Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
440 PM akdt Monday Jun 29 2015
Analysis and upper levels...
there is an upper low centered just to the south of Kodiak Island.
Cyclonic flow from this low covers most of southern Alaska. A few
vorticity maximums rotating (counter-clockwise) around this low
are having an impact on the weather. One of these is over
southwest Alaska and is helping to kick off convection this
afternoon/evening. Another is over south central Alaska...with yet
another to the east of Kodiak Island. There is a blocking ridge
over the the central Bering with the surface high centered over
the Pribilof Islands. There is a low west of the Aleutians with a
front very slowly pushing through the western Aleutians. There is
a surface ridge along the Gulf Coast...and the thermal trough is
the numerical models are in fairly good agreement through the
short term portion of the forecast (60 hours model time...which is
Wednesday afternoon). They begin to diverge a bit on the
locations of two surface lows beginning Wednesday morning...but
nothing to be too concerned about. One of these lows is moving up
from the south into the Gulf of Alaska. The other is a low that is
pushing eastward just south of the Aleutian Islands...cutting
underneath the Bering ridge. Forecast confidence is near normal.
for southcentral Alaska...isolated wet thunderstorm activity will
be the main fire weather concern through Tuesday evening...with
the primary activity remaining confined to the Alaska Range...as
well as the Talkeetna and Wrangell Mountains. Any thunderstorms
that form along the Wrangell Mountains today have a chance to
progress northwestward given the easterly steering flow. This
could push a few of these wet thunderstorms over the lower
elevations of the Copper River basin this evening. Gap winds will
continue to develop this evening along Turnagain Arm eastward to
the Copper River...with winds also stretching into South/West
Anchorage and the Matanuska Valley this afternoon/evening. These
gap winds will persist through Wednesday...generally weakening as
we move further into the week. Relative humidity values along the Copper River
will also dip into the low to middle 30s during the strongest winds
this evening...however with mostly cloudy skies overhead
conditions should remain above red flag criteria.
For southwest Alaska...lingering moisture from the persistent
upper level low pressure system keeps fire weather concerns at a
minimum. The main concern is any lightning over the Kuskokwim
Mountains and Alaska Range the next couple of afternoons...any
thunderstorms should be of the wet variety. The upper level system
will slowly exit to the east through the week...being replaced by
a slightly cooler system from the northwest.
Short term forecast southcentral Alaska (days 1 and 2)...
an upper low near Kodiak will continue to keep broad cyclonic
flow across much of southcentral Alaska through Wednesday afternoon.
A weakening surface low under the upper low center will continue
to produce a series of weak weather fronts...which are expected to
track northward across the Gulf and into the northern Gulf Coast.
This will keep wet weather in the forecast for areas surrounding
the Gulf Coast...and more showery conditions are expected over
the inland areas. The positioning of the thermal trough continues
to shift northward and weaken each day...along with marginal
instability lingering over the southcentral interior through
Tuesday evening. This continues the potential for some isolated
wet thunderstorms to develop over the Copper River basin and
Susitna Valley through Tuesday evening. The main focus areas will
be along the mountains (wrangell/Talkeetna/AK range)...though an
easterly steering flow in the Copper River basin can carry some of
the isolated wet thunderstorms over the valley this evening. When
the thermal trough weakens and slides north tomorrow...the
steering flow becomes light southerly...which would keep most of
the thunderstorm activity confined to areas along the mountains.
Locally gusty gap flows are expected across southcentral in the
afternoon and evening hours through Wednesday with higher pressures
along the coast and lower pressure with the thermal trough inland.
Short term forecast southwest Alaska (days 1 and 2)...
the Mainland is going to be affected by a couple of upper level
systems. The same pesky system aloft still spins south of Kodiak
Island providing just enough cold air and forcing aloft to keep
diurnal shower activity with mountain enhancement. Expect showers
and thunderstorms along the Kuskokwim Mountains through the Alaska
Range with activity maximizing in the afternoon and evening hours.
The Kuskokwim Delta should remain dry but there is a slight chance
for showers on Wednesday afternoon with an incoming trough from
the northwest. Thunderstorm activity will remain mainly along
higher terrain as a deformation zone sets up between the two
systems along with a thermal trough at the surface. Wednesday
afternoon most of the activity will shift toward the Alaska Range.
Expect a continuation of slightly above normal temperatures and
increasing cloudiness in the afternoon and evening hours.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
the story today...marine layer stratus continues to dominate...or
Summer in the Bering Sea. High pressure remains in control over
most of the Bering Sea with offshore flow near the Mainland
bringing the only clear areas. A stalled frontal boundary over the
western Aleutians will strengthen as a disturbance approaches from
the northwestern Pacific. This front will bring steady rain and
rotate over the central Aleutians the next couple days.
Long term forecast (days 3 through 7)...
Wednesday night another round of convection is expected across the
southern Mainland as the upper low in the Gulf sends shortwaves
as it picks up northeasterly momentum. A weak surface trough will
be a focus for thunderstorms across the western Alaska Range and
Bristol Bay. To the west...ridging continues to keep low stratus
and fog across the Bering into the weekend. The ridge flattens
slowly on Friday as low pressure works another front into the
The challenge heading into the weekend is the pattern shift with
the upper level systems over the Mainland. There is potential for
another stretch of warm days to start on Friday as an upper level
ridge builds from the Bering Strait. The GFS is the most
aggressive model on the warm pattern...while the European model
brings fronts into the northern Gulf which would suppress the
ridge dominance. With this much uncertainty between the
models...the long term forecast was influenced by the conservative
European model (ecmwf). Cool and cloudy conditions continues across the
Aleutians/Bering through the beginning of next week with periods
of light rain.
Synopsis and model discussion...bl
southwest Alaska/Bering Sea/Aleutians...mtl