Short Term Forecast Southcentral Alaska (Days 1 Through 3)
Discussion:
Periods of moderate to heavy rainfall along the immediate northern Gulf coast and eastern Kenai Peninsula will slowly trend lighter this afternoon into this evening, as the vertically stacked low in the southwestern Gulf slowly drifts east and onshore flow weakens. Within the broad longwave troughing stretching from the Bering Sea across the Gulf, shortwaves embedded in the flow rotate inland allowing for periods of light rain to spill over the mountains at times. The western Kenai up through Anchorage and the MatSu, as well as the southern Copper Basin, could see up to a tenth or two of light rain through this evening. Periods of snow will be possible at higher elevations with snow levels fluctuating between 2500 ft to 4000 ft along the Kenai and Chugach Mountains. The strongest of such waves should clear the region by late tonight with lighter, more showery precipitation lingering along the coast through Friday as the low weakens and tracks east across the central Gulf. Gusty winds through favored gaps, along the coast, and within Prince William Sound weaken to small craft or below through this evening in the wake of the wave pushing inland.
The upper trough slowly translates eastwards towards the Panhandle through the remainder of the weekend, with drier conditions expected across Southcentral as weak, transient ridging moves over the region. Temperatures trend cooler through the weekend, with temperatures well below freezing across the Copper Basin and near or just below freezing across the Kenai Peninsula, Anchorage, and the MatSu by Saturday night and Sunday morning.
Short Term Forecast Southwest Alaska/Bering Sea/Aleutians (Days 1 Through 3)
A low is tracking east into the western Gulf of Alaska this afternoon, sending precipitation across the Alaska Range and spreading throughout Southwest Alaska. An additional wave of precipitation will move across the Bristol Bay and Kuskokwim Valley region tonight. Some of this could reach Aniak, but there is a smaller potential (around 30 percent) that some of the this could spill farther west into the Kuskokwim Delta. More likely though (40 to 60 percent), the deformation band will set up in the Kuskokwim Valley and through Bristol Bay. Models are not handling the exact location of this feature well. Therefore, confidence for the location and the timing/duration of the precipitation forecast in Bristol Bay and the Kuskokwim Delta in the near term is relatively low- medium.
Gusty winds and precipitation will linger across the central to eastern Bering Sea as cold air advects south under northerly flow. These winds are expected to gradually decrease throughout the day Friday as high pressure upstream pushes east and weakens. Offshore flow across Southwest coupled with the synoptic setup is yielding to additional cold air advection across Mainland Southwest tonight through this weekend. Temperatures will continue to trend cooler over the next few days. Precipitation type will gradually transition to a mix rain and snow by Friday morning across the Kuskokwim and portions of Bristol Bay. The most probable locations for precipitation Friday afternoon and Saturday morning for Southwest will be closer to higher elevations and the Kuskokwim Valley. Any precipitation lingering Saturday morning should be fully transitioned to snow.
Meanwhile, a gale force front is tracking through the Western Bering Sea and Aleutians this evening and is expected to continue toward the Central Bering Friday night into Saturday morning where it will weaken nearly in place. Behind it, a developing compact low from the North Pacific will track northeast, passing the Adak and Atka area Saturday morning. As it catches up to the degrading front, the features will merge in the Central Bering and continue to press the weakening ridge downstream toward Southwest Alaska. Saturday will be relatively quiet for Southwest Alaska, but by Sunday morning, the Bering system will reach the Southwest coast where rain, snow, and south to southeasterly flow returns.
Long Term Forecast (Days 4 Through 7, Monday Through Thursday)
By Monday morning, there is fair confidence (60 to 70%) for a low pressure system, with a depth near 995 mb, to be moving inland into the Kuskokwim Delta, potentially bringing strong onshore winds and wave action to the coast. Given southerly flow continuing to bring in warmer air, rainfall is expected with this system. Additionally, the front of this low is set to break off into a triple-point low spinning up in the western Gulf of Alaska by Monday morning, pushing inland to Southcentral through Monday bringing similar conditions to the northern Gulf Coast.
Confidence in increasing with regards to the progression of a stronger low moving into our area south of the Alaska Peninsula by Tuesday morning. By Tuesday morning, this low may be near 960 mb and would work to drive stronger winds along the Alaska Peninsula as well as bring a large swath of rainfall across the area. By Tuesday night/Wednesday morning moderate model agreement suggests the progression of this low pressure system into the northern Gulf of Alaska bringing it's round of active weather to the Southcentral coastline. There is poor model agreement regarding the progression of the low beyond Wednesday, with several solutions weakening the low in the northern Gulf, while others hold on to it's strength. However, generally unsettled weather expected for the region through the long-term period.
Aviation
PANC...A series of upper level short-waves will rotate from the Gulf up across Anchorage this evening, which could bring an occasional short period of light rain. With deep southeasterly flow aloft VFR conditions will persist. Meanwhile, northerly winds will shift to southeasterly off of Turnagain Arm between 03Z and 06Z. The strongest winds will remain out over Turnagain Arm and stable air near the surface is not conducive to producing gusty conditions. With that said, there is a chance (30-40%) of a few wind gusts at the terminal in the 12-20 kt range this evening.