Area Forecast Discussion
Issued by NWS Anchorage, AK
Issued by NWS Anchorage, AK
643 FXAK68 PAFC 041318 AFDAFC Southcentral and Southwest Alaska Forecast Discussion National Weather Service Anchorage AK 518 AM AKDT Tue Jun 4 2024 .SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today through Thursday night)... A high amplitude trough encompasses all of Alaska and extends well down into the North Pacific (around 40N latitude). Two deep upper level closed lows are the dominant weather features, one vertically stacked low spinning off the British Columbia coast and another (upper center) dropping southward along the west coast of the mainland. Conditions are quiet at the moment over Southcentral and Kodiak Island, with just a few weak vorticity-maxima transiting overhead producing some mid to high level clouds but little or no precipitation. Temperatures vary from the mid 30s to upper 40s, coldest over inland areas where skies are clear. The low off the B.C. coast will lift northward across the eastern Gulf through tonight. A series of short-waves will rotate across Southeast Alaska, the Yukon, and into Southcentral beginning this afternoon. A short-wave ridge will amplify ahead of the first short-wave and overhead of Southcentral today. Thus, expect a warm day with some sunshine for most of the region. Clouds and showers will develop over the Copper River Basin this afternoon, becoming more widespread and heavier through the afternoon and into the evening. While model runs in prior days had been indicating one fairly strong short-wave, they all now seem to indicate a steady stream of short-waves. Solutions of course vary on exact timing/track of short-waves, though the end result is similar. Showers will transition to steady rain overnight tonight and spread westward across all of Southcentral through Wednesday. There will still be a convective aspect to the precipitation as some atmospheric instability persists. Thus, expect areas of moderate to heavy rain embedded with the larger rain shield. The challenge is trying to hone in on timing and rainfall amounts for any given location. Based on the general east to west track of the short-waves over inland areas of Southcentral tonight through Wednesday, it looks favorable for some of the heaviest rain to fall from the western Copper River Basin to the Mat-Su, with roughly 0.50-0.75" through Wednesday afternoon. Future forecasts will continue to fine-tune rainfall amounts. Model guidance continues to struggle with the track and strength of the Gulf low Wednesday through Wednesday night. Most of the operational runs along with ensemble means shows the same general idea of a weakening surface low tracking westward across the northern Gulf while the upper low opens up into a trough and heads inland across Southcentral. The operational GFS is a consistent outlier with a deeper surface low lingering in the northwestern Gulf into Thursday and a much slower process of opening into a trough aloft. In any case, it becomes even more challenging to track specific features Wednesday night through Thursday. The pattern generally favors continued cool and wet conditions, especially along the coast where southerly upslope flow will strengthen. By Thursday night, upper level forcing looks very weak, which should lead to a drying trend. Showers could linger along the coast with continued upslope flow. As far as the wind forecast, there will be some robust sea breezes along the coast today as inland areas warm up and a thermal trough strengthens over the Interior. As short-waves cross southern Alaska Wednesday the thermal trough will retreat northward across the Mainland with a surface ridge building over Southcentral ahead of the Gulf low. This will lead to gusty winds spreading onshore and particularly through the usual gaps - and likely spreading further inland due to the synoptic support. While it will be a much cooler (and wetter) day, forecast soundings indicate some instability right off the surface during the afternoon and evening hours which will help promote the gusty winds. There could be a repeat on Thursday, depending on where the remnants of the Gulf low track. -SEB && SHORT TERM FORECAST SOUTHWEST ALASKA, THE BERING SEA AND THE ALEUTIANS (Days 1 through 3)... Radar this morning shows a persistent convergence band of precipitation just south and east of the Kuskokwim mountains. Scattered showers are also lingering across portions of the Kuskokwim Valley and Bristol Bay, near Iliamna. There is a chance that the band of precipitation will linger into this afternoon, though showers are expected to taper off until later in the day when the foothills of the western Alaska Range bring more cumulus convection showers. Showers look to linger across the Alaska Peninsula today and partly cloudy to mostly sunny skies are expected in the western Bering Sea, western Aleutians to the central Aleutians. Temperatures will be cooler today, especially for the Kuskokwim Delta and the Bering Sea. An artic upper low is expected to track south, ushering in an anomalously cold airmass with it. Midlevel temperatures at its coldest will be around minus 12 to 13 degrees Celsius. A surface shortwave trough originating out of eastern Russia will accompany on the western periphery of this upper low. As it skirts west of the Pribilofs today, the trough will develop into a weak closed low at the surface as it continues into the southern Bering Sea. Energy distribution within the developing low will determine the location of the gustiest winds and the highest chances for precipitation. Models have slight variations in the outcome, but the core of the weather will be over the Bering Sea through tonight. The Pribilof Islands would be cold enough for snow, should a shower develop later this afternoon and evening. By Wednesday morning, the low will be over the eastern Aleutians and the Alaska Peninsula, bringing the cool air with it under northerly flow. Snow showers are expected behind the main trough for the Pribilofs and could potentially extend as far west as the central Aleutians Wednesday night into Thursday morning. Precipitation for the Aleutians and the Alaska Peninsula will likely start off as rain, but as the cool air advects southward, rain will switch to snow sometime Wednesday night and Thursday morning. Timing of the switch is a little uncertain as some models vary and it will mostly depend on the development of the surface low. A weak, easterly shortwave could swinging towards the western Alaska Range from Southcentral Thursday. Some chances of precipitation could spill over those mountains into the Kuskokwim Valley. Overall though, showers will gradually decrease throughout Thursday for most places across the Bering, Aleutians, and Southwest Alaska as the low wobbles as it weakens. High temperatures for the Pribilofs and Aleutians will struggle to make it out of the upper 30s to lower 40s. Temperatures for Southwest Alaska however, will trend warmer. Highs today across Southwest Alaska will be in the 40s to lower 50s (mid to upper 30s near the Kuskokwim Delta coast), and for this weekend, nudge into the 50s and 60s for this weekend. rux && .LONG TERM FORECAST (Days 4 through 7) Friday through Monday... A generally unsettled pattern remains for much of southern Alaska for the beginning of the period as a large upper low sits over the Bering. Through the period, this low will pull another upper- level low up from the North Pacific, into the Gulf of Alaska. At the surface Friday morning, a rainy pattern across mainland Southern Alaska is expected, with models advertising varying levels of intensity. However, by Saturday morning the showers are expected to ease across the area, while a surface low pressure system builds and moves into the Gulf of Alaska. Generally, this system will maintain the unsettled pattern across the Southcentral coastline this weekend. More specifically, deterministic model solutions by early this weekend disagree regarding the location of the surface low, and to what degree it influences the Southcentral coastline. The GFS/EC solutions bring the precipitation into Kodiak Island as early as Saturday afternoon, while the Canadian solution keeps it in the eastern Gulf of Alaska near Sitka at that time. Model solutions continue to diverge beyond Saturday afternoon, leading to low confidence regarding the timing and intensity of escalated rain impacts. -CL && .AVIATION... PANC...VFR conditions and light southwesterly winds are expected to persist through the TAF period. && $$