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Alaska Area Forecast Discussions


548
FXAK69 PAFG 292329
AFDAFG

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
329 PM AKDT Tue Sep 29 2020


.SYNOPSIS...
Northern Alaska will be entering a southerly flow pattern tonight
that will continue into the weekend. This will cause a warm up to
well above normal by Wed in all of Northern AK that will continue
into the weekend. Expect periods of moderate to possibly strong
chinook winds across the Alaska Range, along with windy
conditions in other areas, from Wed into the weekend. Expect
periods of rain over the West Coast and North Slope/Northern
Interior from Wed into the weekend with a chance of snow north of
from the Brooks Range north.

Aloft...
The long wave pattern consists of a high amplitude trough from the
Chukchi Sea south over Western Alaska and into the Wrn Gulf of
Alaska, and a high amplitude ridge extending from the WRN CONUS
and Wrn Canada north over Ern Alaska. A series of very strong
short wave troughs will make their way north out of the long wave
to the SE Bering Sea this week. This will cause the long wave
trough and ridge pattern to retrograde over the next several
days, and cause a moderate to strong southerly flow aloft over
Alaska from Wed night through Sat AM. This will bring the warm and
windy conditions to Northern Alaska Wed through Sat.

The long wave trough will progress east over Alaska next week
bringing a cooler and wetter pattern, more similar to normal, next
week.

850 mb temperatures range from +2C over the SE Interior to -4C
along the West Coast and Arctic Coast. Temps warm to above 0C for
all of of Alaska by Wed PM with temps near +8C over the Southern
Interior, then remain warm through the weekend.

Surface...
High pressure north of the Arctic Coast of Alaska will persist
through Thu. This is causing light east winds, stratus and fog
along the Arctic Coast that will persist into Thu, but the winds
will increase as weather fronts approach from the south.

A weak weather front over the Northern Interior of Alaska will
dissipate along the Brooks Range early Wed. This has mid-level
clouds, and isolated rain showers that will taper off on early
Wed.

A strong low south of Kodiak will move north into Bristol Bay on
Wed, and to Nunivak Island as a 990 mb low by 4pm Thu, then move
west and dissipate. A warm front extending east from this low will
move along the Alaska Range by 4am Wed, to Anvik to Tanana and
Eagle by 4pm Wed, to the YK Delta to Nome to Ft Yukon by 4am Thu,
and to Gambell to Point Hope to Barter Island by 4pm Thu, then
moving north and west of the Arctic and West coasts and weakening
Thu night. Expect east to northeast winds 15-30 mph ahead of this
front over the Interior and West Coast from late tonight through
Wed night, and east winds 25-35 mph along the Arctic Coast on Wed
night and Thu. Expect a chance of rain with the front along the
West Coast/SW Interior and Northern Interior Wed and Thu, with
rain more likely over Norton Sound and the South Slopes of the
Brooks Range Wed night and Thu. This will bring a chance of rain
and snow to the North Slope Thu. Winds will decrease southeast of
the front.

Weak chinook winds over the Alaska Range will increase to moderate
on Wed night and Thu. This will cause warming and drying over the
Southern Interior and cause little if any rain to hit the ground
Wed-Thu. Could see south winds gust 50-60 mph through Alaska Range
Pass, and 40-50 mph near Delta Junction Wed night.

A second strong low will move north into Bristol Bay on Fri and to
the YK Delta on Sat. This will bring another round of moderate to
strong chinook winds across the Alaska Range late Fri into Sat,
along with areas of rain to the West Coast/Western Interior late
Fri into Sat.

&&

.DISCUSSION...
Models initialize well aloft. Models show similar solutions aloft
through 4am Thu. After that time, the ECMF differ from other
models in deepening the low aloft over the Bering Sea much more
than GFS/NAM and Canadian models on Thu and Fri. The ECMF is also
faster than the other models in bringing the next low aloft over
SW Alaska Thu night and Fri. Since there does not appear to be a
significant injection of energy into the Bering low on Thu and
Fri, the deepening shown by the ECMF does not seem as likely as
the GFS/NAM and Canadian solutions. Will use a blend of the
GFS/NAM and Canadians solutions aloft through Fri.

