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FXAK67 PAJK 171419
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
619 AM AKDT Tue Jul 17 2018

.SHORT TERM...Showers continue across the panhandle this morning,
becoming more scattered in the northern half. A weak low in the
NE gulf is helping to enhance some of the showers along the outer
coast between Yakutat and Sitka this morning. It will slowly
dissipate through the day as will the showers. The southern
panhandle will see most of the action today. A trough over the SE
Gulf will develop into a weak closed low as it moves onshore over
POW this morning. This will cause a period of moderate rain
showers from Petersburg south. The low/trough will move through to
the east fairly quickly, with widespread showers ending in the
late afternoon to early evening. Expect some sunny breaks behind
the trough.

There is a weak pressure gradient over the area this morning
between the two lows. This will keep most of the winds light and
variable. As the trough/low moves into the southern panhandle,
expect winds to increase out of the south 10-15mph then shift out
of the NW once the trough passes. Tonight a broad ridge will start
to form over the Gulf of Alaska and keep W-NWLY winds of up to 15
kt in the forecast through Wednesday.

An upper level low looks to drop south over the region on
Wednesday. This will keep the risk of showers in the forecast, but
the building ridge will help to keep them widely scattered. Will
be watching for some showers to move out of Canada from the N-E.
The upper level low will also bring some cooler air with it from
the north. One of the trickier parts to this forecast is how many
sunny breaks and how much heating will occur to warm things up on
our diurnal curve. Warmed temperatures by a degree or two for the
northern half of the Panhandle for today.

With good model agreement forecast confidence is average. Used a
blend of the GFS/NAM for updating the forecast, with most of the
focus on winds and POP.

.LONG TERM.../Thursday through next Tuesday/ The good tidings we
bring for Southeast Alaskans is a dry week's end and above normal
temperatures.

Ensembles are lining up with the message that the omega block,
consisting of upper lows over both Southeast Alaska and the
Aleutians with a ridge separating the two, will be slowly led
eastward by strong westerly jet energy across the Arctic. Thus by
this weekend we should be firmly positioned under the ridge. As a
result, precipitation probability has been lowered to such
extent, we now forecast a bona fide dry weekend Panhandle-wide,
with showers diminishing this Thursday. Yesterday ECMWF hinted at
a thermal trough developing across the southern Panhandle, and now
the GFS hints at this feature as well. A thermal trough is exactly
what it sounds like, thermally induced low pressure, which means
warm surface temperatures. In addition, ECMWF raises freezing
levels up around 10K feet. With much stronger support and pattern
predictability, we have significantly raised temperatures as well
beginning Friday into early next week. While we do not see a
widespread record event, 2 to 3 standard deviations above normal
temperatures indicated by ensembles should place much of our area
in the 70s. While we conservatively characterize skies as either
mostly cloudy or partly cloudy due to easterly wave potential and
potential surprise onshore flow at mid-levels, we may be able to
emphasize sun as the weekend draws closer.

Given better confidence for stronger ridging and greater
signatures for a thermal trough in the interior southern
Panhandle, we significantly raised northwest winds in the gulf
and Inner Passage gateways like Cross Sound and Cape Decision to
20 to 25 kt. We also lightened Lynn Canal southerlies through
much of the period. For most of the Inner Channels, it should be a
nice period for marine activities with winds out of the north and
west.

We were able to use ECMWF and GFS (and early SREF for pops) along
with WPC to lower pops, raise temperatures, and solidify a strong
gulf ridge-thermal trough over Southern Panhandle type weather
pattern. The results are stunningly satisfying; we think most of
you will agree.

&&

.HYDROLOGY...The Salmon Glacier near Hyder is experiencing a
glacial dam release. At this time no impacts have been reported
and only minor impacts are expected. Some minor flooding near 9
mile is possible. As of 6am the river level was just above 23 feet
and is expected to reach 25-27 feet today or Wednesday, then
fall quickly.

The other Glacier Dam we are watching is Suicide Basin above the
Mendenhall Glacier. The water level has reached the top of the
basin and is now spilling over the top and flowing down along the
side of the Mendenhall Glacier. Lake and river levels have begun
to fall after recent heavy rain. We will continue to monitor both
the lake and basin levels for signs of a full glacier dam release
or other developments that could have impacts.

&&

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

&&

$$

KV/JWA

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