Small Craft Advisory 3, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
531 PM AKST Mon Dec 31 2018

.ANALYSIS AND UPPER LEVELS...
There is an upper level low over the central and eastern Bering
Sea. There is a large ridge over the eastern Gulf extending
eastward into Canada and southward through the far eastern Pacific
off the west coast of the lower 48. South to southwest flow aloft
is present between these two features. There is a strong surface
low (964 mb) centered just north of the central Aleutians that is
moving towards the east and slowly filling. There is a rapidly
developing surface low south of the Alaska Peninsula that is
moving northward. The air mass over southern Alaska is quite warm,
with temperatures in most areas above freezing this afternoon. A
colder air mass is over the western Bering sea and moving eastward
behind the central Aleutian low.

&&

.MODEL DISCUSSION...
The numerical models are in fairly good agreement for the next 24
hours. The Nam begins to lag with the surface low as it moves into
southwest Alaska on Tuesday, being further south than the other
models by afternoon. The GFS and ECMWF are in good agreement on
the path and timing of this feature. Tuesday night through
Wednesday afternoon the models continue in fairly good agreement
over the southern mainland, but differ considerably on a number of
small surface lows over the Bering. Thus forecast confidence is
fairly high for the southern Alaska mainland through Wednesday,
and fairly low over the Bering beginning Tuesday night.

&&

.AVIATION...
PANC...VFR conditions are expected. Wind shear is expected this
evening as the southerly surface winds end and strong easterlies
aloft return. Southerly winds will push back into Anchorage
Tuesday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Tue and
Wed)...
The brief "lull in the pattern" is upon us this evening. It will
be short lived. A new storm is developing south of Sand Point and
will "bomb" out over the course of this forecast period (this
means it will drop at least 24mb in 24 hours). As this system
rapidly deepens, it will spread another very strong front across
Southcentral AK. This will be very reminiscent of the front that
came through Sun night into Mon morning, but there are some key
differences.
First, nearly all locations are starting warmer this
evening. In fact, most places across Southcentral are above
freezing with the exception of the higher elevations (Turnagain
Pass, Thompson Pass, etc). This will likely lend to more of the
precipitation type starting out as rain or more of a rain/snow
mix, especially across the Portage Valley area. Cold air will try
to hang on and keep things snow initially, especially with heavier
precipitation rates. But it will be battling strong easterly flow
ushering in more warm air. The next effect should be lower snow
totals across most of the area than we saw from the Sun night
system. There is a healthy amount of instability with this system,
so this could make for brief periods of very intense precipitation
rates (and a slight chance for thunderstorms over the coastal
waters).

Winds will be the other big story with this storm. Again, in many
ways it will be quite similar to our last system. However, the
orientation of the front will be more of an east to west (or even
northeast to southwest) direction vs the southeast to northwest of
its predecessor. This is not as favorable for our channeled wind
gaps and should ultimately lead to winds being slightly lower than
Sun night. We will still see strong gusts through places like the
Turnagain Arm and Prince William Sound with a storm-warning in
effect for the North Gulf Coast. The Anchorage hillside is tricky
in this set-up. The geostrophic flow would lend to some strong
southeast winds blasting over the mountains. However, the down-
inlet ageostrophic component will be very strong. So what will
likely happen is that winds over the Central Chugach and Turnagain
Arm will take a "left turn" down Cook Inlet as they push off the
terrain/inlet.

By Wed, we see the entire pattern shift. A deep trough of cold air
starts to push in from the west and brings all temperatures back
below freezing. This cold air will also bring some gusty winds
from the west. This will favor the typical gaps in the AK Range
pushing gusts across Cook Inlet and Shelikof Strait. There does
appear to be enough moisture across Southcentral that this cold
air could wring out a few inches of snow before the moisture moves
to the east. The models are still sorting out the details of how
this evolves, so stay tuned. However, there is high certainty that
temperatures will return to below freezing for nearly every
location for the end of the week.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Today
through Wednesday)...

A large upper level trough over the northwest Bering is the main
feature guiding the progression and movement of our surface lows.
A fast moving intensifying storm is bearing down on the Alaska
Peninsula tonight and bombing to around 955 mb from its current
central pressure of around 980 mb. This will produce a brief
period of hurricane force wind gust south of the Peninsula and
across the Aleutian Mountains. Warm air will push into southwest
Alaska with this storm. This will make precipitation be rain or a
rain snow mix across the Bristol Bay zone. Cold air remains in
place over the Kuskokwim Delta, therefore precipitation will
remain snow. The QPF for the Kuskokwim Delta is low, with the
heaviest amounts along the upslope side of the Kilbuck Mountains
in the northwest flow, thus there should be some reduced
visibility in blowing snow, but generally not widespread or
significant enough to warrant an advisory or blizzard warning
Tuesday night or Wednesday, except in the unpopulated regions of
the mountains where no impact is anticipated. The low will then
continue northeast as a weak ridge builds into southwest Alaska
Wednesday. This will return the cold air back to southwest Alaska
dropping temperatures around 20 to 30 degrees by Wednesday and
Wednesday night from the current temperature regime.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Today
through Wednesday)...

A strong but decaying surface low over the central Aleutians is
tracking east and will cross through False Pass by Tuesday
morning. This low has good cold advection wrapping around the west
side of it that will result in a gusty showery regime as it moves
along the Aleutians, likely resulting in sporadic hurricane force
gusts in the Dutch Harbor area Tuesday, although these will be
minimally reaching warning criteria, if at all, thus we have
canceled the high wind watch. The low then rapidly decays to a
trough as it continues east into the Gulf of Alaska Tuesday night.

Behind this low cold air settles back over the Bering Sea with
weaker low centers rotating around the persisting broad upper
level low and trough. Expect frequent snow showers over most of
the Bering Sea and Aleutians into Wednesday.

&&

.LONG TERM FORECAST (Days 4 through 7)...

Southcentral:

Beginning the extended period forecast Wednesday night, there
will be a series of low amplitude shortwaves in southwest flow
aloft moving across the region. These will not be strong lows, but
with much colder air aloft (850 hpa temps around -10 to -12C),
there will be the possibility of snow across all parts of
Southcentral, although snow will generally be light. The threat
ends late Thursday with cold high pressure developing for the
weekend. This will result in cooling temperatures and eventually
below average temperatures,in conjunction with clearing skies. By
Monday, threats for coastal precipitation increase, but it looks
like colder high pressure may dominate inland weather heading into
early next week.

Southwest/Bering Sea:

Over the mainland, Arctic air will push
southwestward through the weekend, and conditions will trend
colder and drier day after day. An upper trough will persist over
eastern Siberia, and expect weak shortwaves to continue rotating
around the base of the trough Thursday. This will bring showers
over the Bering and Aleutians, possibly even into Bristol Bay and
the Y-K Delta Thursday. The upper trough will amplify Thursday
into Friday and a cut off low will form, bringing potential for
gale force winds over the western Aleutians Friday into Saturday.
The forecast becomes more uncertain Saturday, with run to run
consistency poor. In general, models point to a stronger low
developing north of the jet stream, bringing another round of
strong winds to the Aleutians Sunday or Monday. That said, much
will depend on the track of the low which models have had a tough
time resolving.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales...125 132 136 137 138 139 140 141 150 160 165 174
				 175 176.
		 Storms...119 120 130 131 155 170 171 172 173.
		 Heavy Freezing Spray...185.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SA
LONG TERM...JRA/MM