Scientific Forecaster Discussion
fxak68 pafc 210159
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
459 PM akst Wed Nov 20 2019
Analysis and upper levels...
a 977 mb low continues to sit near Kipnuk, along the southwest
alaskan coast, with another developing low around 977 mb located
across the west central Aleutians. Ahead of these features, a
surface front attacked to the Kipnuk low is pushing east across
the eastern Gulf. Breezy and warm conditions are over most of
southcentral, with the exception of the Susitna Valley. An Arctic
front is draped across the Kuskokwim Delta with cold conditions,
with warm air over the lower Kuskokwim valley and Bristol Bay.
the models remain in relatively good agreement through the
weekend. In general, they agree in the overall theme of multiple
systems trekking from west to east, with a general cooling trend
noted. How much cooling depends on the various numerical models,
as the differences in their height fields arise.
panc...gusty southerly winds are expected through late Thursday
morning. Late Thursday morning the surface winds will become light
northwest, and strong winds aloft will create low level wind
shear. VFR conditions are expected, but some showers this evening
could drop ceilings to MVFR range.
Short term forecast southcentral Alaska (days 1 and 2)...
rain will be ongoing across the northern Gulf tonight as a weak
boundary remains stalled from Yakutat to the western Prince
William Sound. An upper level low over the southwest lifts north
tonight bringing increased precipitation chances from Kodiak
Island north to the Alaska Range. A moderated warmer air across
the region will support mainly rain, but snow will also occur as
temperatures cool slightly overnight. The biggest challenge
tonight is across the Susitna Valley, as there is a boundary
between a warm and cold air masses draped along or just south of
the Alaska Range. The Susitna Valley is in the path of stronger
energy from the upper level wave so better organized precipitation
is expected. A Winter Weather Advisory was issued as freezing
rain will be in the forecast through late tonight. This upper wave
should lift north of the Alaska Range by Thursday morning.
The next front to monitor moves to the western Gulf early
Thursday morning. This system will be a fast mover, making it to
the northern coast by the afternoon. Expect gusty winds across
most of the region Thursday through Friday with peak strength
Thursday night. The upper low driving this front develops along
the Alaska Peninsula as energy races south of the Aleutian chain,
and then closes off a new center while the upper jet stream takes
a northward turn across the Gulf. At the same time a surface low
develops over the Alaska Peninsula at 960 mb. The pressure
gradient rapidly tights along the northeast Gulf to Turnagain Arm
as the upper level low lifts toward the southern Mainland. This
system will track across the region through Friday with the most
impactful time frame on Thursday night when the system dynamically
organizes. High wind watches were issued for Portage Valley,
Turnagain Arm and higher elevations for Thursday night as winds
have the potential to gust between 70-80 mph.
The eastern Kenai and coasts along Prince William Sound are
expected to receive the heaviest rainfall amounts Thursday into
Friday. Accumulations for these areas will range between 2-4
inches. Thompson Pass is another area of concern as there is
potential for heavy snow as several upper level waves track across
from the Gulf. A watch for heavy snow was issued for Thursday
night which may last through Friday afternoon.
Short term forecast southwest Alaska (days 1 and 2)...
a weakening low is moving inland, currently centered near the
Kilbuck Mountains and Nushagak hills. Behind the low, cold return
flow has brought a cooling trend and scattered showers to the
Bristol Bay area into the Kuskokwim Delta. Showers will generally
diminish tonight, but there is quite a bit of forecast
uncertainty. Models have struggled significantly with the
progression of a series of compact lows along a front associated
with a broader Kamchatka low. The 18z NAM and GFS both initialized
well in general, thus this forecast package leaned towards those
solutions. A low will drift towards Cape Newenham tonight into
tomorrow, and swing a warm front into Bristol Bay. This should
bring a swath of precipitation Thursday morning from south to
north. Expect snow initially in the greater Bristol Bay area, then
changing over to rain or a rain/snow mix along the coast. Along
the akpen, precipitation will mainly be rain. The low will
progress inland and precipitation will turn showery and diminish
in coverage Friday, though not clear up completely.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
a series of compact lows continue to bring areas of gales to the
Aleutians and southern Bering. In addition, strong cold air
advection on the back side of a low positioned over the southwest
Mainland has maintained gales in the northern Bering. Unsettled
weather will continue in the eastern Aleutians/akpen Thursday,
with periods of gales likely. Northerly gales will spread from the
northern Bering into the central Bering as a ridge upstream
Marine (days 3 through 5)...
the period begins with a vertically stacked low moving ashore
somewhere between 60n and Bristol Bay Saturday afternoon in the
neighborhood of 990 mb with a surface front moving into the
northwest Gulf and coastal southcentral Alaska. Northerly winds on
the backside of the low appear to be just below the gale mark in
the coastal waters of southwest Alaska and the Alaska Peninsula.
Coastal locations of the northern Gulf appear to remain below gale
in the 30kt range. However, some gales will appear in the
southern Gulf on Sunday afternoon, along with the waters of Kodiak
Island. With winds hovering in the near gale range, will want to
watch the next couple of model runs for changes. Localized gales
are also possible near the Standard favored offshore flow terrain
gaps along the Gulf Coast Sunday night through Monday. High
pressure in the central Bering follows the system on Sunday as it
moves towards the Alaska Panhandle. The next gale force frontal
boundary enters the western Bering and Aleutians Sunday night and
will strengthen to storm force by Monday afternoon in the central
Bering. Again, model discrepancies will need to be addressed, but
with a potential parent 960 mb low, strong winds will be on the
menu as this system comes ashore Tuesday morning.
Long term forecast (days 3 through 7)...
by Saturday morning an upper level low will be centered over the
southern Bering and Bristol Bay with an associated surface low
offshore over the Kuskokwim Delta. The upper level low will move
inland over the greater Bristol Bay area through Saturday
afternoon and evening while the surface low rotates southeastward
along the coast and moves into Bristol Bay. In the Gulf, a frontal
system stretching across the western and northern Gulf will swing
onshore across the Prince William Sound area and northern Gulf
Coast on Saturday.
The upper level low will continue to the east northeast, pushing
into southcentral Alaska Saturday night through Sunday bringing
significantly colder air aloft and some snow showers with it.
Upper level ridging will move across the Bering from west to east
Saturday night through Sunday. As the ridging amplifies over the
eastern Bering Sunday night and southwest Alaska on Monday,
offshore flow will strengthen rapidly over southcentral Alaska
with snow showers tapering off and skies clearing.
A Kamchatka low will spin up to a rather strong frontal system in
the western Bering Sunday and Monday. The front will reach the
central Bering Monday afternoon and evening and eastern Bering
and Kuskokwim Delta Monday night. A trailing portion of the front
will reach the eastern Aleutians Tuesday morning where it will
merge with a North Pacific frontal wave and then continue east
over the Alaska Peninsula through Tuesday afternoon. The front
will move further inland across southwest Alaska Tuesday night and
into southcentral Wednesday. The southern frontal wave will spin
up into surface low over the southwest Gulf on Tuesday night and
track into central Gulf Wednesday through Wednesday night.
public...high wind watch 101 125.
Winter Storm Watch 131.
Winter Weather Advisory 145.
Marine...Storm Warning 119 125.
Gale Warning 120 130>132 139 150 155 174 176 185.