Gale Warning 839, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
752 PM AKST Sun Dec 30 2018

.ANALYSIS AND UPPER LEVELS...
There is a mature vertically stacked low centered just south of
the western Alaska Peninsula. This low is moving rapidly northward
and will be a major player in the weather across the areas for
the next 24 hours. A number of High Wind and Winter Weather
Warnings have been issued for the area. The surface gradient with
this low is tight, and very strong winds are occurring from the
eastern Aleutians eastward across most of southern Alaska and the
Gulf. Precipitation is pushing northward into southern Alaska
with the leading front. Strong downsloping is keeping the northern
Cook Inlet area fairly dry, with heavy precipitation in upslope
areas along the Gulf coast and eastern Kenai Peninsula. Another
vertically stacked low is centered just south of the Rat Islands.
This low is moving towards the east. There is a strong blocking
ridge along about 135 degrees west longitude.

&&

.MODEL DISCUSSION...
The numerical models are in fairly good agreement today.
Confidence in the forecast is high for the first 24 hours. The
models are now coming into better agreement on the timing and
path of another strong low that will move north into the area
Monday night through Tuesday afternoon. They are all stronger and
further west with the surface low, taking it across the Alaska
Peninsula Monday night and into southwest Alaska Tuesday morning.
There is some disagreement on timing and exact path, but there is
enough agreement to justify extensively modifying the forecast for
this feature. However, confidence is only moderate with this
system.

&&

.AVIATION...
PANC...VFR conditions are likely to persist. Strong wind shear is
expected this evening as the southeasterly winds move aloft.
Strong southerlies are then expected late tonight into Monday
afternoon as a front works its way north of the area.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Tonight through
Tuesday)...

The first of two strong frontal systems is still on track to bring
an increase in southeasterly winds overnight to much of
Southcentral as well as snow to the higher elevations, including
Turnagain Pass and Thompson Pass. With the front continuing its
northward push, the low-level southeasterly flow has increased
over the region. For locations in the lee of the Kenai and Chugach
Mountains, this has helped produce a downsloping wind which has
increasing winds along the ridgeline and kept precipitation at bay
for the Anchorage Bowl, western Kenai Peninsula, and Mat-Su
Valleys. The southeasterly fetch has also brought warmer air into
the region which which has allowed coastal locations to warm up
significantly, with snow mixing with and changing to rain. Winds
will ramp up overnight as the front moves into the interior. Winds
confined to the hillside and Turnagain Arm will mix down and
become more southerly, allowing Anchorage to see a period of
strong winds with gusts at times around 60 mph. The winds will
subside from southwest to northeast across the Southcentral
mainland tomorrow.

The respite from the first system will be brief, however, as a
second powerful frontal system in association with a strong low
to the west sweeps in from the western Gulf. This system will act
much like the previous, with southeasterly winds rapidly
increasing late tomorrow, peaking over the Anchorage area
overnight Monday into early Tuesday morning before diminishing.
Snow will also develop once again for Turnagain and Thompson
Passes, where the combination of snow and gusty winds could cause
near blizzard conditions at times. Another surge of warmer air
will bring rain along the coast again, and potentially further
inland due to the erosion of cold air from the previous system.
Precipitation along the lee of the Kenai and Chugach Mountains
will also be limited due to a repeat of the downslope winds.
Conditions slowly improve late Tuesday as the front and the low
both track north of the area.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2: Tonight
through Tuesday)...

The southwest mainland is in for a wild ride over the next couple
of days as two strong low pressure systems move through the
region. First up, a low currently just south of Cold Bay is
rapidly intensifying as it tracks northeast. The low center will
cross the AKPEN this evening and move into Bristol Bay at around
960 mb. The biggest impact from the system will be very strong
winds, with the highest winds seen on the south side of the low.
Cold air wrapping around the backside of the low from the north
will help to mix down winds to the surface, leading to a strong
southwesterly low level jet. This jet will bring hurricane force
winds to the Pacific side of the AKPEN this evening before moving
up the southern Bristol Bay coast from Port Heiden to Levelock
overnight. Southwesterly winds gusting to 80 mph at times will
have a direct fetch up Bristol Bay and into Kvichak Bay. Luckily,
sea ice will help to diminish waves and surge some, though winds
could act to pile up ice onto the shore where the ice is not
shorefast. Further inland, strong southeasterly winds will be seen
through terrain gaps on the west side of the Alaska Range and
down across Lake Iliamna, gusting up to 70 mph before midnight.
Warm air being pulled north by the low will keep precipitation as
mostly rain for sea level locations.

