Gale Warning 54, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
251 PM AKST Mon Feb 26 2018

.SYNOPSIS...As strong low pressure resides in the Canadian Yukon a
new storm force low will track E along the northern gulf Monday
night into Tuesday.

&&

.SHORT TERM...Rain showers will begin to remix with snow across
much of the Panhandle tonight as colder air pours eastward aloft.
While onshore flow and orographic enhancement will keep showers
going in unstable air, another wave tomorrow affiliated with the
storm force low moving in Tuesday will potentially yield some
light snow accumulations tomorrow and increasing Tuesday night
with potentially more impactful amounts. Wednesday and Wednesday
night we still expect drier conditions as the energy supporting
precipitation will have dug southward. Temperatures, marginal for
snow tonight will cool by a few degrees over the next several days
with more dramatic cooling once drying ensues.

Strong winds with Skagway leading the way with gusts
to 50 mph are steadily trying to diminish this afternoon. Yet breezy
conditions from the south and southwest will remain in the
forecast through the short- term period with a directional flip
to north likely occurring sometime Tuesday night/Wednesday
period. It is debatable at this time whether the outflow event
Wednesday/Wednesday night will be more local or a little more
widespread with southern Panhandle locations becoming gusty, but
this we can determine at a later time.

Few changes were made to the forecast as some disagreement Tuesday
night into Wednesday prevented us from making any confident
changes.

.LONG TERM.../Thursday through Monday/ As of 11 pm Sunday;
Digging mid level trough over the eastern AK gulf with a ridge
building from the N Pacific into the Chukchi sea will start the
long range forecast. The building ridge will be main feature for
most of the long range as it forms a blocking pattern that is
expected to last until at least Sunday before breaking down. Still
some uncertainty regarding when this happens. At the surface
shower activity will diminish through Wednesday night as low
pressure over the eastern gulf tracks to the south. Latest model
runs still differ on the position of the low center with the NAM
further east than the GFS. Main result from these differences is
extent of precip, with NAM further east having highest pop over
the coast mountains while the GFS keeps precip more over Baranof
island. Either way as the day progress Se AK transitions to a dry
pattern with increasing out flow winds as the low center sets up
over Haida Gwaii and an areas of high pressure develops over the
Yukon. Northerly winds over the inner channels at least hitting
gale forces with possible higher gusts. Due to the more N-S
orientation of the pressure gradient not expecting a Taku wind
event for Juneau, just outflow winds with gust of 40 to 50 mph. In
addition to the strong northerly outflow winds temperatures will
drop into the teens to possible single digits as cold arctic
airmass moves in. 850 mb temps fall into the -15 to -20 Celsius
range. Clear skies at night will help lower temps in wind
sheltered areas due to additional radiational cooling. By the
weekend pressure gradient begins to weaken as both the Haida Gwaii
low and Yukon high shift to the southeast. Next potential for
precip will be Sunday, if the ridge breaks down according to last
GFS.

Beside the difference between position of the surface low on
Wednesday models were in good agreement until Sunday. Used a blend
of 00z NAM with inherited forecast then kept with WPC guidance
for the rest of the long range. Besides starting adjustments for
local effects for temps and winds overall changes to the forecast
were minor. forecast confidence is average until it drops on
Sunday due to timing of the blocking pattern breakdown.

&&

.MARINE...The operative terms of the day are "winds!" and "seas!"
with exclamation points attached for emphasis. As a strong front
has blown through the Panhandle, pressure rises throughout our
region has set up very strong southerly and southwesterly winds,
especially through the northern and north-central Inner Channels.
We expect these winds to diminish fairly quickly this evening, as
a new storm force low treks across the northern gulf Monday night
into Tuesday. Thus, some gradients across the Panhandle will
diminish slightly tonight before re-engaging Tuesday with winds up
to 30 kt. Meanwhile very strong westerlies with cold air
advection will support storm force winds to 50 kts across the
central gulf. These winds will ease somewhat Tuesday. Seas of 20
to 30 feet will plague the gulf as well with such broad fetches.
Tuesday night, another low will develop across the central gulf
and it, too, will support winds and high seas. However, a deeper
low will form well to our southeast and thus flip directions
northerly. High pressure ridging to across the mainland will
support colder air developing to our north. This will likely set
up another outflow event by Wednesday night. Winds across the
north-central channels were increased yet again.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind until 3 PM AKST this afternoon for AKZ017-019-023-
     025.
     Strong Wind until 6 PM AKST this evening for AKZ018.
MARINE...Storm Warning for PKZ022.
     Gale Warning for PKZ011>013-021-031-032-041>043-051-052.
     Small Craft Advisory for PKZ033>036-053.

&&

JWA/PRB

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