Gale Warning 147, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
457 AM AKST Sat Feb 11 2017

.ANALYSIS AND UPPER LEVELS...

On the morning weather map there is an upper low on top of
Southcentral Alaska with a very cold arctic airmass in place
across nearly all of southern Alaska. A weakening surface low near
Prince William Sound continues to kick off scattered snow showers
across most of the area, though the majority are confined to the
North Gulf Coast. These showers will be on the rapid decline this
morning as the upper Low weakens and moves off to the northeast.
This will prove to be the opening act of a fairly dramatic
amplification of the upper level pattern that is going to bring
markedly different weather to southern Alaska over much of the
upcoming week. We can already start to see on satellite imagery
the beginning of a dramatic amplification of ridging (high
pressure) over the west coast of North America and a high-
amplitude trough digging southward in the Bering Sea. If this
sounds familiar, it's because we spent much of the last 2 winters
in this pattern. And, much like those winters, we're going to see
above-normal temperatures and the return of steady rainfall to the
coastal areas as this pattern unfolds.

But in the meantime, we remain firmly in the grip of an Arctic
airmass, with bitterly cold temperatures and gusty north winds
producing continued wind chill advisory conditions over parts of
the Kuskokwim Delta. In addition, gusty west winds combined with
ongoing snowfall in Whittier continue to produce blowing snow and
significantly reduced visibilities in that area.

&&

.MODEL DISCUSSION...

The models are in pretty good agreement in the first 36 hours or
so of the forecast, as the first low ushering in the broad
southerly flow makes its way into the northern Panhandle today
and tonight. However, by Monday morning, as the ridge to our east
continues to amplify, the models are still really struggling with
the timing, placement, and intensity of the numerous low pressure
centers that will be working their way toward our area for at
least the first half of the upcoming week. The main impacts from
this uncertainty will be just how warm the temperatures get early
in the week and, accordingly, pinpointing when the precipitation
type will change to rain at any given location. The GFS remains
the warmest and most aggressive solution bringing energy northward
past 60N, although some of the other models have begun to hint
this direction as well. this would make it easier for well-above-
freezing temperatures to surge into Anchorage. For now, we're
still considering the GFS a bit of an outlier, and utilized a
NAM/ECMWF blend for the morning forecast. Regardless of which
solution you choose, precipitation starts as snow along the Gulf
Coast but does change to rain for most of the storm cycle (which
will span most of the week).

&&

.AVIATION...
PANC...Light north winds will persist throughout the forecast
period. Persistent MVFR ceilings should fairly rapidly give way to
VFR conditions by mid morning as the weakening storm moves away.
By tonight, there is high confidence in clear and cold VFR
conditions through Sunday morning. You can never completely rule
out some patchy fog forming this evening off Knik Arm and floating
around the area, but the inversion strength will be much weaker
than many previous fog events, so the most likely scenario at
this point is no fog.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Sat and Sun night)...

A cold upper low over the northern Gulf lifts north this morning
to over the Copper River Basin then moves into the upper Canadian
Yukon tonight. With the exception of the eastern Copper River
Basin, snow/snow showers should taper off west of the Chugach.
Snow showers will likely hold on across some locales on the North
Gulf Coast as a rapidly north-moving low heads toward the
panhandle. Marine winds over the western Gulf will slowly diminish
through this afternoon. Brisk outflow winds across the eastern
Kenai/Western Prince Sound will be slow to diminish and will
likely hold on until the evening hours. Gap flows in Valdez/Thompson
pass will increase this afternoon with a brief up tick in the
pressure gradients and supporting 850 mb low, but should diminish
by late evening. With cold air in place and some cloud breaks
tonight, expect quite a few areas inland to see overnight temps
below zero.

