Dense Fog Advisory 2, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
304 PM AKDT Sun Apr 25 2021

.SHORT TERM.../Sunday evening through Tuesday night/

Another blue sky day across the panhandle today with seasonal
temperatures and very light winds. A light sea breeze was finally
able to overcome the northerly gradient in Skagway making for cooler
temperatures there than yesterday. We have one more sunny day in
the forecast tomorrow before clouds begin increasing in earnest.
There is a hint in some of the models (NAMDNG, Canadian) that as
onshore flow begins to develop, a marine layer with low stratus
may encroach upon the coast (about Sitka to Yakutat) starting as
early as tonight. This feature was added to the forecast grids,
mainly offshore, but confidence is low regarding if clouds will
increase this quickly. For Monday, gradually increasing clouds
and moisture will limit the daytime warming, with high temperatures
running generally 5 degrees cooler than the weekend.

As mentioned yesterday, a couple of weather systems, a wave from
the northeast and a front approaching from the southwest put a
squeeze play on the panhandle, forcing our high pressure off to
the southeast Monday night. The front will start moving rain
onshore early Tuesday. The northern wave moves south out of the
Canadian Arctic approaching the northern panhandle with an airmass
of colder air Tuesday morning. These features appear to phase and
bring cooler air across the northern panhandle and draw moisture
from the south. 1000-850mb thicknesses and model snow levels
point towards a 6-12 hour window Tuesday night into Wednesday
morning for snow to fall across the Icy Strait corridor and north
to Upper Lynn Canal. Any accumulations will be light and slushy
given the warm surface. For the southern panhandle, the
precipitation will be all rain, and 24-hour precipitation amounts
are in the 0.75-1.5 inch range. 30-35 kt winds will accompany the
front as it approaches the coast.

.LONG TERM.../Tuesday through Sunday as of 10 pm Saturday/

***SNOW POSSIBLE AT SEA LEVEL TUESDAY NIGHT***

A complicated forecast begins the long term period as a major
pattern change takes place across Southeast Alaska. Cooler
temperatures, rainfall... and yes, even the potential for snow are
back in the forecast.

The set up features a broad area of low pressure in the southwestern
gulf with multiple fronts and waves rotating into the Panhandle. The
first wave, which is more like an elongated E-W oriented front/low,
pushes into the Southern Panhandle Tuesday morning, then slowly
lifts northward through Wednesday. At the same time, an arctic front
will be dropping south across NW Canada with its associated lower
pressure remaining on the BC side of the Coast Mountains. Model
guidance today shows the possibility of these two systems phasing,
which would allow cold arctic air to spill south while warmer gulf
moisture overruns it... an ideal set up for snow in the north. With
a strong NE gradient setting up in the northern Panhandle, colder
air spilling south from Canada will be locked in place until the
main front pushes northward Wednesday. Deterministic models show
1000-850 mb thicknesses dropping well below 1290m from 06z Wednesday
through 18z Wednesday with 850 mb temps -4C to -10C along and north
of the Icy Strait Corridor. GFS, NAM, and EURO all show snow levels
dropping to zero during this timeframe as well increasing confidence
of the snow potential at sea level late Tuesday night. Since the
cold air moves in at night, strong heating from the late April sun
will not be an issue, creating the possibility for minor snow
accumulations. Currently have an inch or less for Juneau, Hoonah,
Gustavus, and Upper Lynn Canal Tuesday night through Wednesday
morning. This is a very complicated set up and confidence remains
low on the snow potential. Confidence is high that the southern
Panhandle will remain wet (rainy) during this time frame... ending
their 2 week streak of dry weather!

Once the front lifts north Wednesday morning, warmer air will push
in from the south changing all precipitation over to rain across the
north. Higher confidence exists in the precipitation forecast
through the weekend as multiple waves/systems move northward in the
eastern gulf. There are still model discrepancies regarding the
exact track of each feature; however the GFS and EURO and ensembles
are hinting at a much stronger area of low pressure moving north in
the eastern gulf Thursday into Friday. The current forecast reflects
this and keeps a high chance for rain across the entire region.
Southerly flow, rising snow levels, and rising 1000-850mb thickness
values raise confidence that precipitation type will be all rain for
the rest of the week.

For winds, expect an increase in winds across marine areas as each
front/wave passes by. Current forecast has small craft conditions
developing Tuesday as the first front moves through, then again
Thursday into Friday with the next system. Expect a possible
increase in the wind forecast for the end of the week as the
Thursday/Friday system could be stronger than the early week front.
Once there is a better consensus on the timing, placement, and
strength on the late week system, there will be higher confidence on
how strong winds will be. Currently have low end small crafts in the
forecast for that time frame, but this will likely be updated in the
coming days.

Used GFS/EURO for changes during the first half of the long term
then strongly blended toward WPC towards next weekend. Confidence
remains low through the long term with how dynamic and complex the
set up is. Higher confidence though in a pattern change back to
wetter, cooler weather.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

BFL/CM

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