Flood Watch 5, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
443 AM AKDT Sat Aug 18 2018

.ANALYSIS AND UPPER LEVELS...

A mature low stemming from the North Pacific is advancing slowly
toward the southern Alaska Peninsula, its warm front extending
from the eastern Aleutians into the northern Gulf. This can be
seen on radar with light rain beginning to fall from Port Heiden
east across the Barren Islands to south of Homer and Seward. The
low also continues to have favorable upper-level support this
morning as it sits under the left exit region of a 130kt jet
streak rounding the base of the long-wave trough. Strong gales
have been observed along the Pacific side of the AKpen with this
system. Colder air wrapping in along the northwest side of the low
has also produced gusty northerly winds approaching 35 knots
across much of the central and eastern Aleutians. The ridge that
dominated the weather across Southcentral yesterday is breaking
down in advance of the warm front and trailing cold front. This is
evident from the steady increase in mid-and upper-level clouds
stretching from the western gulf along Southcentral Alaska to the
Alaska Panhandle. The increase in cloud cover and easterly winds
along the coast is indicative of a return to the rainy and
unsettled weather pattern we saw earlier in the week.

&&

.MODEL DISCUSSION...

Models continue to improve with the placement of a North Pacific
low just south of the eastern Aleutians. Consistency also persists
in regards to the front in the west as well as the front entering
the Gulf of Alaska. Towards the end of the period, as a front
moves over the western Aleutians, there is some misalignment as
the EC is progressing eastward slightly quicker than the NAM and
GFS.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. Scattered
light rain and ceilings dropping to 5,000ft are possible between
18z and 00z as a warm front moves across the region.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The first of two fronts is moving into Southcentral today. This
warm front will bring rain to the coastal areas and a few areas
inland as well. It will mostly wash out as it heads northward
and crosses the Alaska Range tonight.

A second, stronger, front will move over Kodiak Island tonight
and then over Southcentral Alaska Sunday. This will bring
widespread gale-force winds to marine areas around the Gulf as
well as some gusty winds to Turnagain Arm and into the Copper
River Basin.

There will be enough rain along the Gulf Coast to raise river and
stream levels Sunday and Monday. At this time it does not look
like there will be enough to cause flooding concerns, but that
will be re-evaluated with the next forecast package.

As is typical in this situation, the big question is whether
there will be enough lifting and moisture to override the
downslope and bring some rain to the leeward side of the
mountains. It looks like there should be a period of rain Sunday
in Anchorage and the Mat-Su valleys, but the Copper River Basin
will likely remain mostly dry.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (through Monday night)...

A warm front from the Pacific will graze the mainland today, with
Greater Bristol Bay taking the brunt followed by the Kuskokwim
Delta. Light rain will spread over most areas, except for
downslope areas of the Alaska Peninsula/Aleutians. Along with the
spreading precipitation, gusty southeasterly winds will rise,
especially through Kamishak Gap and along the western capes. The
front will stall out overhead late tonight into Sunday, getting
reinforced by a disturbance aloft and moving it northward during
the day. Winds will drop off pretty quickly on Sunday behind the
front. Late Sunday into Monday, the area will be left under
weaker but moist cyclonic flow, keeping at least a chance of
showers in the forecast. A stronger disturbance will bring more
steady rain Monday evening into Tuesday. Temperatures will remain
right around climatological normals for this time of year with
mostly cloudy conditions.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (through Monday night)...

The gale-force front will continue to move from the north Pacific
into the Bering Sea today. Widespread rain will also spread across
the area along with the front. The front, bringing both winds and
rain, will approach the Pribilof Islands late tonight and
promptly weaken in place. A separate warm front will enter the
western Bering Sea today and slowly progress to the east through
Monday night.

&&

.LONG TERM FORECAST (Days 3 through 7)...

The long term forecast beginning Tuesday features a high amplitude ridge
extending from the Western US coast north into the Yukon, across
far Eastern Alaska and up into the North Slope. A negatively
tilted upper level trough with associated surface low will be
swinging NE across the Gulf of Alaska and into Southcentral,
supported by a strong southerly jet stream. The southerly jet will
tap into a good moisture fetch bringing periods of heavy rain to
the Gulf Coast. The low weakens as it moves ashore during the day
on Tuesday though showers will linger across most of the area
through Wednesday morning.

The low will act to push the down stream ridge further east,
allowing the persistent upper level low that has been dominating
the area the past couple of weeks to move east. This will allow a
more progressive pattern to set up across the region next week
into the following weekend. A ridge will move over the southern
mainland by Wednesday bringing a brief break in the weather before
another broad low moves in from the Bering Thursday bringing
another round of typical wet August weather to the region. Models
begin to diverge after this point with the EC quickly moving the
low east while the GFS keeps it somewhat stationary over the
Eastern Bering/Western Alaska. There is some consensus that
another ridge builds in over the mainland by next weekend but
obviously a lot can change in 7 days. Regardless, the region will
continue to see periods of cloudy and wet weather though
intermixed with brief breaks.

&&

AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 130 131 132 137 138 139 150 155 160 165 170 171
172 173 174 176 180 352 414.

&&

SYNOPSIS AND MODEL DISCUSSION...RJA/TM
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ML
LONG TERM...KVP