Area Forecast Discussion

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: This afternoon through Thursday afternoon)...

Key messages:

- A strong fall storm system moving into western Alaska will bring strong southerly winds and periods of heavy rain to Southcentral during the first half of the week.

- A Wind Advisory is in effect for Anchorage starting Tuesday morning running all the way through midday Wednesday.

- A Wind Advisory is also in effect for coastal portions of the western Kenai Peninsula from Tuesday afternoon through midday Wednesday.

- A Hydrologic Outlook is in effect for Susitna Valley, as steady rain could cause localized ponding and result in rivers reaching bankfull conditions.

Discussion:

The forecast remains on track, with few substantial changes since the morning package. As predicted, cloud cover is beginning to increase across Southcentral as a warm front (extending from the Bering Sea low) lifts north from the Gulf and up across the Cook Inlet and Kenai Peninsula. Light rain still looks likely to fill in along the front as it moves north this evening into tonight, mainly from Kenai and Anchorage north to the Mat-Su Valleys and Talkeetna Mountains. Except for some gusty southwest winds picking up behind the front near Kodiak, winds will stay on the light side through most of the overnight tonight. Expect that to change quickly by the start of the day on Tuesday.

By Tuesday morning, the surface low out west will reach maximum intensity as it moves along the Yukon Delta towards Norton Sound. Meanwhile, a strong southeast to northwest pressure gradient will develop between the low and higher pressure lingering over the Gulf, supporting strong southerly flow pushing into Southcentral. Gap winds will quickly pick up through the Turnagain Arm, Knik Valley and Copper Valley as the day progresses, reaching maximum strength during the afternoon hours on Tuesday. Peak gusts between 40 and 50 mph are anticipated across the western and southern parts of Anchorage and near Glennallen, with slightly weaker winds making it into Palmer and Wasilla. There will be a break in the rain lasting several hours during the morning hours for most places away from the mountains, but the break will not last long.

By Tuesday afternoon, the low will begin to weaken as it heads into the western Interior. A trailing cold front will begin to sweep across Southcentral from west to east, sending another batch of moderate to heavy rainfall through the region as it passes through. Gap winds across the Mat Valley and Turnagain Arm will shut off quickly behind the frontal passage by Tuesday night. However, a second corridor of strong winds will extend from Kamishak Bay up the Cook Inlet, sending another round of strong south winds into Anchorage and much of the coastline of the Kenai Peninsula, with gusts again topping out around the 40 to 50 mph range overnight on Tuesday night into early Wednesday morning. It's worth noting that we still have yet to see most of our trees truly start to turn yellow, so they will be more susceptible to damage from the gusty winds than what we would expect later in the fall.

By early Wednesday, rain will begin to taper off as a drier and cooler air mass works in behind the front as it exits to the east. A few lingering showers could continue on and off throughout the day, but overall it should be much drier compared to Tuesday. Gusty south to southwest winds will slowly relax through Wednesday afternoon as cold air advection wanes and as a transient upper level ridge begins to build back overhead. Showers will linger mostly at higher elevation, but an incoming shortwave could produce a broader region of rain around Prince William Sound Wednesday night into Thursday. By Thursday afternoon, another low moves from the Bering Sea into Southwest Alaska, bringing a return of showers to Kodiak Island.

-AS/KC

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days 1 through 3)...

Key Messages:

* A Coastal Flood Advisory is now in effect until 10AM Wednesday for the Kuskokwim Delta Coast. Water levels are expected to rise anywhere from 1.5 to 4.0 feet above the normal high tide line.

* A Wind Advisory is in effect from 10PM this evening to 10PM Tuesday for the Kuskokwim Delta Coast and Interior Kuskokwim Delta for south winds of 35 to 45 mph with gusts to 60 mph expected.

Discussion:

A steadily deepening, large, ~989 mb low pressure system currently situated between Saint Paul and Saint Matthew Island is driving a strong corridor of southerly winds towards the Kuskokwim Delta Coast this afternoon, accompanied with a round of moderate rainfall along its front's leading edge. We are also keeping an eye on Cold Bay this afternoon as their winds begin to peak in intensity. The highest wind gust so far today as of 4PM has been 54 mph, but given their peak is not until this afternoon as the low's front closes in from the west, and there is upper level support from a digging, positively tilted longwave trough with weaker waves embedded, and any rainfall could mix down stronger winds to the surface, Cold Bay could tick up to high wind territory before winds start to decrease later this evening and overnight.

Overall, models have finally converged on a much stronger solution for this Bering low, mainly due to higher confidence in the models of phasing happening in the upper levels. This consolidation of 500 mb energy is on track to continue strengthening the low as it moves northeastward through tonight towards just offshore of Hooper Bay and eventually to the Norton Sound. Winds will begin to increase for the Interior Kuskokwim Delta for locations like Bethel tonight, as these winds will not be confined to just coastal locations. The low is progged to become a robust gale force low with storm force gusts near the coast overnight into Tuesday morning, with the center of the low located offshore of the Yukon-Kuskokwim Delta Tuesday morning.

The peak in wind and onshore flow with some coastal flooding is most likely to start occurring early Tuesday morning, with wind gusts up to 60 mph and water levels up to 4 feet above the normal high tide line. To compound coastal impacts, rainfall and increased water levels will be coinciding with two high tide cycles, with the most impactful one likely to be Tuesday morning. Again, winds to a lesser extent will be making it to the Bethel area, where winds are on track to gust to 50 to 60 mph by Tuesday afternoon. As the system takes a more eastward bend and moves onshore sometime Tuesday afternoon, winds will shift from out of the south and southwest to out of the west, and become more directly impactful to Toksook Bay and Tununak with winds blowing perpendicular to the coastline there. By Wednesday morning, westerly winds begin to decrease in intensity, but elevated winds (sub-advisory level winds) are expected to continue through Wednesday afternoon.

In the wake of this system is another much weaker Bering low with small craft winds associated with it on its southern flank on track to sweep the Western and Central Aleutians late Wednesday morning, mostly due to the enhanced pressure gradient between a strong high located over the North Pacific and lower pressure closer to the surface low in the Bering. Models are in pretty solid agreement that the low tracks eastward across the Bering and just north of the Pribilof Islands through Thursday morning.

-AM

LONG TERM FORECAST (Days 4 to 7)

Ridging across the Eastern Aleutians and Southwest Alaska breaks down later this week as a low and its associated trough moves through the Bering Sea. This Bering low and its associated front brings rain showers and a return to cloudy skies. As the front moves toward the Mainland, persistent northwest flow will overtake the Aleutians bringing continued rain showers, fog and cloudy sky. Models are in better agreement on the general track of a second low crossing the Bering Sea and reaching the Central Aleutians this weekend. In its wake, it will bring a change to northerly winds for the AK Pen, Eastern Aleutians and Southwest Alaska early next week.

A receding ridge of high pressure in the Southern Gulf of Alaska brings the return of southwest to northeast flow into Southcentral for the week's end. This flow results in rainy and cloudy conditions for the Prince William Sound and eastern Kenai Peninsula form late this week into the end of next weekend. Timing and exact location differences are present though a weak low moves through the Northern Gulf of Alaska in the latter part of this weekend bringing rain to already saturated Prince William Sound coastline.

AVIATION

PANC...VFR conditions expected this afternoon and evening. Winds will remain light through the afternoon, shifting to southeasterly and increasing Tuesday morning. By midnight tonight, MVFR ceilings may move over the terminal along with light rain showers. Through Tuesday morning, expect winds to increase quickly out of the southeast, with gusts to 40 knots expected by 18Z.

CL