contrast, the GFS scenario wraps mid-latitude Pacific low pressure into the upper low to yield a deep storm system near the eastern Aleutians. The new 18Z GFS looks like it is starting to nudge toward the majority in that it does not drop the upper low as far south and retrogrades a little more, though it still wraps Pacific low pressure into the feature as in previous runs. Preferences side with the majority ECMWF cluster overall. Farther east, the specifics of surface lows tracking into the northeastern Pacific toward the Alaska Peninsula/Gulf of Alaska have minimal confidence due to the small scale of supporting shortwave impulses. There are at least hints of a couple defined waves/frontal systems tracking toward the southern coast in the Monday-Wednesday period, with the favored blend reflecting these features. The 12Z UKMET strays on its own with a wave nearing the southern Panhandle by the end of its run early Wednesday. It is likely that details will change over future forecast cycles. During the latter half of the week there is some convergence in the guidance toward a mean upper low reaching near or a little south of the eastern Aleutians/Alaska Peninsula, with corresponding surface low pressure. The ensemble means have been fairly consistent and agreeable with the overall evolution by late in the period but of course there is still considerable spread and variability among the operational models for surface specifics from the North Pacific into and near the Gulf of Alaska. Preference by this time frame transitions toward primary emphasis on the larger scale mean system rather than on any potential waves to the east. Finally, guidance shows an emerging western Pacific system whose influence could reach into the western Aleutians by next Friday. However most solutions keep it fairly suppressed, with only recent GFS runs showing meaningful effects reaching the Aleutians. The new 18Z GFS has trended southward. The early part of the forecast started with a blend of half ECMWF (split between 12Z and 00Z/28 runs) and the rest 12Z CMC and UKMET based on preferences near the Aleutians and to reflect the most common themes farther eastward. As solutions converge somewhat over the Aleutians and North Pacific around/after midweek, the rest of the forecast added some 12Z GFS into the operational model part of the blend while increasing ensemble mean input (12Z GEFS/CMCens and 00Z ECens) to 30-40 percent. CMC input was nudged down some by next Friday as the model's Aleutians upper low became a western extreme while dampening the Panhandle/western Canada upper ridge some versus most other solutions. The resulting forecast maintained reasonable continuity for the large scale pattern while refining details for surface systems. ...Weather/Hazards Highlights... The overall pattern will be favorable for periodically enhanced precipitation along and near the southern coast (including the Alaska Peninsula) and Panhandle next week, with southerly mean flow aloft between an Aleutians/North Pacific upper low and Panhandle/western Canada upper ridge as well as surface systems tracking toward the southern coast. A lot of the specifics in terms of timing and intensity will take additional time to resolve though. Coastal and nearby low elevations should tend to see rain, with snow elsewhere. Milder conditions over the southern Panhandle will likely yield broader coverage of rain with snow confined to higher elevations. The one detail with some degree of confidence is a fairly coherent signal for more intense precipitation over the southern half to three-fourths of the Panhandle early next week. Therefore the Days 3-7 Hazards Outlook depicts a heavy precipitation area for this activity from Sunday (late in the short range period) through Tuesday. Rainfall may produce enhanced runoff and rises on area rivers. Most of the mainland north of the Alaska Range should remain dry next week. One or more of the potential surface lows could generate areas of brisk to strong winds but with minimal confidence in details at this time. The northern three-fourths of the state will start next week on the cold side with below normal readings, including some overnight lows in the -20s over eastern interior areas. Aside from valley locations where cold air typically persists longer, the overall pattern will support a pronounced warming trend with time. Most areas should see above to well above normal temperatures by next Friday, with perhaps parts of the North Slope still a little below normal. 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