Small Craft Advisory 554, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
527 AM AKDT Tue Jul 19 2022

.ANALYSIS AND UPPER LEVELS...

A longwave trough sits stationary over the Seward Peninsula this
morning. Cyclonic flow around the low will usher in cooler
temperatures as well as multiple lows into Southwest and
Southcentral through the end of the week. On satellite a front is
noted over the southern Bering. This front is slowly weakening and
should dissipate upon moving over the Aleutian Chain this
afternoon. A shortwave trough is also digging across Southcentral
with scattered showers over Prince William Sound up into the Mat-
Su Valley. The remnants of a 95 knot upper level jet are moving
across Bristol Bay and the Alaska Peninsula, and making their way
into the North Pacific. Behind it, a more potent 120 knot upper
level jet is developing over Northeast Russia, which will
accompany Wednesday morning's upper low into the YK-Delta over the
next 24 hours. Rain chances will remain in the forecast for
Southwest and Southcentral through Friday.

&&

.MODEL DISCUSSION...

Models are in loose agreement this morning with synoptic
features. The biggest forecast challenge for Tuesday is timing and
placement of rainfall as each model has small, subtle differences
with the timing of each shortwave trough. Hi-res guidance is
showing heavier rain developing over the foothills of the Chugach
Mountains from Eagle River up into Palmer beginning at midday and
lingering through the evening hours. Hourly amounts of 0.15 to
0.25 inches may be possible for a short period of time. However,
light rain chances will exist across most of Southcentral
throughout the day for which operational models are in agreement.
Into Wednesday models are also coming into better agreement with
the progression of a stronger upper low into the YK-Delta,though
the GFS remains the faster solution with the GEM Regional and EC a
hair slower.

&&

.AVIATION...
PANC...Brief VFR conditions to start the morning are expected
give way to MVFR conditions as the next round of rain moves into
Anchorage. Off and on rain chances will keep ceilings at or below
3000' for much of the day. Winds will once again have the biggest
impact on the region with SE'ly gusts to 25+ knots. A gradual
shift to a S'ly direction is expected with some SW'ly winds by
midday. These winds may relax somewhat this afternoon before SE'ly
Turnagain Arm winds redevelop later this evening.

&&

.FIRE WEATHER...

The pattern will continue to favor active rain showers over
Southwest and Southcentral through the end of the week. During the
overnight hours rain has tapered off across Southwest, but rain
should redevelop later this morning and move into Southcentral
this afternoon. The arrival of a large upper low into mainland
Alaska early Wednesday will see widespread showers develop over
southern Alaska with yet another wave of low pressure to move into
the region on Friday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Tuesday through Thursday night)...

A vertically stacked, occluded low is situated just north of Nome
this morning. This feature anchors a large, upper-level Arctic
trough that extends from the Beaufort Sea to the Central
Aleutians. The upper-level ridge that brought a few breaks in both
the precipitation and cloud cover yesterday is now over the Al-Can
border. Between these two features, multiple shortwaves are
tracking across the Southern Mainland, embedded within the west-
southwesterly flow. The first of these waves will move northeast
into the Copper River Basin this morning with a transient ridge
moving over Cook Inlet in its wake. The widespread precipitation
overnight will track with the wave, moving toward Prince
William Sound and across the mountains this morning.

A second wave associated with a frontal boundary over Southwest
Alaska will move into Southcentral later this morning. As it does,
widespread rain will redevelop across Cook Inlet north to the
Mat-Su Valleys. The rain could be heavy at times, especially along
upslope regions of the Chugach and Talkeetna Mountains and across
the immediate Southcentral coast. This wave will slow and become
negatively tilted as it approaches the Kenai Peninsula this
afternoon. A trailing shortwave upstream is then expected to catch
up to the previous wave and allow precipitation to linger across
the western half of Southcentral as the downstream wave lifts to
the Copper River Basin. The timing of this and whether is any
phasing of these features to help further enhance precipitation
is still a bit uncertain. As such, there remains some discrepancy
as to precipitation amounts from Kenai to Talkeetna through early
Wednesday morning. The forecast continues to take a middle ground
between the lesser NAM QPF solution and the higher GFS/GEM
solutions. Expect southerly gap winds to continue through the
period with occasional lulls between systems.

