Graphical Hydro Meteorological Discussion

365
FXAK68 PAFC 161248
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
448 AM AKDT Mon Sep 16 2024

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

A shortwave trough is moving across Southcentral and the Gulf
this morning. Isolated to scattered showers are continuing across
the northern Susitna Valley, eastern Prince William Sound, and the
mountains along the Chugach and the Talkeetna. Gusty winds
through Kenai and Anchorage from southwesterly flow up Cook Inlet
should start to decrease this afternoon along with the showery
activity. The exception to this will be along the Alaska Range and
the Wrangells north of Gakona. Precipitation will likely linger
until early Tuesday morning for those locations as a central low
passes through the central Interior of the state and a weak
shortwave dips slightly into the Susitna Valley and Copper River
Basin. A brief weak ridge will help to clear some skies later
tonight as the next front reaches the western Gulf and Kodiak
Island.

Cloud cover will become more important tonight into Tuesday
morning as cooler, drier air filters into Southcentral. For areas
that clear, light winds and weak ridging will allow for good
radiational cooling overnight and Tuesday morning temperatures may
feel a bit chilly as temperatures drop into the 30s to lower 40s
for most areas in Southcentral. The Copper River Basin will likely
see the coldest morning temperatures will lows looking to bottom
out in the low 30s. Patchy valley fog and patchy Cook Inlet fog is
possible for some of the locations that manage to clear.
Increasing mid and upper level cloud cover, moving in from the
south, should keep temperatures from dropping too low for areas
along the Cook Inlet up through the Mat-Su Valley, but timing of
this will factor heavily into how low temperatures drop. One thing
to note, is that we are now reaching that time of year when we
start seeing our first temperatures of 32 degrees for the upcoming
cold season. Now is a good time to start thinking about how to
protect those cherished warm season plants that are still outside
as we start to increase our chances reaching low temperatures of
32 degrees or lower.

The next system will be another wet one with south to
southeasterly flow for Southcentral Tuesday and Wednesday. Winds
could be gusty through the Copper River and Turnagain Arm. The
front will spread across the Kenai Peninsula late Tuesday morning,
then to Anchorage, Prince William Sound, the Matanuska and
Susitna Valleys in the early afternoon. Rainfall intensity will
increase Tuesday night into Wednesday for eastern Prince William
Sound. Precipitation will gradually diminish for much of
Southcentral later Wednesday as the low starts to move across the
northern Gulf. Thursday looks to be a little bit quieter, with
light showers lingering around the mountains. Overall,
temperatures will remain on the cooler side through the rest of
the week.

- PP / rux

&&


.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 3)...

Forecast remains on track for a stronger system to move into the
Bering Sea this afternoon and trek eastward towards Southwest
Alaska through Tuesday. Models are slow to agree on the track for
this system and some uncertainty remains. There has been a
gradual southerly trend, pushing somewhere onshore into Greater
Bristol Bay. The current forecast leans toward the EC and
Canadian, which are reasonably aligned. The GFS, however,
continues to be the southern outlier, though consistently
southern. The NAM is more of a middle ground solution but does
ultimately trend closest to the GFS. There is better agreement on
higher winds pushing into Bristol Bay, with gusts to storm force
likely, and waves reaching 13 to 17 ft. Questions that need to be
resolved, however, are the placement of the low and duration of
winds. A further north and deeper solution of the low would extend
the duration of winds into Bristol Bay and possibly necessitate
some flood products. A more southerly track, such as the GFS
solution, would still bring very strong winds into Bristol Bay,
but for a shorter duration as the GFS solution is more progressive
with the low.

As low as it occludes, it does so in a bent-back fashion, pulling
colder, northerly air along the backside and increasing winds to
storm force. There is decent agreement for the Pribilof Islands to
be impacted by these storm force winds Tuesday afternoon. Saint
George has the greatest chance for strongest winds.

&&


.LONG TERM FORECAST (Days 4 through 7: Thursday through
Sunday)...

By the start of Thursday morning a building ridge over the
eastern Bering Sea appears to settle in. The resiliency of ridges
is usually a point of contention in the model solutions, however
agreement is currently strong regarding a fairly strong ridge
remaining over the area, warding several low pressure systems off
south of the Chain into the early portions of the weekend. Model
uncertainty is much higher to the east in Southcentral and the
Gulf of Alaska. By Friday morning, the GFS solution pulls a fairly
strong low pressure system up from the northern Pacific into the
Central Gulf, spreading wind and rain across the coast, whereas
the Euro and Canadian models keep a mild trough in the northern
Pacific, with Southcentral staying clear. By Sunday morning,
solutions comes back into better agreement with the ridging over
the eastern Bering weakening as a strong low pressure system fills
over the western Aleutians and a new low over the the Gulf of
Alaska. A broad troughing pattern looks to resume thereafter,
which will keep shower chances in play for both Southwest and
Southcentral Alaska through the remainder of the long term period.

