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Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
444 am akdt Wed Oct 18 2017

Analysis and upper levels...
on satellite imagery there is a mature low south of Kodiak Island
and a second mature low south of Kamchatka. Between systems, there
is a ridge axis bisecting the Aleutians. A tight pressure gradient
has developed along the back side of the low that is south of
Kodiak resulting in widespread gusty winds over the Gulf of
Alaska and south of the Alaska Peninsula. The occluded front spans
from south of Kodiak Island to haida gwaii. To the north, there is
an Arctic vortex that is digging down south the Alaska Range. A
weak midlevel impulse has brought cloudy skies and showers over
portions of the Copper River basin and the Talkeetna Mountains.

&&

Model discussion...
the models are in sync with the synoptic features through Thursday.
On Friday, the Canadian Gem/ECMWF/GFS are tightly clustered with
the Arctic vortex and there appears to be more spread with the low
that moves from Kamchatka and across the Aleutians. By Saturday,
the GFS and the European model (ecmwf) have a similar placement of the low in the
Gulf of Alaska while the Canadian is too bullish and definitely an
outlier this run. The glaring issue is the low near Bristol Bay
that the GFS advertises 00z Sunday, that the European model (ecmwf) doesn't have
and of course the timing of the triple point low. Given the
vagaries of the models, Saturday and Sunday seem to have a fair
amount of uncertainty from the Alaska Peninsula to south central.



&&

Aviation...
panc...VFR conditions and light winds will persist.

&&

Short term forecast southcentral Alaska (days 1 and 2)...
the upper level trough has stalled over the eastern Susitna Valley
and will dig a little southward tonight before finally pushing
into the Copper River basin tomorrow. Where this will have an
impact is the areas east of this trough will have increased
cloudiness instead of clear skies west of the trough. There will
also be a slight chance of snow in the Copper River basin with
this trough. The eastern Copper River basin should see some
accumulating snowfall tonight into Thursday as the front in the
Gulf will push in more moisture to that area. Gusty northerly
winds will increase today along the north Gulf Coast as the front
pulls away from the area and cooler air moves in from the north.
Cook Inlet areas will also see these winds increase and that will
keep temperatures near the inlet warmer tonight as the winds will
prevent an inversion from developing.

&&

Short term forecast southwest Alaska (days 1 through 3)...

A colder and drier air mass has begun to situate across southwest
Alaska as evident by most sites minus the few sites that are being
impacted by some low to mid clouds along the Kuskokwim Delta. An
Arctic trough continues to build over much of Alaska with cold air
advection continuing to usher in slightly cooler temperatures each
of the next several days. It looks like temperatures today will be
near normal while Thursday and Friday will likely be slightly
below normal. This will be the first time most sites will report
below normal temperatures since the first of the month when
temperatures were just below normal across much of the region. A
few impulses will meander along the western periphery of the
upper level trough over the next several days which could help
produce some scattered snow showers across the region but the
greatest chances for snowfall will be along the immediate coast.

&&

Short term forecast Bering Sea/Aleutians (days 1 through 3)...

A relatively active pattern will continue across the Bering Sea as
the next frontal system to move into the western Bering Sea is
already knocking on the door. Unlike previous models that showed
the parent low pressure system decaying as it moved into the
Bering, it now looks like it will tap into more of the colder air
over Russia which will allow it to continue strengthening as it
moves toward Shemya late tonight and early Thursday morning. It
still looks like the front will still see sub-storm force winds
with sustained winds maxing out in the 40 to 45 kt range but some
gusts to storm force are quite likely until the system occludes
Thursday morning. The low pressure system will take a similar
track to the previous two as it moves north of Shemya and Adak
before moving into the northern Pacific Ocean and ultimately the
southern Gulf of Alaska. Very strong cold air advection on the back side of the
low (850 mb temperatures dropping to -12c) will create numerous
snow showers in the wake of the low as low-level lapse rates
steepen on Friday.

&&

Long term forecast (days 3 through 7: Friday through tuesday)...

Cold weather remains the primary weather story to start out the
extended forecast on Friday with a very wintry pattern in
place. Strong polar high pressure persists over interior Alaska
and Lee-side low pressure over the Gulf leads to gusty outflow
winds through all of the gaps of southcentral into Friday. Despite
model disagreements as to where the low is, they all agree there
will be one somewhere over the northern Gulf early Friday,
weakening through the day.

A much warmer, stronger low moves into the Gulf on Saturday,
effectively ending the recent cool-down over southcentral.
Despite continued disagreement between the models on the track of
the low, they all agree the low will be strong enough to change
the precipitation type to rain along the coast, with likely
downsloping through Anchorage and the Cook Inlet region.

Behind the low on Sunday is where things get interesting for
those eagerly awaiting the first snow in Anchorage. Cold air
rushing in from the southwest following the low will coincide with
a decently strong shortwave trough in the upper levels. If this
plays out as currently indicated, then rain early in the day may
change over to snow as the trough moves through. Cold advection
and southwest winds with the precipitation would all favor a
quick, but potentially heavy hit of snow during the day. There is
still a lot of disagreement in timing and potential amounts, so
stay tuned for future updates as those details get hashed out.

&&



Afc watches/warnings/advisories...
public...none.
Marine...gale 150 175 176 177 178 411 413 351 352 130 131 132.

&&

$$

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