Scientific Forecaster Discussion
fxak68 pafc 291206
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
406 am akdt Thu Jun 29 2017
Analysis and upper levels...
Two upper level lows are the driving systems controlling the
weather across southern Alaska this morning. The expansive upper
low over the northwestern Bering controls much of the steering for
the disturbances moving across the area, as it has done for the
past several days. For Mainland Alaska, that flow is out of the
south, off the Pacific and the Gulf. As as result, nearly all of
the area is socked in cloud cover, except for a few breaks here
The second low is spinning across the Gulf, with the central
approaching Kodiak Island. Its influence doesn't cover nearly the
area as the first low, but it is solely responsible for much of
the precipitation occurring across southcentral this morning. This
is because the dynamics for precipitation generation are much
better with this low, which is much younger than its counterpart
in the Bering. With the flow circulating around the low fairly
weak, the southeast flow associated with the low has not resulted
in the usual downsloping across the Kenai Peninsula. The broad
precipitation shield associated with the low has yet to reach
Anchorage and points north. Thus, communities on both sides of the
Kenai Peninsula are picking up wetting rains this morning. There
are a few showers occurring over the southern Susitna Valley, but
those should fill in as the morning progresses with steady light
to occasionally moderate rains for much of the area.
The models remain consistent enough with previous runs that few
changes were needed as updates to the current forecast package.
Regarding the low approaching Kodiak, the models start to differ
by Friday morning as to where and when the low dissipates or gets
absorbed into another weak low that looks to track across the
eastern Gulf Friday night. Thus, there is a pretty quick
transition from high confidence and model agreement to low
confidence and model dissonance. For southwest Alaska and the
Bering, the models remain in good agreement for far longer, so
forecast confidence is better there. For southcentral, the broad
pattern of wave after wave of moisture moving northward into the
Gulf and then wrapping up and dissipating/being absorbed into the
next wave looks to continue, making a cloudy, cool, and wet
forecast a good bet regardless of which model ends up closest to
Panc...off-and-on rain showers will develop this morning and will
become a steady light rain through the afternoon into the evening.
MVFR conditions will be possible at times, though for the most
part VFR conditions are expected. The Turnagain Arm winds will
subside and move south out of Anchorage as northerly flow
develops with the approach of the Gulf low responsible for the
today's expected rainfall. Tonight, the rain should taper to
isolated showers as stronger southeast flow leads to more
downsloping and thus, drier conditions.
Short term forecast southcentral Alaska (days 1 and 2)...
the low in the Gulf centered south of Kodiak Island will slowly
drift north today. Satellite imagery depicts a surge of Pacific
moisture moving north to the Gulf Coast in association with a
left-exit region of the subtropical jet stream. The western
portion of a front over the northern Gulf containing this
moisture will remain nearly stationary through this afternoon,
while the eastern portion pivots northward. While most of the
precipitation will fall along the coast, rain is likely inland
from Anchorage southward as the deep moisture mitigates downslope
impacts. The front will weaken along the coast tonight and
Friday with precipitation tending to favor the coast and tapering
off. Inland drying will result in slightly warming temperatures
Friday, with possibly some sun breaks Friday afternoon and
evening. On Friday night an easterly wave arriving from the
Panhandle will move into the Cordova and Valdez areas, and then
push into the Copper River basin and Kenai Peninsula Saturday
Short term forecast southwest Alaska (days 1 and 2)...
the longwave pattern features a broad area of troughing that is
centered over the Bering Sea with multiple impulses embedded and
getting ejected. The flow aloft over southwest Alaska will be
southerly. Expecting showers to materialize over portions of
southwest Alaska today and again Friday. With time, the upper
level low will drift to the north and the models hang on to the
concept of bringing additional weak shortwave energy across the
Kilbuck Mountains. At this time, the progged stability indices are
marginal for this forecast period and did not introduce lightning
to day 1 nor day 2.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
The Bering Sea low will remain steadfast and continue to influence
the sensible weather pattern. Multiple shortwaves will emanate
from the Bering low including one that will track along the
Aleutians from Adak to Kodiak this period. The main impact of
this system will be increase rain potential. In its wake...
another storm will move into the western Bering Sea Friday. This
second storm will have better dynamics fueling it and expect gale
force winds to be ushered into the western Aleutians Friday.
Long term forecast (days 3 through 7)...
Long term models have not come into any closer agreement than they
were yesterday on the timing or placement of the low pressure
system in the Gulf of Alaska. The same general pattern continues
with a strong jet streak diving into the western Gulf of Alaska
Sunday night, however the models diverge greatly on surface low
development and movement. GFS favors a further north solution
while ec pushes the low further south along the Gulf of Alaska
border. Both models continue to favor a wet Sunday and Monday for
most of the Mainland but GFS does Harbor some hope that things
might dry out a little bit for the Tuesday Holiday. Meanwhile in
the Aleutians, model variability is high, with ec indicating a
rainy low pressure system moving through, and GFS indicating broad
westerly flow with significantly less precip. By Tuesday
afternoon, both models move a ridge into the eastern Bering, which
would dry things out in western Alaska for the evening of the 4th
of July. Current thinking is to not adjust the long term
forecast, which would continue to bring rain through the early
part of next week.
Marine...gale: 119 120 130 131 139