673 FXAK68 PAFC 091259 AFDAFC Southcentral and Southwest Alaska Forecast Discussion National Weather Service Anchorage AK 459 AM AKDT Thu Sep 9 2021 .ANALYSIS AND UPPER LEVELS... A series of shortwaves cycle around a broad longwave trough pattern that is set up over the northern Bering. The first of which makes its way onshore to Southwest Alaska today. Light rain and low clouds will likely expand across Bristol Bay and parts of the Kuskokwim Valley, as well as spilling over the Alaska Range to the Kenai Peninsula. The second and third waves enter the western Bering rather close together by Friday afternoon. There is good jet support on the southern edge of the system that will help drive moisture across the Bering and Aleutian Islands. Additionally, a swath of colder air advects across the Bering towards the Southwest Coast, which may aid in some convective development. A separate system moves into the southern Gulf today and pushes off towards the Southeast Panhandle over the next couple of days. This system looks like it will keep far enough south to keep the bulk of the precipitation along the Coastal regions, with little to no precipitation spilling into the Mainland. && .MODEL DISCUSSION... Models remain in good overall synoptic agreement through the short-term. There are, however, few discrepancies associated with a developing surface low moving towards the western Bering, mostly with the placement of the low and how quickly it moves eastward across the Bering. The NAM/EC place the low much further north than the Canadian/GFS and move at a much faster pace towards the Southwest Coast. The Canadian dissipates this surface low before pushing onshore, merging it with a separate low developing behind the first. The NAM also develops a second low, however, it is much weaker and the initial low continues pushing northeastward. && .AVIATION... PANC...VFR conditions and light winds will persist. && .SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Today and Friday)... The well-advertised strong Gulf low has developed and is beginning to track across the Gulf this morning. The low will track from it's current position over the southwestern Gulf to the eastern Gulf by Friday evening. Along this track, high-end gale force winds (45 kt) with higher gusts are expected along the northern periphery of the low from Cape Suckling to the Barren Islands this afternoon. With the low track expected to parallel the coast, these winds are going to be significantly weaker than they would have been had the low taken a track into the coast. That said, the winds will also last longer than that alternative track as well. Few forecast changes were made this morning related to the track of this low. Still expect around 2 inches of new rainfall in Kodiak City by the time the precipitation ends late tonight. The strong easterly winds that will enhance precipitation rates along the east coast of Kodiak Island will diminish precipitation rates around Seward, as the prevailing flow runs perpendicular to the valley. Storm total rain around Seward is expected to be anywhere from a half to three quarters of an inch. Strong northeasterly flow will enhance the winds along Cook Inlet and Shelikof Strait today, with waves around 15 feet likely. The cross-barrier flow will develop a upslope-downslope couplet, with heavy rain in the Chugach and Kenai Mountains near Whittier coupled with next to no rain to the lee of those mountains in the western Kenai, Anchorage, and the Mat-Su. As the low peaks in intensity tonight and begins weakening, it will draw much cooler air down from the stratosphere. This will increase the instability around the center of the low, and isolated thunderstorms are possible across the offshore waters of the southern Gulf into Friday morning. On Friday, gusty northwesterly winds will allow the skies to clear from southwest to northeast as the center of the now weakening low moves into the eastern Gulf. Meanwhile a separate upper level front and associated troughing will graze along the northern and western Susitna Valley. Some of that rain could impact Anchorage as the winds turn southwesterly late in the day. Rainfall amounts are very likely to remain under one tenth of an inch in most populated areas. The disturbance will lift to the north and out of the area by early Saturday morning taking the rainfall with it. Looking ahead, high pressure will build into Southcentral, making for a nice day on Saturday. The next low and a renewed round of rain will move into Southcentral from the southwest Sunday afternoon and evening. && .SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Today through Saturday morning)... Today a low pressure system south the AKPEN will bring areas of light rain to the Bristol Bay region while the rest of the area will remain mostly dry. A cold front will move across Southwest Alaska Friday