Flood in Alaska, June 29 to 30, 2021

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
554 AM AKDT Tue Jun 29 2021

.SHORT TERM...Change is on its way today, but how much change
depends on where one resides. An upper level trough taps some
momentum from a passing shortwave across northern Alaska and
begins to slowly nudge the massive high pressure ridge over
southern BC into Alberta over the next 24 hours. For Southeast
Alaska all of this allows more mid to high level cloudiness
across the area. At the low levels, cooler air behind the trough
strengthens surface high pressure across the Gulf of Alaska. With
stronger high pressure, we do expect stronger east- west pressure
gradients through the Inner Channels today: up to 25 kt through
Cross Sound and Icy Strait this afternoon. A ragged marine layer
expanded across the gulf yesterday evening and became even more
robust and impressive overnight as it swallowed Yakutat and lies
along the coast of Baranof Island. This morning it knocks on the
door to Gustavus, too. Winds at Rocky Island have blown 20 to 25
kt from the west all night. We expect enhanced westerlies in Sitka
for much of Tuesday as well. Lift from passing vorticity tonight,
coupled with the marine layer, may produce some drizzle along the
eastern gulf coast tonight into Wednesday morning. With much
cloudier skies along the coast, high temperatures will come
crashing down by 10 to 15 degrees this afternoon, in comparison to
Monday. Westerlies should spread a bit more moist air into the
Inner Channels even as skies will be partly cloudy at worst. This
will help lower temperatures back into the 70s from near 80
through the Inner Channels, but will not manifest itself as such a
stark change as that along the coast.

Switching gears, a low off the Oregon Coast failed to phase with
the upper trough over the gulf. This leads to more dry weather
for the south, instead of the chances of showers we feared for
Tuesday during this past weekend. But as the center of the upper
low treks eastward across southern Alaska, a chance of showers
spreads into Upper Lynn Canal as early as tonight. By Wednesday
afternoon, instability parameters rise to a level where we felt
comfortable to place a slight risk of thunder. Even Juneau may
experience a slight chance of a shower Wednesday night underneath
the passing low, but confidence is not high.

Cloud cover increases and thickens across the region Wednesday,
and temperatures fall below 70 even throughout the Inner
Channels. Even so, unless you are along the coast of Baranof
Island and possibly in Upper Lynn Canal, the forecast remains dry
for the rest of the Panhandle.

.LONG TERM.../ Thursday through Tuesday / A weak trough aloft
will be exiting the panhandle and moving into British Columbia
Thursday, and the few showers that remains associated with it
will be tapering off. A sharper ridge exists behind that trough
and ahead of the deep low near the eastern Aleutians. The low
moves northeast to be near Nunivak Island Saturday morning. A
trough from the eastern feature tries to push through the
panhandle. The models think that this may happen Saturday,
however, with a blocking ridge, models have a bias for trying to
break them down too soon. Have adjusted for that and stopped that
trough off the coast. A follow on trough now in the zonal flow
from South of the Aleutians across the gulf should make land fall
later Sunday or Monday.

This zonal flow pattern will be the start of the shift in the
weather pattern, as an upper level low is moving to occupy the
northern gulf late Tuesday. This will shift the feel of the
weather from summer to near fall, just on the warmer side.

Temperatures for much of the SE Alaska panhandle will be in the
60s near 70, with Yakutat on the cooler side of that range. Could
possibly see some light rain Thursday as the trough moves through
the panhandle. Saturday for the Northeast gulf coast would be the
earliest any rain is anticipated, and it is beginning to look as
if it may not be until Sunday or even Monday that something gets
to the forecast area.

One of the other big concerns especially over the gulf with a
nearly persistent surface high in place, will be the potential
for a marine layer to form and start moving about the gulf coast
areas. This will play havoc with temperatures, and sky for the
following week. Confidence is average for the extended time frame.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Areal Flood Advisory until 2 PM AKDT Sunday for AKZ026.
     Areal Flood Warning until 2 PM AKDT Sunday for AKZ025.
     Areal Flood Advisory until 10 AM AKDT Thursday for AKZ019.
     Flood Watch through late tonight for AKZ018.
MARINE...Small Craft Advisory for PKZ021-022-041>043.

&&

JWA/Bezenek

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