Marine Weather and Tide Forecast for Kalifornsky, AK

FXAK68 PAFC 240350 AFDAFC

Southcentral and Southwest Alaska Forecast Discussion National Weather Service Anchorage AK 650 PM AKST Thu Jan 23 2025

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Tonight through Sunday)...

Key Point:

* Flood Watches have been issued for most of Southcentral Alaska for Friday morning through Monday Morning. A long duration rainfall, heavy at times, will fall on a frozen ground, limiting absorption of water into the ground. There is high potential for widespread areas of ponding/standing water, especially for low lying or poor drainage areas. Flooding along area rivers and streams is also possible.

* An approximation of Storm Total Rainfall Amounts Friday morning through Monday Morning:

-Western Kenai/Anchorage/Matanuska Valley/Kodiak Island: 1 to 2.5"

-Susitna Valley/Eastern Kenai/Prince William Sound: 2 to 6"

* Gusty southeasterly winds will accompany the rainfall, strongest through Turnagain Arm into parts of Anchorage, the Knik River Valley into Palmer/Wasilla, and along the Copper River. Wind gusts could reach 70 mph along the Anchorage Hillside. The combination of wind and rain could bring down additional trees, particularly in areas where the ground is not frozen or where trees are already weakened from the recent windstorms.

* Snow levels will rise Friday through Saturday, reaching as high as 3000 feet in the Mat-Su and 3000 to 6000 feet Saturday afternoon from Anchorage down to the western Kenai. The only areas along the road system that could remain mostly snow throughout this event are the north end of Broad Pass up to Cantwell; the top of Thompson Pass; and Paxson up to Isabel Pass.

* Colder air will move into the region sometime Monday, possibly changing precipitation to snow. At a minimum, there is good potential for temperatures to drop below freezing early next week, causing any standing water to freeze.

A look at satellite imagery shows ongoing amplification of the upper level flow, which is the key to what will be significant impactful weather over Southcentral Alaska over the next 3 to 4 days. A deep upper level low anchors a trough over the western Bering Sea. A corresponding strong ridge is centered over the Northeast Pacific extending northward to the eastern AK mainland.
A strong jet stream (150 kts+) and atmospheric river can be found in between the low and ridge, from the Aleutians northeastward to Interior AK. A low amplitude short-wave and surface low are riding the jet northward up the west side of the ridge, which is amplifying out ahead of this low.

This sets the stage for upcoming weather. First, focusing on precipitation, a short-wave over the eastern Bering will ridge over top of the ridge this evening, leading to increasing precipitation along south to west facing mountains of Southcentral. This will be in the form of rain for lower elevations where temperatures are well above freezing. The primary area of impactful snowfall will be Hatcher Bass up to the northern Susitna Valley - especially through the Broad pass area.
Precipitation will diminish behind this short-wave late tonight/early Friday.

As the upper level ridge continues to amplify, the atmospheric river will reform and reposition on Friday, tapping into deeper moisture and warmer air from a low well down in the Pacific (~40N Latitude). Rain will gradually overspread Kodiak Island/western Gulf and most of Southcentral during the day Friday. There will be little change in the pattern through Sunday - with periods of rain across lower elevations of Southcentral through this entire time period. Of note, there is a lack of strong dynamical forcing aloft, with just some weak low amplitude waves transiting the west side of the upper ridge. However, the moisture content of the airmass will be extremely high, so it will take very little forcing to produce rain. What looks more impressive is low level forcing, with warm air advection and strong (southerly) upslope flow.

For this forecast package, have tried to hone in on approximate storm total rainfall Friday through Sunday night, which will be significant for all areas - likely setting new January records for rainfall. While snowpack is quiet meager at lower elevations of Southcentral, the ground is frozen. This means water will not be absorbed and will tend to drain to lower spots and sit on top of the ground. This will likely lead to widespread standing water and flooding of low lying areas - from roads to yards to rivers. With snow levels expected to rise quite high on Saturday (as high as 3000 feet for Hatcher Pass and in the 3000 to 6000 foot range for Anchorage and the western Kenai) snowmelt will also contribute to runoff and lead to rising water levels in area rivers/streams.
Bottom line: this is a rare event for January which will likely cause significant impacts.

As for the wind, relatively lower pressure will persists to the west from the eastern Bering Sea to Southwest AK as weak lows transit northward through that corridor. This will lead to gusty southeasterly gap winds over the next few days. As is usual, the strongest winds will be found along Turnagain Arm into Anchorage, Knik River Valley into Palmer, and along the Copper River. Of some concern, the warm air advection will lead to development of a temperature inversion, which is conducive to mountain waves along the Front Range Chugach in Anchorage. This event does not look nearly as strong as the January 12th event. However, winds could still be quite strong Friday night through Saturday night. For now, have capped wind gusts as around 70 mph for the Anchorage Hillside and 45 mph for lower elevations of Anchorage. Will continue to evaluate stability profile and mountain wave potential to determine if winds could be a little stronger than that.

Forecast confidence for what follows this rain event is low.
Model spread is quite large for early next week with progression of the upper trough. Two potential scenarios are much colder and mostly dry or somewhat colder with snowfall. In either case, the colder temperatures will lead to freezing of any water sitting on the ground.

-SEB
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