textproduct: Anchorage

SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)

Key Messages:

* Thunderstorms have developed across Southcentral this afternoon, with the focus of activity being across the Susitna Valley. In the strongest storms, frequent lightning, small hail, and brief heavy downpours could cause localized flooding. If you hear thunder or see lightning, seek shelter.

* A Red Flag Warning remains in effect for LIGHTNING for the Southern, Central, Western, and Northern Susitna Valley, and the Lower Matanuska Valley until 1AM AKDT Saturday.

Discussion:

As of 4PM this afternoon, scattered thunderstorms have developed across the Susitna Valley. Most of the activity is situated along a corridor from just north of Chickaloon to the Talkeetna area. Storm motion is slow with weak steering flow in place, but storms are moving to the northwest with time. The main concern through late this evening is frequent lightning strikes in the vicinity of storm activity and outside of areas that see rainfall for potential fire starts. If caught under one of these storms this afternoon/evening, also expect locally heavy downpours due to the slow storm motion. With the freezing level sitting at around 8500 feet across the Susitna Valley, small hail is also possible with more intense storms.

Aloft, a reinforced southerly flow from a ridge anchored over the Al/Can border and a digging trough over Southwest Alaska is sending a series of weak shortwaves over the Matanuska and Susitna Valleys. This upper level support has aligned with peak daytime heating to initiate storm activity today. As temperatures begin to cool mid to late evening, the atmosphere will stabilize itself and diminish storms into areas of showers that could linger through mid Saturday morning. There is also a more robust shortwave currently located over the north-central Gulf and east of Kodiak Island. With a secondary ridge axis building over the eastern Gulf, this feature will quickly be pulled northward and over Southcentral for Saturday afternoon. As this ridge builds, it will also slightly reorient the mean flow from southerly to southwesterly, which will likely shift the focus of thunderstorm activity Saturday to the Copper River Basin. Models have come into better agreement today in regards to positioning and timing of this wave, and the consensus is to bring the wave over the southern Copper River Basin during mid afternoon/peak daytime heating hours. Therefore, there is medium to high (70-80%) confidence of isolated to widely scattered thunderstorms Saturday afternoon for this region.

Meanwhile, a weak North Pacific low's front will lift northward across the western Gulf through the end of Saturday. There are still some minor timing/positioning differences among the global models, but the general idea is for periods of rain showers to begin to fill in across Kodiak Island midday Saturday to Saturday afternoon. With the low ever so slowly moving northward and weakening through Sunday, rain showers will likely remain across Kodiak Island through Sunday afternoon.

-AM/Rux

AVIATION

PANC...VFR conditions and light winds will persist. There is a chance of showers, and potential thunderstorm activity in the vicinity of the terminal this afternoon and evening. If a shower moves over the terminal, expect a brief heavy downpour and gusty winds. Any activity will be winding down by 06-09Z.

FIRE WEATHER

For Southcentral Alaska:

A Red Flag Warning remains in effect for Southern, Central, Western and Northern Susitna Valley and the Lower Matanuska Valley due to lightning until 1 AM AKDT Saturday.

Discussion: Scattered showers and wet thunderstorms have developed this afternoon across the Susitna Valley, mainly along the eastern slopes of the Talkeetnas and west of the Talkeetnas where temperatures have warmed well into the 70s today. Storm motion is slow today and to the northwest with time. Although these storms are producing locally heavy downpours and could drop some small hail, the primary concern is frequent lightning strikes outside of areas that see rainfall for potential fire starts. Storm coverage will likely increase as the afternoon wears on and begin to decrease late this evening as we lose daytime heating and associated upper level energy exits the region. A few remnant showers could linger early Saturday morning. There is medium to high (70-80%) confidence that the focus for thunderstorm activity Saturday afternoon will likely shift eastward to the Copper River Basin, meaning less storm coverage across the Susitna Valley and lower Matanuska Valley.

For Southwest Alaska:

A Red Flag Warning remains in effect for Lime Village due to lightning until 1 AM AKDT Saturday.

Discussion: The forecast looks on track regarding the Red Flag Warning in effect through tonight. Scattered wet thunderstorms are currently developing off the foothills of the Western Alaska Range in Lime Village, with the expectation that 10 to 15 kt of steering flow will lead to northwest propagation of thunderstorms through the evening. Given modeled CAPE ranging from 300-700 J/kg, these thunderstorms may produce hail and gusty winds in addition to frequent lightning. For tomorrow, have increased the footprint of thunderstorms across the Kuskokwim River corridor from the coast to Lime Village, but confidence is lower regarding where there may be widely scattered or perhaps even scattered thunderstorms. While max surface-based CAPE looks lower for tomorrow, there looks to be a larger footprint of 200-400 J/kg, which would likely be sufficient to produce widely scattered thunderstorms. However, there is poor model agreement regarding where the highest CAPE will be; as such, have put in a preliminary area of widely-scattered thunderstorms in Kuskokwim Valley East and Lime Village where temperatures will likely be warmest and skies clearest.

-AM/KC