Area Forecast Discussion
Issued by NWS Anchorage, AK

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
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
FXAK68 PAFC 201525

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
544 AM AKDT Tue Mar 20 2018


A colder air mass has overspread all of southern Alaska this
morning. The result is a lot less cloud cover since the air mass
is also much drier. However, a few small-scale features remain,
which are causing some localized clouds. These include the last
remnants of the old air mass moving through the eastern Copper
River Basin, and a fast moving mesolow over the Lower Kuskokwim
Valley that is likely to dissipate once it reaches the Alaska
Range later this morning. Finally, with broad west to northwest
flow, the west and northwest faces of the Kuskokwim Mountains and
Alaska Range remain cloudy due to upslope. The Anchorage sounding
remains very dry compared to 24 hours ago, so any passing cloud
cover is not expected to develop any shower activity except possibly
in the mountains where upslope and less dry air below in the
atmosphere for any precipitation to fall through, may allow it to
reach the ground. The primary weather hazard today, and for the next
several days, will be strong winds gusting through the gaps making
for hazardous travel conditions in those areas.

In the upper levels, a broad trough axis is right over
Southcentral this morning. It is amplifying southeastward as an
upper low forms over the northern Gulf. Meanwhile, ridging over
the Bering is also amplifying. The result will be a pattern that
holds as is for the next several days as any features moving
through the area get sent poleward or equatorward, resulting in
little eastward movement.



The models remain in excellent agreement for the next few days.
Thus, forecast confidence is very high. The primary forecast
challenge will be the winds, as minute features in the flow will
act to locally enhance or diminish the broad offshore winds. The
high resolution models were preferred for this forecast package
since they much better handle the local effect winds.



PANC...VFR conditions and light winds will persist for the next
several days. Periods with stronger winds will be possible, and
represents the main forecast challenge. Currently, the first
period of stronger northwest winds looks to impact the airport
this afternoon.


The primary weather concern through mid week will continue to be
on the strength of the outflow winds that are developing
throughout the Gulf Coast. The one exception to this will be for
an outside chance of snow showers over the Cook Inlet region this
morning as a weak shortwave moves overhead, which much of the
model guidance is indicating in some way. The 12z Anchorage sounding
however indicates very dry low levels and as a result this has
been left out of the forecast outside of western faces of the
Chugach Range. Radar will need to be closely monitored this
morning to confirm this is actually the case.

Back to the main focus of the forecast, strong northwesterly winds
have already developed around Kodiak Island and the Barren Islands
and will continue to spread to the east today as the trough slides
off to the east. This event still looks to favor northerly gap winds
throughout the Gulf Coast, such as through Resurrection Bay and the
Valdez area, though gusty westerly winds will occur today from
Turnagain Arm into Passage Canal while there is still some westerly
support aloft. The strongest winds still look to hold off until at
least Wednesday when high pressure will approach from the interior,
reinforcing the tight pressure and thermal gradient, especially in
the Valdez/Thompson Pass area.
There is an outside chance of catching a few warning level gusts
especially through Thompson Pass as this occurs, however with the
strongest upper level support further to the west and limited cold
air in the Copper River Basin it still appears that these winds
will likely peak at around 70 mph.



The METAR at Sparrevohn has snow showers, which is a function of
upsloping. Both the radars at King Salmon and Bethel are pretty
uneventful. A dome of high pressure has set up over Unalaska and
upper level heights are rebounding across Southwest. Predominately
dry and mild weather is expected today, Wednesday and into



The latest ASCAT (advanced scatterometer) pass has the surface
high over Unalaska. This forecast period expect the high to build
and become more amplified. The 570 DAM will be just south of the
Central Aleutians by this afternoon bringing a touch of spring to
the region. Expect widespread subsidence and warming across the
Central and Eastern Aleutians. Looking upstream, the low over
Central Kamchatka continues to skirt the peninsula. The jet that
is driving this storm is just west of Attu Island and it is well
over 120 knots. The latest ASCAT also pings into gale force winds
near Attu. Wednesday, the models bring the cold front into the
Western Aluetians and those gales will push eastward. Expect
another round of precipitation and in the wake of the frontal
passage a distinct cooling trend.


.LONG TERM FORECAST (Days 3 through 7)...
On Thursday night a large upper low will be situated over the
Bering Sea, Aleutians, and the west coast of Alaska. There will be
a full latitude blocking high along 155 degrees west longitude,
with another upper low centered to the west of Vancouver Island.
This pattern will begin to break down by the weekend, but exactly
how this occurs is not well agreed upon by the models. The 00Z
ECMWF supports a compact upper low which will peel off from mean
trough near the Yukon Territory that would undercut the ridge and
form a Rex block. Meanwhile, the past few runs of the GFS suggest
maintaining more of an Omega block signature as downstream energy
does not show any retrogression back toward mainland Alaska.
Underneath this blocking regime, a chain of shortwaves appear
primed to sweep across the Aleutians into the Gulf of Alaska
during much of the forecast. This will be a fairly active pattern
for the Aleutians and southern Alaska. Due to the model
differences, timing of any individual systems is problematic.
Therefore the WPC forecast relies heavily on the ensemble means,
with the strongest weighting towards the 00z ECMWF mean.


MARINE...Gale 119 120 121 127 128 130 131 132 136 137 138 178 185
351 352 411.



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