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Southcentral and southwest Alaska forecast discussion 
National Weather Service Anchorage Alaska
451 am akst Monday Dec 22 2014

Analysis and upper levels...

A weak low near Prince William Sound continues to spin snow and
rain showers this morning as it drifts north. Over the southwest
portion of the state...a weak and mostly dry shortwave is sweeping
through the area. Further out front is weakening over
the western Bering while another is strengthening over the North
Pacific and driving towards the western Aleutians. This
strengthening front is riding a 180 knots North Pacific jet from
southwest to northeast. This jet will help to deliver the biggest
weather maker to the area that we will see over the next few days.
A low is developing off the Kamchatka this morning and will
rapidly deepen to around 955 mb as it rides the jet through the
western Aleutians into Tuesday morning. The jet then weakens as it
parallels the Aleutians for a bit before diving down to around 35n
south of the akpen. It then makes a very sharp turn toward the
north over top of the North American ridge and barrels into the
Panhandle coast near haida gwaii at around 140 kts.


Model discussion...

Tough work cut out for the models in this forecast package. With
the deepening low out west...the models are in good agreement for
only about 24 hours through Tuesday morning...then diverge quite a bit
on the placement of the low center. This is not surprising
considering that it is prognosticated to be a rather strong low coming
from a data sparse region. The ec continues to be the furthest
east and elongates the system the fastest while the NAM is now the
furthest west. Discrepancies continue to arise through the week as
this low will dominate much of the pattern for the region. All
models show the initial warm front associated with this system
spreading precipitation into Bristol Bay and Kodiak Island by the
middle of the week. Problem is that the ec solution would have it
tracking up Cook Inlet by Wednesday morning while the American
models are essentially 24 hours later. We did lean towards the ec
with this package...but mainly only through Tuesday as forecast
confidence was low beyond that.

For southcentral for the next 24 to 36 hours...several small
features are making for a tricky forecast. The low near Prince
William Sound is filling as it lifts north and wraps some moisture
into the Anchorage-matsu area this morning. A broad upper level
trough remains over the northwest portion of the Mainland which is
trying to help provide some additional convergence. Then the
remnants of an upper level low near Kodiak is forecasted to move
right through the area later today. Couple all these small
features with weak flow aloft and the result is a forecast based
on a model consensus with a good amount of forecaster discretion.
Either way...this should not be a big weather maker with the most
likely impact being some snow bands setting up and putting a few
inches of snow in localized areas...very similar to Sunday.


Short term forecast southcentral Alaska (days 1 and 2)...
the forecast remains pretty low confidence over the next couple of
days due to continued poor model handling of key features.
Overall flow will be southerly bringing in moisture from the Gulf.
That combined with instability over the area will result in
continued showers through tonight. These showers will be light for
the most part but there is a possibility of some banding that
would result in small areas seeing higher amounts. Things will
start to lighten up later on Tuesday as weak ridging moves in over
the area.


Short term forecast southwest Alaska (days 1 and 2)...
the western Bering Sea will have a storm force low tracking into
the Bering Sea by late Tuesday evening. An associated weather
front from the low pressure system will approach the southwest
coastline by early Wednesday morning. The winds will shift from
light offshore to onshore flow as the front moves inland through
Wednesday. This will bring warmer temperatures throughout
southwest Alaska. The warmest temperatures will be along the
coastline in the middle 20s to middle teens inland. The precipitation
will predominantly be snow due to cold temperatures already in
place in the region. There is a slight potential for light
freezing rain with the warm air advection aloft moving over the
colder temperatures at the surface.


Short term forecast Bering Sea/Aleutians (days 1 and 2)...
an upper level closed low moves into western Aleutians by early
Tuesday morning. This synoptic feature will be reflected at the
surface with a storm force low positioned over Shemya. The
associated weather front will extend from the low pressure
southward between Adak and Atka into the Pacific Ocean. This
system will track into the western Bering Sea and track just west
of Shemya by Wednesday evening before transitioning to an occluded
low pressure system. This will bring warmer temperatures with
precipitation starting out as a wintry mix before transitioning to
all rain with the warm air advection into the western and central
Bering Sea through Wednesday. This strong system will bring very
windy conditions through the western Aleutians before spreading
into the Bering Sea. There will be a high wind watch issued this
morning for the western Aleutians from Shemya eastward into Adak
for Tuesday morning through Tuesday afternoon. The aforementioned
weather front located through the western Aleutians will track
eastward before moving through the Pribilof Islands and extend
southward just west of Unalaska by Tuesday afternoon. This weather
front will reach the southwest Alaska coastline by early Wednesday
morning before moving inland and dissipating. This weather front
will have a swath of gale to storm force winds along and ahead of
the front as it moves through the Aleutian chain and through the
Bering Sea.


Long term forecast (days 3 through 7)...
the main feature impacting the region Wednesday into Thursday
will be the front associated with the storm force low over the
western Bering and a triple point low that forms along the front
and then moves into the Gulf. The biggest forecast challenge with
these features will be the aforementioned timing issues with the
ec/Gem solutions running well ahead of the GFS/NAM. Have a slight
preference towards the faster ec solution for now...but will have
to continue to closely monitor the timing of these features over
the coming days.

Generally showery conditions will then persist across the region
into the weekend in the wake of the front as a longwave trough
remains centered over the western Bering and a series of
disturbances track towards the Gulf in broad south to
southwesterly flow. Guidance is hinting at another deep North
Pacific low moving into the Bering towards the end of the
week...albeit with significant discrepancies regarding placement
and timing. Despite these differences...this would generally
result in another round of windy and rainy conditions across the


Afc watches/warnings/advisories...
public...high wind watch 187 191
marine...Storm Warning 175 176 177 178 185
Gale Warning 155 165 170 171 172 173 174 179 180 181
heavy freezing spray 180 181
fire weather...none



Synopsis and model discussion...mso
southwest Alaska/Bering Sea/Aleutians...pld
long term...cumulonimbus

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