Area Forecast Discussion
Issued by NWS Anchorage, AK
Issued by NWS Anchorage, AK
816 FXAK68 PAFC 170029 AFDAFC Southcentral and Southwest Alaska Forecast Discussion National Weather Service Anchorage AK 429 PM AKDT Tue Jul 16 2024 .SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Tuesday evening through Friday)... A cold upper-level trough continues to spin over the Mainland. There is actually a large/deep parent system spinning over the Pole that connects to a weaker center over the west-central Interior. The whole trough extends well into the Bering Sea, and Gulf of Alaska. The jet stream influence remains south of the Aleutians, but rounds the base of the trough and is currently pointing at the Panhandle. The setup leaves Southcentral Alaska within the trough which will be the controlling feature for active weather in the next few days. For sensible weather, a weakly unstable air mass on the downstream side of the trough is producing widespread rain showers across most of the area. The Copper River Basin is the exception, though some showers are forming along the Alaska Range. Expect shower coverage to decrease over night tonight from west to east as the trough pushes through the area. Showers will pop back up again on Wednesday as we remain within the broad scale trough and cold pool aloft, but without a focusing mechanism, activity should remain along the mountains in most areas. Forecast uncertainty increases on Thursday as the upper low center drops over the Kuskokwim Valley. The position of the system should allow for both forcing and moisture to be in place for the Cook Inlet region, but guidance is struggling with placement and amount of precipitation. While these things are uncertain, late Wednesday into Thursday does look more certain for periods of rain for Cook Inlet through the Susitna Valley. Late week looks to be drier and warmer for all areas, however, as the upper low drops into the Gulf of Alaska with Mainland ridging, a very low predictability easterly wave pattern sets up for late weekend into next week. Stay tuned. MTL && .SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days 1 through 3: Today through Thursday)... Overall, fairly benign weather expected as weak weather systems move across Southwest Alaska, the Bering Sea, and the Aleutian Islands over the coming days. Expect light to moderate rain and rain showers, with minor uncertainty in rain timing and totals. As a ridge builds over the Western Bering/Aleutians today and transits eastward through Thursday, rain will taper off and give way to fog and low stratus, especially in the Bering Sea and Aleutians. The slowly evolving pattern is supported by an anchored upper low currently located near the Seward Peninsula. Weak shortwaves moving around it are currently bringing spotty rain showers to Southwest Alaska. A more robust shortwave is expected to move first over Nunivak Island before entering the Kuskokwim Delta coast. The wave is expected to spread rainfall quickly inland, bringing light rain from Kipnuk to Aniak through Wednesday morning. Up to a quarter-of-an-inch of rainfall is possible for this area and time. By Wednesday morning, the boundary is expected to push across the remainder of Southwest, reaching from Bristol Bay to Lime Village, with similar rainfall totals across mostly the Lower Kuskokwim Valley. Rain from this system expected to taper off by Wednesday afternoon. For the next few afternoons, through midweek this week, there is the chance for some isolated thunderstorms to develop along the western side of the Western Alaska Range. Synoptic wind flow will likely keep any cells over elevated terrain. The Bering Sea and Aleutian Islands have increased rain chances as a low skirts south of the Aleutian Islands and Alaska Peninsula. As the ridge of high pressure increases in amplitude Tuesday night into Wednesday, expect fog and low stratus to develop along the Bering Sea and Aleutian Islands. The Pribilof Islands will likely return to fog Wednesday and Thursday morning, with the deck lifting in the afternoon. A likely target for some fog development will be along the central axis of the ridge building in the Central Aleutians Wednesday night/Thursday morning. By Thursday, another low moves into the Western Aleutians/Bering, bringing winds to small craft advisory speeds and another round of rain. A front along the leading edge of the low is expected to cross the Chain Thursday and push into the Central Bering, reaching the Pribilofs by Friday morning. -CL && .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...Weak flow through the atmospheric column and persistent cyclonic flow aloft out ahead of a closed upper low over SW AK will maintain a fairly steady state ceiling, with just minor fluctuations in height. VFR conditions should prevail, though low level flow will gradually veer from SE to SW overnight tonight and persisting through Wednesday. This will lead to upslope flow along the Chugach and likely lead to lowering ceilings. The bulk of lower clouds look like they will remain banked up against the mountains, though the terminal could see conditions temporally lower to MVFR sometime late tonight or Wednesday morning. Meanwhile, gusty SSE winds will die down overnight, similar to last night. For Wednesday, the coastal ridge will be replaced by a much broader ridge over the Gulf. As a result, W to SW winds will prevail for Wednesday. && $$