With precipitation, models show similar solutions through 4am
Thu, then differ slightly. Models miss areas of light isolated to
scattered showers now occuring in the southern Interior, NW
Alaska, and the North Slope. Will use a blend of models through
4am Thu but accounting for areas of current precip in the Southern
Interior, NW Alaska and North Slope for today. Will use a blend
of the GFS and NAM precip for into Fri.

At the surface at 15Z, models all verify well. Models show similar
solutions through 4AM Thu, then differ with next low moving into
SW Alaska Thu night and Fri. Will use a blend of models at the
surface through 4am Thu, then use a blend of the NAM/GFS and
Canadian at the surface for Thu and Fri to reflect the preference
for those models aloft.

Bottom line for models, we will use a blend of models for precip,
temps, winds through 4am Thu, then use a blend of the NAM and GFS
for Thu into Fri.

Coastal Hazard Potential Days 3 and 4...
Offshore winds causing below normal sea levels Fri and Sat.

&&

.HYDROLOGY...
Rivers steady or slowly falling.

&&

.AFG WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory for PKZ200-PKZ210-PKZ240-PKZ245.
&&

$$

JB SEP 20



000
FXAK68 PAFC 300057
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
457 PM AKDT Tue Sep 29 2020

.ANALYSIS AND UPPER LEVELS...

The low south of Kodiak Island is continuing to intensify as it
moves northward today. The moisture tap from this system is
nearly down to 30N and feeds directly into this system. Satellite
imagery shows a triple point a little south of Kodiak Island. The
strongest winds with this system will be along the warm front
portion of this storm as it crosses Kodiak Island and vicinity
this evening. This is also a set up where a barrier jet will
develop along the southern part of the Kenai Peninsula and that is
the feature that is expected to enhance the winds to hurricane-
force over parts of the marine area near Kodiak this evening
ahead of this front.

Northerly flow is over the Bering Sea with the approaching low
heading for Bristol Bay overnight. A brief break in between
systems has brought a few hours of sunshine to parts of southwest
Alaska today, but this will be short-lived with the new low
approaching.

&&

.MODEL DISCUSSION...
Models are in very good agreement with the strength and track of
the low cutting to the southwest of Kodiak and into Bristol Bay
tonight. The next low in the parade should move through Thursday
night into Friday on a similar, but slightly more northerly track.
Models are in agreement on the main features, but the actual
track and strength are different enough that there may be some
adjustments on the Kodiak and Alaska Peninsula forecasts over the
next few days. Yet another low will move into the Gulf this
weekend, but models are indicating this one will eventually make
it into Prince William Sound. Timing and strength are similar on
the GFS and ECMWF models so there is reasonable confidence in this
solution at this point.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through Wednesday
afternoon. Gusty Southeast winds are likely to develop Wednesday
evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

Two separate storms from the North Pacific will keep most of
Southcentral and the Gulf of Alaska under hazardous weather
conditions through the next few days. The first complex storm low
is still parked more than 400 miles south of Kodiak Island, but
the frontal zone from the low is lifting heavy moisture and
extensive areas of rain across the Gulf, with more rainfall
spreading over Kodiak Island, Kenai and Prince William Sound this
evening through Wednesday afternoon. Storm and hurricane force
winds will be strongly felt south of Kenai Peninsula and much of
the western Gulf this evening, while gale force winds will pass
right through Cook Inlet tonight and Wednesday along the axis of
the frontal boundary. As soon as the stronger feature of the
frontal boundary passes westward of Shelikof Strait and Cook
Inlet, winds and wave heights will gradually subside Wednesday
morning through Wednesday afternoon. Rainfall, however, will
continue to spread over the coastal region and farther inland as
cloud remnants trek north-northwestward the next couple days.

The next developing storm from the North Pacific will re-introduce
hazardous weather conditions back into the forecast for much of
western Gulf starting Thursday night. This expected system will
generate inclement weather including heavy rainfall, strong winds
and hazardous waves. Although not a quite a lot of rainfall is
expected over inland, but strong gusty winds are a concern near
Gaps and Passes should this next storm shift more northward before
curving northwestward on Friday afternoon. Overall, anticipate a
very wet pattern and hazardous conditions over the Gulf, with only
minimal showers north of Anchorage and over the Copper River Basin
the next several days.

