National Weather Service United States Department of Commerce


000
FXAK68 PAFC 220045
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
445 PM AKDT Sat Apr 21 2018

.ANALYSIS AND UPPER LEVELS...
The two main players today are a northward moving trough over
Southwest Alaska/Southcentral and a developing surface low and
associated front approaching the Southern Gulf/Kodiak Island. The
first of the two features is producing widespread cloud cover and
shower activity across the Gulf and Southern Mainland. Surface
pressure gradients across Southcentral have increased and given
rise to gusty gap flows along the Copper River, Turnagain arm,
and over the Knik Valley. The second of features is bringing rain
along the Alaska Peninsula and over Southern Kodiak Islands.
Easterly winds are also increasing over the Southern and Western
Gulf in advance of the frontal trough and developing surface low.

Generally cloudy conditions with some areas of light rain continue
across much of the Bering Sea and Aleutians. Conditions are
becoming more showery over the eastern Bering Sea as an upper low
retrograding over the area combined with low level cold advection
increases instability over the region.

&&

.MODEL DISCUSSION (through Monday afternoon)...Overall, good
synoptic agreement.

&&

.AVIATION...
PANC...VFR conditions with showers this evening. LLWS potential
will develop overnight with the surface winds shift to the north
and winds above the surface remain southeasterly.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Sunday and
Monday)...

The scattered shower activity over much of the Cook Inlet region
this afternoon will diminish tonight with the loss of solar
heating. Broad southeasterly flow will keep the precipitation
ongoing through the southern Kenai Peninsula and Prince William
Sound as the winds are upsloped over the Chugach and Kenai
Mountains. A front currently entering the southern Gulf will push
northward, resulting in gale force easterly winds and locally
heavy rain through Kodiak, the Barren Islands, and across much of
the rest of the Gulf tonight. The Knik River Valley and Matanuska
Valley winds will continue overnight, moving southward along the
Chugach down Knik Arm.

On Sunday, the front will move into the northern Gulf and the
north Gulf Coast, reinvigorating the rainfall and higher elevation
snow throughout the day. The front`s presence will locally diminish
the off-and-on showers during the day in favor of a steady rain.
With the increased southeasterly flow, the Cook Inlet region will
return to downsloped conditions with mostly cloudy skies and
channeled gap winds. As the front moves into the coast, stronger
northerly down- Valley flow will push the Turnagain Arm winds to
the Kenai Peninsula side of the Arm. However, the Matanuska Valley
wind will contribute to the northerly winds blowing through much
of Anchorage during the day, especially along the Knik Arm. The
next low moving into the southwestern Gulf will restart the
steadier rainfall across Kodiak Island late in the day. A 40 kt
barrier jet developing ahead of the front early in the day will
persist along the north Gulf Coast into Prince William Sound
throughout the day.

By Sunday night, the low over the southwestern Gulf will weaken as
it moves northward off the Gulf coast side of Kodiak Island. The
barrier jet will persist throughout the night, along with heavy
driving rain through Prince William Sound. This will also support
the continuation of the Turnagain Arm and Knik/Matanuska valley
winds. The low rapidly weakens during the day Monday, resulting in
the weakening of the barrier jet. However, because the low turned
remnant trough moves into the southern Kenai Peninsula during the
day, the weakened downsloping will result in an increased chance
of some scattered shower development across the Cook Inlet region
and Susitna Valley, especially Monday afternoon and evening.
The remnant Turnagain Arm and other downsloped channeled wind may
keep Anchorage and immediate leeside communities dry.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Sat and Sun)...
The longwave pattern features a dome of high pressure over NE
Russia with a deep vortex over Western and NW Alaska. Some very
cold temperatures aloft are associated with this vortex over
Hooper Bay. Today both the radars at Bethel and King Salmon have
been very unstable with a report of small hail in Levelock. More
isolated to possibly scattered convection is anticipated this
evening. Sunday, this vortex move into the Eastern Bering and then
the path will recurve. This recurvature will then bring a more
organized band of precipitation into southwest. Additionally,
expect gale force winds in Bristol Bay as the pressure gradient
force intensifies.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