A brief break in the precipitation in a corridor from Cook Inlet
northeast to the Copper River Basin is possible Wednesday morning
as the aforementioned wave exits the region and a transient ridge
moves over Southcentral. A deep area of low pressure will then
move onshore along the west coast by early Wednesday afternoon.
This feature will extend a front over the Alaska Range, bringing
more widespread rain to Southcentral through late Wednesday
night. Again, a break in the action, or at least a transition from
widespread rain to scattered showers, is possible for Thursday
morning. However, models are indicating a surface low will spin
up somewhere near the Barren Island, lifting toward the Kenai
Peninsula by Thursday afternoon. This will likely bring more rain
to much of Southcentral through Thursday night. Gap winds through
favored terrain will also ramp up for Thursday as a front
associated with the low lifts to the Southcentral coast.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

A cloudy and wet pattern is expected across Southwest Alaska over
the next few days. An upper-trough will continue to bring clouds,
showers and cool temperatures today. A low pressure system will
then move over Southwest Alaska on Wednesday and bring wetting
rain across the region. About 0.50-1.00" of rain is expected
across the Kuskokwim Delta from 10 pm tonight through 10 pm
Wednesday night. Other areas across Southwest are more likely to
see 0.25-0.75" of rain. Showers and cool temperatures will then
continue on Thursday.

Another impact aside from the rain this low will have on Southwest
Alaska are high water levels along coastal areas of Bristol Bay,
specifically the Togiak area. The current storm track has the odds
tilted in favor of brisk southerly flow coinciding at the time of
high tide. This will allow for high water levels where coastal
flooding may occur. Forecast confidence is low with respect to the
details on this potential storm surge.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

A fairly active pattern is expected across the central and eastern
Bering/Aleutians over the next couple days as an upper-level
trough moves onshore today followed by a low moving across the
eastern Bering Sea on Wednesday. This low will likely bring
localized gale force winds due to cold air advection on the back
side of the low. This low will then move inland Wednesday
afternoon/evening. Quieter weather is expected for Thursday as
winds will remain below gale force with no high seas expected.

&&

.MARINE (Days 3 through 5: Thursday through Saturday)...

Gulf of Alaska: A low moves along its front across the Northern
Gulf through Saturday. Forecast confidence is good. Gusty
Southwest winds spread across the Gulf, with gusty Westerly winds
through Southern Cook Inlet, Kamishak Bay and Barren Islands,
diminishing Saturday. Seas to 10 feet, subsiding.

Aleutians/Bering: High pressure spreads across the Northern
Bering through Saturday. A well developed North pacific low moves
over the Aleutians by Saturday. Forecast confidence is good.
Generally light winds over the Bering through Saturday. Gusty
Westerly winds South of the Alaska Peninsula moves into the Gulf
of Alaska for Saturday. Gusty Easterly winds extends over the
Aleutians by Saturday. Seas to 10 feet South of the Alaska
Peninsula, lower elsewhere.

&&

.LONG TERM FORECAST (Days 4 through 7: Friday through Monday)...

A cool, active pattern will likely continue through the rest of
this week and through the weekend. Model agreement falls out of
alignment over the weekend and while the details are less clear on
the strength, evolution, and timing of the synoptic features, the
general upper level synoptic pattern trends similarly with an
upper low descending into Southwest Alaska Friday. Zonal flow is
apparent over the Aleutians but an upper level disturbance
develops over the western Aleutians that could have some influence
in the forecast for the Aleutian chain. Also, the subtleties in
the evolution of the trough over mainland could leave cooler air
over the AKPen and eastern Aleutians a little longer and warmer
air for the eastern portions of Southcentral Alaska Friday. The
evolution and any amplification of the synoptic pattern by Monday
afternoon remains uncertain into next week and will affect the
temperature and precipitation outlook at specific locations.
Temperatures should begin to trend towards normal for many
locations by Monday and unsettled weather is more likely to move
along the Aleutians Sunday towards Southwest and perhaps as far as
Southcentral by Monday afternoon.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 180, 181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ED
MARINE/LONG TERM...KZ/VR