Area Forecast Discussion
Issued by NWS Juneau, AK

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FXAK67 PAJK 202243
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
243 PM AKDT Tue Apr 20 2021

.SHORT TERM.../Through Wednesday night/...An extensive marine
layer present along the outer coast early today largely
diminished by late morning. Farther inland, broken to overcast
conditions gave way to sunny skies by this afternoon. Clouds over
the eastern Gulf continue to flow west towards the outer coast,
but have the appearance of a more convective environment rather
than a stratiform marine layer. Higher up in the atmosphere,
mostly cloudy conditions at the start of the day gave way to
partly cloudy or even sunny skies. Current forecast calls for
mostly cloudy skies north of Frederick Sound tonight, but
satellite imagery shows the present cloud deck to be somewhat
thin. This could result in another night with a large diurnal
spread and overnight lows in the low to mid 30s. Complicating
this, models are indicating a short wave dropping southeast out of
the interior and the Yukon later tonight. Models are in good
agreement that this feature will contain some measurable
precipitation. This feature can be seen easily on IR satellite
imagery, but current visible channel imagery shows the
corresponding colder clouds to be rather thin and not very
representative of precipitation bearing clouds. The afternoon
forecast issuance reflects the idea that models are correct, but
no precipitation being reported where those clouds currently are
does not instill a sense of confidence. If, in fact, the
precipitation does happen as modeled, the outcome could likely be
snow for White Pass. Other locations are not expected to get cold
enough for snow before the short wave passes into British
Columbia.

In the wake of this short wave, strong high pressure over the
interior will build southeast in the Yukon and create a stiff
northerly pressure gradient over the far northern inner channels.
Have included small craft advisories for Lynn Canal, Stephens
Passage, and southern Chatham Strait for tomorrow as a result.
Clearing skies early tomorrow should provide for slightly warmer
daytime highs in spite of the weak cold air advection that will
accompany the short wave tonight.


.LONG TERM.../Thursday through Tuesday as of 10 pm Monday
evening/...A large blocking ridge over the eastern Pacific
extending into the Northwest portion of Alaska continues Thursday.
Part of an upper level (500 mb) low, south of the central
Aleutians, slides southeast. As it does, the low continues to move
southeast of the CONUS into Saturday. The upper level ridge
repositions to extend from the north central pacific, through the
gulf to Alaskan interior for Sunday and Monday. The pattern Monday
starts to look more chaotic as another low from the Bering moves
into the western gulf.

With the blocking pattern in place aloft, the forecast for SEAK
is expected to be dry. There may be a possibility of rain around
Friday night/early Saturday from a low in the gulf, or maybe some
drizzle along the coast from the marine layer on Wednesday.

Although conditions remain dry, sky cover may not remain as clear.
A marine layer is expected to push into Southeast Alaska Tuesday
night into early Wednesday. With this, temperatures are expected
to be cooler. Seasonal normals include Highs in the 50s to 60s.
The marine layer is expected to move offshore or be eroded by
Thursday and late Thursday into weekend as the potential for
sunshine returns. This being said don`t think the temps will push
back to the warmth of the last couple days for SEAK.

Did not need to make many changes to the forecast package.
Forecast confidence is average to above average as a more stable
forecast pattern develops.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ012-013-031-033-041>043-051-052.

&&

$$

FCF/CDC/Bezenek

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