Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
400 PM akst Friday Nov 27 2015
Short term...occluded front over the eastern Gulf will move into
the northern Panhandle tonight...then reach the Central Area Sat
morning before stalling. The front will begin to drift back to the
northwest during the afternoon. Looks like one or more small low pressure
waves will move along the front tonight and Sat. Used blend of the
12z GFS/NAM for tonight and Sat.
Main forecast concerns will be precipitation potential/amounts...ptype
over the far north...winds...and threat for thunderstorms. Right now...precipitation
is falling generally west of a pajn-pasi line...and will spread eastward
through the night as front moves in. Think it will get to the
S-central and southern coast around midnight tonight...then the pakt
area toward morning. Think precipitation should remain west of the Hyder
area through Sat but a few sprinkles or flurries cannot be ruled out
there on Sat. There will be a lull in the precipitation behind the front
over the northwest third of the area Sat...but not enough to lower probability of precipitation
below likely at this point. Precipitation will increase over the northern area
again in the afternoon as front begins to head back to the northwest in
the afternoon. Rainfall will be generally 2 to 4 inches over the
central Panhandle through Sat...but considerable variations over a
short distance will occur due to terrain influences. This will
cause the smaller streams to rise a decent amount but they are not
expected to get above bankfull levels. Further north...rainfall up to
1.5 inches is expected through Sat...with similar amounts over the
southern coast. Pakt area will probably see around 0.25 to 0.50 inch
Precipitation should be all rain at sea level...with only the western parts
of the chilkat valley and higher elevations around pagy getting
snow out of this system. The western chilkat valley will start as snow
this evening...but should change to rain by late Sat morning. Snow
levels around pagy will be around 1700 feet to start
tonight...then rise to about 2500 feet Sat. Think the western chilkat
valley will get 1 to 3 inches of snow tonight. White Pass will see
locally heavy snow tonight...then this will diminish some Sat
morning before picking back up toward late afternoon. Decided to
keep the Winter Storm Warning going for White Pass through the
short term period...even with the diminishing trend Sat. Think
around 6 inches of snow will fall near White Pass tonight...with
around 4 inches for Sat.
Strongest winds will be just ahead of the front. Expect gale force
S-southeast winds ahead of the front tonight...with this area shrinking
in areal coverage to the southeast Gulf Sat morning. Most Inner Channels
will see winds peak at Small Craft Advisory levels...with some gales for parts of
Stephens Passage SW of Douglas Island...eastern Frederick Sound...and
Clarence Strait tonight. Stephens Passage should drop below gales
late tonight. Will have strong wind headlines for the central and
southern coastal areas for at least part of tonight before winds drop
off toward late tonight. The southern coast will see locally strong
winds again later Sat morning and afternoon as low pressure waves
move along the front to their west.
Looks like far western parts of the Gulf marine waters will have risk
of thunderstorms late tonight and Sat morning as shortwave passes through
them with cold air aloft. This will diminish later Sat as warmer
air aloft diminishes instability.
Long term...wet weather and above normal temperature pattern
continues through the extended range but some increased
uncertainty for the start of next week between upper level
troughs moving over the region. Through the weekend upper level
trough over Kodiak moves eastward with long fetch of moisture on
Lee side. This will keep the warm southerly flow pointed at the
coastal and central Panhandle. Frontal boundary will again move
over the Panhandle Sunday into Monday under 500 mb vorticity maximum with
rainfall rates beginning to taper off. As this 500 mb trough
tracks to the northeast a second closed low will moves in toward
Kodiak from the Kamchatka peninsula, but system in between in the
question. Indications of a wave moving up the western Continental U.S.
Coast, then merge in with the second Kodiak upper level low. At
the surface if the Pacific low forms then it can 1) bring in some
colder air from the north and 2) with offshore flow produce a dry
break for the central and northern Panhandle. However large spread
in operational models for this time period. While model spread
increases still expecting a warm and wet pattern to continue,
sorry to all skiers no big snow events expected. May see some mix
of rain and snow over the local peaks but snow levels still near
the peak tops. The exception being snow along The Klondike
Highway with the Winter Storm Warning lasting until midday Sunday.
Over the southern Panhandle winds will remain strong as the next
waves moves Saturday night then diminish Sunday, but quickly pick
up again for the all of the coastal waters by Monday. Gradient for
the far northern Inner Channels will flip north on Monday with the
advancing surface low but then shift back. Rest of the Panhandle
will still be south to southeast due to geostrophic flow and
either parallel or weak northerly gradient. Next flip in winds
will occur middle week with next advancing system and more
pronounced northerly gradient. This is in conjunction with some
potential cooler air moving in, but as middle and upper level flow is
still southerly do not expect any Arctic air moving in.
Continued with a 12z NAM/GFS blend to link with short term then
transitioned to a GFS/European model (ecmwf) blend through Monday. Not enough
confidence to pick one model with the individual waves. For Monday
Onward as spread in operational models becomes rather large used
wpc which was a blend of ensembles. Forecast confidence is
average until Monday then drops.
Hydrology...the heavy rain bands will transition over to the
northern and central Panhandle tonight and with the front stalling
Saturday morning, the heavier rain will linger through the
weekend. Around 3 to 5 inches of rain is expected through midday
Sunday. While rivers levels will rise in response to the heavy
rain no flooding is expected at this time.
public...strong wind until 3 am akst Saturday for akz027.
Strong wind from Saturday morning through Saturday afternoon for
Strong wind until midnight akst tonight for akz017-022.
Strong wind until 3 am akst Saturday for akz023.
Winter Storm Warning until noon akst Sunday for akz018.
Marine...Gale Warning for pkz031-034-036-041>043-051.
Small Craft Advisory for pkz011>013-021-022-032-033-035-052-053.
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