Area Forecast Discussion


960 
FXAK68 PAFC 190138
AFDAFC
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
438 PM AKST Sun Feb 18 2018
.ANALYSIS AND UPPER LEVELS...
The remnants of the warm front that brought snow and rain to the
Southwest Mainland overnight into this afternoon is continuing to
spread eastward into the Gulf early this afternoon. This development
is becoming more and more apparent with each scan from the Kenai 
radar, and this trend is expected to continue and spread northward
through Cook Inlet by this evening. Behind the front, the cold/dry
air is readily apparent on satellite and radar imagery with an 
abrupt shift from widespread snow to clearing and showers. Over 
the Western-Central Aleutians a mature storm force front brought 
a brief round of hurricane force wind gusts to the Adak area 
earlier this morning. Cold/dry air is wrapping in behind the low 
and heading toward the Central Aleutians, which is now expected 
to bring another round of high winds to the area this evening.
&&
.MODEL DISCUSSION...
The models remain in generally good synoptic agreement through
Monday with the general pattern, though there remains some
uncertainty in particular with how much precipitation will move 
into Northern Cook Inlet and the North Gulf Coast this evening 
from the weakening front begins to fall apart. Radar trends this 
afternoon however are beginning to bring snowfall slightly faster
than many of the models, though based on this trend there is 
still good confidence in deteriorating conditions throughout Cook 
Inlet this afternoon into this evening. The other question is with
how much precipitation is expected, as there is a significant 
spread between the GFS which is mostly dry in the Anchorage Bowl 
and the rest of the models that bring in light amounts. With good 
southwest flow and moisture advection ahead of the snow and no 
downsloping present to limit precipitation from reaching the 
ground this dry GFS solution was ignored for the afternoon package
in favor of the light amounts from the rest of the guidance.
&&
.AVIATION...
PANC...A brief period of snow will bring temporary IFR
visibilities, though ceilings are expected to remain MVFR. After
04Z more unstable air arrives and expect a period of VFR conditions.
As this air mass settles in overnight, an upper level ridge will 
build overhead, which will bring a quick return to stratus potential.
Depending on how much snow falls, this could come in earlier than
the current taf indicates. While fog can't completely be ruled 
out, the cooler and drier airmass will limit temperature 
inversion, so it seems more favorable for stratus with an IFR 
ceiling. 
&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A weak front across the central Gulf to the Susitna Valley moves
east overnight bringing a brief period of snow across the southern
mainland. Ridging builds in behind this front with drier
conditions returning through Wednesday afternoon. Patchy fog 
along Cook Inlet is expected to develop each night with a stable 
air mass in place, and should diminish during the late morning 
hours. Winds will generally be light through mid week with 
offshore flow gusty at times through Whittier and Seward from 
weak gradient forcing.
&&
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Wednesday evening)
An unsettled pattern will continue over the next few days. This 
afternoon a weak front is currently moving across the southwest 
mainland, and this front looks to reach the Alaska Range in the 
next few hours, with drying conditions following in its wake. The 
real question tonight will be patchy fog coverage, mainly south of
a Cape Newenham to Port Heiden line. Here, decreased coverage of 
mid to high-level cloud cover may allow enough breaks for 
radiational fog to develop. As such, some patchy fog was included 
in the forecast. Further north, the cloud shield looks more robust
in coverage, mitigating the fog potential enough not to include 
any mention of it.
Another front looks to reach the coast tomorrow afternoon, 
sweeping eastward through the evening and into the overnight 
hours, with fairly widespread rain and snow accompanying it. This 
system will clear the area by Tuesday morning, as another brief 
ridge of high pressure brings clearing skies with drier conditions.
This will be short-lived however (sound familiar?), as a third 
front approaches by Tuesday evening, with rain and snow potential 
once again on the increase.
&&
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Wednesday evening)
A familiar pattern looks to continue through the next few days, 
as a series of low pressure systems and their associated fronts 
cross the region. The first such storm is crossing the western 
Aleutians right now, with ship observations and the latest 
scatterometer pass showing storm force winds accompanying the 
low/cold front across the western Bering, through the western 
chain, into the north Pacific. Winds just below storm force are 
also seen along a northward advancing warm front which looks to 
continue affecting the central portions of the Bering Sea/Pribilof
Islands through this evening. Widespread precipitation will 
accompany both fronts as well.
The real story will be the air mass/wind gust potential
immediately behind the cold front. For this evening into the 
overnight hours, the low, currently near 960 mb, will generally 
remain at about the same strength as it treks northward toward 
the eastern Kamchatka Peninsula. As it does so, a 120 kt mid-level
jet will cross the front across the west-central portions of the 
Bering Sea and Aleutian chain. At the same time, a strong vort 
max will rotate through the region with strong dry and cold 
advection continuing, aided by a decent PV anomaly (on the order 
of 500mb). This anomaly will just compress the air more, and when 
combined with the dry/cold advection, enhanced downward mixing 
potential will be enhanced. With this in mind and based on the 
observations that winds over-achieved earlier this morning at Atka
in the typically weaker warm advection pattern, the high wind 
warming for this area which was issued earlier is being maintained.
Hurricane force gusts are expected, with winds topping out in the
80 mph range. Further east toward the eastern portions of the 
chain, storm force wind gusts are likely, with even weaker winds 
(gales) toward the AKpen. 
As this system moves northward through much of the north central
Bering tomorrow, the disturbance aloft substantially weakens, 
with just storm force winds expected. However, a favorable fetch 
length will keep seas elevated in the 30 to 35 feet range. A pair 
of systems looks to approach the region for Tuesday and Wednesday,
but they will be considerably weaker. Widespread precipitation 
will accompany each system from tonight through Wednesday, mainly
along the associated fronts crossing the region from west to east
through the period.
&&
.LONG TERM FORECAST (Days 3 through 7)...
The general trend for the long-term forecast beginning the middle
of this week will be for continued active weather moving from the
Bering Sea and across Mainland Alaska into next weekend. This 
will be produced by a flattening of the ridge that has set-up 
over the North Pacific, allowing for a broad westerly jet to 
stretch from the Aleutians through the AK mainland and into 
British Columbia. With the more progressive zonal flow, the
primary challenge in the extended will be timing of each storm
system. With lower amplitude waves, this is a favorable pattern
for snow over Southcentral Alaska, including Anchorage. Southwest
will be more exposed to a brief warm-up with each system, so will
likely see a mix of rain and snow. 
&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm Warning 177 178 185.
 Gale Warning 165 170 171 172 173 174 175 176 179 180 181.
&&
$$
SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...DEK/SEB