Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxak68 pafc 231309 
afdafc

Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
509 am akdt Mon Oct 23 2017

Analysis and upper levels...
satellite shows a very active winter-like pattern across Alaska
this morning. Working from west to east, the first feature is a
developing low near the Kamchatka peninsula. This low is primed
for serious strengthening over the next 24 hours as a 145 kt jet
is moving over top of it. It is also bringing up some very
warm/moist 14c air from the Pacific. Behind this low, there is
some very cold air over Siberia approaching -10c. This sort of
thermal gradient sets the stage for the system to pack a very
strong front as well. It is just about to start the deepening
phase and there is high confidence that it will drop nearly 50mb
in the next 24 hours.

The second feature is the parent low along the West Coast of the
Mainland. This continues to drag cold air down the Chukchi Sea
producing widespread cold air showers across the central and
eastern Bering. Some of these showers are wrapping into the Alaska
Peninsula and into the western Mainland. It is not unusual for a
location to go from clear skies and good visibility to heavy snow
and gusty winds in matter of minutes with a passing shower.

Feature number three is a compact, but intensifying low near
Kodiak. Models have been struggling to resolve this thing for a
few runs now. And it appears that it is exceeded expectations
still. The King Salmon and Kenai radars are both starting to show
more and more returns as this system works north right along The
Spine of the Aleutian Range. A Winter Weather Advisory for the
eastern portion of Bristol Bay is in effect to address this.

The fourth and final feature is a developing low well south of
Cold Bay. It does not look like much on satellite at this time.
However, this is the system that we will watch through the day as
it moves north and tries to interact with the cold low over the
West Coast. If these 2 systems can interact and consolidate their
energy coming through the Cook Inlet, it could bring snow to many
of the population centers. However, overnight models have trended
lower on snow accumulations for Anchorage.

&&

Model discussion...
while models are in much better agreement than they were yesterday
morning, they are still struggling mightily with some of the
details. The GFS was preferred on both domains for consistency and
resolution. The NAM has been the outlier with the system moving
over Kodiak this morning, so it was mostly discounted. We have
manually sped up this system and its associated
winds/precipitation as models were a bit too slow overnight. There
is still a fair amount of uncertainty as to how much snow this
feature brings over the Glenn Highway this evening, but there
should be at least some. The other item of interest (mentioned
above) is the interaction with the low near 48n and the trough
along the West Coast. How these 2 features interact will have
large consequences on how much snow falls over the Cook Inlet Tue
afternoon-evening. The models are handling the largest, strongest
system over the western Bering quite well. They all agree that it
will be even deeper than initially progged and that its associated
front will hold together further east.

&&

Aviation...
panc...VFR conditions will persist through most of the afternoon
with increasing down-inlet flow. Winds will become southeasterly
this afternoon as a weak front moves through. Winds will then
shift to more southerly as the front slides east. Timing of the
winds could vary by a few hours and the speeds could be a bit
stronger than forecasted. Lower stratus could bring the ceilings
to MVFR behind some snow showers. Conditions will then improve
once again in the late evening.

&&

Short term forecast southcentral Alaska (days 1 and 2...Mon and
tue)... a tightly compact low centered over Kamishak Bay this
morning will move north through the Cook Inlet area today while
weakening. This system will bring chances of snow to the Kenai
Peninsula this morning, then to Anchorage and areas northward this
afternoon and evening. This system will quickly exit the area
tonight. The next system will push into Kodiak from the south
early Monday night, with precipitation spreading rapidly northward
and spreading across much of southcentral Tuesday morning. For
the Anchorage and Matanuska areas, it looks like there will be
enough easterly flow that some down-slope conditions will limit
snow amounts. Decent amounts of snow are expected over the western
Susitna Valley and over inland portions of the eastern Kenai.

On Wednesday, a pattern shift is in store. Southerly flow aloft
will develop ahead of a large upper low over the Aleutians. Much
warmer air will surge northward. Many areas will see a transition
from snow to rain as temperatures climb well above freezing.

