textproduct: Anchorage

LONG TERM FORECAST (Days 4 through 7: Wednesday through Saturday)

Key messages:

- A very strong low pressure system is likely to develop over the Bering Sea between Wednesday and Friday.

- The post-tropical remnants of Typhoon Dujuan will likely be absorbed into an initial weaker system moving into the western Bering Sea, causing the low to rapidly intensify from Wednesday to Thursday.

- Very strong winds, large waves and heavy rainfall are likely near this storm system across much of the Bering Sea, Pribilof Islands, and portions of Southwest towards the latter half of the week ahead.

- A threat for coastal flooding will exist along the Southwest coast sometime between Wednesday evening and Saturday, but this potential is highly dependent on the still uncertain track of the low as it heads across the Bering Sea.

Discussion:

Much activity will be brewing at the start of the long term period on Wednesday morning, mainly across parts of the Bering Sea and Aleutian Chain. A complex trough with multiple embedded shortwaves will be shifting away from Kamchatka and into the western Bering Sea, sending an initially weak attendant surface low into the western/central Bering. Meanwhile, the remnants of Typhoon Dujuan are anticipated to be curving northeast away from Japan by this point and drifting south of the western parts of the Aleutian Chain. Much calmer conditions should be ongoing across most of the southern Mainland by the middle of next week as a longwave ridge over the eastern Bering begins to shift east into Southwest and Southcentral.

A complex interaction between the incoming trough/low heading into the western Bering and the remnants of Dujuan moving over the North Pacific is projected to unfold starting around the middle of next week. The consensus from deterministic and ensemble model solutions in terms of how this plays out has shifted quite a bit compared to indications from yesterday. It now looks likely (about a 60-70% chance) that Dujuan's remnant center will curve north across the Aleutians and phase with one or more shortwaves drifting across the western Bering. As these features merge, this will result in a period of rapid deepening of the resulting single surface low, perhaps bottoming out in the 940-960 mb range or lower as it reaches maximum intensity somewhere over the central to western Bering Sea on Thursday.

A large field of Gale to Storm Force winds is anticipated to develop around the low as it deepens, with a swath of Hurricane Force gusts possible on the south side of the low. Much of the Aleutian Chain, including Adak, Atka and Unalaska, will have the potential to see strong to damaging wind gusts south of the low center as early as Wednesday evening. Large waves of up to 20 to 30 ft or more are possible as well, particularly across the southern Bering Sea between Thursday and Friday. Strong winds will also affect the Pribilofs from late Wednesday through at least Friday, where large breaking waves reaching the shoreline will also be a possible hazard. Heavy rainfall rates will also be a concern across much of the Aleutians as a deep band of tropical moisture surges north into the low, especially between Wednesday and Thursday.

Turning attention to Southwest, the first impacts from this system will arrive along the low's occluding frontal boundary as it moves up the AKPen and reaches the Southwest coast from Wednesday night into Thursday. Intense southerly gap winds will ramp up ahead of the frontal passage along the AKPen, including spots like Cold Bay, Port Heiden, and Pilot Point. Strong southeast winds will shift up to the coast of Bristol Bay and the Kuskokwim Delta later on Thursday, with early indications suggesting the front will weaken as it heads inland, limiting impacts from strong winds to along/near the coastline. Some sections of coastline that face southeast, namely in and around Kwigillingok, could see minor coastal flooding with this first round of strong southeast winds on Thursday.

Potential for more widespread coastal impacts in terms of erosion and coastal flooding beyond Thursday is more uncertain. Models are in relatively good agreement that the low will nearly stall as it reaches maximum intensity over the western Bering as it detaches from stronger steering flow along the Pacific jet. However, the weaker steering for the low thereafter is contributing to much lower confidence for the track of the low as it drifts east and closer to Southwest through Saturday. Another period of strong onshore winds will be possible if the low moves in a more northeast direction towards the Y-K Delta after it begins to weaken, but this highly unclear at this time. Land and marine interested alike are advised to closely monitor the forecast for updates as we follow this strong Bering storm system over the next several days.

Looking briefly to Southcentral, much calmer conditions are expected up until Friday morning as the upper ridge moves directly overhead. The front associated with the massive Bering low will arrive from the west on Friday, sending gusty southerly gap winds into parts of the region. Rain moving in with the moisture tap along the front will be mostly concentrated on the southern faces of the mountains as strong southerly flop upslopes into parts of the Chugach Range, Kenai Mountains and Alaska Range. Drier conditions will mostly persist in the lee of the mountains over much of the interior valleys through Saturday, while the heavy rainfall potential will remain confined mainly to the coastal mountains across the western Prince William Sound and eastern Kenai Peninsula.

-AS

AVIATION

PANC...VFR ceilings with occasional MVFR ceilings are likely throughout the TAF period. The most likely period for MVFR ceilings will be this morning and this evening as multiple rounds of rain move over the area. Rain tapers off by late this morning into the early afternoon. The next round of rain and potential MVFR ceilings arrives this evening and overnight as a low in the Gulf moves into Prince William Sound. Showers with this round will persist through the rest of the TAF period into Monday morning. Expect light and variable winds with majority of wind being out of the north and at or less than 10 kts.