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Southeast Alaska forecast discussion 
National Weather Service Juneau Alaska
515 am akdt Wednesday Jul 29 2015

Short term...early morning water vapor satellite image shows
broad upper level low over Alaska interior and the Yukon territory
with a shortwave trough remains across the western and northern
Gulf...maintaining a healthy jet stream heading toward Southeast
Alaska. Infra red image reveals a weak ridge of high pressure
builds over Southeast Alaska with continued moisture feeding from
the Gulf waters under persisting onshore low to middle level flow. As
the ridge builds...southerly pressure gradient remains tight
enough to produce small craft level winds over northern Lynn Canal
and 15 to 25 miles per hour winds across Skagway area. Extensive cloud cover
and showers have kept overnight temperatures mostly in the lower
to middle 50s.

Models remain in fair agreement for the weak ridge of high
pressure remains across Southeast Alaska through tonight. The
ridge will weaken and move southeast late tonight or early
Thursday morning as a developing low in the south central Gulf
tracks northeastward into the eastern Gulf on Thursday. Refreshed
the inherited forecast with the 06z run nam12 for its good

With continued deep onshore flow...numerous showers will continue
across the Panhandle today...then decreasing a little tonight as
the weak ridge starts moving south. Southerly pressure gradient
will consequently relax and the winds over the northern Inner
Channels will drop below small craft level late this afternoon
into early evening hours. Continued gloomy skies and showers will
maintain diurnal temperature fluctuation in a minimal range.
Expect daytime highs in the upper 50s to the lower 60s...and
overnight lows in the lower 50s across the northern and central
Panhandle to the middle 50s over the southern areas.

Long term...seemingly endless string of July rainy days continues
into the weekend (august) for most of the Panhandle. Culprit continues
to be an elongated west-southwesterly jet that has set up over the southern
Gulf and Panhandle. Dew points in the middle 50s and southeasterly
flow continue on Thursday...creating widespread rains despite modest
upper forcing. Latest model output delays the eventual drying
trend until early next week and this agrees with the trend
mentioned in the previous forecast package.

For Thursday inherited grids served nicely as only minor updates were
introduced with the new model thermodynamic updates. Upper wave
entering the southern Panhandle serves to increase rain rates
during the afternoon over the central and southern Panhandle. By
Friday morning the wave has led to a weak closed low aloft on the
forecasts over the northeastern Gulf. This could trigger the record-
breaker for total July precipitation in Juneau as 0.1-0.2/hr continues
there into Friday...only need an inch or so for the record as of 12z
today. Meanwhile gradients at the surface remain very weak as only
wind problems will likely be in northern Lynn Canal with 20kts
late on Friday as well as similar wind speeds on Thursday over the far
southern channels and adjacent outer waters on Thursday associated with
the surface low associated with incoming upper wave.

Chose showers as the initial Mode of precipitation over the northern
Panhandle until Thursday night with the rest of the Panhandle in
expanding stratiform forced by the upper wave. Then behind the
wave showers again take over for the north on Friday, spreading to
all areas by Friday night. Showers appear likely to continue in a
scattered fashion through Saturday, most numerous over the south
as offshore middle-level flow begins to take over the northern
sections and spreads slowly southward on the north side of the
upper trough. On days 5-6 (sun/sun pm) a chance of rain is
included for most areas as residual troughing now hangs on over
the Panhandle enough to prevent eliminating the possibility of
rain at this point. This is a new development not in the 12z wpc is the trend in the latest GFS/ec runs and have
included it in the grids for day 5. Even a subtle error in the
exact trough positioning could make the difference between wet and
dry conditions for a large part of the Panhandle at that time. By
the same token..assigned the Monday PM/Tuesday grids to near 0 probability of precipitation as
models agree on the drying trend at that point. Did this because
the model agreement is very strong at 6 days with the strong
Aleutian low persisting with ridging to the east over the eastern
Gulf and Panhandle. Out at even day 7 remarkable agreement between
the 00z ec/06z GFS exists on this scenario. Worth noting that the
southern Panhandle regions are a bit less certain of a dry early-
week scenario as the troughing persists just south of the
Panhandle until Tuesday morning on the GFS/ec updates...this feature
is even a bit slower to exit the southern areas on the GFS.


Ajk watches/warnings/advisories...
Marine...Small Craft Advisory for pkz012.




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