-CL

&&


.AVIATION...

PANC...VFR conditions persist. Southerly winds around 25 to 35
kts this morning will remain gusty but gradually diminish through
today. Isolated to scattered showers move through this morning
but should be mostly focused along the Chugach Front Range, away
from the terminal. From this afternoon through Tuesday morning,
high confidence in a spell of fairer conditions with no rain,
lighter winds, and VFR conditions expected. The is a small
possibility for patchy fog after midnight into Tuesday morning if
skies clear and overnight temperatures cool. Later on Tuesday, a
front will bring a return of rain and stronger winds.

&&


$$

149
FXAK69 PAFG 160923
AFDAFG

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
123 AM AKDT Mon Sep 16 2024

.SYNOPSIS...
A coastal low is moving inland across the Yukon Valley this
morning, and will shift east today along with more showers. A very
strong low in the Bering Sea develops Tuesday, although the track
will be over Bristol Bay, keeping any coastal storm impacts well
south of the Yukon Delta. This storm will bring rain and gales to
the YK Delta, and more chances for rainfall to the Interior
Wednesday and Thursday.

&&

.DISCUSSION...

Upper Levels and Analysis...
A weakening occluded low is moving inland over the Yukon Valley,
with the low currently situated over Galena, with a central
pressure around 989 mb. The upper low center is dominating the
flow over Alaska, with the height of that low center around 528
dam. A subtropical jet stream persists over the Bering Sea, with
another developing low over the W Aleutians. Widespread moderate
rain showers are wrapping around the low, with the heaviest
rainfall falling over the YK Delta as expected, with 24 hour
rainfall totals in excess of 1.0 inches. The associated occluded
front is moving over the Brooks Range, bringing rain mixed with
snow above 4000 feet.

Forecast and Model Discussion...
Numerical models are fairly locked into the pattern as the strong
low moving inland over the Yukon Valley shifts east through
Tuesday, and another strong low moves into Bristol Bay. The latter
will bring offshore winds to the YK Delta and more rainfall, with
light rainfall chances for the remainder of the Interior Wednesday
and Thursday. This forecast prefers a blended NBM and NAM nest.

Central and Eastern Interior...
Rain showers move through the Interior today as the low out west
shifts east. An isolated thunderstorm is possible again today
across the Fortymile and Yukon-Tanana Uplands, including Eagle.
Showers continue into Tuesday then things dry out as a low over
Bristol Bay induces downslope drying. The low dives into the Gulf
and more rain/showers will move through the Interior Wednesday and
Thursday. With the clouds and showers temps over the Interior will
be running a few degrees below seasonal norms.

West Coast and Western Interior...
Showers will continue into Tuesday as the low currently over the
Yukon Valley shifts eastward. Gusty west-northwest winds will move
onshore into E Norton Sound behind that low. A strong low will
move into Bristol Bay Tuesday, and this will bring strong
northeast offshore winds to the YK Delta along with more rainfall
of 0.5 to 0.75 inches from the Yukon Valley southward. Rain tapers
off Tuesday night and turns more showery as northwest winds move
in behind that low.

North Slope and Brooks Range...
Snow levels are falling to 2500 feet by Wednesday, with continued
chances for rain/snow showers over the Brooks Range. Rain and
snow showers with some drizzle will also continue over the Arctic
Coast. Extensive fog over the E Arctic continues this morning and
will be possible into midweek with a weak area of low pressure
persisting along the immediate coast.

Extended Forecast Days 4-7...
A building ridge over the Bering Sea will induce some upper
troughing over the Mainland Thursday through the weekend, which
likely means it will be getting colder as the main storm track is
shunted south. Rain showers mixed with higher elevation snows will
be possible through Friday night with a drying trend noted for the
weekend.

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

&&

.HYDROLOGY...
0.5 to 1.0 inches of rainfall fell over the Lower Yukon watershed
and down to the Delta. More rainfall is expected for Tuesday and
Wednesday with another 0.5 to 0.75 inches over the same area.
River rises are likely, but will not be significant. Elsewhere, no
concerns as rivers are running low due to fairly low snow levels.

