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Southeast Alaska forecast discussion 
National Weather Service Juneau Alaska
308 PM akdt Tuesday Jul 28 2015

Short term...persistent onshore flow has resulted in a cloudy and
showery day across Southeast Alaska. This will remain the case
over the next 24 hours with little change in the overall weather
pattern. There will be a shortwave moving into the southeast Gulf
late Wednesday afternoon which will increase shower coverage and
intensity over the southern Panhandle. For the remainder of the
Panhandle, cloudy skies and scattered to numerous showers will be
the rule of thumb through Wednesday afternoon. Rainfall totals are
not expected to be overly impressive with up to 1 inch possible.
Highest amounts will be over the higher elevations of the Coast

Weak ridge of high pressure building over the Panhandle has
resulted in gusty southerly winds this afternoon especially in the
northern Panhandle. Skagway and Haines have gusting to 30 to 35
miles per hour with small craft winds at Eldred Rock. Expect this to
slightly diminish later this evening as the pressure gradient
relaxes a bit. Think the pressure gradient will then restrengthen
again Wednesday morning and into the afternoon as the surface
ridge sharpens in response to the wave moving into the southeast
Gulf. So have increased winds back to small craft level for
northern Lynn during this time frame. Has been the case for the
past several days there will not be much in the way of a diurnal
temperature range. Overnight temperatures will be between 50 to 55
degrees with high temperatures on Wednesday only reaching around
60 degrees.

Used the 12z NAM and hires arw/nmm for updates through the first
24 hours of the forecast. Changes to the forecast were minimal.
Forecaster confidence is above average on continued cloudy and
showery conditions.

Long term...long range forecast remains rather damp through the
first half of the weekend this is followed by the possibility of a
few day dry spell. The primary cause is the variations in the
structure of a broad upper level trough over most of Alaska. We
start out the long range with the trough axis aligned north to
south around 155 west while a broad weak ridge sits over the western
U.S. That pattern leaves the Panhandle under SW onshore flow
through at least Friday or Sat with one or two very weak short waves
embedded in the flow. Into the weekend and early next week
confidence in the pattern becomes lower. For the past 24 hours long
range models have been suggesting that a piece of energy plunging
south through the Bering Sea would deepen the trough and slightly
retrograde it farther west allowing a ridge to build over the
eastern Gulf and Panhandle. The ensemble models are still
suggesting this but the new 12z runs from both the European model (ecmwf) and GFS
has gone back to the wet scenario that they were supporting two
days ago with the trough farther east. With the ensemble models
still in support of the trough farther west and this change in the
operational models being rather recent, I am still going for the
trough farther west with a building weak ridge over the Panhandle
trend but confidence is lower then yesterday.

At the surface, rain continues through at least Friday under
persistent and deep onshore flow. A weak short wave embedded in
the flow will make the Middle Range forecast somewhat difficult as
models are at odds on whether to be progressive and send the wave
right through the southern Panhandle (nam) or have it get wrapped
northward to stall in the eastern Gulf for a day or two (ecmwf
and gem). The better consensus is the wrap north and stall which
will keep rain around about a day longer than the progressive
solution. In any case the southern Panhandle will be the one to
take the brunt of the precipitation from this system the rest of the
Panhandle south of Haines and Skagway will see some rain as well
but will not be in the main moisture feed

Farther out, confidence is still decent for a drier late weekend
into next week as a ridge builds over the Panhandle and eastern
Gulf. Confidence on this took a hit with the 12z runs of the GFS
and European model (ecmwf) going back to a wetter solution after a few runs of
going for the dry solution. However, ensemble solutions still
support a drier scenario and the newest 18z run of the GFS went
back to supporting a drier solution. In light of this decided to
stick with a drier and a little warmer forecast but still retain
the slight chance of precipitation in the extended.


Hydrology...water levels continue to rise on Mendenhall Lake due
to recent heavy rainfall and a suspected glacial dam release. The
lake is at 8.3 feet as of 2 PM. While much uncertainty exists in
how much water is in the basin, current forecasts indicate the
lake will crest this evening and fall steadily overnight.
Persistent rainfall has also led to heightened water levels on the
taiya river which has reached minor flood stage at 16.5 feet as of
1 PM. A Flood Advisory has been issued until midnight tonight for
the taiya. Rainfall rates will decrease this evening in the far
northern Panhandle with water levels expected to slowly recede
overnight. Refer to the advisory statements for the latest
information on Mendenhall Lake and the taiya river.


Ajk watches/warnings/advisories...
public...areal Flood Advisory until midnight akdt tonight for akz018.
Marine...Small Craft Advisory for pkz012.
Small Craft Advisory due to seas for pkz041-042.




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