Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 191231

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
431 AM AKDT Wed Sep 19 2018


The upper air pattern features a closed upper level low over the
Bering Strait with the trough axis extending into Northern Alaska.
There is a shortwave ridge over the Bering Sea ahead of a series
of low amplitude shortwave troughs ejecting off a larger trough
over the Kamchatka. Over Southcentral, there is a weak leftover
remnant of a once stronger shortwave trough moving through, and
this seems to be enhancing mid level cloud cover and even a few
stray showers over parts of the interior.

Finally, over the western Bering, there is a rapidly weakening
warm front which is moving toward the Pribilof Islands.



The numerical models have gone through quite a few adjustments the
past few days (i.e., they have really been terrible) with respect
to a closed upper low (the one noted above which is currently over
the Bering Strait) and the associated track of this feature over
the next few days. Previously, most guidance was depicting this
low taking a southward track which would put most of Southcentral
Alaska under the influence of dry northwest upper level flow. Now,
all the models track this feature farther north which allows for a
much more rapid transition to moist southwest flow from the Bering
Sea. Long story short, this means clouds and moisture will be
moving into Southern Alaska much sooner, and it also keeps a more
moist and cloudy pattern in for the weekend. The favored models
were the GFS and NAM, although all models are quite similar now
with the general pattern.


PANC...Satellite and ground based obs are showing quite a bit of
low stratus around Cook Inlet this morning. The general thought is
this should clear through daytime heating and reach VFR by this
afternoon. Other than that, expect winds less than 10 knots
through the period with dry weather.


The upper level low that is centered near Kotzebue Sound this
morning will slowly slide east over the next couple of days, being
centered near Anaktuvuk Pass Friday morning. As this low moves
eastward, the flow aloft will become more westerly. Much of the
moisture from this system will thus be blocked by the Alaska
Range, but plenty of clouds will still be over the area. Showers
will mostly be limited to the Susitna Valley and Copper River
basin, although a few could be seen further south. The lingering
low level moisture and longer nights are also allowing some fog to
form this morning, mainly near Cook Inlet.


A surface ridge currently across the Southwest holds across the
region today; however, a low over the Bering Strait sends short
waves south... bringing increased rain chances today through
Thursday. A brief break from showers is expected Thursday night
with the upper low exiting east across the central interior. The
next front moves to the coast early Friday morning with gusty
southeasterly flow along the boundary and rain. The pushes across
the Kuskokwim Valley and Bristol Bay by late afternoon. Rain
continues through Friday night with the associated surface low
approaching the Kuskokwim Delta.



The ridge that has been present over the Bering and Aleutians for
the past few days is losing influence as a weak front pushes
across from a Kamchatka low. The next front moves quickly up
from the North Pacific tonight, with the low center crossing over
the central Aleutians Thursday by morning. Gale force winds
increase over the central Aleutians as the front pushes further
north into the Bering. The surface low continues to deepen on its
eastward track into Friday. This subtropical system spreads
moderate rain and gusty gale winds as it moves across to the west
coast on Friday. Westerly winds develop in the wake of the low
with with gusty flow along the eastern waters with the low center
nearing Nunivak island.


LONG TERM FORECAST (Days 4 through 7...Sat through Tue)...The
pattern will start to "feel" much more like traditional Alaska
Fall as we head into the long term portion of the forecast. Models
have sorted out some of the discrepancies over the past few days
and are now in much better agreement through the weekend. The
ridge of high pressure from the Bering will drift south and set up
shop over the southern Gulf of AK. This will leave the southern
half of the state in zonal flow and open to systems/moisture
moving in from the west.

The low pressure center that will push the front through SW and
Southcentral AK on Fri will itself follow a similar track on Sat.
This system will keep things cloudy and showery. Then a slightly
stronger system will roll up into the area along the Alaska
Peninsula on Sun. With broad and fairly deep SW flow, this pattern
should make for more steady rain Sun and Mon.

By early next week, the models really being to diverge, which
leads to decreasing forecast confidence. The primary feature that
will drive the synoptic pattern is the ridge of high pressure
centered near 150W. Some models show this feature building back
over mainland AK and bringing another period of dry (and
potentially sunny) weather by mid-week. But other models keep the
ridge flatter and allow more energy to spill over the top. This
set-up would spell continue active weather (rain, clouds, seasonal
temperatures) across Southern AK. Thus, for this latter portion
of the forecast, we have elected to make few changes until there
is more clarity.


MARINE...Gale 173-177 413.



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