Area Forecast Discussion
Issued by NWS Anchorage, AK
Issued by NWS Anchorage, AK
465 FXAK68 PAFC 231216 AFDAFC Southcentral and Southwest Alaska Forecast Discussion National Weather Service Anchorage AK 416 AM AKDT Mon Sep 23 2024 .SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Monday through Wednesday evening)... Unsettled weather will remain over much of Southcentral through mid-week as a quasi-stationary upper-level trough and surface low sit anchored over the northern Gulf. The extent of cloud cover and precipitation will be largely driven by a series of upper- level waves rotating around the base of the trough combined with their respective surface troughs driving north into the coastal mountains. The first of these features will move from east of the Barren Islands to an area just southeast of Seward today. The upper-level wave will move northwest from Prince William Sound over the Chugach and Talkeetna Mountains. An area of rain is expected to move inland across the Copper River Basin and Cook Inlet later this morning and into early afternoon. The strong winds across the northern Gulf and gusty northerly gap winds from Seward to Homer and Wasilla will diminish in intensity as the wave moves north and weakens and the surface front falls apart along the coastal mountains. The forecast challenge comes later this evening through Tuesday as the subsequent waves and troughs approach the Southcentral coast. Models continue to struggle with which features will be the dominant players, due not only with the position of various vorticity maxima, but with the extent of cold air advection in the wake of the main trough. The advection of colder air across Kodiak Island and the Barren Islands will likely help to spin up mesoscale surface features driven by the aforementioned upper- level vorticity advection. How each moves north and affects the mid-level wind field will dictate the extent of precipitation beyond today. Currently, the forecast leans toward the NAM/NAMnest and GEM/REM. This solution lingers a mid-level trough axis over the central and southern Kenai Peninsula, extending into the western half of the Susitna Valley through Tuesday, resulting in an extended period of rainfall. This solution also brings two waves over the Copper River Basin with likely rain this morning and Tuesday morning and scattered showers in between. For other inland locations, including Anchorage, Talkeetna, and Palmer, occasional bouts of light rain are expected through the next 24 to 36 hours. By Wednesday, most of the area continues to remain within cyclonic flow and thus will continue to see cloudy conditions with most of the showery activity confined to the coast and higher elevations. The aforementioned trough over the Kenai Peninsula may also drift south by Wednesday, bringing another round of rain to Kodiak Island. -TM && .SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days 1 through 3)... Broad longwave troughing stretches from Interior Alaska across Southwest Alaska and over the Eastern Aleutians, with higher pressure moving into the western Bering. Cold northerly flow advects across the eastern Bering Sea and and Southwest, elevating winds through the gaps and passes south of the Eastern Aleutians and southern Alaska Peninsula. Winds will gradually diminish from west to east through Tuesday evening. Morning low temperatures across Southwest Alaska will continue in the low to mid 30`s, and low 40`s along the Alaska Peninsula and Eastern Aleutians. Monday into Tuesday mostly drier weather is anticipated for Southwest Alaska. While a few showers will be possible during the day on Monday, within weak northerly flow, the bulk of any precipitation should largely be confined to the higher elevations. There is potential for some light snow from Sleetmute to Iliamna and areas north and east Tuesday night into Wednesday morning with the development of a weak deformation band. The next upper low will emerge from Asia into the western Bering late Tuesday into Wednesday. An attendant front may reach the Western Aleutians by Wednesday afternoon with renewed shower activity and perhaps some gustier winds with any cold air advection behind the arriving low. -KM && .LONG TERM FORECAST (Days 4 through 7: Thursday through Sunday)... Broad longwave troughing over the Gulf of Alaska and subsequent ridging over most of the Bering Sea will persist from the start of the extended period into Friday morning. Unsettled weather will be experienced by most of Southcentral while portions of Southwest near the coastline could remain dry as cold and dry air moves in from the north. By midday Friday, a new shortwave low enters the western Bering and tracks southeastward towards the North Pacific by the middle of the weekend. The shortwave phases with the upper level trough on Saturday, jetting moderate precipitation towards Kodiak Island and much of the gulf coast into Sunday. While a week out, a system that currently has our attention is a subtropical Pacific low that will move northwards towards the Gulf of Alaska. Most ensemble and operation model guidance is trending towards a stronger system moving towards the gulf, but the mechanisms in place to get it there vary by model. While there is not major concern for heavy rainfall, unusually strong winds for the gulf are not out of the question at this point in time. Continue to follow the forecast for continued updates on this system. && .AVIATION... PANC...VFR conditions and northerly winds will persist through this morning. A strong occluded front will lift northward across the Gulf today which will lead to increasing north winds for the terminal this morning along with light rain. The winds are expected to diminish in intensity through the afternoon and evening as the front weakens and the pressure gradient relaxes. Light rain developing this morning is expected to continue through the afternoon and evening. There may be times when the rain tapers to sprinkles or shuts off entirely. The diminishing winds both at the surface and aloft will also allow for more moisture to advect over the coastal mountains today. This should cause ceilings to gradually lower throughout the day, with MVFR ceilings likely by early evening. -MK/TM && $$