Scientific Forecaster Discussion
fxak68 pafc 240040
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
440 PM akdt sun Jul 23 2017
Analysis and upper levels...
a fairly Flat Ridge across southcentral Alaska continues to provide
abundant sunshine. Marine stratus did form along the north Gulf
Coast last night. With onshore flow, it is pushing onshore across
Prince William Sound and the Kenai Peninsula. Flow is also taking
stratus into southern Cook Inlet via Kennedy and Stevenson entrances.
With strengthening southerlies in the inlet the stratus is poised
to envelop the Cook Inlet region later today through tonight.
Weak ridging also persists along the Alaska Peninsula and up into
Bristol Bay and the Kuskokwim valley, out ahead of an approaching
trough and surface front. Fog and stratus is well established
across Bristol Bay, supported by strong onshore flow. Light rain
is pushing onshore along the Kuskokwim Delta, helping to mix up
things a bit resulting in higher ceilings.
Out west a strong ridge centered south of the Aleutian chain with
warm advection over top of it is helping maintain extensive fog
and stratus region-wide.
models are in good agreement with large-scale features through
Tuesday and forecast confidence is generally high. Some of the
forecast challenges include determining extent to which fog and
stratus push onshore across southcentral Alaska tonight; as well
as the precip forecast for southcentral with dry low levels and
weak upper level forcing moving through tonight through Monday
panc...gusty southeast winds will continue to come in off Turnagain
Arm through about mid-day Monday at which point gradients will
weaken and flow will shift around to up inlet. Stratus will make
it's way up Cook Inlet tonight, but the turnagain winds should
keep it all south and west of Fire Island. Flow will become more
favorable Monday night for a possible push of stratus (low MVFR
or IFR ceilings) into the terminal.
Short term forecast southcentral Alaska (days 1 and 2)...
expect a cooler and cloudier regime over the next couple of days
with increasing chances for rain in most areas. The ridge aloft
promoting very warm conditions today will be pushed out overnight tonight
by a trough and front coming in from the west. The front is
weakening as it stalls out overhead from southwest to northeast
across the area, and will mainly bring an increase in clouds on
Monday. However, guidance is suggesting a weak trough over the
front will bring light rain to northern Cook Inlet through the
mat-su late Monday night/early Tuesday morning. In the upcoming
pattern there are no strong features to key in on. Increased cloud
cover and cooler temperatures are the only certainty. While
precipitation chances are not great, they are still there.
An area of marine layer stratus is currently pulsating in the
northern Gulf. High pressure in the Gulf should be conducive to
further development over the next couple days. Coupled with
falling pressure inland, should promote the marine layer coming up
Cook Inlet the next 2 nights.
Short term forecast southwest Alaska...
A front associated with an Arctic low has moved onshore over the
northern Kuskokwim Delta, spreading rain over the coast. The front
will progress over the Kuskokwim Delta tonight, weakening as it
moves to the northern Bristol Bay area tomorrow afternoon. Monday
night, a developing shortwave off the Kuskokwim coast will move to
Bristol Bay and reinvigorate precipitation overnight over the Bristol
Bay area and inland.
The marine layer of low stratus and fog remains a forecast
challenge over coastal areas today. Parts of interior Bristol Bay
have remained socked in low stratus and fog most of the day, only
improving in the mid afternoon hours. After a brief improvement
this afternoon, conditions will deteriorate again overnight as
southwest onshore flow continues. With the passage of the front
tomorrow, fog and low stratus will remain in the area, with only
brief improvements expected during the afternoon hours.
Short term forecast Bering Sea/Aleutians...
An east-west oriented ridge of high pressure is holding over the
Aleutians. Under this ridge, fog and low stratus persist. Warm
air advection behind the front over the Bering is only compounding
the problem by further increasing the low-level inversion. This
increasing stability as high pressure builds behind the front will
allow the marine layer to persist for the next few days. The
front over the Bering will continue to move southeast through
Monday night, spreading rain over the area. Small craft winds
associated with the front near Nunivak Island and over the
northern Kuskokwim Delta coast will weaken tonight as the front's
progression is slowed by the steady ridge holding strong over the
Long term forecast (days 3 through 7)...
The forecast period commences with a potent low over the akpen and
a ridge axis bisecting the Bering Sea. Generally speaking, the
synoptic pattern is amplified...very dynamic and progressive.
Anticipate the possibility of precipitation for the western and
south central portions of the state Wednesday night through
Saturday. All the while, the western and central Aleutians will be
under the influence of the ridge keeping much of the region dry,
however low stratus clouds and fog will persist under the stable
air mass. Late in the period, the European model (ecmwf) and GFS models are in
relatively good agreement with the concept of another cyclone
getting spun up over Russia and traversing the Bering once again
bringing a fetch of maritime moisture into western Alaska.