textproduct: Anchorage

SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)

Key message:

* Flood Advisory in effect for the Yetna River near Lake Creek until 4 PM Wednesday. A flood watch remains in effect for the Susitna Valley for areas upstream of Lake Creek and the Yetna River until 4 PM tomorrow.

Discussion:

The large low that has been tracking through the Bering Sea is now in Southwest Alaska. The main front associated with it has swept through Southcentral and is in the Interior of the state. The result is that there are strong pressure rises behind this front that are spurring gusty southerly winds up Cook Inlet and also though the Copper River Basin gap areas. Southcentral is also in the showery sector of the low with multiple shortwaves kicking off periods of showers. Both winds and shower activity should diminish throughout tonight into tomorrow morning, with some gusty winds persisting in the Copper River Basin. The Susitna Valley is the one region that will likely really hold onto the rainfall as the south-to- southwest flow aloft is favorable for upsloping along the Alaska Range and Talkeetna Mountains. In addition, the Susitna Valley is closer to the upper low center which is keeping it more unstable as well. This is a major reason there is a Flood Watch in effect for some portions of the Susitna Valley. This low center moves slowly from Southwest to the interior on Tuesday as it weakens, fully dissipating late Tuesday night. The northern Gulf Coast and high elevations will also see lingering showers on Tuesday, with lower elevations seeing less of a chance.

As is the case in August, large longwave troughing persists across the Bering Sea with another upper low embedded at the base of the trough over the Aleutians. This low will move eastward throughout the week, crossing the AKPen Wednesday night and moving into the northern Gulf on Thursday. With southerly flow ahead of the low pulling in more moisture, another round of widespread precipitation is expected for for Southcentral Wednesday and Thursday, starting with Kodiak Island early Wednesday morning and the rest of Southcentral Wednesday afternoon. Ahead of the upper level low, a surface low develops in the North Pacific Tuesday night and moves into the Gulf on Wednesday. With a coastal ridge building in, winds will strengthen to small- craft levels offshore the northern Gulf Coast Wednesday evening into early Thursday morning as the low moves onshore and quickly weakens.

Widespread precipitation will continue across Southcentral on Thursday as the upper low slowly moves into the northern Gulf. The heaviest precipitation Wednesday-Thursday will be along the northern Gulf Coast and around Prince William Sound.

- PA

LONG TERM FORECAST (Days 4 through 7: Friday through Monday)

The long term begins with a strong zonal jet between Kamchatka and western Alaska that will amplify into a broad trough over the state on Saturday and Sunday. The jet streak over the Bering will help to generate a fairly strong and broad low pressure system that will impact southwest Alaska starting early Friday. Depending on the track and timing of this system, which has a fairly large spread, there are concerns regarding the potential for coastal flooding, high surf, and/or erosion. It's difficult to pinpoint where this may occur at this time with guidance highlighting a broad area of concern from the northern Alaska Peninsula all the way to the Y-K Delta.

The other aspect to this system is significant rainfall. Although there isn't a deep connection to the pacific and tropical moisture, a decent slug of warm, moist air will accompany the system. Models are indicating 1-2 inches of rain may fall on Southwest Friday and Saturday. The storm will pass into Southcentral and then the Gulf over the weekend, with a widespread 1-3 inches of rain possible. Given the wet environment, particularly in the Susitna where flooding is currently occurring, flood concerns will rise once again. Winds look strong through the Bering, Southwest, and Gulf of Alaska/Kodiak Island.

AVIATION

PANC...VFR conditions are expected to remain through the TAF period, with ceilings hovering around 5000 ft. Gusty southerly winds will persist through the day and start to diminish tomorrow morning. As the surface winds and winds aloft are both from the south, low level wind shear is no longer expected.