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000
FXAK69 PAFG 130031
AFDAFG

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
331 PM AKST Tue Dec 12 2017

.DISCUSSION...
An active southerly flow pattern continues across northern
Alaska. Overall, the models are in good agreement though Thursday
in terms of the overall synoptic scale pattern.

Central and Eastern Interior: The biggest issue in the short term
will be the winds in the Alaska Range. High Wind Warnings are out
for the Alaska Range with a Wind Advisory for the Delta Junction
area. We expect winds to continue to increase this afternoon and
peak this evening. There will be a brief break in the winds
overnight; however, current thinking is that winds will increase
again tomorrow with the strongest winds expected in the evening.
Little to no precipitation is expected out through Thursday. For
areas from Fairbanks south any precipitation that does fall
during this time will likely be freezing rain or rain due to warm
temperatures aloft; however, because of the downsloping winds off
of the Alaska Range precipitation making it to the ground is
fairly unlikely.. Warm surface temperatures are expected to
continue tomorrow with a cooling trend beginning on Thursday.

North Slope and Brooks Range: Winter Weather Advisories are out
all along the coast mainly for blowing snow. Current expectation
is that winds will increase all along the coast tonight and
especially tomorrow causing the visibility to decrease. Winds
decrease a bit Thursday; however, a band of snow is expected to go
through around the same time that the winds begin to decrease.
This snow band will bring an inch or two of snow as it moves
across.

West coast and Western Interior: A northward moving weather
system will bing precipitation to portions of the west coast and
western Interior tonight into tomorrow. Warmer temperatures aloft
will result in a mixed bag of precipitation types. Winter weather
advisories remain in effect for zones 211, 212 and 214. Winter
storm warnings for freezing rain are out for zones 215 and 216
where higher QPF values are expected during the period where the
sounding supports freezing rain. Snow is expected north of the
Seward Peninsula Wednesday night into Thursday morning as a
weather front move to the north. Up to 4 inches of snow is
possible in zones 207, 208 and 217.

Coastal Hazard Potential Days 3 and 4...None.

Coastal Hazard Potential Days 3 and 4...None.

&&

.AFG WATCHES/WARNINGS/ADVISORIES...
Wind Advisory for AKZ223.

High Wind Warning for AKZ225-AKZ226.

Winter Storm Warning for AKZ215-AKZ216.

Winter Weather Advisory for AKZ201-AKZ202-AKZ203-AKZ204-AKZ211-
AKZ212-AKZ214.

Heavy Freezing Spray Warning for PKZ210.

Gale Warning for PKZ230.

Brisk Wind Advisory for PKZ200-PKZ215-PKZ220-PKZ225-PKZ235-
PKZ240-PKZ245.
&&

$$

DEC 17


000
FXAK68 PAFC 130100
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
400 PM AKST Tue Dec 12 2017

.ANALYSIS AND UPPER LEVELS...
The train of lows continues with deep southerly flow from the
eastern Pacific up to the southern mainland of Alaska. A weak
low is lifting northward across Southwest Alaska today, with a
secondary low nearly stationary south of Cold Bay. Temperatures
have warmed enough across Southwest Alaska to produce mainly rain.
Showers persists along the Gulf coast behind a front which moved
inland this morning. Otherwise, dry and warm conditions continue
across Southcentral.

Satellite imagery shows signs of a frontal wave several hundred
miles south of Kodiak Island. With weak upper level support and
low level baroclinicity expect the process of cyclogenesis to be
slow.

Broad northerly flow and cold advection across the Bering Sea and
Aleutians continues to produce snow showers. However, a pattern
change is in store with a strong jet over the northwest Pacific
beginning to drive eastward south of the Aleutian chain. A deep
mature low north of the jet is slowly making its way toward the
western Aleutians.

Ultimately, the eastward progression of this low/trough will
cause the high amplitude trough over the north central Pacific
to lift northward and lead to a pattern change across the entire
Alaska region as we head through the remainder of the week.

