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Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
523 am akst Mon Jan 21 2019

Analysis and upper levels...
an upper level ridge is retreating as a low pressure system south
of the Aleutians is pushing a front north across the southern
Mainland. Satellite imagery shows high and mid level cloud cover
streaming north across the the akpen and Gulf of Alaska as
precipitation is beginning to show up on the King Salmon radar
just to the south of King Salmon. A weak surface low is currently
spinning over the Gulf bringing snow showers to the price William
sound and northern Gulf Coast. Radar imagery also shows snow
showers pushing across the western Kenai Peninsula and even across
the Anchorage area, though local observations and webcams show
this is mostly evaporating before reaching the ground. Meanwhile,
the surface low further west over the Aleutians is bringing
strong winds and snow showers to the area, with scatterometer
passes showing a wide swath of gale force winds over most of the
southern Bering Sea.

&&

Model discussion...

Models are in good agreement through Tuesday as a series of fronts
pushes north across the akpen and Gulf of Alaska. By Wednesday, a
secondary low moves up from the North Pacific and crosses over
the akpen and into Bristol Bay. Models have recently been
struggling with how far east or west the low would track, which
would have a significant impact to weather across the southern
Mainland. The NAM has been the outlier tracking the low further
east across the SW Mainland while the GFS and ec have been keeping
the low further west over Bristol Bay. Recent NAM runs have been
trending further west towards the GFS/ec solutions, thus trended
towards that solution as well.

&&

Aviation...
panc...an approaching front from the south is helping to
destabilize the atmosphere and has eroded all remaining fog and
low stratus in the area. VFR conditions and light northeasterly
winds will prevail for the taf period with just a chance of some
light snow this afternoon.

&&

Short term forecast southcentral Alaska (days 1 and 2/today
through Tuesday night)...
broad southerly flow above the surface combined with a weak upper
level wave will produce areas of very light snow across
southcentral today. Generally expect flurries or a dusting inland.
Low level instability and upslope flow along the coast and Prince
William Sound will lead to snow showers and some minor snow
accumulation. Further south, a weakening occluded front extending
from a deep low along the Aleutians will slowly lift northward
toward Kodiak Island today, then fall apart before reaching the
northern Gulf tonight. Another upper level short-wave will lift
northward, but will weaken as it crosses southcentral due to
rapidly rising heights aloft as the upper level flow amplifies.
Thus, expect little in the way of precipitation with it. Low level
southeast flow will favor upslope precipitation in western Prince
William Sound as well as the west side of Cook Inlet and the
western Susitna Valley.

Another occluded front will make its way to Kodiak Island on
Tuesday, with temperatures warm enough to produce all rain. This
front will also dissipate as it lifts northward toward southcentral.
A much more significant storm system is shaping up for Tuesday
night into Wednesday. While there is uncertainty in the storm
track, there is high confidence this system will track into the
Bering Sea. Deep southerly flow will pump up warm air from the
Pacific to southcentral, so as precipitation spreads to the Gulf
Coast on Wednesday it will fall as mostly rain. Strong low level
southeast flow will lead to downslope drying for the population
centers, from the western Kenai to Anchorage and the Matanuska
Valley. The exact track of the storm will ultimately determine how
significant of a warm-up these areas see.

&&

Short term forecast southwest Alaska (days 1 and 2...today and
tuesday)... an occluded front associated with a North Pacific low
nearing the central Aleutians will bring precipitation to much of
southwest over the next two days and, in its wake, a shift to
warmer weather.

The front is currently stretched from the akpen to the central
Aleutians. High pressure over eastern Siberia and an Arctic upper
low over northern Alaska have maintained offshore flow and very cold,
dry conditions in the lower Kuskokwim valley/Kuskokwim Delta,
however farther south along the akpen temperatures have started to
increase in southerly flow behind the front. Expect blowing snow
in the Bristol Bay area later today, then a transition to rain and
temperatures above freezing Tuesday. Looking farther north, the
lower Kuskokwim valley and Kuskokwim Delta will hang on to
northeast flow at the surface until later tonight, and expect
blowing snow along the kusko Delta coast tonight. These areas will
likely hang on to cold air a bit longer than farther south, but
even inland valleys will warm to above freezing by midweek.

&&

Short term forecast Bering Sea/Aleutians (days 1 and 2...today and
tuesday)... a gale force North Pacific low is slowly shifting
north/northwest toward the central Aleutians. This low will
continue to bring areas of gales to the western Aleutians and
widespread gales to much of the Bering, including the pribilofs,
throughout the day. Over the eastern Aleutians, winds will slowly
diminish today as the low gradually shifts westward.

An occluded front associated with the low currently stretches
from the akpen to the central Aleutians, and ptypes over the
central and eastern Aleutians have changed over to rain.
Temperatures in the upper 30s to low 40s will persist into
Tuesday. The western Aleutians remain on the cold side of the low,
thus will continue snow or rain/snow mix. A deformation band
nearing the pribilofs is evident on water vapor satellite which
will enhance precipitation rates. With northeast gales at the
surface, expect blowing snow today into tonight. As the front
lifts north of the area, winds will veer to southerly and
precipitation will transition to rain.





&&

Marine (days 3 through 5)...
(wednesday through friday)

The marine outlook remains challenging as model continuity remains
modest. The general trend over the last 24 hours has been for a
northwestward shift in the surface and upper air pattern, as a
ridge of high pressure build northwest across the east central
Gulf, then across southcentral Alaska into the North Slope. This
will also allow a series of lows lifting across the western and
central Aleutians on Wednesday to retrograde towards the western
Aleutians and Kamchatka peninsula by Friday. How fast this occurs
and where the low pressure centers will be is yet to be
determined.

However, there has been some improvement with this since
yesterday. The GFS and Canadian models have similar forecast
solutions, where as the European model (ecmwf) seems somewhat in
agreement, but to a much lesser extent. This causes surface low
strength differences of up to 20 mb. Overall, the most likely
scenario at this point is for each system and their associated
fronts to bring sustained gale force winds with some gusts perhaps
approaching lower-end storm force. Some of these fronts may reach
the western Gulf before weakening as The Bump into the downstream
ridge.

&&

Long term forecast (days 3 through 7)...

Model uncertainty remains moderate in the Wednesday through Friday
time frame, before the numerical forecasts continuity degrades
more towards the weekend. In general the surface and upper pattern
favors a ridge of high pressure gradually building northwestward
from the eastern Pacific, into the east central Gulf, continuing
towards the North Slope and eastern Siberia. This would allow for
moderating temperatures as we head through the period with some
precipitation type issues for southcentral possibly developing,
due to warmer air aloft advecting over the colder surface
conditions, as a weakening front from the Bering lows tries to
push into the region before weakening somewhat while slowing down.
Should this occur, the over-running warm air around 850 mb would
cause at least some precipitation type concerns, most likely to
develop in the late Wednesday through Thursday time frame.

Meanwhile, a series of lows will continue to lift north across
the central Aleutians before stalling as they build into the
building ridge, causing each low to rotate around each other in a
fujiwhara-like fashion early on. As the ridge continues to build
towards the weekend, its associated effect will be to shift this
area of disturbed weather further west towards the western
Aleutians, extending northwestward towards the Kamchatka
peninsula.

&&



Afc watches/warnings/advisories...
public...none.
Marine...gales 130 139 172 173 174 175 176 177 179 185 411 412 413 414
heavy freezing spray 160 185 412 414.
Fire weather...none.

&&

$$

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