Area Forecast Discussion
Issued by NWS Anchorage, AK
Issued by NWS Anchorage, AK
753 FXAK68 PAFC 170133 AFDAFC Southcentral and Southwest Alaska Forecast Discussion National Weather Service Anchorage AK 533 PM AKDT Mon Sep 16 2024 .SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: This afternoon through Thursday evening )... Today featured a relatively nice day across Southcentral Alaska, albeit windy at times. A weak shortwave trough rotating around an upper-level low in the central interior of Alaska is helping to pop off a couple of showers mostly across the Mat-Su Valleys, but also in the vicinity of the Anchorage area as well. This shower activity should begin to wane into this evening with the loss of day-time heating and instability. Low temperatures tonight across portions of the Copper River Basin look to fall to near freezing overnight while most other locations across Southcentral see mid 30s to low 40s for lows. Attention then turns to the next system in the form of a Bering Sea front lifting through the Gulf for Tuesday. Rain spreads from Kodiak Island northward to the north Gulf Coast and Prince William Sound Tuesday morning through the afternoon. Initially, Anchorage, the Matanuska Valley, and portions of the Western Kenai Peninsula will enter into southeasterly cross-barrier flow late Tuesday afternoon which will cause rain to be more sporadic in nature. Early Tuesday afternoon, these areas could see some light over- running precipitation with flow through a good portion of the atmosphere still south of southwesterly before the downsloping ensues. Southeast winds through Turnagain Arm will also develop Tuesday and bend into Anchorage through Tuesday evening. The strongest winds are expected to remain out over the Arm. The Anchorage Hillside can also expect gusty southeast winds as well and on the order of 25 to 35 mph at times Tuesday afternoon before diminishing through Tuesday evening. Meanwhile, steady rain, moderate to heavy at times, is likely across the Eastern Kenai Peninsula and Prince William Sound for Tuesday into Tuesday evening. Rain looks to fill in for Anchorage and the Western Kenai Peninsula late Tuesday evening into Wednesday morning as the southeasterly flow weakens and eventually turns south to southwest. Moderate to heavy also looks to linger across Prince William Sound as multiple upper- level waves moves across the area aided by southerly and southeasterly upslope flow. The system pulls away to east Wednesday afternoon as winds through the favored gaps in terrain turn to northerly and off- shore. This will continue to be the case through the day on Thursday as well with quieter yet breezy weather expected. Any showers for Thursday look to confined to the terrain along the Talkeetna Mountains and Alaska Range as another weak upper-level system moves through. && .SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days 1 through 3: This afternoon through Thursday afternoon)... The focus in the coming days will be a low currently north of Adak, which will strengthen as it moves across the Bering Sea and into the Alaska Peninsula for Tuesday night. Much of Southwest Alaska, the Bering Sea, and the Aleutian Islands will see strong, gusty winds from this low, with the potential for high surf and coastal erosion. If you live in or will be traveling through these areas in the coming days, please stay tuned to the forecast. While there`s high confidence in the overall picture, finer details such as maximum wind gusts continue to be refined. Here are the products currently in effect: * High Surf Advisory is in effect for Bristol Bay, primarily the from the Kvichak River mouth south to Port Heiden. Confidence is lower for areas west of the Kvichak River mouth, which includes Dillingham and Togiak. * Special Weather Statement is in effect for the Pribilof Islands for strong winds and high seas. Diving into the details...the low currently north of Adak will strengthen through early tomorrow morning as it phases with a robust upper low moving in from Kamchatka. Models came into excellent agreement this morning, lending higher confidence to the forecast. While there may still be changes to the forecast if the low`s track changes, the overall strength of the system is unlikely to change. As such, expect widespread gales across much of the Bering Sea and Aleutian Islands, as well as the Southwest Alaska coastline. The strongest winds continue to be expected near the Pribilof Islands tomorrow as cold air wraps back around the low. There is a chance that gusts could come in at hurricane force, which is 75 mph or greater. However, the current forecast has gusts remaining just below this threshold as confidence is low that the strongest winds aloft will mix down to the surface. Regardless, it will be a very windy day for Pribilof Islands and surrounding waters. Strong winds are also expected across the Eastern Aleutians, Alaska Peninsula, and Bristol Bay. Winds will be southwesterly for much of Tuesday, generally parallel to the shoreline. However, as the low moves inland near Naknek/King Salmon early Wednesday morning, flow will become more westerly. This will focus wind and waves directly onshore, leading to the potential for high surf and coastal erosion. As such, a High Surf Advisory is in effect for Bristol Bay. While high surf/coastal flood products are not in effect elsewhere, there still may be minor impacts. For the south-facing shore of Kuskokwim Delta (ex: Kipnuk to Kongiganak), southeasterly onshore flow along the front on Tuesday morning could bring a brief period of higher water levels. However, this is expected to be short-lived and not as impactful as this past weekend`s storm. Similarly, the south-facing shore of Bristol Bay (from the Kvichak River mouth to Cape Newenham) could also see higher water levels from surge as there will be decently long duration of onshore winds. However, these winds will be weaker than those impacting the portion of Bristol Bay under the High Surf Advisory. Finally, the Bering side of the Alaska Peninsula (especially Nelson Lagoon) will also get their turn at strong winds from Tuesday evening through early Wednesday morning, though the winds will not be directly perpendicular to the shoreline. Overall, confidence remains low when it comes to the potential for high surf and coastal flooding due to lack of observations and modeling methods that are still relatively new. Looking ahead, northerly flow behind the departing low will usher in cooler air, with much of Southwest Alaska seeing temperatures in the 30s from Wednesday night through Thursday morning. This, in combination with weak upper level shortwaves moving in behind the low, could result in a fresh dusting of snow along area mountains and even on lower foothills. -Chen && .LONG TERM FORECAST (Days 4 through 7: Friday through Monday)... There is little agreement amongst operational models by the end of the work week on Friday. Even the ensemble means largely differ from each other during the same time period. From a pattern perspective, the general consensus is for a broad trough or upper low over the Western Aleutians and western Bering to gradually slide eastward through the beginning of next week on Monday. The operational GFS, in particular, is more aggressive with yet another strong low overspreading the Western and Central Aleutians. Ensembles eventually have the low positioned over the Gulf of Alaska by the end of the long term forecast. As far as potential impacts, the track of the trough should keep the threat of shortwaves over the Aleutians on Friday and Saturday. As the general trough continues eastward, the track of disturbances moves south of the AKPEN and eventually well south of the Gulf coast. Therefore, moderate confidence exists in somewhat of a quieter pattern setting in for the long term for Southwest and Southcentral Alaska with any upper-level jet support remaining south of mainland Alaska. What this will do however is present the opportunity for colder air to drop southward into both Southwest and Southcentral Alaska. Thus, cooler temperatures are likely along with lower precipitation chances. -BL && .AVIATION... PANC...VFR conditions persist. Southerly winds around 25 kts will continue to gradually diminish through this evening. Through Tuesday morning, high confidence in a spell of fairer conditions with no rain, lighter winds, and VFR conditions expected. Later on Tuesday, a front will bring a return of rain and stronger winds. && $$