The first low pulls north out of the area tomorrow afternoon,
providing a quick break before the second low approaches the area
by tomorrow evening from the North Pacific. This system will
track a bit further east than the first low, so strong
southeasterly winds will really ramp up through the Alaska Range
terrain gaps, especially through the Kamishak Gap and across Lake
Iliamna, where winds could be approaching hurricane force by
Monday night. The big challenge with this system will be
precipitation type, especially along the Bristol Bay Coast. Heavy
precipitation will spread north across the Kuskokwim River Valley
and Delta through the area Monday night and Tuesday morning.
While temperatures should remain cold enough to keep precip as all
snow in the Lower Kuskokwim River Valley and Delta (e.g., Bethel,
Aniak, Sleetmute), warmer air near the coast will make
precipitation type much more challenging to predict. If the low
center tracks a bit further east, then colder air would filter in
quicker leading to more snow, and vice versa. Right now, looks
like precip should be mostly rain along the coast then change over
to snow near the tail end of the storm on Tuesday
afternoon/evening as the low moves north and colder air filters in
behind it. Confidence is higher for the Kuskokwim Delta, where
heavy snow combining with gusty NE'ly winds will bring widespread
blowing snow during the day on Tuesday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2: Tonight
through Tuesday)...

An active pattern continues across the Bering as several strong
lows move through the area. The first low will cross the eastern
AKPEN tonight bringing a quick burst of SW'ly hurricane force
winds to the Bering and Pacific side of the AKPEN this evening.
Winds will relax some during the day tomorrow before another North
Pacific low moves across the AKPEN, bringing another round of
rain and winds, though winds should be weaker due to the further
east track of the system. The Western Aleutian low will continue
to track east across the island chain bringing a period of gale to
even storm force NW'ly winds from Atka across to Cold Bay Monday
night through Tuesday, enhanced by cold air wrapping around the
backside of the low. As the low tracks south of the Pribilofs,
strong NW'ly winds combined with snow will lead to periods of
blowing snow Tuesday afternoon to St. Paul and St. George. Cold
northerly flow looks to return to the Western Bering/Aleutians by
Tuesday as a deep upper level low moves off of Siberia.

&&

.LONG TERM FORECAST (Days 3 through 7)...

The run to run consistency with the global models continues to be
declasse. Generally speaking, anticipate a wet pattern for this
forecast period for the Bering and the southern tier of Alaska
with the possibility of a wintry mix for Southwest Alaska and
eventually creeping into Southcentral Alaska. The forecast period
commences Tuesday January 1st with a broad trough over NE Russia,
the Bering and Aleutians with southwest or southerly flow aloft
over most of mainland Alaska. There is a high amplitude ridge
from Hawaii to the Eastern Pacific to British Columbia. Meanwhile
at the surface at 12z Tuesday, both the GFS and the ECMWF have a
low north of Unalaska and a second surface low near Bristol Bay.
Neither model has either of these lows in the same place, with the
positions ranging 50-10 NM from the other solution. Yesterday's
eye sore courtesy of the GFS is still in the latest run of the
GFS but it is less intense and more plausible. The timing of the
precipitation shield and the gale/storm force winds are dependent
on the intensity and the placement of the lows that develop and on
the track of the low. There was more blending and hedging this
forecast package. Stay tuned for more details with these potential
winter weather storms. Many of the important decisions being made
with these storms are occurring one or two days out because the
models have not been doing well this winter.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning...101 161 181.
		 Winter Storm Warning...125 131.
MARINE...Gales...128 129 136 137 170 171 172 174 175 176 177
				 178 185.
		 Storms...119 120 125 130 131 132 138 139 140 141 150
				  165 173 180.
		 Hurricane Force...155 160.
		 Heavy Freezing Spray...185.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP
LONG TERM...PJS