Small Craft Advisory 309, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
840 PM AKDT Sun Apr 15 2018

.ANALYSIS AND UPPER LEVELS...

The high amplitude upper level ridge is holding firm over
Southcentral Alaska and northward into the Arctic. The 250 mb jet
stream has changed a little since yesterday with it running more
zonal over the ALeutians and weakening as it goes up-and-over the
ridge before increasing again as it exits the southeastern Gulf
of Alaska.
The low pressure in the northwestern Bering Sea has become
vertically stacked and is weakening as it heads to the northwest.
The associated front is stalling over Kodiak Island and Southwest
mainland Alaska. The development of a triple point low along this
front near Kodiak Island tonight that then moves southeast and out
of the Gulf of Alaska still looks to be on track.

&&

.MODEL DISCUSSION...
Models remain in excellent synoptic agreement into the middle of
the week and the differences even for the later half of the week
are relatively minor. In spite of the agreement between models,
there is a significant area of concern with them. One of the more
common model errors is to break down a large ridge too fast. The
fact that the breaking down of the ridge during the middle of next
week shown on the models today is different than yesterday when it
generally looked like the ridge would hold over the mainland part
of the state for most of, if not all of next week. Therefore, it
is possible that all models are breaking the ridge too fast and
the high pressure will persist longer than today's model runs
indicate.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
High pressure currently resides over much of the southern
Mainland, which will quickly yield to a series of frontal system
pushing into the Gulf region from the west this evening through
Wednesday. Confidence remains high that these systems will keep
rain over the Kodiak Island region through Tuesday, along with
increasing easterly winds beginning late tonight as a triple point
low develops south of the AKpen and tracks into the Southern
Gulf. As a result of this feature and a second system tracking
into the gulf on Tuesday, a round of gale force easterlies are
expected around the Barren Islands late tonight into early Monday,
and again on Tuesday afternoon.

Impacts with regards to the initial system will remain confined
to the Southern Kenai Peninsula as ridging over the remainder of
the southern Mainland continues to bring dry conditions with only
a risk of increasing mid-high level clouds. However, the ridging
begins to weaken on Tuesday as a broad trough moves into the
Eastern Bering. This should allow for precipitation to spread
back into the northern Gulf areas for mid-week, with downsloping
conditions over the remainder of Southcentral. There is some
uncertainty with regards to how low the freezing levels drop along
the eastern Kenai with this system, yielding lower confidence on
what elevations will experience snow or rain.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Today through Wednesday)

The front stretching across a broad area of the Southwest
dissipates late tonight, however scattered to numerous light rain
and snow continues through Monday across inland areas as a weak
trough lingers in the wake of the front. Gusty flow tapers down
late this evening as the associated surface low lifts northwest.
Patchy fog was added for late tonight across most of the region
as winds go light and temperatures drop toward dewpoint values.
The boundary keeps higher surface moisture across Bristol Bay and
the Kuskokwim Valley which may keep fog slightly longer than
anticipated.

Southeasterly flow returns to across Southwest late Monday and
persists through Wednesday as the next organized storm system
tracks across the Bering. The surface front slides to the west
coast Tuesday night, pushing slowly inland through Wednesday.
Gusty winds develop along the frontal boundary, with the strongest
winds along the coast as the front makes landfall. Since the
front is moving in at night, precipitation starts out as snow but
will change over to all rain as strong southeasterly flow advects
a warmer air mass up from the North Pacific. Flow from this
direction also act to limit precip along west facing slopes and
the valleys on the lee side of the mountains.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
Strong southerly winds across the eastern Bering taper down this
evening as the storm system in the northern Bering lifts north.
The next storm system swings a frontal boundary to the western
Aleutians around midnight. The surface low deepens as it crosses
to the southern Bering late Monday. The strong dynamics of this
organizing low support higher winds than the storm that moved
across over the weekend. This developing storm rides up from the
North Pacific along a 150+ kt jet streak with additional energy
sliding into the base of the low as a short wave dives from the
Kamchatka Peninsula. Storm force winds were added to the marine
waters along the central Aleutians Monday night and Tuesday.
Elsewhere across the Bering, max gale force winds are expected
around the core of this system which continues through Wednesday
morning. Winds tapers down during the afternoon as the mature
system fills.

&&

.LONG TERM FORECAST (Wednesday through Sunday)...

There is good model agreement that the 2nd half of this coming
week will feature a continuation of a deep longwave trough over
the eastern Bering Sea, with a series of low pressure systems
moving to near the AKPEN and stalling. This will result in fairly
cloudy conditions across much of southern Alaska, with multiple
rounds of steady rain/snow affecting the Gulf Coastal areas. The
interior areas look safely downsloped throughout most or all of
these storms, however, so the forecast is cloudy but dry from
Kenai to Anchorage and the Mat-Su.

This weather pattern will also bring unsettled weather to
southwest Alaska and the Bering Sea. No specifically strong storms
are expected during this timeframe, but periods of onshore flow
across the mainland will keep conditions cloudy and fairly wet
(especially for this time of year).

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 130 155 165 170 171 173 174 175 179 180 181 185
         411-414
         Storm 171 172 176 177 178.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...TP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...AD