producing gusty winds to 30 mph across portions of the Y-K Delta especially along the coast. This front will also bring showers to the area. As the cold front moves across the Middle Kuskokwim Valley Friday afternoon and evening there will be a slight chance of a thunderstorm or two as instability develops near the front with CAPE near 200 J/kg and favorable Total Totals and Lifted Indexes. A low pressure system and warm front will approach the Y-K Delta Saturday morning with another round of gusty winds and showers. && .SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Today through Saturday morning)... An active weather pattern will dominate the Bering and Aleutians through the short-term with a series of low pressure systems and fronts developing along the Jet Stream which will remain over the Bering and Aleutians extending into western Alaska. The low south of the AKPEN will move east into the Gulf today, and will produce Gale force winds south of the AKPEN through Friday. Another compact low will move across the Bering today with mainly small craft winds with a very small area of near Gale force wind gusts near the low. This will move near Nunivak Island this evening and move inland tonight. There may also be a few lightning strikes near the low as cold air aloft and instability interact with this low. Another low will develop early Friday near the western Aleutians along a cold front which stretches across the Bering there is some uncertainty on the evolution of this low among models, but there is some confidence that a small area of Gales may develop near the low, but very uncertain on where this will occur so left them out of the forecast until models come into better agreement. However this low will quickly move across the Bering along a similar track as the previous low. And yet another low will likely develop near the western Aleutians Friday afternoon and move in a similar manner, however like the previous low there is a bit of uncertainty on the details. Otherwise very cold air aloft will develop across the northern Bering this afternoon through Friday evening. There is expected to be shortwave energy, instability and good lapse rates which could produce some isolated thunderstorms at times across this area. && .MARINE (Days 3 through 5/Saturday through Monday)... Gulf of Alaska: The deep low pressure will be exiting the Gulf of Alaska to the Panhandle, but a new developing system is quickly coming on its heels from the North Pacific on Saturday afternoon. There is some uncertainty on the exact track and strength of that system but it does look to track into the far western Gulf near Kodiak Island, then toward Cook Inlet on Sunday. The associated front looks to be increasing to gale-force as it approaches Kodiak, strengthening as it moves northward. At this point high- end gales are possible along the north Gulf coast but will have to be monitored as any increase in strength will put it into storms. Behind that system on Monday the Gulf opens up to a broad southwesterly flow ahead of a new weakening system that is approaching from the Bering Sea. Bering: A complex and broad surface low pressure area stretches across most of the Bering Sea. A stalled front on the southern edge of this area will spawn a stronger compact low pressure center, possibly several in a series on Saturday. Timing and strength on these systems are VERY uncertain, it is possible that a strong and compact wind field develops from this scenario with gales and possibly storm-force winds. && .LONG TERM FORECAST (Days 4 through 7/Sunday through Wednesday)... The extended forecast begins on Saturday with one large upper- level low center dominating the long-wave pattern over the northwestern Bering Sea. The temperatures associated with the low are cold even for early fall as it has tapped an unseasonably cold arctic air mass. This tap of cold air shifts a frontal zone over the southern and eastern Bering Sea which will be the driver for development of surface low pressure systems, there looks to be one in the eastern Bering Sea on Sunday but timing and strength is very uncertain at this point. There does look to be a stronger surface system associated with this feature that will move from the north Pacific into the western Gulf on Sunday. That system will bring another round of rain and possibly stronger winds to much of Southcentral Alaska. By the middle of next week, the longwave shifts from the Bering into the Gulf while weakening. The net effect would be to continue the cool, cloudy, and wet conditions, but likely less wind. && .AFC WATCHES/WARNINGS/ADVISORIES... PUBLIC...NONE. MARINE... Gale Warning 119 120 130 131 132 136 137 138 139 150 155 351 352 FIRE WEATHER...NONE. && $$ SYNOPSIS AND MODEL DISCUSSION...KM SOUTHCENTRAL ALASKA...JPW SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...RC MARINE/LONG TERM...MTL