&&

.MODEL DISCUSSION...
Models have finally converged on a similar solution with the
developing low over the northeast Pacific today and its track to
Southwest Alaska. There are still some minor differences in
intensity and track of the front across the Gulf, but forecast
confidence has greatly increased. However, confidence quickly
falls behind this system, with yet another frontal wave or low
tracking northward across the eastern Gulf as the upper trough
begins to lift northward on Wednesday. For now prefer the
ECMWF/GFS/NAM idea of a weak fast moving low and will disregard
the very different Canadian solutions which depict a much slower
and deeper low. The end result of this feature will be to prolong
and enhance precipitation in eastern Prince William Sound and the
Copper River Basin.

Model differences also develop as trough moves to the Bering Sea
and Aleutians the next couple days, with favorable conditions
for development of multiple surface lows rotating around a broad
upper center. Bottom line, forecast confidence drops off
considerably as we head through Wednesday and beyond. As far as
the plan for the forecast, will focus on depicting key aspects
of the pattern change and not worry too much about location of
individual features at this point in time.

&&

.AVIATION...
PANC...The big question early on is whether Turnagain winds will
bend into the airport this afternoon/evening on a consistent basis
or just an occasional gust. Pressure gradients look very marginal.
Have stuck with a bit of wind in the forecast, but it may stay
just to the south over Fire Island. Otherwise, with strong
downslope flow VFR conditions will dominate through the TAF
period. With a marginally deeper and faster moving low headed
inland across Southwest Alaska tomorrow and a much stronger front
crossing Southeast winds will definitely move into the airport
Wednesday afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The pattern over Southcentral Alaska continues with the next front
moving north through the southern Gulf. It will make it to the
north Gulf coast early Wednesday morning bringing more precip to
the coast. Lower elevations look to see mostly rain from the
front.

Some showers will sneak past the coastal ranges bringing rain and
snow to inland locations, with the further inland areas seeing
more snow than rain. Winds will also accompany the front with the
strongest winds through east-west oriented gaps. After the front
pushes inland, flow will remain out of the south which will result
in continued showers along the coast.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

Broad southerly flow over the region continues to transport
moisture and predominantly rain showers (some snow showers along
the Kuskokwim Delta coast) over parts of Southwest this
afternoon. A strengthening shortwave moving north from the Pacific
will plow into Bristol Bay and the Lower Kuskokwim Valley tonight
and tomorrow, bringing rain with a mix of wet snow to those
regions. This system exits tomorrow evening and will be a series
of very weak and subtle shortwaves which will keep snow chances
over the region through Thursday as colder air slowly infiltrates
eastward behind each successive shortwave.

&&

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

A low over Bristol Bay will lift north tonight, keeping an
enhanced northerly pressure gradient over the central Bering
Sea/Eastern Aleutians and the Pribilof Islands. The gradient will
begin to weaken tomorrow with a gale force front entering the far
western Bering Sea this evening and moving east slowly through
Thursday. The air mass should be cold enough to mainly support
snow since it will quickly occlude tonight, although some rain
will mix in at times as the front tracks east.

&&

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

The weather pattern will remain active through day 7 (Tuesday).
We expect a continuation of the train of lows moving into the Gulf
of Alaska and Bering Sea. The models continue to struggle with
this pattern and coming into agreement on the location and
intensity of these storms for the entire extended forecast period.

Beginning Thursday, a strong low will move to the western Alaska
Peninsula area, though where exactly is still uncertain. It will
then push a front into the Gulf of Alaska. The ensemble mean
looks to be the best path forward at this point. By Friday the
deterministic operational model runs differ significantly with
respect to where the low center will be. This storm should be in
the Eastern Bering Sea by Friday, but here again, the models
diverge with the placement of the low center. This has a
significant impact on the sensible weather over the southern
mainland and the Bering Sea/Aleutians. As we progress through the
weekend the models do come into agreement bringing the strong
large low to the Prince William Sound region by Sunday thus
finally breaking down the blocking ridge over the west coast of
North America. The big take away from this will be a trend to
cooler temperatures as we near the weekend and into early next
week. Beyond Sunday the models once again diverge and have
significantly different solutions Monday and Tuesday, thus overall
confidence in the extended forecast is on the low side. The
overall approach to the extended forecast will be to nudge and
trend toward the ensemble solution, but in general we will not be
making any big changes to the forecast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 119 120 125 130 131 178.

