Small Craft Advisory 396, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
406 AM AKDT Mon May 11 2020

.ANALYSIS AND UPPER LEVELS...

A front and a strengthening upper level wave over the Gulf this
morning are moving northwestward into the Kenai Peninsula. Rain
has begun over much of the Kenai Peninsula, and will persist into
the morning hours. Elsewhere, associated cloud cover is helping
keep temperatures warmer from Anchorage and Prince William Sound
northward through the Mat-Su. Higher cloud cover is overspreading
the lower Kuskokwim Valley as well, with much more broken cloud
cover south and west of there. Like last night, temperatures
remain in the 40s for most areas. Over the Bering, a dying
stationary front is keeping moisture entrenched across the
Pribilofs, where fog remains a significant weather issue. A large
low currently well west of the Western Aleutians will move into
the islands this afternoon, and will keep the weather active along
the Chain over the next day or two.

In the upper levels, a large ridge persists along the
Alaska/Canada border. Meanwhile a large negatively tilted longwave
trough extends from Kamchatka southeastward across the Bering and
into the Alaska Peninsula. A rather weak jet aligned from
southeast to northwest is over the Gulf into the Lower Kuskokwim
Valley. The jet is the track along which the precipitation ongoing
and expected through the day today will move.

&&

.MODEL DISCUSSION...

Excellent agreement between the models continues through the
short-term this morning, so forecast confidence remains high.
Forecast challenges are mainly focused on a local level. As the
wave over the Gulf moves into Southcentral and Southwest Alaska
today, precipitation amounts associated with the wave are still
somewhat uncertain. All models have trended stronger with the
forcing with the upper level wave, so more rain is expected today,
especially from the Cook Inlet region through the Lower Kuskokwim
Valley. There will be some minor cross-barrier flow, so downslope
regions may see a bit less than other locations further
downstream.

Finally, the threat of thunderstorms remains over portions of
Southwest Alaska. The challenge is determining whether the strong
forcing and plentiful moisture can overcome plentiful cloud cover,
and thus, limited instability. Thunderstorm coverage is quite
uncertain for this afternoon.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist through the
morning. This afternoon, light rain moving over the area may cause
MVFR ceilings. VFR conditions return in the evening after the rain
moves off.

&&

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

A weak occluded front moving north across the Gulf today will usher
in cooler temperatures and increased chances of rain to
Southcentral. Rain currently over the Kenai Peninsula will spread
north to the North Gulf Coast and to the Anchorage Bowl and MAT-SU
Valleys this afternoon and evening. A few showers should be the
extent of precipitation over the Copper River Basin. As
aforementioned, temperatures will be cooler and RH values higher
today, though the Copper River Basin will still hit afternoon
highs today in the upper 60s to near 70 with RH values in the mid
20s. The front moves inland and weakens overnight tonight. Expect
decreasing clouds with broad offshore flow as high pressure builds
over the mainland and a weak inverted trough sets up over the
Gulf Tuesday. Temperatures will start to slowly rebound on
Tuesday.

&&

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

An upper level wave is approaching Southwest Alaska this morning.
As a result, clouds will be on the increase with rain showers
moving in this morning. The steadiest rain is expected to fall in
a stripe from near Port Alsworth west to the Kilbuck Mountains and
north towards Aniak. Lesser rain amounts are expected near
Sleetmute and Lime Village and on the leeward side of the Alaska
Range due to the downslope component expected.

Latest model guidance is mainly in agreement that any convective
activity will remain across the southern Bristol Bay region
including Dillingham, Quinhagak, Tuntutuliak, and King Salmon.
Instability values are enough to put into the forecast the risk
for an isolated wet thunderstorm with a possible rumble of thunder
or flash of lightning. The best chance time for this activity
will be Monday afternoon into Monday evening. However, there is
one aspect of the forecast that remains uncertain even with the
event taking place today. Areas near Sleetmute to Aniak to Lime
Village also have a potential risk for a shower or thunderstorm
Monday afternoon and evening. There are several factors that went
into the decision of not placing this in the forecast for this
package update. They included a good deal of cloud cover will be
present throughout the day and the main energy with the upper
level shortwave having already passed by the region. Also, only
piece of model guidance (HRRR- Hi-Resolution Rapid Refresh model)
showed the activity robust with the other hi-res model guidance
keeping any activity showery and not indicative of thunderstorms.
There is a chance this will need to be put into the forecast
package during the day if the next model cycles continue to show
it. Rain shower chances will continue into Tuesday morning before
tapering off and ending.

Heading into the middle of the work week and beyond, drier air
will move into the region bringing lower RH values for the
afternoons. Temperatures will also warm due to upper level
ridging that builds into the region with southeasterly flow aloft
continuing.
&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

Very weak flow aloft across the Bering is leading to fog across
the Pribilofs this morning. The region is in between two
dissipating frontal systems that are remaining nearly stationary
to the east and west. Any wind flow that would possibly help to
mix out the fog will continue to remain away from the region. With
perhaps some daytime heating today the fog may lift some but the
fog may stick around the region beyond the morning hours today.
With the same pattern in place through Tuesday, fog will be
expected again Monday night into Tuesday morning.

The aforementioned fronts remain nearly stationary across the
eastern Aleutians and the central Aleutians leading to showery
activity continuing through today.

The next big weather system will begin to move into the western
Aleutians and western Bering later this morning with a gale force
front. Expect strong winds and wind driven rain to affect the
area from later this morning through early Wednesday morning. The
steadiest and heaviest precipitation is expected to fall across
Shemya this afternoon into tonight with more in the way of showers
after tonight through early Wednesday. The Adak area will receive
the steadiest and heaviest tonight through Tuesday morning before
becoming more showery in nature.

&&

.MARINE (Days 3-5 Wednesday through Friday)...
A low south of the Aleutians on Wednesday will move east into the
Gulf of Alaska by Saturday. Currently no gales are forecast with
this low, but they are possible over the Aleutians on Wednesday
and over the Gulf by late Friday.

&&

.LONG TERM FORECAST (Days 4-7 Thursday through Sunday)...
On Thursday there will be an upper level low centered south of
the Alaska Peninsula and an upper high north of the Bering Strait.
Southern Alaska will be in a trough that is the westward
extension of a low over northern Canada. This trough will slowly
slide westward through Sunday. There are differences amongst the
models as to how strong this trough will be and where exactly it
will set up, so increasing amounts of ensemble means were used in
the forecast blend by next weekend. Even with the trough over
southern Alaska, temperatures are still expected to be above
normal.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 174 175 177 178 411 413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MV
MARINE/LONG TERM...BL