Sunday through Sunday night will usher in a change in the pattern
toward windy, warmer, and wet. The culprit: a strong weather
front which moves into the southern Gulf Sunday afternoon and to
the northern Gulf Sunday night. This feature will bring abundant
warm air with a mix of rain and snow to Kodiak Sunday afternoon
which should transition to rain in the evening. The warming may
evening bring mixed precipitation types to some coastal locations
as early as Monday morning. Strong winds will also accompany this
front and bring gales to the western Gulf waters and brisk Gap
flows to favored locales across Southcentral.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Cold and dry conditions will persist over the bulk of the
southwest mainland through the weekend as an arctic trough remains
entrenched over the western half of Alaska. A weakening pressure
gradient will allow winds to diminish enough to bring an end to
wind chill concerns over the Kuskokwim Delta by later this
morning. An elongating North-South oriented front associated with
a deep North Pacific low south of the Alaska Peninsula will bring
a chance of snow to the Aleutian Range and perhaps other portions
of Bristol Bay beginning Sunday night. This will initiate a slow
warming trend area-wide, although most locations will still remain
below zero into the start of the work week.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Cold northerly flow on the western periphery of the longwave
trough will maintain a regime of scattered snow showers
accompanied by widespread Small Craft Advisory conditions and
freezing spray through the weekend. A weak disturbance dropping
through the Central Bering will possibly serve to enhance shower
activity over the Pribilof Islands late tonight into Sunday. The
deep North Pacific low then begins to approach the Alaska
Peninsula on Sunday bringing an increase in northeasterly winds,
however at this point in time it appears that the bulk of the
impacts will be confined to the coastal waters south of the Akpen
and Kodiak Island in the form of gale force winds and heavier
precipitation. Will continue to monitor this system for changes in
track that would shift these conditions closer to the Akpen.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Confidence continues to increase in the significant pattern change
early next week as warm, moist southerly flow and an active storm
track pushes into the Gulf of Alaska. The initial surface low
pushing into the southwestern Gulf Sunday afternoon will continue
north-northeast Monday night to track along the outer coast of
Kodiak Island Tuesday morning. This surface low track, combined
with an inverted trough extending north across the Kenai
Peninsula and into the Susitna Valley will act to reduce
downsloping as the front lifts north Sunday night and Monday.
While downsloping should still generally dominate, it's likely the
typically downsloped areas of Cook Inlet will see some light
precipitation at some point.

With cold northerly winds increasing out of Broad Pass and into
the Susitna Valley as the low approaches during the day Monday
confidence is higher in precipitation remaining snow with amounts
in the range of a foot or more possible. The second low coming up
Tuesday looks warmer with strong downsloping southeasterly winds
also increasing temperatures, so rain rather than snow looks more
likely for most low elevation locations. A third deep low follows
a similar track for a round of strong winds and rain. Areas along
the Gulf coast will get hit hardest in terms of precipitation
totals with between 5 and 7 inches of rain possible between Sunday
night and Wednesday.

Along the coast snow will quickly turn over to rain at the lower
elevations but do have concerns in the Coastal Mountains for snow
becoming increasingly wet and heavy on top of significant new
accumulations. Snow levels will increase to around 1500 ft along
the immediate Gulf Coast on Monday and then to 1500 to 2000 ft
over a broader area Monday night. Tuesday looks to be the warmest
portion of the storm with snow levels around 2500 ft. By the
third frontal system Wednesday snow levels will fall back to
around 2000 ft and then return to sea level as cooler air pushes
back in on Thursday.

Southwest Alaska and the Bering will remain on the cold side of
the baroclinic zone through much of the time period with
temperatures solidly in the snow range. As the various frontal
systems from Southcentral Alaska rotate west across the
Alaska/Aleutian Range and into Southwest Alaska, periods of snow
will be likely through the week with moderate to heavy snow
possible at times where deformations bands set up. Over the
Bering, persistent northerly flow will continue instability driven
snow showers from the contrast between the cold air temperatures
and warmer sea surface.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...blowing snow advisory 125
         wind chill advisory 155
MARINE...heavy freezing spray 121 129 130 131 132 136 138 139 150
                              160 165 170 180 185 414
         gale warning 155

&&

SYNOPSIS AND MODEL DISCUSSION...AMD
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
LONG TERM...JR