Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 758 PM EDT Tue Sep 10 2024 Valid 12Z Sat Sep 14 2024 - 12Z Wed Sep 18 2024 ...Bering Sea storm expected to bring areas of heavy rain, strong winds, and coastal flooding to portions of the mainland this weekend... ...Overview... Most guidance shows a fairly progressive and energetic pattern during the period, with at least one and possibly two significant storm systems affecting portions of the mainland. The leading and more confident storm should track over the Bering Sea this weekend and is most likely to reach near the northwestern mainland by early Monday. Expect this storm to bring areas of heavy rainfall to some western and southern areas, along with strong winds over the west plus coastal flooding concerns along the southwestern coast. Guidance suggests another developing system may track somewhere between the Bering Sea and North Pacific during the first half of next week, with corresponding uncertainty in effects on the mainland. The current majority cluster shows a Bering Sea low track but generally at a lower latitude than the first system. Another low emerging over the far northwestern Pacific may begin to influence weather across the western Aleutians by next Wednesday. ...Guidance Evaluation/Preferences... For the leading storm that should be deepest over the Bering Sea this weekend and trend weaker as it nears/reaches the northwestern mainland, there are still some questions regarding detail/track/timing but with a reasonable majority scenario to provide a coherent forecast. Among dynamical guidance, the ensemble means have been showing a fairly consistent track and the 12Z ECMWF/UKMET/CMC offer the closest comparisons in principle. GFS runs have been a lot more inconsistent. The 12Z version takes longer to deepen the system and then tracks the low farther south than other models/means. The new 18Z GFS offers yet another idea, placing more emphasis on trailing dynamics aloft and yielding a much weaker low than other guidance. The most common theme in latest runs is for a depth reaching into the 960s mb, most likely around late Saturday or Saturday night. The 12Z ICON reaches the 950s mb though. Looking at latest machine learning (ML) guidance, the 00Z/06Z runs offered potential for some elongation to exist over the Bering Sea as of early Saturday before the dynamics aloft and surface low pressure consolidate farther eastward. New 12Z ML runs seem to be trending toward the non-GFS dynamical models with a better defined system already as of 12Z Saturday, albeit with some continued north-south track spread thereafter. The second potential system has a lot more spread/model variability and thus much lower confidence in specifics. Latest ensemble means (with the ECens being relatively more consistent with the signal in recent runs) suggest that western Pacific low pressure will track northeastward across the Aleutians and into the Bering Sea in association with a phased upper trough, with the surface low reaching near the south-central western coast by early next Wednesday. The past couple ECMWF runs are closest to this idea, while the CMC has switched from a Bristol Bay track in the 00Z run to a slower/northwest track near St Lawrence Island in the 12Z run. The 12Z ICON is between the 12Z ECMWF/CMC at the end of its run late Tuesday. In contrast, latest GFS runs area tracking a southern tier shortwave ahead of any northern stream dynamics, leading to a weak North Pacific wave that has little impact on the state. 00Z through 12Z ML models add to the intrigue, with most of them favoring a defined wave across the eastern Aleutians/Alaska Peninsula--but latest AIFS means have tilted farther north closer to the dynamical means. For the system nearing the western Aleutians by next Wednesday, operational models generally side with latest ECens means while the GEFS/CMCens means are slower. However the 12Z/18Z GFS runs represent a slightly slower trend versus earlier runs that strayed a little faster than most other solutions. ML models encompass this east-west spread among the dynamical models/means. Overall guidance preferences were for the non-GFS majority with the leading Bering Sea storm and for the ECMWF/CMC (more the former) plus 12Z CMCens/00Z ECens means for the second system, as the 12Z GEFS was weaker than the other means. This led to a starting blend consisting of the 12Z ECMWF/CMC/UKMET from early Saturday into early Monday, followed by a quick transition to half ECMWF/CMC (35/15 weight) and half total CMCens/ECens means by Tuesday-Wednesday. ...Weather/Hazards Highlights... Expect strong low pressure tracking across the northern Bering Sea during the weekend to bring areas of enhanced precipitation to western areas, and then west to east across southern areas in association with the storm's frontal system and embedded southern coast wave. The Day 3-7 Hazards Outlook depicts the areas most likely to reach hazardous criteria for rainfall, from portions of the Seward Peninsula south to the eastern Alaska Peninsula on Saturday and then along/just inland from the Southcentral coast on Sunday. Meanwhile higher elevations will likely see snow but with less pronounced totals. This storm will also produce strong winds over the Bering Sea and into parts of the mainland, but with speeds currently forecast to remain below hazardous criteria. However the long fetch of southwesterly flow across the Bering Sea will bring a threat of coastal flooding, especially to the southwestern coast of the mainland. By Monday, lingering low pressure over or near the northwestern mainland and shortwave energy aloft may support persistence of lighter precipitation. Confidence is below average regarding the details of the next potential storm during the early-middle part of next week. Current guidance preference would bring the low track through the Aleutians and southern Bering Sea, then into the southwestern mainland--south and somewhat weaker versus the leading system. This would confine the majority of precipitation to the southwestern mainland and southern coast. Possible alternatives include a farther south track that would reduce effects across the southern mainland, or less likely a farther north track. The forecast pattern will favor daytime highs ranging from mostly below normal across the south and Panhandle to a mix of above/below normal readings over the interior and mostly above normal closer to the northern coast. Morning lows should be mostly above normal through the period, aside from some modestly below normal readings over the southwest interior on Saturday and in parts of the Panhandle on multiple days. 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