&&

.AFG WATCHES/WARNINGS/ADVISORIES...
AK...High Surf Advisory for AKZ825.
PK...Small Craft Advisory for PKZ801-817-850.
Small Craft Advisory for PKZ802-806-807-853-854-856.
Small Craft Advisory for PKZ803.
Small Craft Advisory for PKZ804.
Small Craft Advisory for PKZ805.
Small Craft Advisory for PKZ852.
&&

$$

Ahsenmacher

728
FXAK67 PAJK 161239
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
439 AM AKDT Mon Sep 16 2024

.SHORT TERM...Chances of rain showers continue across the northern
half of the area through the day on Monday, while a stronger low
passing across the southern half of the panhandle brings
widespread rain to areas south of Sumner Strait. Drier weather is
in store Monday night into Tuesday.

Satellite and radar imagery as of the time of writing (12z)
depict a developing low moving into the southern panhandle, with
its associated cloud shield reaching as far N as the Icy Strait
Corridor. Upper level analysis reveals the culprit behind the low
as a weak upper level shortwave trough moving within a broader
area of troughing across the gulf. Overall dynamics aren`t all
that conducive to a strong system developing, and the primary
impact of this system will be the rainfall it produces across the
southern half of the area through the day. The rainfall itself
will also be less than outstanding, with amounts near sea level
between 0.25 and 0.75 inches of rain expected through the daytime
hours. Across the northern panhandle, chances of rain showers are
expected given continued onshore flow, although these will be
less widespread in nature.

Chances of rain will diminish through the evening and overnight
timeframe on Monday across most of the area as northerly flow
develops, with most locations drying out save for a few passing
showers by Monday night. The exception to this will be parts of
the far southern panhandle like Revillagigedo Island and Hyder,
where PoP chances will remain more robust until Tuesday morning.

As PoPs diminish, increasing breaks in the clouds will set the
stage for fog or low stratus development across parts of the
area, though this will largely dissipate through the morning
hours on Tuesday.

Winds will continue to diminish through Monday, with Lynn Canal,
the Icy Strait Corridor, and Clarence Strait likely remaining the
last holdouts for elevated winds, through even these areas,
barring Clarence Strait, will drop below 15 kt by late Monday
night. Primary changes to the forecast were to add some additional
detail to the PoP and QPF fields for the low moving in across the
south, as well as to extend the duration of elevated winds for a
few locations given a continued sharp pressure gradient.

.LONG TERM.../Tuesday through Friday/...
Quick Notes:
-Rainy weather continues.
-24 hour rainfall amounts are not out of the ordinary for this
time of year.
-Marine winds will be elevated late Tuesday night through
Wednesday night with sustained winds around 15-25 kts for some
of the inner channels.

Details:
The active pattern will continue across the panhandle through
next week.

To start off, an exiting system on Tuesday with brief 500mb ridging
may give the area a short break in the rainy weather, and even some
breaks in the clouds, on Tuesday.

The short break will come to an end Wednesday as the next front
pushes through. There will be both wind and rain with this front.
That being said, rainfall amounts aren`t all the impressive for this
time of year, with 24 hour rainfall amounts around 0.50 to 1 inch.

Wind speeds will increase as the front tracks through, but nothing
too out of the ordinary with frontal passes this time of year.
Beginning late Tuesday night, most inner channels will see speeds
increase to around 10 to 20 knots for Wednesday. The exception
will be Lynn Canal where sustained speeds upwards of 25 knots are
possible based on forecasted pressure gradients.

Once the front clears the area, lingering light, on/off rain will
last through Friday with decreasing wind speeds.

Ultimately, what is currently on the radar as the next impactful
rain event could possibly be next weekend, into early next week.
There are early indications of a long duration rain event. At this
time, there is 40-60% chance of IVT values exceeding 250 kg/m/s for
three days. Unfortunately, details are scarce, but it will be
monitored closely.

&&

.AVIATION...Areas of rain and some fog/low clouds are making for
a mixed bag of aviation conditions this morning. Ceilings and
visibility range from VFR (6000 ft ceilings and 10 mile vis) all
the way down to LIFR (200 ft ceilings and 1.5 mile vis) in some
areas. Winds are mostly light except for the northern Lynn Canal
area where gusts to 40 mph are being observed. The forecast is for
general improving conditions through the day with most areas
becoming VFR by late afternoon. The southern panhandle will be the
slowest to improve as there is a disturbance moving through that
area this morning. Winds near Skagway will likely persist through
the day before diminishing this evening. Tonight will possibly
bring a mixed bag of ceilings and vis conditions again. This time
due to fog possibly developing in many areas and persisting into
Tuesday morning. Expect areas of VFR with IFR or lower conditions
where fog develops.



&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Strong Wind until 7 AM AKDT this morning for AKZ318.
MARINE...Small Craft Advisory for PKZ012-013-022-641>644-651-652-661>664-
671-672.

&&

$$

SHORT TERM...GFS
LONG TERM....GJS
AVIATION...EAL

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