Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
405 am akst Friday Dec 19 2014
Analysis and upper levels...
a long-wave trough encompasses all of Alaska with the subtropical
jet suppressed well to the south over the North Pacific
(generally between 30n and 50n latitude). North of the jet there
are a couple cut-off lows impacting the forecast area. One low is
nearly stationary just south of the Gulf of Alaska. An upper level
short-wave and surface low embedded in the circulation around this
low are tracking westward across the Gulf and toward Kodiak
Island. Middleton Island radar shows a band of precipitation crossing the
northern Gulf and moving into Prince William Sound. Dry offshore
flow has dominated the past 12 hours...so it will take a bit to
saturate the low levels and any precipitation that does fall will be
The second cut-off low is located just south of Adak along the
central Aleutians. It is moving very slowly to the east while the
occluded front lifts northward into the Bering Sea. Colder air
moving down the back side of this system has changed precipitation to
snow along the western Aleutians...but everywhere else will remain
warm enough for all rain.
satellite imagery and a scatterometer pass at around 06z shows the
GFS is notable too far south and too weak with surface low crossing
the Gulf. The rest of the solutions are pretty close...perhaps just
a tad too far south. All solutions are now in lock step with the
Aleutian low...but differences quickly show up with the many lows
strung out across the North Pacific and Gulf of Alaska the next
couple days. In particular models are all over the place with a
stronger low moving into the eastern Gulf Friday night and
rotating up toward southcentral Alaska Saturday. This will greatly
affect the wind forecast over the Gulf and precipitation forecast for
southcentral. For now will go with the majority of solutions which
weaken this low over the northeast Gulf with little or no precipitation
spreading westward to Prince William Sound.
Forecast confidence is below average along the southern tier of
the forecast area close to where the surface lows are tracking.
Elsewhere forecast confidence remains fairly high.
Short term forecast southcentral Alaska (days 1 through 3)...
portions of Prince William Sound will see a brief shot of precipitation
this morning...with low level dry air limiting how much actually
reaches the ground. Other than that it looks like the various
weak lows rotating around the Gulf through Sunday will largely
remain to the south. Thus...will back off probability of precipitation for this area. The
rest of southcentral Alaska will remain dry with just some high
clouds. Pressure gradients along the north Gulf Coast will have a
few ups and Downs as lows pass to the south...so expect occasional
gusty gap winds...but nothing very strong.
Short term forecast southwest Alaska (days 1 through 3)...
the Alaska Peninsula may occasionally see a few showers associated
with lows rotating around the Gulf of Alaska. Otherwise...dry
conditions will persist across the region through the weekend. A
short-wave ridge building ahead of a trough moving out of Asia
will help amplify the upper level flow this weekend allowing
colder air to gradually make its way down from the north. Expect
temperatures to drop a little more each day. By Sunday high temperatures
across much of the region will be below normal...with widespread
single digits from the Kuskokwim Delta to the Kuskokwim valley.
Short term forecast Bering Sea/Aleutians (days 1 through 3)...
the low along the Aleutians will be slow to depart...but as it
heads south into the Pacific Saturday night a pattern change will
commence. Colder air will first work its way southward across the
Bering Sea and Aleutians leading to an increase in showers and a
transition to a predominant precipitation-type of snow for most areas.
This will be short-lived as a dynamic upper level pattern sets up
early next week...bringing more active weather to the Bering Sea.
Long term forecast (days 4 through 7)...
the big story next week is the deep trough moving out of Asia
and into the Bering Sea...supported by a 200+ knot upper level
jet. Numerical guidance continues to show rapid development of a
deep surface low. However...run-to-run continuity in the track of
this low is poor. The latest GFS/European model (ecmwf) solutions track the low
into The Heart of the Bering Sea where it stalls out. Some prior
solutions along with the latest parallel GFS track the low much
farther west...closer to the Kamchatka peninsula. With such a
strong upper jet the further east solutions seem to make more
sense...but since this is still several days out the more prudent
decision is to stick with the current forecast which depicts a
track over the far western Bering. No matter what the ultimate
track of the low expect much more active weather to return to the
Bering Sea early next week and spread eastward to Mainland Alaska
by Wednesday or Thursday. The track of the low will have significant
impact on how much warm air moves into Mainland Alaska next
week...but with a fairly cold air mass in place prior to arrival
of precipitation expect snow to be more widespread than it has
There is one feature of interest out ahead of the trough moving
out of Asia. As a short-wave ridge builds ahead of that trough
models have been indicating a short-wave over eastern Siberia
diving southward down the east side of the ridge into the Bering
Sea Sunday night and Monday and then lifting northeastward across
Mainland Alaska on Tuesday. This trough could interact with any
remaining surface low over the Gulf to produce precipitation over
southcentral. The air mass in place would likely be cold enough
for all snow except along the Gulf Coast. With model differences
in the amplitude of the upper trough and uncertainty about whether
there is still a low in the Gulf and where it might be...forecast
confidence is low. Still...this is Worth keeping an eye on as we
get closer in time.
Marine...Gale Warning 119 120 130 131 132 136 137 138 177 178.
Heavy freezing spray warning...180 181 185.
Synopsis and model discussion...seb
southwest Alaska/Bering Sea/Aleutians...seb