Gale Warning 163, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
214 PM AKDT Wed May 9 2018

.SHORT TERM...Another warm day across southeast Alaska in spite of
the increased cloud cover since yesterday. Warmest conditions
observed as of early this afternoon were in Skagway where the
mercury rose to 75 degrees shortly after noon. Precipitation
associated with a weak frontal boundary has been making slow but
steady progress to the north through the day as well. Rain along
the southern outer coast has been going on since last night and
has been on and off over the central outer coast since morning.
Rain over the southern inner channels began early this morning and
progressed as far north as Frederick Sound by noon. Precipitation
is expected to continue moving north with the front, but total
rainfall should diminish as the front encounters a drier airmass.
Winds associated with this front have not been excessive, but a
subtle gust front is expected to accompany it as it moves north.
This has already been observed over Clarence Strait where light
and variable winds organized out of the south and increased to 20
kts soon after noon. Small craft intensity only expected over
northern Lynn Canal and for only a few hours later this evening.

A slightly cooler airmass behind the front should yield cooler daytime
highs tomorrow over the northern half of the panhandle with
similar conditions over the south as were experienced today.
Friday's highs will be cooler still, ranging from the upper 40s to
mid 50s. Overnight lows should bottom out in the low 40s, but
there is a concern about expected partial clearing of the skies
Thursday night over the central and southern inner channels. If
that in fact does happen, temps may fall into the mid 30s. Model
guidance not indicating this at the moment, nor is ensemble spread
guidance when compared to existing forecast. For now, have made no
downward adjustments to Thursday night's lows, but stay abreast of
forecast updates if you have frost-sensitive plants outdoors
already.

By Friday morning, another front will arrive over Southeast Alaska
from the west. This front has the characteristics of a weak
atmospheric river, with IVT values of 250 to 500 kg/m/s as it
arrives on the outer coast. That translates in rain, heavy at
times, along the northeast gulf coast beginning early Friday
morning and spreading inland over the northern half of the
panhandle through the day. Gale force winds over the gulf will
also accompany this front, beginning west of Icy Bay Thursday
night and extending east and south to the Cross Sound area by late
Friday morning. Expect small craft winds over the inner channels
as well with most likely locations being the east-west
channels...and of course Lynn Canal.

Adjustments to the forecast  limited to temperatures for today.
PoP was decreased over central and southern locations for Thursday
and the advance of the Friday front was sped up by 3 to 6 hours.
Remaining elements unchanged. Overall forecast confidence is
average to above average.

.LONG TERM.../Friday through Wednesday as of 10 pm Tuesday/
Large scale upper level amplification will be underway at start
of long range period. Deep trof will develop along 160W, while
ridge builds just E of SE AK. This will set up yet another
atmospheric river (AR) event for the area, with the main core of
it aimed at the N gulf coast. Due to differences between the
models handling of the S end of the upper trof, they differ on
how long the more significant rain lasts. Toward the end of the
period, there could be another deep trof moving into the same area
as the one at the start of the long range. There are significant
differences between the models handling the energy within the trof
though. For the long range, used blend of 18z GFS/NAM for 15z
Fri-06z Sat, then went with mainly WPC from 12z Sat onward.

Did increase winds over the gulf and much of NW half of the inner
channels for Fri with the front moving in. Looks like gales for
the N gulf ahead of the front. Could be SCA level winds over
mainly the N inner channels as well. Still some model disagreement
on how far E the rain gets, but they agree the NE gulf coast
should get hit, so kept highest POPs there Fri. The far SE area
may end up missing precip from this system as the upper ridge axis
will be very close to them.

Once this system moves out, looks like a break between systems,
at least for early next week, as some ridging moves in. Late in
the period, the large model differences handling the next major
trof make for low confidence in how long the break will last.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ012-041-052.
     Small Craft Advisory due to Seas for PKZ042-043.

&&

Fritsch/RWT

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