Small Craft Advisory 556, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
506 PM AKDT Mon Jul 20 2020

.ANALYSIS AND UPPER LEVELS...

The latest analysis shows a weak low moving slowly east across the
Gulf, with deformation-type precipitation continuing across the
eastern Susitna Valley, extending south and east through the Kenai
Peninsula and the southern half of the Copper Basin. Further
west, clouds were thinning somewhat across the western Susitna
Valley. Then across the Bering and Aleutians, a pair of lows were
noted, with one approaching Nunivak Island with a trough axis
extending south towards Cold Bay, and another out near Shemya.
The eastern Bering low was likely producing showers based on
cloud top temperatures noted on the water vapor imagery, with
Shemya reporting rain as well.

&&

.MODEL DISCUSSION...

The models remain in excellent agreement in the synoptic pattern
over the next few days, with very tight clustering seen on the 500
mb spaghetti plots, with a similar continuity seen on the surface
progs. In general, look for a short-wave trough to establish
itself over the mainland by Wednesday, with a short-wave ridge on
its flank across the east central Being and Aleutians, followed
by a deeper trough anchored across the far western Bering and
Aleutians.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

An upper level trough over Southcentral is slowly tracking toward
Southeast. A lot of the low lying stratus that has persisted this
morning will gradually clear from west to east as the low
continues to depart this evening. With this clearing, enough
instability may be present to generate thunderstorms this
afternoon and evening, especially over the Susitna Valley. A few
isolated storms may form over the western Kenai Peninsula as well.
By Tuesday, we'll see a transition between systems as a weak ridge
to the south attempts to build in. Enough instability may be
present over the Copper River Basin tomorrow afternoon, which
could generate thunderstorms during the afternoon and evening
hours. While skies may clear briefly Tuesday afternoon, expect
increasing cloud cover by the evening tomorrow night as another
upper level trough moves in from the northwest. This next system
will bring another round of rainfall to Southcentral for
Wednesday. By Wednesday evening, precipitation should taper from
west to east as the low quickly departs the area leading to
improved conditions for Thursday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

Relatively dry conditions persists across the Southwest mainland
this afternoon, aside from the western Alaska Range, which could
possibly see convective activity through the evening hours. By
Tuesday morning, an upper level trough will drop down over the
Kuskokwim Delta, which will bring cooler temperatures, cloudier
skies, and another round of showers to the region. As the trough
pushes farther into Southwest Alaska, these showers will spread
across the Kuskokwim Valley and Bristol Bay through Tuesday
evening. This system will move out of the region by Wednesday,
bringing a brief break in the precipitation before the pattern
shifts later this week with an approaching front.

&&

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

A front crossing over the western Aleutians and Bering continues
to be the weather story over the next few days as this systems
progresses eastward. This system should keep small craft advisory
level winds through Tuesday morning as it works its way to the
central Bering. By Tuesday afternoon, a developing triple point
low will keep the elevated winds, with the potential for embedded
gale force gusts, in the forecast. Another round of widespread
rain will accompany this system stretched across the Bering,
Pribilofs, and along the Aleutian Chain through the end of the
week as the associated front moves toward the Southwest coast.

&&

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

...Bering Sea and Aleutian Coastal Waters...

A low complex and its front moves across the Bering through Sat.
The front completely wraps the low complex. While the models are
mixed and uncertain on the development of the low complex as it
moves East, confidence is good that the Eastern Bering small
craft winds and waves will dissipate Thu night, the Northern and
Western Bering portions diminish on Fri, but Westerly small craft
winds and waves will persist over the Aleutians through Sat.

...Gulf of Alaska...

The offshore flow pattern yields to a front entering the Western
Gulf Fri morning that moves to the Eastern Gulf by Sat. The
Northwesterly small craft wind and waves through the Barren
Islands and Sitkinak areas diminish Thu night. Confidence is good
for an area of Southwesterly small craft winds and waves to move
East of Kodiak Island and Southeasterly small craft along the
North Gulf Coast and Prince William Sound through Sat.

&&

.LONG TERM FORECAST (Days 3 through 7: Thursday through Monday)...

A cool and unsettled pattern is in-store for the Alaskan mainland
during the next 3-7 days. Teleconnections support broad upper
level troughing over the Alaskan mainland during the next 3-7 days
as both the West Pacific Oscillation and East Pacific Oscillation
trend stronger into the positive phase. At the same time, Climate
Forecast System show the placement of the Madden Julian
Oscillation (fluctuations in tropical rainfall occuring in a 30-60
day cycle, often influencing atmospheric circulations for North
America) will help enhance the upper troughing over the Alaskan
mainland as the upper ridge displaced north of the convection will
displace an upper trough in the Rossby Waves (series of troughs
and ridges in the upper level westerlies) over mainland Alaska.
The upper troughing that is causing these cool temperatures will
remain in place during the entire 3-7 day period as 200mb zonal
winds remain easterly in the polar- jet region and thus, decrease
the strength of the jet. ECMWF/GFS/GEM ensembles all show a high
degree of consistency in the placement and intensity of this upper
troughing over Alaska. Ensembles also show consistency with the
upper trough extending over Alaska for this 3-7 day period.
Moreover, ensembles show weak to moderate cold 850mb temperature
anomalies over Alaska during this period. There is a high
confidence that temperatures will be cooler than normal during
this period with an unsettled pattern

On the synoptic scale, deterministic models show different
solutions with the placement and intensity of an area of low
pressure that will move through the Bering/Aleutians on Friday and
Saturday, then moving through the Gulf of Alaska on Sunday into
Monday morning. The only consistency is the low tracking
northeastward from the central Gulf of Alaska and into either
Southeast Alaska or Yukon Territory. Due to the track of this
low, Turnagain winds are possible on Saturday and Sunday, though
there is low confidence in the strength of these potential
Turnagain winds due to a high degree of uncertainty with how far
offshore the low will track.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...MK/ED