Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/05/19

Southeast Alaska Forecast Discussion

National Weather Service Juneau AK

 245 PM AKDT Sun Aug 4 2019

.SYNOPSIS...Low pressure persists over western Canada through
Tuesday, weakening at night and strengthening during the day. High
pressure remains over the Gulf of Alaska. A weak weather front
presses into the northern Inner Channels Tuesday from the
northwest.

&&

.SHORT TERM...Tracking the marine layer and its associated low
clouds and fog continues to be priority number one. With a broad
ridge over the Gulf of Alaska deflecting a deep moisture plume to
our friends over southwest Alaska and the Alaska Range and into
the Canadian Yukon, we are left with mainly dry weather, fog/low
clouds along and near the coast, and where sunny, fairly warm
conditions. Fog persisted longer than originally forecast over the
far southern Inner Channels and a morning dense fog advisory was
extended for a couple of hours. Mixing has eroded this layer early
this afternoon, but despite the late start to insolation we are
witnessing a temperature rebound that only such a warm ridge can
provide. In fact, 850 mb temperatures were near record high for
the date on the Annette 12Z sounding. And freezing levels measured
by MDCRS and soundings come in higher than 13,000 feet. Farther
north, a mature and robust marine layer advanced squarely into the
north- central Inner Channels last night and persisted over
Chichagof Island, Glacier Bay, the west side of Douglas Island and
western Admiralty Island most of the day. As a result,
temperatures are approaching 70 in Ketchikan, Sitka, and even
Juneau despite the added humidity and specific heat capacity. And
Wrangell has already reached 72.

What will the marine layer do tonight? We are thinking it will
invade our region in much the same spots. However, due to
shortwave activity tracking northwest to southeast across the
region, we are not expecting dense fog. However, some patchy fog
cannot be ruled out, and that was added. Short-wave activity may
also add the threat of an isolated shower in the far north during
the afternoon, however, a more significant front arrives from
Interior Alaska on Tuesday. Some models are indicating more pop in
this region, but given such a dry direction, we are not buying
wholesale.

Now finally we discuss temperatures. Ensembles have been
advertising such a warm period for days. Gradients are not
forecast to be as strong tomorrow, and this leads us to believe
that the marine layer may not advance any farther, and will likely
persist for shorter durations. Thus, we are more confident in
many of our 70s Monday and even Tuesday, particularly those
inland of the direct coast.

We made little change to the forecast. Our overall forecast
confidence is moderate.


.LONG TERM.../Tuesday through Sunday as of 10 pm Saturday
night/...Strong high pressure will remain firmly entrenched over
the southwestern Gulf of Alaska through mid week. By Wednesday
afternoon, current models are depicting an inverted trough of low
pressure as pushing west out into the Gulf of Alaska and
displacing the surface high further west. By Thursday, models are
developing a surface low over the northeastern Gulf and then
maintain a broad and weak area of low pressure over the majority
of the Gulf through the next weekend. This is a drastic departure
from model depictions yesterday and does not seem very probable.
Overall confidence in this extended range forecast is low as a
result. Thus, little change made to the extended range forecast
this evening. Accordingly, the present forecast still reflects
warmer than normal temperatures, non-impactful winds, easterly
wave showers, and variable cloudiness resulting from northwest
flow at the surface over the eastern Gulf for the coming week.

&&

.AVIATION...Aviation issues continue to be confined to coastal and
near coastal locations with a pesky marine layer limiting ceilings
and in some locations visibility. Ketchikan this morning suffered
under one quarter mile visibility in dense fog. An increase in
shortwave activity crossing our region beginning tonight during
the next two or three days, will in theory keep low clouds high
enough so as not to hinder aviation as badly as Ketchikan this
morning. However, confidence is poor, and our ridge over the gulf,
and hence all its fog/low cloud formation capabilities, is still
strong. While conditions along the coast look to fall tonight to
either MVMC or IMC, we likewise expect improvement Monday morning.
Many of our inland location will remain VMC through the 24 hour
period.


&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...None.

&&

$$

JWA/Fritsch

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