The latest Himawari satellite image shows a mature low near
Kamchatka and GOES-West has a mature system with an elongated
occlusion south of the Aleutians. There is a vast swath of cold
air strato-cu over the Bering along with some mid-level clouds.
The latest ASCAT (advanced scatterometer) pass depicts gales with
the storm south of the chain. For the low south of the chain, the
models propagate the main low eastward, but an inverted trough
develops tonight. A second piece of energy will move into the
region virtually marrying up with remnant energy from the southern
low. And by Sunday morning there is a new surface low near the
Pribilof Islands also producing gale force winds. Meanwhile,
upstream the Kamchatka low will continue to deepen as it pushes
towards Attu Island producing gale force winds for the Western
Aleutians.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning Monday night a broad area of low pressure will continue
to dominate the Bering with its front having pushed through the
southern mainland bringing rain to the north Gulf coast and
southwest Alaska. Current model runs are in better agreement than
previous runs with a triple point low forming in the eastern
Bering Monday night, but there does remain some discrepancies with
placement of this feature. These discrepancies grow through
Tuesday as the triple point low moves into the northern Bering
while the parent low tracks east. Impacts will remain fairly
similar for the southern mainland and Aleutians even though
guidance is struggling with where exactly to put these two lows.
On Wednesday high pressure will build in from the west and the
models fall apart with how to handle low pressure over the Bering
with widely varying solutions. This resulted in ensembles being
preferred for the remainder of the week into the weekend.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale warning 119 120 130 131 132 136 137 138 139 150 160
176 177 178 180 181 185.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...BC
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS
LONG TERM...DK


965
FXAK69 PAFG 212236
AFDAFG

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
236 PM AKDT Sat Apr 21 2018

.SYNOPSIS...
Aloft...
A low near Nunivak Island will move near the Pribilof Islands by
4am Sun, persist through Sun PM, then move to the central Bering
by 4pm Mon, and persist there into Tue.

A weak short wave south of the Alaska Range will move over the
southern Interior tonight and weaken. A second and stronger
short wave will move north over SW Alaska Mon. Weather with these
features will be addressed with surface frontal systems below.
South flow aloft will gradually increase Sun and Mon causing weak
chinook conditions and warming temperatures over the SE Interior.

A third and much stronger short wave moving north over the West
Coast and Western Interior Tue will bring stormy conditions to the
West Coast and Western Interior Tue, and strong Chinook
conditions to the Eastern Interior Tue.

The low aloft will weaken and progress east late next week causing
cooling conditions late next week.

Surface...
A 1028 mb high over the Chukchi Sea will build east over the
Beaufort Sea Sun and Mon, then persist into Tue.

A low pressure trough from Barrow northeast will move northeast
tonight then weaken. Patchy stratus and fog is associated with
the trough.

A 995 mb low near Nunivak Island will move tot he southeast Bering
Sea tonight, then strengthen to 983 mb by 4pm Sun, then move to
the Central Bering as a 970 mb low by 4pm Mon and persist into
Tue. An arctic front stretching from the low to Minchumina to
Chicken will move to Unalakleet to Galena to Chicken by 4am Sun,
to Gambell to Huslia to Eagle by 4pm Sun, to the Bering Strait to
Hughes to Eagle by 4am Mon, to the Bering Strait to Coldfoot to
Old Crow by 4pm Mon then moving along the Brooks range Mon night.

East winds 15-25kt are occuring just north of the front with gusts
to 35 kt along the West Coast. Winds will decrease slowly tonight
and Sun. Snow just north of the front in the southwest Interior
will mix with rain this evening then diminish. Little if any snow
expected in the Central and Eastern Interior with this front due
to increasing chinook conditions. Could see a few flurries along
the front Sun into Mon.

An occluded front moving into SW Alaska Sun, will move north over
Norton Sound and the SW Interior Mon, and then dissipate. This
will bring east winds back up to 20-30kt along the West Coast and
10-20 kt over the Interior. Rain with the front will change to
mixed rain and snow along and south of the front.

A frontal wave developing in Bristol Bay Mon night will move to
the Yukon Delta as a 977 mb low by 4pm Tue. This will bring more
rain and snow to to the West Coast and Western Interior Tue.
This will likely cause a strong chinook event to develop over the
Alaska Range and SE Interior Tue.