&&
$$

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...SA


000
FXAK67 PAJK 122314
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
214 PM AKST Tue Dec 12 2017

.SHORT TERM...Persistent, anomalously deep troughing continues
over the western Gulf of Alaska through Wednesday night. Deep
layer moist southerly flow will continue to direct a series of
fronts across the Panhandle. The first in the series of fronts
will track across the central and southern Panhandle tonight. Rain
will be widespread and heavy at times south of Frederick Sound
tonight with small craft level winds over the outside waters and
spreading into Clarence Strait later this evening.

More substantial shortwave and associated strong front will
slowly approach the panhandle Wednesday and Wednesday night. This
front will once again be accompanied by strong moisture transport
with NAEFS ensemble integrated moisture transport values between 4
and 5 standard deviations above normal. Southerly flow in the H85
to H7 layer progged around 50-60kt and these favorable
orographics will combine with strong WAA to produce yet another
period of heavy rainfall from late Wednesday through Wednesday
night.

Winds will also be on the increase Wednesday especially over the
outside waters. Barrier jet formation Wednesday will lead to gale
force easterlies along the northeast Gulf coast. As the front
pushes east on Wednesday night, gales will spread into the
remainder of the offshore waters with small craft SE`ly winds on
the inside waters. Gusty winds will also develop along the outer
coastal land areas with 40 mph gusts likely in places such as
Yakutat and Sitka Wednesday evening.

Model spread becomes more significant as we head into Thursday
regarding the position of the front but still looks to be a very
warm and wet period.

.LONG TERM...Thursday through Tuesday...The upper level ridge
extending into British Columbia, which has contributed to the
blocking pattern and warmer temperatures we have seen as of late,
will finally begin to break down and push east. This should
provide an opportunity for additional shortwaves to transition
east and impact the panhandle, leading to a fairly active pattern
and one that should remain wet; the position of these shortwaves
will continue to pull in subtropical moisture. Model analysis
continues to show anomalously high precipitable water values with
each of these passing systems, so we`re not seeing any significant
periods of drying out in the long term.

Most of the major models agree that a strong shortwave will
develop near the Aleutian Islands on Friday, and an associated
weather front will lift northeast and impact the panhandle on
Saturday. As this shortwave continues to lift northeast, an upper
level trough will begin to build in aloft over our area and
should provide some assistance with lowering temperatures across
our area. The NAEFS currently shows a decreasing trend in
temperatures continuing into the early part of next week, with
temperatures nearing normal or even slightly below normal for
this time of year. The GEFS is also following this trend with
temperatures as well, which led to higher confidence in adjusting
our temperature forecast.

After the Saturday timeframe, most of the major model solutions
disagree with one another, so we opted not to make any
significant changes to the forecast after this point. However, we
decided that we would lower temperatures a few degrees in the
early part of next week to fit with the pattern shift, as
mentioned previously. For the Thursday through early Saturday
time period, we used a 50/50 blend of the GFS and ECMWF to make
adjustments to the winds and PoP fields.

&&

.HYDROLOGY...Strong warm advection on Wednesday will result in
freezing levels once again rising to between 4 and 6 kft. While
recent warm temps have likely decreased the mountain snow pack
significantly, there will likely be at least some additional snow
melt which will combine with storm total rainfall amounts of 2 to
4 inches over the northern panhandle to produce rises rivers and
streams. At this time no flooding issues are expected but will
need to monitor smaller streams such as Jordan Creek near Juneau
should heavier rainfall rates develop.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Gale Warning for PKZ041>043-051-052.
Small Craft Advisory for PKZ012-013-021-022-031>036-053.

&&

$$

DEL/JJV

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