Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
459 am akdt Friday Jul 3 2015
Analysis and upper levels...
an upper level closed low remains positioned east of the Seward
Peninsula. This synoptic feature with have shortwave troughs
rotating around the low and will push through the upper southwest
Alaska region and spillover into the matsu valley region. The
radar imagery is showing numerous showers moving eastward through
the upper Kuskokwim valley and approaching the McGrath area this
morning. The southwest Alaska region has weak onshore flow at the
surface and a 70 knot zonal jet streak through the Bristol Bay
interior. The satellite imagery is showing low stratus sitting
offshore in the Kuskokwim Bay and starting to gradually move
onshore. While...the Alaska Peninsula region through the Gulf has
a Flat Ridge of high pressure with another small ridge extending
into the eastern Copper River basin. The majority of the
southcentral Alaska region at the surface has weak offshore flow
and the central Gulf has southwesterly flow. The sport modis
nighttime microphysics satellite imagery is picking up the areas
of fog over the Kenai area and the matsu valley region. The
central Aleutians near Adak has a weak North Pacific low and a
secondary low south of the Dutch Harbor region this morning. This
is resulting in east to southeasterly small craft winds through
the eastern aleutains extending into the Bering Sea. The satellite
imagery is showing a large swath of moisture over the
aforementioned region with stratus and patchy fog streaming into
the central Bering Sea.
the models are in good agreement through the short term forecast
except for the ecwmf bringing the North Pacific low farther east
along the chain than the GFS/NAM models. The preferred model of
choice was the NAM for the southcentral Alaska region and the GFS
for the southwest Alaska/Bering Sea region. By Friday afternoon
the two North Pacific lows begin to merge together over the
central Aleutians. There will be a frontal boundary associated
with this system approaching the eastern Aleutians and the Kodiak
Island region. This system will move into the souhtwest and the
southcentral Alaska interior before dissipating over the weekend.
This will bring south to southeasterly winds through the Gulf and
onshore flow to the northern Gulf marine areas. The Turnagain Arm
and Portage/Whittier winds will strengthen with this frontal
boundary over the weekend before diminishing by late Saturday as
the pressure gradient weakens. This will bring rain and gusty
winds to the coastal marine areas with scattered showers inland.
The eastern Gulf will have the remains of the upper level ridge of
high pressure that will remain through the weekend.
the onshore flow over the southwest Alaska region will help to
produce a stable environment over the region. This is due to
different areas of offshore stratus beginning to move onshore
this morning. The satellite imagery is also showing areas fog in
the lower Kuskokwim valley. By Saturday a weather front will move
into the area. This will bring wetting rains and gusty winds
through Saturday before tapering off to scattered showers by
Sunday. The humidity values will climb well above any critical
fire weather levels through the weekend.
The southcentral Alaska region will have isolated wet thunderstorms
Friday afternoon/evening hours. These wet thunderstorms will
mainly be over the Alaska Range...Talkeetna Mountains and along
the Wrangell Mountains in the Copper River basin. These wet
thunderstorms will gradually move off the mountains and become
showers inland over the southcentral. The upper level ridge over
the region will keep the humidities fairly low but with the
arrival of the weather front by Saturday the humidities will
recover. The marine coastal humidities will see the largest
recoveries. The inland areas will have humidity recoveries just
not as much due to mountains keeping majority of precipitation
along the coast. The humidities will continue to rise above any
critical fire weather level through the weekend.
Short term forecast southcentral Alaska (days 1 and 2)...
Broad southwesterly flow aloft and minor short waves will provide
for the development of showers across southcentral this afternoon
and tonight. The positioning of the thermal trough...draped
across the Copper River basin and Susitna Valley...combined with
forcing from upper short waves and marginal stability indices
should allow for development of isolated afternoon and evening
thunderstorms over the northern and eastern Susitna Valley and
mainly mountains of the Copper River basin. Saturday should be
more stable overall as cooler air and abundant cloud cover spread
north across southcentral ahead of an approaching weather front.
A weather front moving north across the Gulf will bring rain to
Kodiak Island this evening. Rain will spread to the north Gulf
Coast and Kenai Peninsula Saturday. Minimum easterly gales are expected
with and ahead of the front as it moves north through the Gulf.
Brisk gap winds are expected across Turnagain Arm/Portage Valley
this afternoon and to a lesser extent the Copper River and Knik
Arm. Gap flows will increase across these areas on Saturday as
the coastal ridge builds.
Short term forecast southwest Alaska (days 1 and 2)...
a wet and windy pattern will be returning to southwest Alaska
starting late tonight as the front currently south of the Alaska
Peninsula will push northward into the region. Scattered showers
are expected along the lower Kuskokwim valley today as a weak
shortwave passes through the area...but otherwise partly sunny
skies with warmer temperatures are expected. Areas of dense fog
over the Kuskokwim Delta will dissipate by middle morning although
patchy coastal fog may continue through the day along the western
coast. As with previous days...areas of smoke are expected in the
overnight and early morning hours with the heaviest smoke in
valley locations near active fires.
The front will begin to impact the Bristol Bay region in earnest
early Saturday morning before rapidly moving northward to the
Kuskokwim valley/Delta by Saturday afternoon. There will be moist
southerly flow with this system...so the heaviest rainfall is
expected over the Ahklun Mountains with lesser amounts in the
leeward side of the Aleutian Range and the Kilbuck Mountains.
As this front pushes through...winds will begin to increase over
the western two-thirds of the area. Local gusts to 35 miles per hour are
possible starting this afternoon with the strongest gusts through
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
the front that is associated with the low near Adak has pushed into
the Bering Sea overnight while the next front...associated with
the low in the North Pacific...will begin to move into the Alaska
Peninsula this afternoon and to the Pribilof Islands by late
tonight. Rain and winds to near gale force are expected to
continue along the front as it moves northward through early
Saturday morning. With the deep southerly flow...rain will
continue over the Alaska Peninsula through Saturday night. By late
Saturday night...another front will approach the eastern
Aleutians and Alaska Peninsula...bringing more rain to the area.
Long term forecast (days 3 through 7)...
for the start of the weekend a ridge transits the Gulf and south
central suppressing rain except for along the mountain ranges of
the Susitna and and Copper River valleys where afternoon showers
develop along a trough that moves across the interior. To the
west...a low pressure system over the western Aleutian chain pulls
a front north from the Pacific. This front is expected to move to
the Alaska Peninsula and southern Gulf early Saturday morning. The
northward progression still remains uncertain as model solutions
still lack agreement. The Alaska Peninsula...Kodiak Island and the
eastern Kenai will see the highest precipitation amounts from this
front during the weekend. Models do agree on bringing a swath of
rain across Bristol Bay Saturday night and into the Kuskokwim on
Sunday. Increased cloud cover can be expected across the rest of
the southern Mainland this weekend with scattered showers mainly
along the mountains.
Overall forecast confidence decreases in time as models have
struggled with handling the upper trough deepening across the
interior through the weekend. The GFS and European model (ecmwf) were used to
update the weekend forecast with no significant changes made
beyond Monday. The beginning of next week models are suggesting
that a ridge amplifies across eastern Alaska keeping southcentral
dry on Monday. Low pressure in the Bering brings another
reinforcing front late Sunday spreading rain to the southwest
Mainland and Bering into Monday. This low pressure system will be
the pattern to watch next week as it may become the dominating
system that brings a regular pattern of clouds and rain.
Marine...Gale Warning...120 130 131 132 138.
Synopsis and model discussion...pld
southcentral Alaska...British Columbia
southwest Alaska/Bering Sea/Aleutians...rf