&&

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

A pair of strong lows following nearly identical paths (one
following the other) will be impacting Southwest Alaska through
the short term. Both lows are expected to be of similar strength,
as the centers of both track northwestward out of the North
Pacific, cross the Alaska Peninsula near Chignik, then track up
the coast of Southwest Alaska.

The first low is already well developed over the North Pacific
south of the Gulf. Its leading front is already approaching the
coast and will be the more impactful aspect of the storm, causing
winds to hurricane force in the waters along the Pacific side of
the Alaska Peninsula this afternoon through the first half of
tonight. As the front lifts northwestward across Southwest Alaska,
the predominately northeasterly flow should reduce (but not
entirely eliminate) the downsloping to the lee of the Aleutian
Range. Thus, most of Southwest Alaska can expect wetting rains
from the front. The strongest winds will be in the typical areas,
primarily through Kamishak Gap and into Lake Iliamna. The front
will rapidly weaken with time Wednesday morning as it pivots to
track more towards the west, so little rain is expected to survive
the downsloping from the Kuskokwim Mountains to reach the
Kuskokwim Delta midday Wednesday.

Another concern of note will be potential for low water along
coastal Bristol Bay tonight. While fortunately the strongest
northeasterly winds of 55 to 65 mph will not correspond well with
the low tide (the tide will be coming in as the strongest winds
are occurring), it may make for water levels dropping several feet
below mean low tide this evening. In addition, the incoming tide
and strong offshore flow will make for very choppy seas in Bristol
Bay tonight.

The center of the first low will cross the Alaska Peninsula
Wednesday evening in a much weakened state, with gusty winds
through Kamishak Gap and into inland Bristol Bay and continuing
upslope rain along the Kuskokwim Mountains being the primary
lingering effects by Thursday. The second low will rapidly
approach the AKPEN Thursday evening, with another round of strong
easterly winds, heavy rain along the coast, particularly along the
south side of the Alaska Peninsula, and favored east facing
slopes of the Kuskokwim Mountains expected. There will be more on
this second low in upcoming discussions. Temperature-wise, both
lows will reinforce the warm air over the area, making for well
above average temperatures, especially with overnight lows.

&&

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

A strong low moving out of the North Pacific will track up the
coast over the eastern Bering through Thursday, weakening as it
goes. The low will work to bring seasonably cool air southward out
of Siberia and across the central and western Bering. A large, but
weak area of high pressure over the western Bering this morning
will slowly shift south into the North Pacific by Wednesday, with
the low eventually needing to do all the work of bringing cooler
air south across the central Bering by Thursday. This means a
continuation of the current pattern of widespread cumulus over the
area, some of them producing shower activity, but otherwise
remaining a broken cloud field, allowing many areas to get at
least a little sunshine between the clouds the next several days.
Gales to occasionally storm force northerly winds will impact the
southern Alaska Peninsula and eastern Aleutians starting tonight
and slowly diminishing through the day on Wednesday.

&&

.MARINE (Days 3 through 5/Friday through Sunday)...

Bering/Aleutians

A storm system will move northwestward across the AKPEN into the
eastern Bering Sea on Friday. Winds will remain under gale force
over the open water. There is a chance of gale force winds along
Cape Constantine and Cape Newenham due to the localized funneling
of winds. Confidence is low in the exact forecast of these winds
as there are significant differences in solutions with regards to
the placement and strength of the low center. The general trend is
this storm system will move into the northeastern Bering Sea and
weaken while remaining stationary through Sunday.

Gulf of Alaska

A storm system will move northwestward through the western Gulf
and cross the AKPEN into the Bering on Friday. Widespread gale
force south-southeasterly winds associated with this storm system are
expected in the Gulf on Friday. These winds will ease to below
gale force Friday night and on Saturday. However, a second storm
system will move into the southern Gulf on Sunday. Confidence is
very low in the forecast for Sunday as there are significant
discrepancies between solutions on the placement and intensity of
the low center.

&&

.LONG TERM FORECAST (Days 3 through 7/Friday through Tuesday)...

