Area Forecast Discussion
Issued by NWS Anchorage, AK

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737
FXAK68 PAFC 210021
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
421 PM AKDT Sat Jul 20 2019

.ANALYSIS AND UPPER LEVELS...
The upper level pattern over the region is quite stagnant at the
moment, with a col over Southcentral and a broad ridge over the
Bering Sea and Southwest Alaska. Conditions are very stable across
the Bering Sea, Aleutians, and Southwest Alaska. This is producing
widespread low clouds and patchy fog. For Southcentral, the
atmosphere can be characterized as weakly unstable. Compared to
the last few days there is much more cloud cover which is leading
to cooler temperatures and greater stability. With less heating
expect most of the diurnal shower activity to form over the
mountains. The only location unstable enough for a few
thunderstorms is the Copper River Basin. Aside from air mass
instability the only forcing mechanism for precipitation is a few
upper waves moving through the upper ridge, out of the Yukon and
into the eastern Copper River Basin. The low which has been
spinning over the Gulf has sunk southward and the primary impact
on the Gulf coast is widespread clouds.

The quiet conditions in the Bering Sea are poised to change over
the next 24 to 48 hours, with a strong short-wave trough digging
southeastward across eastern Russia this morning and headed for
the Bering Sea.

&&

.MODEL DISCUSSION...
Despite the changing pattern, models are in good agreement with
large scale features through Tuesday. With a dynamic trough
moving into the Bering Sea, will have to keep a close eye on models
there, as they tend to struggle with systems coming out of
Russia. For Southcentral, there are some minor differences with
the track of an "easterly wave" Monday night through Tuesday.
Precipitation with these waves is often difficult to pinpoint
until about 12 hours prior to arrival. For now, the forecast will
trend toward more cloud cover and higher chances of rain or rain
showers.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. With stable
conditions and light flow, there may be some MVFR or lower clouds
in the vicinity. More likely, the VFR ceiling will drop below 5000
feet at times, especially Sunday morning.

&&

.FIRE WEATHER...
Fire weather concerns are minimal across southern Alaska thanks to
more stable conditions and extensive cloud cover. Will see a bit
of a change over Southcentral on Monday as a ridge noses in and
offshore flow helps clear out some of the clouds. This will lead
to warmer conditions, but still marginal as far as thunderstorm
potential. Tuesday looks a little more interesting as an easterly
wave crosses through. The combination of increasing warmth and
instability combined with forcing from the upper wave will likely
lead to a much more active thunderstorm day, particularly for
inland areas (Copper River Basin/Susitna Valley).

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Through Tuesday)...
Looks like a couple nice days are on tap for southern Alaska. The
area will remain between major synoptic weather systems, with
relatively weak flow aloft. The low-level air mass is quite warm,
resulting in a couple days of above-average temperatures for this
time of year on Monday and Tuesday. The only chance for showers
or thunderstorms will be over the mountains in the late afternoon
and evening. Look for high temperatures in the 70s with mostly
clear conditions, save for some low clouds around Cook Inlet in
the mornings. On late Monday into Tuesday, an easterly wave aloft
will move along the coastal mountains, likely sparking showers and
thunderstorms along with it. These easterly wave events are
usually poorly modeled and can come in much faster/slower than
progged. At the very least, expect an increase in mid and high
level cloud cover, along with an increase in showers and
thunderstorms.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3; Tonight
through Tuesday evening)...

A ridge will slowly track eastward across the Bering through
Monday bringing mostly cloudy skies and possible morning fog to
Southwest. Brief onshore showers are possible along coastal areas.
The onshore flow will weaken by Tuesday, potentially leading to
partial clearing over the region.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3; Tonight
through Tuesday evening)...

A front descending southward from Siberia will likely bring gales
to the western Bering on Sunday before departing back northward. A
weak North Pacific low will approach Adak on Sunday as well,
leading to widespread rain and gusty southeasterly winds to areas
generally west of Unimak Island through Monday afternoon. Further
east, a ridge will slowly track over the region bringing marine
stratus and fog to the AKPEN and eastern Aleutians. Though the
ridge will weaken by Tuesday, weak onshore flow will cause the
cloud cover to persist.

&&

.MARINE (Days 3 through 5/Tuesday through Thursday)...
A series of fairly strong storm systems (for this time of year)
will cross the Bering Sea Tuesday through Thursday. There is high
probability of small craft winds with each of these systems, with
some potential for gales as well. Based on the expected storm
track across the northern Bering Sea, the strongest winds will
be most likely across the heart of the Bering Sea on over to the
western Alaska coastal waters.

&&

.LONG TERM FORECAST (Days 4 through 7/Wednesday through Saturday)...
There is quite a bit of spread in model solutions, which grows
significantly as we head through the extended forecast period.
Thus, forecast confidence is low. A deep trough will be sitting
over the Bering Sea, with active weather likely as short-
waves/lows move through the trough. The big question is how far
east does the long-wave trough extend, with some solutions driving
the trough across much of mainland Alaska. This would represent a
change from the typical summer-time diurnal convection to more
organized dynamically induced precipitation with cooler/cloudier
conditions. For now, the forecast will continue to reflect an in-
between solution, with the trough reaching Southwest Alaska and
upper ridging (with associated warmer temperatures and instability
induced convection) holding across Southcentral Alaska.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 185 411.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...MTL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...SEB



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