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fxak68 pafc 270101 

Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
501 PM akdt Thu Apr 26 2018

Analysis and upper levels...
there is a large upper level low centered just west of the Bering
Strait. An upper level short-wave low with closed circulation is
just to the south of this low and moving rapidly eastward. The
associated surface low is nearly vertically stacked, and has a
front extending southward through the pribilofs to the eastern
Aleutians. An upper level trough extends across southwest Alaska
into the northern Pacific just west of Kodiak Island. This
feature is moving northeast and weakening. There is an upper level
ridge over the central and western Aleutians extending into the
south central Bering, and another ridge over British Columbia. An
eastward moving upper low is near the Kamchatka peninsula, with
its associated front approaching the western Aleutians. Showery
weather is evident across most of the southern Mainland, with more
stratiform precipitation near the pribilof/eastern Aleutian front.


Model discussion...
the models are in surprisingly good synoptic agreement through
the short term portion of the forecast (saturday afternoon)
considering the rather progressive pattern. The models do differ
somewhat on precipitation. This is most notable with the more
showery, convective driven precip. Forecast confidence is thus a
little above normal.


panc...VFR conditions will prevail, though there could be very
brief periods of marginal conditions as heavier showers move
through. Ceilings will be challenging on Friday with a persistent
southeasterly turnagain wind in the low levels and south to
South-West flow aloft. Will the southeast flow be enough to keep
steady precip and lower ceilings just to the west of the terminal?
Have largely decided yes on this, keeping a low end VFR ceiling in
for the first half of the day. Ceilings will then drop as steady
rain develops right ahead of the upper trough around 00z. There is
a chance precipitation could mix with or change to all snow as
colder air arrives with the trough. This would produce lower


Short term forecast southcentral Alaska (days 1 and 2: Friday and
numerous showers have redeveloped this afternoon across portions
of southcentral. This is largely the result of both sufficient
atmospheric moisture and decent instability as evidenced by the
00z Anchorage sounding. With plentiful cold air at the mid and
upper levels, this has allowed the stronger showers to produce
graupel and small hail at times, with freezing levels under 2,000
feet. As the upper level flow becomes more linear with the loss of
daytime heating tonight, the showers will diminish once again
this evening. A lack of strong forcing near the area today should
make this afternoon the weakest the winds will be the next couple

On Friday, the first of two upper level atmospheric disturbances
moves across the area. For the Anchorage bowl and Knik River
Valley, along with other gap-wind-prone areas, this will be
countered by the continued downsloping from the various gap winds.
Thus, the precipitation forecast is especially tricky in these
areas. Elsewhere, the moist southwest flow and additional moisture
being advected into the area by the disturbance itself will set
off another round of afternoon showers again. The stronger forcing
may be enough for the strongest showers to set off a few rumbles
of thunder at times, though no such wording has been added to the
forecast due to the expected isolated nature of any thunderstorms.
As has been the case the past several afternoons, the stronger
showers will be capable of producing small hail. The disturbance
will move towards the northeast, so the Kenai Peninsula will see
the steadier precipitation activity in the early afternoon, then
will move northeastward into Anchorage and the mat-su valleys in
the afternoon and evening.

Behind the wave Friday night, warming upper level temperatures
will greatly diminish the overall atmospheric stability. Along
with a lack of solar heating should see most areas going
precipitation-free, with at least some areas getting the clouds to
clear as well.

On Saturday, a low will move into the Kuskokwim Delta in the
morning, continuing inland through the day. This storm track will
further invigorate The Gap winds through Turnagain Arm and the
Knik River Valley. The winds through Turnagain Arm are very
likely to impact Anchorage once again with the low tracking north
of the area. In addition, the associated front will bring yet
another round of precipitation to much of southcentral, which in
turn will set off the perennial battle between the drying
downslope winds verses the moist southwest flow above them
supporting rain. Warmer air aloft should help to diminish the
chances anything other than rain is observed.


Short term forecast southwest Alaska (days 1 and 2)...
showery conditions along the southwest Mainland will persist
through tonight when another shortwave tracks into the area from
the northeastern Bering. This will bring another quick round of
snow to the area through the early morning hours. By mid-morning
conditions will transition back to showers with some shortwave
ridging passing over the area through Friday afternoon. However,
right on the heels of this ridging is the next system tracking
inland from the Bering on Friday night. This front will pull up a
lot of warm air from the south, which will help transition much of
the snow to rain as it pushes inland Friday night through Saturday.


Short term forecast Bering Sea/Aleutians (days 1 and 2)...
a front from the North Pacific begins to push into the western
Aleutians/Bering tonight, forcing the weak ridging over the area
eastward over the central/eastern Bering and Aleutians through
Friday afternoon. The front is pulling up a significant amount of
warm air and will primarily bring rain as it tracks across the
Aleutians through Friday. By Friday evening it will have made it
to the southern Mainland leaving the Bering and Aleutians in broad
westerly flow through Saturday. The next North Pacific system
approaches the western Aleutians from the southwest Saturday
morning, and will push through to the central Bering/Aleutians by
Saturday evening.


Long term forecast (days 3 through 7)...
an active pattern continues at the start of the long range
forecast beginning Saturday night. This period starts out with a
weakening front slowly tracking west to east across the southern
Mainland with another storm system swiftly moving across the
Bering to the southwest coast. This pattern keeps higher
precipitation chances across the southern Mainland and surrounding
waters through the weekend. Yet another low moves across the
Aleutians on Monday staying on a more southerly track, but pushes
across the southern Mainland and Gulf by Tuesday.

The jet stream pattern shifts from zonal flow along the Aleutians
and Gulf to an amplified which should change up the speed and
track of the storm systems heading into the middle of next week.
Models continue to struggle with the extended forecast leading to
a low confidence and conservative edits for Sunday through
Wednesday. A blend of the operational models and ensembles were
used to make any updates necessary.


Afc watches/warnings/advisories...
Marine...Gale Warning 150 155 165 170 172 173 174 175 176 177 178



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