Area Forecast Discussion
Issued by NWS Anchorage, AK

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

188
FXAK68 PAFC 101342
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
442 AM AKST Sun Dec 10 2017

.ANALYSIS AND UPPER LEVELS...
The very active weather pattern that has dominated the weather
over the Gulf and South Mainland over the course of the last week
continues as three separate and potentially impactful systems can
be seen on satellite imagery rotating around the area. The first
of which brought rain and gale force winds throughout the Gulf
coast overnight, and is beginning to bring gusty winds through gap
wind prone areas of Southcentral as well as rain/snow to the
Southwest Mainland. The strongest winds so far has occurred
through Portage Valley which has had a few gusts approaching
warning level. This system is beginning to make landfall, however
there will only provide a very brief break in the wind and rain
as a second stronger system is quickly approaching behind it. This
system will track into the Gulf as it strengthens to storm force
this evening, and bring rain and wind to most areas through
Monday. The last system currently is a weak low centered well
south of the Aleutians, but it to will strengthen as it moves
northward into the Gulf under a strong southerly jet early next
week. This persistent track of these systems continues to be
dominated by a stout blocking ridge stretching from the Panhandle
to well south of the California coast and a broad trough over the
Bering/North Pacific.

&&

.MODEL DISCUSSION...
The models remain in very good synoptic agreement into Monday considering
the very active and rapidly developing series of lows expected to
track into the Gulf. In particular, agreement in both the strength
and intensity of the incoming low through Monday has improved
significantly since yesterday, allowing for some additional wind
detail to be added to the forecast and the issuance of 3 High Wind
Watches. Starting Tuesday (and has been the case beyond 48 hours
in the current pattern), model agreement begins to break down with
the track of yet another low that now looks to track further west
towards the AKpen, and especially in whether a secondary low will
quickly develop off of it in the Gulf late Tuesday. This is a new
feature from both the NAM and GFS that is being treated as an
outlier for now, and is the main culprit in diminishing forecast
confidence towards mid week.

&&

.AVIATION...
PANC...Rain showers should clear out of the area early this
morning as gusty SE winds and downsloping begins to take over.
These winds will then briefly diminish later this afternoon
before gusty N-NE winds with rain and ceilings falling to around
5000 ft develop this evening. Another round of SE winds which are
expected to be much stronger will then develop Monday morning-
afternoon. Confidence is low in exactly how strong Monday`s winds
will develop, but at this point gusts from 35-45 kt appear likely.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The active pattern continues through Monday. A strong low near
Kodiak Island this morning moves west and weakens. Brisk and gusty
gap/terrain driven winds are expected across much of Southcentral
today. The next and stronger in a series of lows, moves to over
Kodiak Island the evening. This system in combination with its
frontal system will spread storms to many of the Gulf marine areas
this afternoon and tonight. A barrier jet should develop in
advance of the front along the North Gulf Coast and could kick
southeast winds up to warning levels near Cordova tonight.
Pressure gradients will also become fairly strong across Portage
Valley and Turnagain Arm and could produce warning level gusts
beginning around midnight tonight. As the low moves toward
Bristol Bay on Monday, brisk to strong southerly winds will
develop across much of Cook Inlet/Kenai Peninsula, Anchorage Bowl,
Matanuska Valley, and Copper River. However, Turnagain arm and
Portage Valley winds will diminish from near warning levels.

As has been the case with the last couple of system, the brunt of
the precipitation, mainly in the form of rain, will be confined to
the coasts. Inland areas, except for upslope along the Aleutian/Alaska
Ranges, should be mitigated to a large extent by the strong
cross-barrier flow over the Chugach and Kenai Mountains. The
degree of warming would indicate mainly rain across Cook Inlet,
Anchorage Bowl, and Matanuska Valley with mixed elsewhere. That
said, there may be some areas around the Anchorage Bowl which will
some very light freezing rain/sprinkles this morning.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A weakening trough of low pressure is moving across the Aleutian
Mountains and into Southwest Alaska this morning. The trough will
spread some precipitation across Bristol Bay this morning,
spreading northwestward into the Lower Kuskokwim Valley this
afternoon. The King Salmon 12Z sounding shows an atmosphere primed
for freezing rain, with an extensive warm layer a couple thousand
feet above the ground, with temperatures at the surface in the
20s. Thus, once precipitation begins this morning, it is likely to
be all freezing rain. Fortunately, the forcing for precipitation
will be weak and moving quickly, so the threat should be over by
afternoon as the trough moves westward and temperatures warm with
downsloping behind the trough. Elsewhere from Dillingham north and
westward, the downsloping will not be present, so there should not
be a warm layer, meaning precipitation should mainly fall as snow
or a plain rain where temperatures warm above freezing.

The next round of precipitation will be quick on its heels
starting tonight. It should fall as mainly snow, albeit a wet
snow, from Dillingham north and west, while King Salmon and points
east should be mostly liquid. This much stronger low will bring in
additional warm air, with easterly winds through the Alaska Range
contributing to downsloping. Thus, Iliamna and north into the
Lower Kuskokwim Valley along the foothills of the Alaska Range
will potentially see lower 40s by Monday from the downsloping. The
weakened low moves across Bristol Bay Monday morning, then tracks
northwestward towards Norton Sound by Monday evening. The
associated precipitation will be heaviest during the morning, but
quickly taper to scattered rain and snow showers during the day.
Yet another low tracks into the southern Alaska Peninsula Monday
night, keeping precipitation going for portions of Southwest
Alaska, particularly the eastern slopes of the Kuskokwim
Mountains, which will benefit from upsloping.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A decaying front over the western Aleutians will continue to
spread rain and snow to the area today, before finally drifting
back into the Pacific tonight. As a series of lows move into
Southwest Alaska, a strong northerly flow will develop over much
of the Bering by Monday, the lows themselves and the passage of
upper level shortwaves will cause snow showers to develop across
the central Bering. The resulting snow will blow around through
the Pribilofs as the winds gust to gale force. The strong
northerly flow will continue through Monday night before another
front approaches the western Aleutians for Tuesday, diminishing
the area under the influence of the cold north winds and embedded
snow showers.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The subtropical jet continues to keep an active pattern over
southern Alaska through the next work week. However the upper
level pattern shifts from high amplitude to a zonal flow which
sets up just south of the Aleutian chain heading into the middle
of next week. This upper level change is significant as it will
lead to a cooler trend Wednesday through Friday.

Models are struggling with the pattern shift mid week as they vary
on how quickly they break down the high amplitude ridge over the
Southeast. Kept forecast updates conservative as models tend to
break ridging down to quickly. The most notable change arrives
with cold air advection across the Bering early Monday with a
front plunging south across the Aleutian Chain. The front gains
eastward momentum on Wednesday allowing colder air to wrap into
the following lows moving toward the southern mainland during the
second half of the week.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Watches 101 125 135
MARINE...Storm 119 120 125 130 131 132 137 138 139.
         Gale 121 127 128 129 136 140 141 150 177 178 179 181 185.
         Heavy Freezing Spray 181.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM...KH



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.