&&

.DISCUSSION...
Models initialize well above the surface and show similar
solutions through 4pm Mon, then see differences on strength and
timing of short wave troughs and surface centers after that.
Prefer the ECMF solution for Monday night Tue as the strong short
wave moving north over southwest Alaska should keep the main
energy further east per the ECMF solution, not move energy west
as the GFS does on Tue.

At the surface, all models initialize several mb too weak at 15z
on high over Chukchi Sea, and as a result have winds 3-5 mb too
weak over the West Coast and NW Interior. Expect the stronger
winds to persist today, but differences should become less
noticeable tonight as the surface low moves southwest and the
gradient relaxes. Significant differences in model surface fields
show up after 4pm Mon, and we are using the ECMF after that time
for reasons stated above.

Coastal Hazard Potential Days 3 and 4...
Elevated surf North Side of St Lawrence Island now through Mon,
but waves should be breaking offshore due to reduced sea height.

&&

.FIRE WEATHER...
None.

&&

.AFG WATCHES/WARNINGS/ADVISORIES...
Heavy Freezing Spray Warning for PKZ200-PKZ210.

Small Craft Advisory for PKZ200-PKZ210-PKZ220.

Brisk Wind Advisory for PKZ225.

&&

$$

JB APR 18



000
FXAK67 PAJK 212251
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
251 PM AKDT Sat Apr 21 2018

.SHORT TERM...Some dense areas of fog arose this early morning in
Peril Strait, Gustavus, and Hoonah, before burning off mid-
morning. Residual showers persisted throughout the afternoon (and
even some ice pellets), as yesterday`s frontal system exits and
drags the precip with it. Our next system we`re looking at will
make its entrance late Sunday into Monday, followed by another
quick band of frontal precip late Monday night into early Tuesday.

Temperatures at the 500 mb level are still in the mid -20C range,
which are below normal for this time of year. Temperatures Sunday
into early Monday morning won`t be following a diurnal curve, as
some warmer, borderline-unstable air sneaks in. Maximum
temperatures were lowered into Monday, following rain soaked
cooling and some pockets of CAA that slip in behind the front.
Minimum temperatures were raised for tonight especially, as cloud
cover prevails for most of the short term. Temperature edits were
done through a GFS blend and some local edits using MOS Guidance.

Rain showers will become more widespread throughout the panhandle
into tonight with some snow showers mixing in over the northern
panhandle. QPF will be 2-3 standard deviations above normal during
this timeframe. POP was blended with some SREF, AKHIRESWarw, and
GFS and QPF the AKHIRESWarw and GFS, respectively.

Pressure adjustments were made using a NAM/ECMWF blend to better
capture the "waves" associated with the second front. The biggest
change to the short term was increasing winds locally late sunday
night into Monday and increasing to gales along the outer coast.
Model agreement has been doing relatively well, however, the exact
timing and details of these next waves may need further fine
tuning.

.LONG TERM.../Monday through Saturday as of 10 pm Friday/ Upper
trof will move NE across the area Mon-Tue. Another trof will
amplify over the N-central PAC into the E Bering Sea. Ridging will
build downstream over western Canada. Models have trended toward
a stronger ridge, and this will likely have an effect on a system
moving toward the area for late Tue into early Thu. Still some
important model differences to work out for this system and any
following systems later in the week. Ended up basically staying
with current forecast in long range, waiting to see if we get
better agreement especially on Tue-Wed system.

Occluded front will be moving slowly E across the area Mon and Mon
night. Looking like most precip with this front will fall over the
NW half of the area. May have some gale force winds over the NE
gulf early Mon but front will be weakening. Weak trof should move
in behind the front Tue.

Following occluded front appears to be weaker and holding up
further W in latest model runs, but models have been inconsistent
with it. Big problem seems to be how strong ridging builds over
and E of the area. If the current models maintain the stronger
ridge idea, later shifts may need to keep the front further W
along with the stronger winds and precip. After Wed night,
forecast confidence drops off even more as the model differences
only increase. Overall, looking more like the southern panhandle
will not get as much rain as previously thought, which may not
alleviate the abnormally dry conditions there.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Gale Warning for PKZ041>043-051-052.
Small Craft Advisory for PKZ022-033-035-036-053.

&&

$$

SS/RWT

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