&&

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

Snow showers continue to be observed across the West Coast of
Alaska this morning as a decaying area of low pressure over the
yk-Delta continues to pump moisture inland. These showers are
expected to dissipate throughout the day today ahead of rather
impressive front moving toward the area. In the meantime, a rather
deepening low pressure system has developed west of Kodiak and
will move into Cook Inlet by late this morning. This system will
help bring 4 to 8 inches of snowfall to the Bristol Bay area east
of King Salmon through this afternoon as the low tracks to the
northeast. With the cold pattern observed over the past week, it
seems plausible that the warmer air advecting northward will
overrun the colder air at the surface and help enhance
precipitation processes (isentropic lift).

Going back to the aforementioned front, a strong frontal boundary
associated with a deep low pressure system over the western Bering
Sea will move inland Tuesday night. Strong and gusty winds in the
range of 45 to 60 mph are expected along immediate coastal
locations of western Alaska Tuesday night before winds gradually
weaken on Tuesday. Coastal flooding does not appear to be an issue
with this system due to the track of the low and the fact that it
will be weakening by Wednesday. Models are hinting at some
tropical moisture being advected northward which should help
create some decent rain/snow as the front moves inland. Warmer air
will also be advected northward so while precipitation may begin
as snow, it should quickly transition to rain by Wednesday
morning. The period of below normal temperatures has ended and
above normal temperatures are expected for the second half of the
work week.

&&

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

The calm before the storm is being observed across the western
Bering Sea as high pressure has situated across the region. At the
same time, cold air from the Arctic continues to funnel over the
eastern Bering Sea with scattered to numerous snow showers being
observed. The low pressure system expected to bring hurricane
force winds gusts to much of the Bering Sea is beginning to take
shape near the Kamchatka peninsula and is now beginning to show
on satellite imagery. By early this afternoon, a strengthening
frontal boundary will accompany the rapidly developing cyclone as
it moves into the southwest Bering Sea. A High Wind Warning
remains for the western and central Aleutians starting early this
afternoon and continuing through 7 am Tuesday for winds in the 75
to 80 mph range.

Models have been trending slightly stronger with the low pressure
system over the past 24 hours which causes the low to reach peak
intensity later than originally expected. This should help keep
the front from stretching out and shearing apart before it
transitions into the eastern Bering Sea. That being said,
confidence is starting to increase that hurricane force winds
gusts may be observed across the Pribilof Islands, the eastern
Aleutians and the Alaska Peninsula. For the eastern Aleutians
(dutch harbor), it looks like a modest stable layer around 6kft
to 8kft could help produce some rather impressive mountain waves
which could create some nice gusts spreads. Winds will likely be
in the 45 to 60 mph range with occasional gusts upwards to 80
mph. For the Alaska Peninsula, there is a lot of southeast/northwest oriented
gaps for the winds to channel through which could create some
localized very gusty winds. The Pribilof Islands are the hardest
to predict due to the lack of terrain but if the front holds
together, they could see a few hurricane force wind gusts. High
wind watches have been issues for all of these locations for
Tuesday.

&&

Long term forecast (days 3 through 7...Wed through sun)...
an active pattern is expected to continue during the long range
forecast through next weekend. The focus remains on a very strong
low tracking through the Bering and southern Mainland through the
end of the week.

The leading warm front associated with the remnants of typhoon
lan will track eastward through the eastern Aleutians, Alaska
Peninsula, and southwest Mainland into Wednesday. This will begin
to usher in warmer temperatures over the eastern Bering and
southwest Mainland by Wednesday morning, and build over much of
the southern Mainland through the end of the week. This system
will also bring some moderate to heavy precipitation to these
areas, as well as strong gusty winds. A mixture of rain and snow
is expected as the dominant precipitation type along the Aleutians
and Alaska Peninsula given the warming southerly flow and heavy
marine influence. However, the southern Mainland will start off as
snow early Wed and transition to rain or a rain snow mix along
the Gulf and southwest coast Thursday. There is a possibility that
colder areas in higher terrain such as the northern Susitna Valley
and Thompson Pass could remain snow longer and accumulate
significant amounts. Models remain in good agreement through the
later half of the week regarding this system, but confidence
begins to wane as the models begin struggling more with the track
and timing as the low center tracks into the eastern Bering on
Thursday.

&&



Afc watches/warnings/advisories...
public...High Wind Warning 187 191.
High wind watch 181 185 195.
Marine...Storm Warning 155 165 170 172-179 185.
Gale Warning 130 131 171 180 181.

&&

$$

National Weather Service Glossary of Abbreviations