Gale Warning 116, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
439 PM AKST Fri Feb 15 2019

.ANALYSIS AND UPPER LEVELS...
A strong vertically stacked low positioned over the northern
Bering is the major weather maker today. The low is wrapped in
cold air and is advecting that cold air over the western and
central Bering. The meridional jet is experiencing the highest
wind speeds over the central Aleutians. These south-southwesterly
winds are reaching 160 kts at 30,000 ft. Southerly flow is
dominant over the southwest which is advecting slightly warmer
air into the eastern Bering. Ahead of the Bering low, a ridge is
building over the Gulf of Alaska and Southcentral.

At the surface, a front is pushing over the western mainland
bringing blowing snow conditions to the Kuskokwim Delta coastline.
Looking at visible satellite, the front can easily be identified
as a swath of clouds that spreads over the western mainland and
pushes into the northern Pacific. The front is trekking eastward
and is beginning to eat away at the weak surface ridge over the
Gulf. Dry, calm, and clear conditions prevail over Southcentral
which is helping to keep temperatures cooler. However, with the
approach of the incoming front, a more active pattern is
imminent. The next surface low pressure is advancing towards the
western Aleutians this evening from the North Pacific.

&&

.MODEL DISCUSSION...
The models continue in general agreement on the synoptic pattern
and timing of systems. The area where they struggle are in the
surface waves embedded in the flow. This is particularity
noticeable in the QPF pattern as fronts pass through Southcentral
over the weekend. The NAM appears to have a better handle on
these fields and is used in part for the afternoon package. Out
west, Model differences overall are not as pronounced. Stay tuned.

&&

.AVIATION...
PANC...VFR conditions and light winds through tonight. MVFR or
potentially lower conditions are expected to develop Saturday
morning as snow moves into the area.

&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
A pattern change is on the way for Southcentral as a series of
frontal systems will move across the mainland from the south and
west and will usher in a return to more wintry conditions across
Southcentral through the weekend.

The first frontal system is lifting over Kodiak Island this
evening and will bring an initial burst of snow across the island
before quickly transitioning to rain as a southerly flow develops
and warmer air works its way in. This system then lifts over the
Kenai Peninsula overnight with areas of light snow overspreading
Southcentral from Cook Inlet to western Prince William Sound
through tomorrow as a southwesterly flow aloft overruns colder air
remaining entrenched at the surface. The southwesterly flow,
coupled with upper-level support in the form of the short wave to
the north, will help enhance snowfall along the Alaska Range north
into the Susitna Valley. The precipitation may briefly diminish
in intensity a bit over the Susitna Valley tomorrow afternoon as
the upper-level shortwave lifts away from the region; however
precipitation along the coast should continue as the first wave
stalls and becomes absorbed into a second frontal system stemming
from a low moving north near the AKPEN.

This second system will have a strong moist and southerly
component, ushering in another round of heavier snow across the
most of Southcentral late Saturday through Sunday before lifting
north. Warmer air advecting in from the Gulf will likely change
precipitation to rain for locations from Kachemak Bay to portions
of coastal eastern Kenai Peninsula and western Price William
Sound. Areas of enhanced snowfall are also expected along the
western Cook Inlet north through the Susitna Valley

For areas in the lee of the Chugach and Kenai Mountains,
precipitation will be limited as a southeasterly downsloping wind
develops ahead of each front. Winds in the favored gap areas will
also become quite gusty Saturday into Sunday as the pressure
gradient tightens.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Saturday and
Sunday)...
An active weather pattern will persist through the weekend for
Southwest Alaska. A front associated with a Bering low continues
to advance inland, bringing snow and gusty south to southeasterly
winds along the Southwest coast through this evening. By Sat
morning, another low moves up from the North Pacific, centered
near Cold Bay by Sat afternoon. Its associated front will lift
over Southwest Alaska, bringing another round of snow, with a mix
of snow and rain possible over the AKPEN. As southerly flow
advects warm air northward, precipitation will likely fall as rain
along the southern AKPEN. As the low tracks to the north through
Sat evening, cold air will wrap around the backside of the low
allowing any remaining precipitation to likely fall as snow. The
greatest snowfall amounts are expected north and west of King
Salmon, thus a Winter Weather Advisory was issued for that area
beginning Saturday afternoon. The one fly in the ointment is that
the latest run of the GFS model is showing snowfall over Bristol
Bay starting as early as tonight and continuing through Saturday.
This would lead to increased accumulations across interior Bristol
Bay. We will monitor the situation for possible updates.

