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308
FXAK67 PAJK 210004
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
304 PM AKST Wed Feb 20 2019

.SHORT TERM...A low pressure system and associated front is
tracking across the gulf. Overall the system has been sped up
compared to the previous forecast and timing of winter weather
advisories have been adjusted accordingly. Snowfall is expected
to begin in Yakutat between 9pm and midnight, with rates
increasing thereafter. Similar to the last weather system, expect
Yakutat to see a change to rain in the morning, limiting
additional snow accumulation and compacting what fell overnight.
As the front spreads into the panhandle after midnight, the
northern and central inner channels will see a period of heavy
snow. Pelican, Gustavus, Hoonah, Angoon and Kake will start
mixing with rain in the morning then change to all rain in the
afternoon. Expect Haines/Skagway and the Mendenhall Valley to see
snow slightly longer into the late afternoon/early evening, but
downtown Juneau and the airport will likely mix with in the
afternoon. See the latest statements for updated timing and
snowfall amounts.

The low pressure center will track SE along the coast through
Thursday and be just over Prince of Wales Island by Friday
morning. After the main portion of the front passes, expect a
transition to showers, mostly mixed. The precipitation will
decrease from north to south as the low passes by and winds will
see a shift back to a northerly direction.

CAA behind the low and increasing north winds will be the story
through Friday and into the extended forecast period. Confidence
in high POP on Thursday and becoming dry on Friday is high.
Confidence is lower on timing of change over to rain, and how much
that affects snowfall amounts.


.LONG TERM.../Friday through Wednesday as of 10 pm Tuesday/
Long range period will be dominated by a big upper ridge or closed
off upper high over the gulf and interior AK. The developing blocking
pattern could last a week or more. There are some model
differences on how far W the ridge axis will develop this weekend
and its evolution next week. Prior to the big ridge building, a
strong shortwave trof will move SE and out of the area during Fri,
with a weaker shortwave dropping SE through the area Sat as the
upper ridge to the W begins its amplification. The biggest
difference that result from the model discrepancies is how much
cold air will move into the area during the weekend into early
next week, and how much moderation might occur later on.

Did make some adjustment to POPs during the period, moving out the
precip faster Fri into Fri evening, then basically going
completely dry for much of the remaining longer range period. Will
need to watch for some higher level clouds moving in from the N as
shortwave moves through Sat, and will also need to monitor
potential for low cloud development later in the weekend and early
next week. This will depend on how warm it gets just above the
sfc, with the GFS warmer in the 925-850 mb layer than the other
models. If the center of the upper ridge/high gets close enough,
could see possibility of fog development that becomes persistent
once formed. Did make some temp tweaks, mainly blending in latest
NBM with previous forecast, but also lowered nighttime temps in
some of the usual cold spots even more with decent amount of
snowcover enhancing radiational cooling. Will see some rather
large diurnal temp ranges during next week as sun angle is high
enough to warm things somewhat during the day. In some places,
would not be surprised if the diurnal range reaches 30 degrees or
more. Forecast confidence is above average for POPs, but a bit
below average for temps, clouds, and winds.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Weather Advisory from 3 AM to 3 PM AKST Thursday for
     AKZ026.
     Winter Weather Advisory from midnight tonight to noon AKST
     Thursday for AKZ020-024.
     Winter Storm Warning from 3 AM to 6 PM AKST Thursday for AKZ025.
     Winter Weather Advisory from midnight tonight to 6 AM AKST
     Thursday for AKZ022.
     Winter Weather Advisory from midnight tonight to 9 AM AKST
     Thursday for AKZ021.
     Winter Weather Advisory from midnight tonight to 6 PM AKST
     Thursday for AKZ018-019.
     Winter Weather Advisory from 9 PM this evening to 9 AM AKST
     Thursday for AKZ017.
MARINE...Gale Warning for PKZ041>043-051-052.
     Small Craft Advisory for PKZ013-021-022-031>034-036-053.

&&

$$

Ferrin/RWT

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