Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxak68 pafc 090203 
afdafc

Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
503 PM akst sun Dec 8 2019

Analysis and upper levels...
a brief clearing over southern Alaska from this morning is
quickly giving way to the next rapidly moving low out of the
North Pacific. Satellite shows this system approaching Kodiak
Island with a front extending to around 25n. This pattern is
strengthening the surface low which will move into the eastern
Bering Sea early tomorrow morning. This will also bring the strong
front associated with this low through the Gulf and into southwest
and southcentral Alaska tomorrow.

&&

Model discussion...
models remain in relatively good synoptic agreement the next few
days as the differences are of the type where they are not
disrupting the forecast very much. What is the biggest question
mark is the way the winds will move around the Anchorage area.
This is due to the complexities of the terrain and therefore the
high-resolution models are preferred.

&&

Aviation...
panc...winds will be the main concern around the terminal
tonight. Specifically the low level wind shear. Winds should
increase from the north at the surface, but winds aloft will
increase from the southeast this evening creating some wind
shear. These southeast winds aloft will increase more after
midnight with northerly winds persisting at the surface therefore
increasing the wind shear. Late Monday afternoon the strong southeast
winds will move over the Airport at the surface.

&&

Short term forecast southcentral Alaska (days 1 and 2: tonight
through tuesday)...

High wind warnings will go into effect along Turnagain Arm, higher
elevations of the Anchorage hillside, and through Portage Valley
starting at midnight tonight and continuing until 9pm Monday
evening. A strong east-west oriented front will move north across
the Gulf tonight. As it approaches the north Gulf Coast, southeast
winds will increase ahead of the front from Prince William Sound
through Portage Valley, Turnagain Arm, and the Anchorage hillside.
For the Anchorage bowl, 10 to 20 mph winds will remain out of the
north or northeast, likely coming into town as remnants from a
gusty Matanuska Valley wind. That mat wind will impact Palmer and
Wasilla with gusts up to 45 mph from around midnight tonight
through the day Monday.

Precipitation-wise, we have drastically reduced expected rainfall
totals across the Anchorage bowl and the mat-su valleys over the
next 24 hours or so. The aforementioned southeast winds moving
down the Chugach Mountains should dry out as they descend, which
will evaporate most of the rainfall. Portage and Whittier will be
the wettest communities from this rainfall event, with around 6
inches of rain possible from this evening through Monday evening.
For Seward, 2 to 4 inches are possible. These reduced amounts are
primarily because the winds with the front will have a significant
easterly component to their direction. Thus, instead of moving up
the valley, the winds will be directed somewhat across the valley,
which will reduce rainfall totals in town. However, higher
precipitation amounts will be possible towards Exit Glacier on the
eastward facing mountain slopes.

The front will move inland Monday evening. After frontal passage,
the winds will diminish dramatically, which should coincide with
the end of the high wind warnings Monday evening. As the front
crosses the area, increased lift will give the Anchorage bowl its
best chance of rain from this event, though amounts should remain
under a quarter of an inch. Monday evening will also be the
Anchorage bowl's best chance of getting the Turnagain Arm wind
into town, which will spike temperatures well into the 40s.
Behind the front on Tuesday, the winds will be significantly
diminished, but the Turnagain Arm wind will continue through the
day, but should not affect the Anchorage bowl. A return to
northerly winds for Anchorage and the mat-su will bring high
temperatures down into the mid to upper 30s on Tuesday. A front
associated with another low will move into the southern Gulf
Tuesday evening. This will be the next chance of precip for much
of the area through midweek.

&&

Short term forecast southwest Alaska (days 1 through 3: tonight
through Wednesday evening)...

A strong low will approach the Alaska Peninsula this evening.
Easterly winds with gusts exceeding 60 mph are likely in terrain
gaps of the Aleutian Range near Iliamna by early Monday morning. A
brief period of snow is possible over the Bristol Bay area over
the same time period, however temperatures will quickly rise
above freezing as the warm front lifts northward, changing
precipitation to rain. Snow and blowing snow are expected across
the Kuskokwim Delta Monday morning as well (see Winter Weather
Advisory for further details). As the associated low departs to
the northwest, strong southerly flow on its east side will lead to
well above average temperatures in the low 40s over much of
southwest Alaska by Monday afternoon. After a brief break between
systems on Tuesday, another front will track near the Alaska
Peninsula by Wednesday with gusty easterly winds redeveloping near
the Aleutian Range though precipitation looks to be minimal at
this time.

&&

Short term forecast Bering Sea/Aleutians (days 1 through 3: tonight
through Wednesday evening)...

A low will track northward over Bristol Bay through Monday
morning, bringing rain and gusty northerly winds to the eastern
Aleutians. Scattered rain or snow showers will continue across the
entire chain through Wednesday in a broad northwesterly flow.

&&

Marine (days 3 through 5)...

..Bering Sea and Aleutian coastal waters...

A pair of low pressure systems will affect the region. The first
low moves from the northern Bering towards the Kamchatka peninsula
on Wednesday as it retrogrades (moves west), with sustained gales
likely north of 60n and west of 175w. Further east, a North
Pacific low moving towards the Alaska Peninsula (akpen) will also
bring sustained gales to the Bristol Bay and coastal waters of
the akpen. The winds will then subside for both locations on
Thursday and Friday. Seas look to generally remain at or below 15
feet through the period.

..Gulf of Alaska...

A front will move from south to north across the region on
Wednesday, with sustained gales preceding it. Given a long fetch
length, seas are expected to build into the 20 to 25 feet range
ahead of this feature. The winds will subside for Thursday and
Friday, but, the waves will take more time. We expect seas to
remain near 20 feet for the southern and eastern Gulf on
Thursday...before decreasing more on Friday.

&&

Long term forecast (days 3 through 7)...

Models continue to struggle with the specifics of the long term
forecast, but some general signals can still be gleaned in the
noise. Synoptically, a progressive pattern will persist over the
Bering and Alaska. A vertically stacked low in the northern Bering
Sea Wednesday will continue steering modified Arctic air over the
Aleutians and Bering. The low will continue to gradually weaken
and shift towards Siberia, but in the meantime expect scattered
showers over much of the Bering/Aleutians through Thursday.
Meanwhile, a low that is currently roughly 400 miles southwest of
Shemya will quickly traverse the North Pacific over the next few
days. As this low tracks northeast of 50n 150w it will push a warm
occluded front towards the north Gulf Coast, likely in the
Wednesday to Thursday timeframe. This should trigger a healthy
round of precipitation along the north Gulf Coast and
Alaska/Aleutian Range. South/southeast facing slopes should get
the heaviest precipitation, while areas in the Lee (e.G.,
Anchorage bowl, western Kenai Peninsula, Bristol bay) will be more
of a challenge with shadowing. While it's early to get into
specifics with respect to precipitation type, this looks like a
fairly warm system... the mountains should get snow but in lower
lying areas precipitation type will be a challenge. Friday into
the weekend, models indicate a strong low or series of lows
traversing the Aleutians, bringing a round of strong southerly
winds and precipitation.

In short, temperatures will largely be near or above average with
a southerly component to the flow over southern Alaska for much
of the Wednesday through Sunday timeframe. In addition,
precipitation will mostly be focused along the north Gulf Coast.

&&



Afc watches/warnings/advisories...
public...npw 101 125. West-southwest 155.
Marine...storms 119 120 130-132 138 160 180 351 352.
Gales 125 129 139-141 150 155 165 179 181 412.
Fire weather...none.

&&

$$

National Weather Service Glossary of Abbreviations