Gale Warning 389, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
456 PM AKDT Tue Aug 13 2019

.ANALYSIS AND UPPER LEVELS...
The upper level ridge over Southcentral Alaska has flattened
considerably over the past day as a trough in the Bering Sea
pushes into it. This is bringing some southwesterly flow aloft to
Southwest Mainland Alaska and the Bering Sea with flow aloft more
westerly in Southcentral Alaska. There is enough of a gradient to
bring in a jet streak into Southwest Alaska which is around 90 kt
at 250 mb. The surface low is near the northwest Alaska coast and
the front is stretched across western Alaska into Kuskokwim Bay
and then through the Eastern Aleutians and shows up well on both
satellite and radar. The next low is approaching the western
Aleutians from the south and is the source of the Gales in the
region.

&&

.MODEL DISCUSSION...
The flattening of the upper level ridge in the short term is
reflected well on all the major models with good consistency from
them into late in the week.

For late in the week, the shift in the models that was discussed
and which brought them into much better agreement looks likely to
occur. This increases the confidence in the forecast for this
weekend and beyond. There is still some refinement in the way the
upper level low tracks across the state late this week, but the
pattern is consistent now and makes meteorological sense.

&&

.AVIATION...
PANC...VFR conditions will persist. Southeast winds will also
persist into tomorrow morning. Some gusts in the 12 to 15 kt
range are possible through this evening but a strong Turnagain
Arm wind is not expected.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
(Tonight through Thursday evening)

The much anticipated rainfall (in the form of showers) continues
to move across much of south central. However, rainfall amounts
are very very light in most areas, with the latest observations
showing all rainfall has been less than 1/10 of an inch. These
showers will continue overnight, as a series of disturbances
aloft pass by just to our north, giving the region bursts of
ascent (rising air).

Heading into your Wednesday and Thursday, the track of the
aforementioned disturbances will take on a more northwest to
southeast track, from the northern Matanuska Valley through the
Copper Basin. However, 850 mb winds also become more
northwesterly as an area of high pressure across the southwestern
Gulf edges further west. This decreases the low-level
moisture content. Thus, any shower activity should be confined to
the Alaska Range (closest to the best source of lift), and the
windward side of the mountains of the MAT-SU Valleys and Copper
Basin. This shows up nicely in the RH fields, as the regions
mentioned above have values near and above 75%, dropping to 35%
for the Anchorage Bowl, and 15% for the Kenai Peninsula. Thus, the
further south you go, the less clouds you have. with that in
mind, we raised afternoon highs for both days, especially for
those locations with lower humidity/cloud cover, with perhaps some
high thin cirrus having a minimal impact n the temperatures.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Tuesday
evening through Friday)...

A band of rain showers stretches across Southwest Alaska and into
Bristol Bay, lying along a nearly stationary boundary. This stream
of moisture will be present into tomorrow when the next low pushes
north through the Bering Sea. This low will move more through the
western Bering, but its eastern edge will clip the southwest
coast and lift the out the nearly stationary boundary currently in
place. Precipitation will hang around the Kuskokwim River valley
though through Thursday night. During this time, high pressure
will again influence the area as ridging builds in from the
southeast, which will warm temperatures mainly around Bristol Bay.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3/
Tuesday evening through Friday)...

Widespread low level cloud cover and fog continues for the Bering
Sea and Aleutians. A nearly stationary front draped across the
Eastern Bering extending from the remains of the low in southwest
Alaska through the central Aleutians is providing some rain showers
as well. Some clearing of clouds and fog is present along the lee
(north) side of the Aleutians with continued southerly flow.
Tomorrow, the next low will enter the Bering Sea from the south,
bringing gale force winds and more rain with it. Low ceilings and
fog will also continue. This low will stay more towards the
central and western Bering, as high pressure will begin to push
back into the eastern Bering, forcing weather to the west.

&&

.MARINE (Days 3 through 5)...

There is above average confidence of an increase in northerly
offshore winds beginning late Friday as a vigorous shortwave moves
from the Southern Mainland into the northern Gulf. Northwesterly
gales with waves in excess of 10 feet are expected Friday and
Saturday across the Barren Islands before diminishing back below
small-craft criteria for Sunday. Northerly small-craft advisory
winds are also possible for a time overnight Friday into Saturday
south of the Copper River Delta. Elsewhere, expect gusty winds out
of bays and passes across the Southcentral coast through Saturday
with a decrease in wind speed on Sunday.

For the Bering and Aleutians, continued ridging across the
eastern Aleutians and Bering will produce an offshore flow across
the channeled bays of the southern AKPEN through the weekend. Low
pressure south of Shemya will produce southeasterly small-craft
advisory winds along a stalled front draped across the western
and central Aleutians.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Guidance has come into much better agreement with the progression
of a vigorous upper-level shortwave dropping from the Southern
Mainland into the northern Gulf by late Friday. This yields above
average confidence in the development of strong outflow winds
across much of the favored channeled locations across both the
interior and coastal locations of Southcentral from Friday
afternoon through at least Saturday. Although temperatures aloft
will be dropping and aiding a general cooling trend across the
interior, coastal locations may stay relatively warm as the gusty
offshore winds negate the chance for cooling seabreezes. Fire
weather concerns will also be on the increase beginning late
Friday as the combination of drier air advecting over the region
coupled with gusty winds will help drop relative humidity values
and further dry out vegetation. RH values could drop to around 25
to 35% through the weekend, especially for the Kenai Peninsula,
Susitna Valley and Copper River Basin. As for precipitation, the
bulk of the moisture with this system will remain north of
Southcentral due to upslope flow along the Alaska Range; however,
areas of light rain showers are possible over the mountains and
within the Copper River Basin through late Friday as the system
passes. Winds are expected to wind down by Sunday with dry
conditions persisting. The continued northerly flow and colder air
aloft will allow for the development of isolated showers over the
mountains into early next week.

Farther west, a ridge over the eastern Bering will keep conditions
rather quiet and slightly cooler over Southwest Alaska and the
eastern half of the Bering through the weekend as a stalled
frontal boundary allows southerly winds and precipitation to
persist across the central and western Aleutians. The ridge is
then expected to retreat northward late Sunday allowing a north
Pacific low to lift toward the western half of the Aleutian Chain.
As it does, this system will sweep a front over the eastern
Aleutians beginning early, possibly impacting areas as far east as
the AKPEN and Kodiak Island by midweek next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales 175-177, 413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
MARINE/LONG TERM...TM