Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 231243
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
443 AM AKDT Tue Oct 23 2018

.ANALYSIS AND UPPER LEVELS...

Low pressure in the eastern Gulf is intensifying this morning as
it moves northwest into the northern Gulf. This is quite evident
on IR satellite with imagery overnight showing not only cooling
cloud tops but a transition from a developing baroclinic leaf to a
more mature storm with the textbook comma head. This storm is also
getting plenty of upper-level support, as it sits underneath the
left exit region of a meridionally amplifying 140kt jet streak.
Widespread rain has developed from Prince William Sound south and
east along the Alaska Panhandle overnight along the associated
warm front. This area of precipitation is lifting to the northwest
as the low tracks closer to the Southcentral coast. Buoy
observations and ASCAT scatterometer winds also show widespread
gales with this system along with storm-force winds wrapping
around the center of circulation.

Farther west, a trough extending across the AKPEN is allowing
rain to persist across the Bristol Bay region this morning. Mid-
and high- level clouds are also filling in over the Southcentral
and Southwestern interior as the mid-level ridge lifts north in
advance of the low in the Gulf.

A second area of low pressure situated over the eastern Bering is
also slowly gaining strength as it drifts southeast toward the
Pribilofs. This system is bringing showers and gusty northwesterly
winds to the central Bering and the Pribilofs, and is helping
reinforce the colder air advecting southward from the northern
Bering.

&&

.MODEL DISCUSSION...

Models remain in decent agreement with the evolution of the
surface low in the eastern Gulf and with its supporting upper-
level dynamics. Guidance has also come into better agreement with
the timing and evolution of the low, bringing the onset of winds
and precipitation across Southcentral about three hours earlier
than suggested in yesterday`s model runs. Given the consistency of
this acceleration in the most recent individual runs and overall
run to run consistency between models, this adjustment to the
timing has been reflected in the latest forecast package. There is
still some uncertainty in the timing and placement of the second
Gulf low on Thursday. Overall, the GFS was favored due to its run
to run consistency. The 00z NAM appears an outlier given its more
northerly track along the coast. However, the latest NAM run (06z)
has aligned better with the GFS. The EC and Canadian guidance
looked to be too fast with this second system, but both did favor
a more southerly track for the low, similar to the GFS.

&&

.AVIATION...
PANC...VFR conditions and light northerly winds are expected to
continue through mid-morning. By late morning, MVFR conditions
will develop as rain moves over the airport complex reducing
visibility and dropping ceilings below 5,000ft. Gusty northerly
winds will also develop by mid-morning prior to and in association
with a frontal passage. Behind the front, the gusty winds will
turn to the southeast and continue through the evening and
overnight hours before diminishing. MVFR conditions will also
persist with ceilings remaining at or below 5,000ft through the
evening as showers develop behind the front.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The biggest item to discuss with this morning`s forecast is the
strong low in the northern Gulf is moving faster and slightly
farther south than previous forecast package. This is bringing in
the precipitation and winds sooner this morning for most of the
area. The more southerly track is now just south of Seward and
then near Kalgin Island and then over the Alaska Range as opposed
to moving over Turnagain Arm and over Anchorage.
This track will lessen rainfall for Anchorage and areas farther
north as there will be more cross-barrier flow with this system
than it appeared yesterday morning when the low was expected to
track nearly over Anchorage.

Snow will be mostly relegated to the Copper River Basin and
especially in the passes. A winter weather advisory for the snow
remains in effect today for the Richardson Highway south of the
Edgerton Highway cutoff.

After today, the forecast is basically on track with (you can stop
reading now if you have heard this before) a break between systems
tomorrow before the next strong low moves out of Southeast Alaska
and across the Gulf to the Kenai Peninsula by Thursday night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

A surface low in the Gulf pushes a front across the Alaska and
Aleutian Ranges later this morning with a northeast track across
the Kuskokwim Valley through the afternoon. Northerly flow shifts
to the south as the surface low moves to Norton Sound tonight...although
winds across the Kuskokwim Delta will prevail out of the west through
Wednesday night. Winds shift again to the north for Thursday as
the next low approaches from the Gulf. Gusty winds develop across
the Kuskokwim Delta during the afternoon as a front diving across
the eastern Bering squeezes the pressure gradient.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

Low pressure across the eastern Bering slides south of the Alaska
Peninsula late today. The shortwave aloft driving the low will
initiate scattered to numerous showers as it moves across the
southern Bering. A strong wave of energy then propagates across
the western Bering late tonight, ushering a colder air mass
toward the Central Aleutians. Northerly flow increases across the
western half of the Bering in response, with winds ramping up
across the central and eastern waters Wednesday through Thursday.
Wind and precipitation taper off on Thursday across the western
Bering with a ridge building across from the North Pacific.

&&

.LONG TERM FORECAST (Days 3 through 7)...
(Thursday through Monday)

A very challenging forecast is in store for us through the
extended period, as the models remain in modestly good agreement
with the overall pattern through Saturday. Thereafter, the train
goes off the tracks as solutions diverge about as much as they
can with really no semblance of agreement. This occurs as the
persistent pattern of a deep trough over the west and multiple
systems rounding the base riding north into Southcentral, which
we`ve had for what seems like a few weeks now, breaks down. This
will allow some potential for cooler Arctic air to try to make
inroads into our neck of the woods. However, given the huge spread
in the numerical guidance, trying to get too specific would be
problematic at best.

For the end of this week, the various forecast solutions agree
with each other, bringing another low northwestward through the
Gulf and across the Kenai Peninsula. As stated yesterday, this low
will be in a slow weakening state, but, it`ll still be in the
upper 960s to mid 970s mb range. This should still be strong
enough for some sustained gales across the northern Gulf and
periods of light to moderate rainfall, with a few pockets of heavy
rain possible on the south and east facing slopes, as orographic
lift helps maximize precipitation efficiency. The main challenge
with this storm is timing. The European (ECMWF) is the fastest of
the models, bringing precipitation into the Copper River
Delta/Valdez area Thursday morning, then spreading it westward
while reaching the Kenai/Anchorage Bowl/Mat-Su Valleys region by
late Thursday evening. Meanwhile, the GFS is about 12 hours slower
with the same precipitation pattern, and the NAM is about 18 to
24 hours slower. The timing has really not been in good agreement
over the last few model runs, so we know the rain is coming, just
when is the question. At this point we`re leaning towards the GFS
solution as it`s a good "middle ground" approach.

For Sunday and Monday, no changes were made to the forecast given
the pattern change and high degree of uncertainty with the model
spread. For now will just lean towards the ensembles (family of
models blended towards climatology), with just some minor tweaks
to POPs (probability of precipitation) and temperatures.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 141.
MARINE...Gale Warning 127 128 129 131 139 140 170 172 173 174 175
176 177 178 185 351 411 412 413 414.
Storm Warning 119 120 125.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...PD


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