textproduct: Anchorage

SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)

Key Messages:

* Flood Watch remains in effect for the Central Susitna Valley through Wednesday afternoon.

* Flood Warning has been issued for the Petersville area through 5PM Wednesday.

* Flood Warning has been issued for the Yentna River including Lake Creek through 1PM Friday.

Discussion:

Much of Southcentral Alaska will remain in a stagnant, cool, cloudy, and somewhat showery pattern through the end of the short term period. Coastal ridging is slowly eroding this afternoon, which is allowing for gap winds in general to decrease across Southcentral, with the exception of the Copper Basin where winds continue to gust up to 40 mph. These winds will diminish through Wednesday as there is high confidence of a decreasing north-south pressure gradient currently in place.

Synoptically speaking, general troughing over Southwest Alaska and the Bering is anchored by an occluded Bering low that has become quasi-stationary over the Norton Sound. Meanwhile, its attendant front has stalled out from Kodiak Island northward to Talkeetna. Its moisture transport has kept widespread rain showers over the region today, though a mid to upper level wave digging over the North Pacific in the wake of the front is starting to effectively undercut the moisture transport in place. To the east, a ridge of high pressure situated across the eastern Gulf continues to remain anchored, preventing further eastward progression of the front. Due to this blocking pattern, scattered, intermittent rain showers will continue in the same areas as the front gradually shears apart and its parent low slowly weakens and meanders across the far northern Bering.

By Thursday afternoon, model guidance depicts the upper level support for the Bering low lifting northward to the North Slope, and thus Southcentral's rain chances greatly decrease. That said, a Flood Watch remains in place for the central Susitna Valley and excessive runoff from rainfall will keep the Yentna River rising through Thursday morning, where a crest near moderate flood stage is expected.

-AM

LONG TERM FORECAST (Days 4 through 7: Saturday through Tuesday)

An upper-level low situated over North-central Alaska will send shortwave energy southward across the Southern Mainland. The resulting weather will be diurnally-driven showers this weekend into next week. This pattern supports more of a scattered shower regime as opposed to widespread stratiform. The challenge will be to gauge instability each afternoon to determine whether or not lightning will be a threat with any robust showers that form. Right now, it looks like the Copper River Basin, along the Talkeetna Mountains, and along the Western Alaska Range feature the most instability through the long-term period.

Farther west, a system will drop south out of Upper Kamchatka and into the Bering Sea by Saturday morning. The point of uncertainty with this system is its progression. The GFS solution is the quickest with the system and places it near the Pribilofs Saturday morning. The Canadian and European guidance is slower with the progression and has the system in the north-central Bering near St. Matthew/St. Lawrence Islands Saturday morning. Regardless of the differences in speed/progression, this system does not look all that strong or impactful; just wet for the Pribilofs and perhaps the Central Aleutian Chain for this weekend. A separate front enters the western Bering on Monday and brings the Western Aleutians a round of rain Monday into Tuesday. A North Pacific low lifts towards the Alaska Peninsula by Tuesday morning. This system will bear further watching and interrogation as a tropical moisture plume may accompany this system.