The pattern remains highly amplified as a deep negatively-tilted
upper trough extends across the Bering/Aleutians and the AKPEN
while an upper ridge extends across the west coast of Canada. A
strong storm system will move northwestward from the Gulf of
Alaska into the eastern Bering/Aleutians. This low will
potentially bring 2-3" of rain to Kodiak and the southern Kenai
Peninsula. Widespread gale force winds are expected in the western
Gulf on Friday as this system moves over Kodiak and the AKPEN.
This storm system will move into the northeastern Bering Sea and
weaken into a trough through Sunday. A second storm system will
move into the southern Gulf of Alaska on Sunday, though forecast
confidence with respect to this second system is low as there are
significant differences between solutions in regards to the
placement and intensity of this low. The overall trend is this low
will move into the northern Gulf/southern Alaska mainland and
weaken into a trough while remaining nearly stationary, however
there is a high degree of uncertainty where this will occur due to
the significant model differences. A third storm system will move
into the western Bering/Aleutians Tuesday afternoon, though
confidence is very low as there are significant discrepancies in
solutions with respect to the placement and intensity of this low.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Hurricane force wind warning 131 132 138 150.
Storm warning 119 120 130 136 137 139 155 160 165 170 171 172 351 352.
Gale warning 127 140 180 414.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...ED


038
FXAK67 PAJK 292254
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
254 PM AKDT Tue Sep 29 2020

.SHORT TERM...A low in the Gulf featuring a warm type occlusion
will be our weather maker Tuesday night into Wednesday. Currently,
the warm front is passing over the region. Though not much rain
was initially expected from this frontal passage, amounts have
been less than previously forecast as the track of the low looks
to be somewhat further west than what was anticipated. This front
should move north of the region tonight and will lead to some
clearing. This could cause temperatures to drop enough for fog
formation in many areas, but best chances look to be in the
southern panhandle as they should clear out first, thus get more
radiational cooling. Speaking of temperature, 925mb and 850mb
temps are incredibly high behind the front which will lead to a
very strong inversion. This set up creates a great amount of
uncertainty regarding the surface temperature forecast, especially
the high temps for Wednesday. At locations where the inversion
manages to break temps could rocket upward in the morning. At
locations that have fog that lingers or see clouds move in earlier
ahead of an approaching cold front, temperature could remain
cooler. Winds will increase overnight into Wednesday with this
approaching front in the Gulf and along coastal areas as well.
This front will bring another round of rain to the region with
amounts of a quarter to a half an inch between Wednesday and
Wednesday night. This front should move through rather quickly
leading to another period of clearing and potential fog overnight
Wednesday. This will likely lead to most locations seeing sunshine
on Thursday, however temperature behind the cold front should
limit the temps to the mid to high 50s, with some 60s in the
southern panhandle.

One additional note: while not all of the ingredients will be in
place for a traditional Taku wind event (cold air), guidance has
suggested the possibility of some mountain wave activity in Juneau
Late Tuesday night/early Wednesday morning. This further
complicated the temperature forecast as noted above and prompted
the raising of the overnight low temp forecast for downtown to
well above normal.



.LONG TERM.../Thursday through Monday/ As of 10pm Monday. Fairly
active pattern, but models are in good agreement that between each
front is a distinct break in the weather of 12-24 hours (shortest
for Yakutat). Often in an active fall pattern the breaks are very
short or not discernible/more like lulls in the rain. These
breaks should make the week more palatable.

On Thursday (break day #1) a front will have just departed with
some lingering showers over the northern gulf coast. High pressure
centered over the southern inner channels will cause light and
variable winds and likely fog early in the day. This will be
followed by clearing skies from south to north for the whole
region. This set up should mean a larger spread between high/low
temperatures.

A low tracks northward into the western gulf Thursday night with
a front extending outward. Rain assoc with the front should arrive
along the outer coast by Friday morning (front #1). Winds over
the outside waters are expected to increase to gale force with
this front. Showers look to linger behind the front into Saturday,
but expect generally improving conditions through the day (break
#2).

A stronger low moves into the western gulf from the SW late
Saturday night. The assoc front (front #2) will likely cause
another round of gale force winds and periods of heavy rain
Sunday. Showers again linger behind the front on Monday under SWLY
onshore flow. Think that there will be breaks on Monday, primarily
over the inside waters. How much improvement there will be is
less certain that far out, so lowered POP to the likely category
below model guidance.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind from late tonight through Wednesday morning for
AKZ025.
MARINE...Gale Warning for PKZ041-043-051-052.
Small Craft Advisory for PKZ012-022-036-042-053.

&&

$$

JDR/Ferrin

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