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fxak68 pafc 241408 

Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
508 am akst Fri Feb 24 2017

Analysis and upper levels...
the "waiting" phase is upon US. A modest shortwave is moving over
southwest Alaska with an associated surface cold front pushing right
up against the Alaska Range. A 130kt jet is providing the upper-
level support to this system. There is a healthy amount of low
and mid- level moisture out ahead of it as is evidenced from
looking at the radar and widespread low stratus across Cook Inlet.
Now we are just "waiting" for these ingredients to work together
to start producing some snow across the area. It has been snowing
much of the morning near Iliamna and near Talkeetna. Satellite
does show better lift now starting to surge through the Barren
Islands area, so it should only be a matter of time. The other
interesting component this morning is the warm "nose" just off the
surface. 800 hpa temperatures were able to climb above freezing
yesterday and some of that warm air has hung on into the morning.
That said, it could allow a little bit of rain to mix in with the
snow at precipitation starts.

Out west, yet another strong Bering low is blasting through
Shemya. This is bringing another round of storm-force winds to the
Aleutians and western Bering Sea. The system will peak in
intensity late this morning or early this afternoon with a central
pressure in the mid-upper 960mb range. Over southwest AK, warm air
riding over the colder surfaces has led to widespread fog. This
should burn off later today as some colder, drier air moves in.


Model discussion...
models are handling the synoptic pattern quite well. The few areas
of uncertainty are regarding timing of the onset of precipitation across
much of south central and what type it will be as it does start.
Thinking continues to be that it will start within a few hours and
should be mostly snow. The bigger question mark for the day
remains snow accumulation. Due to a slightly later onset and some
residual warm air, we have backed off snow amounts across
Anchorage a bit. There remains a chance that areas of the
Matanuska Valley could still see a bit more than forecasted,
especially near Hatcher Pass, if all the dynamics come together.


panc...low stratus will keep at least MVFR ceilings going through
the morning. There is some uncertainty on the start time of the
snow. But once it does start, we should see ceilings lower and
visibility drop to at least MVFR. If the snow intensity picks up,
visibility could easily drop to one half mile. The best chance for
those conditions would be through the late morning into early
afternoon. Drier, colder northwest flow will move in this evening
and should help to improve conditions to VFR. However, there is an
outside chance that some lower stratus could hang on through the


Short term forecast southcentral Alaska (days 1 and 2)...

The challenging part of the forecast is for the precipitation
around Cook Inlet and the mat-su valleys today. One uncertainty we
have been addressing the last few days has been precipitation type
and it looks like the system is beginning to tip its hand on
that. The 12z balloon sounding out of Anchorage showed a small
above freezing layer from about 300 to 1300 ft. Models show
temperatures aloft slowly cooling through the day as well. This
indicates that the predominate precipitation type should be snow,
at least from Anchorage northward today.
The challenge in all this is that surface temperatures are still
likely to get above freezing later today in spite of the cooling
temperatures aloft. There is still a chance for some brief
freezing rain early this morning while the surface is below
freezing and there is the warm layer just above the surface. By
this afternoon when temperatures get above freezing it looks to be
cold enough aloft, and heavy enough precipitation rates to wet-
bulb out the air column and keep precipitation type snow from
Anchorage north. As one GOES farther south in Cook Inlet, chances
of rain mixing in or changing to all rain increase to where it
will be likely all rain in the Homer area by late morning.

Yet another, very similar looking front is in store for the area
Sat night into sun. Initially, this system looked to be drier as
it came across the Alaska Range. However, models have trended more
aggressive with it with more upper-level support. Thus, we have
increased the chances for snow, mainly north of Anchorage, during
this period.


Short term forecast southwest Alaska (days 1 and 2)...

Temperatures will rise above freezing across much of the area
today as the front drifts east. Although some freezing rain is
expected this morning from Dillingham to Iliamna, precipitation
will generally be a mix of rain and snow, with snow more likely
in the northwestern portions of the Kuskokwim Delta. Temperatures
will again fall below freezing tonight beneath a building ridge,
with lingering snow activity near the mountains and fog near the
coast. On Sat a front will send another stream of moisture into
the Mainland with snow and rain, that becomes all light snow sun
into Mon.


Short term forecast Bering Sea/Aleutians (days 1 and 2)...

A series of fronts will bring gale to storm force winds with snow
and rain to the Bering and western Aleutians into Sat. Building
high pressure will then spread cold air over the area as winds diminish
through sun.


Long term forecast (days 3 through 7)...
the long range forecast transitions late weekend, returning colder
conditions across the southern Mainland and much of the Bering as
the upper level jet flattens as it sags southward on Sunday. A new
upper level wave pattern develops on Monday with ridge amplifying
along the west Bering. Cold air advection along the Bering spreads
east across the Mainland and Gulf through the middle of next
week. Models are in relatively good agreement that a closed low
forms along a deepening wave Monday into Tuesday. This pattern
sets up to bring multiple waves from the Arctic to the Gulf
through much of the next week which brings continued chances for
snow for the Mainland. To the west...ridging will bring a quieter
weather pattern over the western areas of the Bering dominated by
low clouds and period showers with generally small craft winds
along the western Aleutian chain.


Afc watches/warnings/advisories...
Marine...Storm Warning 177 178 185.
Gale Warning 120 131 170 173 175 176 179 181.



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