Scientific Forecaster Discussion
fxak68 pafc 221301
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
501 am akdt sun Oct 22 2017
Analysis and upper levels...
the cold, vertically stacked parent low over the West Coast
dominates the pattern as it continues to sink south. As it does
so, it is dragging plenty of cold air from the Bering Strait all
the way into the Cook Inlet region. Several individual pieces of
energy embedded within the cold air are moving counter-clockwise
around the low as well. The first wave was the one that brought
snow to much of the Cook Inlet and Prince William Sound area on
Saturday. This feature is moving off to the north and east as it
falls apart. However, in its wake, there are still quite a few
snow showers courtesy of southwest flow aloft and some low
stratus, especially over the Kenai Peninsula. The story is much
the same across southwest Alaska. Where snow showers are not occurring,
low stratus/fog is trying to fill in. Meanwhile, northerly flow
is making for widespread cold air advection showers across the
entire Bering. A potent 155-165 kt remains well south of the
state near 50n. While it has been pretty much zonal (west to east)
the past week or so, it is starting to show some signs of
amplifying over the western Bering.
models are having fits with this pattern. This is resulting in
low confidence in the forecast beyond 24 hours over Mainland Alaska.
The weak waves rotating around the parent low in the cold air are
proving very challenging for the models to resolve. The first
issue is with the continuing light snow across Cook Inlet this
morning. There is some uncertainty is how much longer it lasts,
but radar is trending weaker and weaker. The next feature will be
a low that moves inland over Bristol Bay tonight. The NAM wants to
paint this feature further west over Dillingham while all other
models hold it east near Iliamna. This forecast is based upon the
further east solutions. Then attention will shift to a low
tracking from south to north in the western Gulf early Tue. The ec
is much faster with this feature than the other models in how it
arrives in the northern Gulf, however, they all end up in roughly
the same spot. The ec was preferred with this. Then there is the
very deep low developing Mon over the western Bering. Thankfully,
models are in good agreement with this feature, so confidence is
much higher. But how this system tracks east and interacts with
the cold air over the Mainland remains hazy.
panc...light snow should continue to dissipate through the
morning. Lower ceilings could hang on for a few hours after it
ends, but visibility should readily improve to VFR. The rest of
the day will feature VFR conditions with a shift from southeast to NE
Short term forecast southcentral Alaska (days 1 and 2...sun and
mon)... the main upper level low will remain along the West Coast
of Alaska through Monday night. This will send a series of weak
upper level disturbances through the northern Gulf and
southcentral Alaska. The exact timing and strength of these
disturbances is not well agreed upon by the models, but the
general consensus is that they will be fairly weak and unlikely to
produce very much precipitation (if any). There is a chance of
snow inland this morning with the departing system that brought
snow yesterday, and this will be generally north of Anchorage.
Another weak system crosses the area later this afternoon into
tonight, with a low probability of snow. On Monday, precipitation
should spread across Kodiak, and some will spread inland Monday
night. Due to the model uncertainty, probabilities of
precipitation were generally kept in the chance to slight chance
Short term forecast southwest Alaska (days 1 through 3...sun
A weakening area of low pressure situated over the yk-Delta will
continue to help create showers along the southwest coast of
Alaska through Tuesday morning. Models are starting to become more
aggressive with a series of shortwaves rotating around this low
pressure system that will impact the Bristol Bay area Monday and
early Tuesday. These waves are expected to bring some light
snowfall to the Bristol Bay area with minor accumulations.
However, one stronger wave on Monday will develop into a closed
low and bring the potential for 3 to 6 inches of snowfall near
Iliamna by Tuesday morning. The snowfall will result from a
deformation band that sets up over the region but as typical
models are struggling on the placement of this feature. At this
moment, model spread varies from Dillingham to Iliamna but models
are clustering on the Iliamna solution. A Winter Weather Advisory
was considered for this system on Monday but with poor confidence
at the moment the decision to not issue with the morning package
was justified. Otherwise, the cold pattern will continue with a
warm up expected by midweek.
Short term forecast Bering Sea/Aleutians (days 1 through 3...sun
High pressure has continued to build across the western Bering Sea
this morning but the period of calmer weather will be rather short
lived. An extratropical low pressure system will develop near the
tip of Kamchatka tonight as typhoon lan weakens over the western
Pacific. While typhoon lan is expected to dissipate over the
northwestern Pacific Ocean on Monday, a lot of the moisture and
energy associated with the low will be absorbed into the
extratropical low moving along the Kamchatka peninsula. The
cyclone near Kamchatka is expected to undergo rapid cyclogenesis
(drops from 979 mb Monday at 12z to 938 mb Tuesday at 12z) as it
moves into the southwest Bering Sea Tuesday morning.
Ahead of the low will be a very impressive front that rapidly
occludes over the Bering Sea. Widespread 50 to 60 kt winds are
expected over the western two-thirds of the Bering Sea Monday and
Tuesday with gusts up to 75 kts over the far western Bering Sea.
A high wind watch has been issued for the western and central
Aleutians where winds upwards to 80-85 mph are possible. The
strongest winds will likely be east of Shemya but a subtle change
in low track will change the frontal orientation and bring
stronger winds into Shemya. On top of this, seas of 25 to 35 feet
will be common in the western Bering on Tuesday while seas upwards
to 45 to 50 feet could be seen on the Pacific side of the Aleutian
Islands on Tuesday. This system should be closely monitored in
the coming days.
Long term forecast (days 3 through 7...from previous discussion)...
an active pattern is expected to continue during the long range
forecast through the middle of next week, with the focus being on
the remnants of typhoon lan entering the Bering and upper level
disturbances over the Gulf tracking into southcentral.
The moisture rich and storm force remnants of typhoon lan will
push into the western Aleutians/Bering Monday evening in the form
of a warm front, also bringing some warmer temperatures and rain
to the area through mid-week. Models remain in good agreement up
until this point, but confidence begins to wane as the models
begin struggling more with the track and timing of the system
central and eastern Bering through Thursday. Thus giving US less
confidence on the timing, placement, and strength of any potential
wind threats and precipitation over the Aleutians and Bering as
the leading front passes over. This system will eventually spread
gusty winds and rain over the southern Mainland Wednesday morning
through the end of the week.
The other features we are monitoring are a series of upper level
waves tracking through the Gulf and into southcentral Monday
evening through Wednesday. Given the cold air already in place
over the area, these waves bring potential for snow to make it
into southcentral, with potential for moderate to heavy snowfall
along the northern Gulf Coast, Susitna Valley, and Copper River
basin areas. The only caveat to that is that the models are
struggling with where and when these smaller systems develop and
when they eventually track.
public...high wind watch 187 191.
Marine...Storm Warning 178 411.
Gale Warning 177.