Area Forecast Discussion
Issued by NWS Anchorage, AK
Issued by NWS Anchorage, AK
156 FXAK68 PAFC 171334 AFDAFC Southcentral and Southwest Alaska Forecast Discussion National Weather Service Anchorage AK 534 AM AKDT Wed Jul 17 2024 .SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)... Discussion: The large complex low continues to spin over west-central Alaska as it meanders inland before progged to start dropping south by late tonight into Thursday morning. While this low has been the main driver of the unsettled weather pattern we have seen over the past week, a change in the pattern is looking more certain by this weekend. For this morning, a shortwave trough rounding the southeast periphery of the parent low brought a few showers to the eastern Kenai overnight and is now lifting towards the Talkeetna Mountains and southwestern Copper Basin. As such, may see a few rain showers across the Susitna Valley and Copper Basin through mid-morning before the wave moves too far east. However, the Susitna Valley will not have too much of a break before the upper low begins to drop south with increasing chances of more widespread rain developing across the Susitna Valley this afternoon and spreading southward with time. Models have struggled the past few days on how to handle this upper low, but have finally come into good agreement with taking the southern track down towards the southern Cook Inlet by Thursday afternoon. A low moving across the central Aleutians this morning will phase with the trough from the western Alaska low as it moves into the southwestern Gulf. This low will then become the dominant low with our southward diving low tracking over Kodiak Island before rounding the the southern half of the Gulf low as a shortwave trough. Models are also agreeing that a band of rain will develop before transitioning to more of a broad deformation band across the western Kenai up through the Mat-Su Valley as the low moves towards the Cook Inlet. Highest precipitation amounts look most favorable for the northwest quadrant of the Kenai, Anchorage Bowl, and into the Mat-Su Valley where a quarter to a half inch of rainfall is possible through Thursday night. However, even a slight deviation in the track of the low will shift the area of heaviest rainfall. Looking into the weekend... The new Gulf low will set up residency placing much of inland Southcentral in a warmer, and somewhat drier pattern. The warmer temperatures filtering in from western Canada will begin to increase instability over the Copper Basin. With low spinning over the Gulf, easterly waves will also move through setting the stage for the return of more convective type weather pattern for the Copper Basin. - PP && .SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days 1 through 3: Today through Friday)... Conditions continue to look fairly benign in the short-term, with weather trending warmer/drier in Southwest Alaska versus windier/wetter across the Bering Sea and Aleutian Islands as we head towards the end of the week. Some uncertainty persists with the timing of rainfall and thunderstorm chances, but forecast confidence generally remains high. Diving into the details...the current weather pattern features a longwave trough over Mainland Alaska and the Eastern Bering Sea, while a longwave ridge extends across the remainder of the Bering Sea and Aleutian Islands. Shortwave troughs embedded within the longwave pattern will bring rain across Southwest Alaska through today, with rain gradually tapering off from west to east. With a robust upper low moving across the Western Alaska Range, there remains a slight chance for isolated thunderstorms in the afternoon and evening. While the energy associated with this upper low could invigorate convection, thermal instability looks relatively poor, especially with lingering cloud cover from the this morning`s rain. As such, thunderstorm chances remain quite marginal. By Thursday, the approaching ridge of high pressure will lead to gradually warming temperatures and a gradual suppression of rain. Daytime highs climb into the 60s and low 70s by Friday, with higher temperatures likely over the weekend. For the Bering Sea and Aleutian Islands, the building ridge will likely lead to widespread fog and low stratus today as moisture becomes trapped near the surface. Fog is inherently difficult to forecast and we forecasters tend to have a low bias (i.e. we under-do fog coverage and severity). Therefore, be aware that fog may become more widespread and thicker across the Bering Sea and Aleutian Islands than is currently reflected in the forecast. By tomorrow, an occluding low moves in from the west, leading to windier and wetter conditions. Forecast confidence is high that winds will remain small craft speed or lower. The front gradually pushes eastward through the remainder of the workweek, reaching the Pribilof Islands by Friday night. -KC && .LONG TERM FORECAST (Days 4 through 7: Saturday through Tuesday)... Much of the long-term period will be characterized by an omega pattern. A large upper-level low will be present in the Bering Sea with another large upper-level low in the North Pacific in close proximity to the Gulf. A high pressure ridge will be between these two features. This type of pattern will keep most of the Bering Sea, Aleutian Chain, and southern Alaska Peninsula active as numerous shortwaves rotate around the Bering low. The ridge, initially over Southwest Alaska Saturday and the first half of Sunday looks to shift eastward towards Southcentral Alaska while it weakens. As the ridge shifts eastward and weakens, this will open the door for a frontal system to move towards Southwest Sunday night. There are still questions on the strength of the front and whether or not it holds together enough to deliver precipitation to the interior locations. The coast has the best chance at seeing rain from this system. Conditions look to remain showery and unsettled over Southwest for Tuesday as additional shortwaves rotate over the area from the Bering. As for Southcentral Alaska, the North Pacific/Gulf low will help to drag in easterly waves from Yukon Canada over the area. This is a good pattern for interior locations of the Copper River Basin and the Susitna Valley to receive precipitation. With riding nudging westward from the Yukon, temperatures also look to warm over Southcentral relative to what is being observed in the short- term period. Depending on day-time heating and instability, potential thunderstorm development is not out of the question across Southcentral each afternoon Saturday through Tuesday. However, confidence is extremely low with where exactly each mesoscale easterly wave will track. && .AVIATION... PANC...MVFR/VFR conditions are expected to continue through at least 03Z with the potential for IFR ceilings overnight Wed. A few light rain showers are possible this morning with more widespread rain setting up by 03-06Z. && $$