Small Craft Advisory 329, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
412 AM AKDT Thu Apr 27 2017

.ANALYSIS AND UPPER LEVELS...
Weak southeasterly flow in the upper levels continues across
nearly all of southern Alaska this morning. The moisture and upper
level waves embedded in the flow originate from a low over the
southeastern Gulf. As those waves move along and interact with the
moisture and instability already present across Southcentral,
showers have been intermittently forming along the mountains. The
Anchorage sounding yesterday afternoon shows light and variable
flow throughout the lower atmosphere, so downsloping is not an
issue. Where the showers have been forming, they remain stationary
or drift.

Due to the instability and drying of the lower levels, there was
quite a bit more sunshine across Southcentral yesterday than the
day before. As a result the shower activity was much more
prevalent in the afternoon. The showers formed during the
mid-afternoon along Knik and Turnagain Arms from a weak sea
breeze that pushed in, then the mountains helped lift some of the
showers as the northwest winds encountered the foot of the
Chugach. After initial shower development, subsequent shower
activity favored the east side of town along the mountains before
diminishing. Towards sunset, shower activity re-emerged along the
mountains and east Anchorage, though not to near the intensity of
their counterparts from earlier in the day. These showers were
better able to spread westward, with traces of precipitation
reported from them at both Merrill Field and Anchorage
International. Shower activity was also present along the Kenai
Mountains, and numerous over much of the Susitna Valley as well
yesterday afternoon.

Shower activity also blossomed across Southwest Alaska, namely
along the Kilbucks east of Bethel, which then moved into Bethel
later in the afternoon. While it was associated with a different
wave, the processes that led to shower formation were largely the
same, with more sunshine and greater instability also present
through that area during the day yesterday. Bristol Bay also saw a
few showers, but were not nearly as plentiful due to a lack of
forcing. A very similar song and dance to the past week's weather
played out across the Bering yet again yesterday as the
persistent stacked low continued to weaken, but still was
responsible for plentiful cloud cover and intermittent rain for
the Aleutians.

&&

.MODEL DISCUSSION...
The models all remain in very good agreement on the synoptic
scale features, or lack thereof for some areas, affecting Southern
Alaska over the next couple days. The NAM was the model of choice
as the GFS was an outlier on some of the details of a front
moving into the Gulf Friday night, and the NAM was closer to many
of the ensembles and the non-American models. Out west most of the
models were in great agreement following the timing of the next
front to move into the Western Aleutians later today, and push
into the central Bering by Friday night. Regarding those large
scale features, forecast confidence is high.

The primary forecast challenge in this pattern will be the
strength and extent of afternoon shower activity expected once
again over Southcentral and Southwest Alaska. As all the models
forecast strengthening ridging along the Canadian border today,
the broad upper level flow should return to a dominant southerly
direction, which may impact the level of instability and in turn,
shower coverage during the day today.

&&

.aviation...
PANC...Remains under VFR conditions through Friday. There will be
showers over the mountains, and look for the winds to shift to
southerly flow with gusty conditions by late this evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Today should be a repeat of yesterday with scattered rain showers
around the area and mostly cloudy skies. The air mass and upper
level flow pattern is essentially remaining unchanged from
yesterday, which makes a persistence forecast the best forecast.
The trend is your friend and all that.

Yesterday afternoon's radiosonde showed about 150 J/kg of CAPE,
more than any model was showing. Given the unchanging nature of
atmosphere, 500-700mb temperatures progged to remain the same, as
well as surface temperatures, another round of convective showers
is in store. The main difference would be a shortwave progged to
move inland tomorrow evening/overnight. The timing of said
shortwave would keep showers going later into the overnight hours
instead of waning in the evening.

Coastal ridging returns late tonight which should bring rise to
some weak gap winds Friday into Saturday. Late Friday a low
develops in the western Gulf, ramping up cross-barrier flow once
again which should hamper any convective showers.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Scattered rain showers will remain in the forecast through
Saturday for parts of the Southwest Mainland. Colder air aloft
has filtered in across the region which has aided in creating a
conditionally unstable air mass, along with multiple weak waves
and moist southerly flow continuously tracking over the area from
the pacific. Scattered to widespread rain showers will be
possible through Saturday, especially near the mountains, but
thunderstorm potential continues to look rather unlikely with not
enough instability or forcing available.

Portions of the Kuskokwim Delta, mainly from Bethel westward,
will see patchy fog develop again early this morning. Some
locations could see visibilities as low as one half of a mile at
times again today, but the nature of radiation fog makes it very
difficult to pinpoint where exactly the fog will be the thickest.
The rest of the week will see clearing skies which will allow
temperatures to warm a few degrees warmer than previous days.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Isolated to scattered rain showers will continue across much of
the Bering Sea through tonight, with an upper level low near the
Central Aleutians slowly meandering eastward. Fog near the
Pribilof Islands should re-develop under a rather unstable air
mass this morning, but should finally mix out by this afternoon.
All eyes then turn to a frontal boundary pushing through the
western half of the Bering Sea later today and into Friday. This
should help bring back a nice swath of gale force winds along the
frontal boundary with periods of light to moderate rainfall. This
front will move to the Alaska Peninsula by late Friday night into
Saturday morning, but will have weakened substantially by the
time it makes it that far east.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long-term forecast looks like a continuation of the rather
cloudy pattern that has spread across much of mainland Alaska. The
main feature for the weekend and into mid next week is a low that
will be tracking through the Bering over the weekend and then move
across the Alaska Peninsula and eventually into the Gulf of Alaska
next week. There is good model agreement with this system through
the weekend giving higher confidence for showers across the
southern mainland and around the Bering for much of the long term.
Early next week areas along the north Gulf coast look to see more
steady rain as the low moves towards the Gulf.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE Warning...177 178 411.
FIRE WEATHER...NONE.

&&

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