By Sunday afternoon, a weak ridge brings a break in the weather
over Southwest before the next front associated with an
approaching low out west moves in for the beginning of the work
week.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A storm force low has exited the Bering to the north. With ample
cold air wrapped around the backside of the low, west to southwest
gales will persist across the Bering through Sat morning.
Meanwhile, a developing North Pacific low will move in from the
south Sat afternoon, producing easterly winds over the eastern
Bering as the leading front lifts over the eastern Aleutians Sat
evening. Some northerly gale force winds are expected on the
backside of this low over the eastern Aleutians early Sun morning
as cold air begins to wrap in once again. Lastly, but most
importantly, yet another storm force low arrives over the western
Aleutians. The low will be centered near Shemya Sun morning. The
leading edge of the front will pack a large swath of southerly
gales with embedded storm force winds lasting through Sun
afternoon. This will produce periods of blowing snow for Atka and
Adak as the leading edge of precipitation moves in. As the low
lifts northward, ample cold air will once again wrap around the
low, enhancing winds on the backside with west to southwest gales
and pockets of storm force winds spreading across the Bering into
Sun evening. With the low currently expected to track east of
Shemya, this would lead to the far western Aleutians being
vulnerable to blizzard conditions. We will closely monitor the
situation for a possible Winter Storm Watch.

&&

.MARINE (Days 3 through 5/Monday through Wednesday)...
A relatively strong ~975mb low will be tracking west to east
across the central Bering Monday bringing widespread westerly
gales (and possible storm force winds) to the north side of the
Aleutians. This low will weaken as it moves onshore over the
western mainland Tuesday and winds and seas are both likely to
diminish through the day. Another low will approach the central
Aleutians from the south by early Wednesday morning. There remains
uncertainty with the exact track and well as intensity. Gales and
higher seas would likely occur on the east side of the low where
winds will be southerly, which would include areas generally east
of Dutch Harbor. Stronger high pressure building in behind the low
should force it out of the region quickly though again the timing
remains uncertain.

Marginal southwesterly gales and higher seas are possible over
the western Gulf near the Barren Islands Monday morning
associated with the same aforementioned low, but are expected to
diminish through the remainder of the day as the low weakens over
land. Gales have the potential to redevelop over the central Gulf
Tuesday as the pressure gradient tightens between a trough over
the mainland and ridge to the south. Model agreement begins to
degrade on Wednesday. One model solution gives widespread gales
over the western Gulf, but others keep them much further west over
the Bering. Confidence will remain low until the low's track is
better agreed upon.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Partly to mostly cloudy skies with light scattered showers are
likely for the Aleutians Monday as a broad low departs to the
north. Strong winds will develop on the south side of the low over
the southern Bering with cold air wrapping around from the
northwest. With regards to the potential coastal surge threat, the
GFS and ECMWF indicate a more southerly flow earlier in the day,
gradually increasing in magnitude and veering to a more westerly
direction by the evening. NAM starts off more southeasterly in the
morning, but also gradually veers westerly by Monday afternoon.
Thus, we cannot rule out possible storm surge and rough surf over
the southwest coastline, especially Kuskokwim Bay where shorefast
ice is mostly absent. Winds will diminish Tuesday as the low
weakens over the mainland. A low and associated warm front will
approach the central Aleutians from the south on Tuesday evening
bringing moderate rain and increased winds to areas generally east
of Dutch Harbor, though confidence is low on its exact track. A
strong high building to the south will quickly push the low out of
the region through the afternoon and bring fair skies for much of
the central and eastern Aleutians Thursday into Friday. Models
are indicating a stronger low developing over the western Bering
Thursday with moderate precipitation possible into Friday along
the associated front.

A relatively active pattern appears likely for the mainland much
of next week. A warm front will lift northward Monday likely
bringing coastal rain and interior snow. The associated low will
move into western Alaska Tuesday and is expected to bring further
accumulating snow to that region. Another low will bring in strong
southwesterly flow to southern and western Alaska on Wednesday
and, as with Monday's system, accumulating rain/snow appears
likely though amounts will depend on the exact track which is
poorly agreed upon at this time. High pressure will begin to build
in from the south on Thursday and looks to somewhat dry
conditions out by Friday, however another low in the Bering may
bring precipitation to the western mainland.

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 125,155(Blowing Snow) 161(Snow).
MARINE...Gale Warning 119 130 131 139 155 172 176>179 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RA/RC
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...AP