Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
444 am akst Thursday Mar 5 2015
Analysis and upper levels...
A trough extends from the Bering Strait across the akpen and into
the North Pacific this morning. It is bisecting a massive ridge
that extends from the Kamchatka peninsula to the Canadian
rockies. Also embedded in this ridge is an upper low over the Gulf
of Alaska. At the surface...a low is spinning and weakening in the
southern Gulf...a front is weakening as it moves across southwest
Alaska with some snow...and the next major player is a front being
driven into the western Bering by a 140 knots jet.
Models are in very good agreement that the initial front over SW
Alaska will reach southcentral today and interact with the Gulf low...before
moving east and dissipating over the Copper River basin Friday.
Ridging behind this first front will be brief. The front entering
the western Bering is stronger than the first and has better
upper support with the aforementioned jet. It will race across the
Bering and reach SW Alaska tonight. The front will then cross the Alaska
and Aleutian ranges on Friday. A stream of moisture will then
interact with the front Friday night into Sat...taking aim at the
northern Susitna Valley with snow...coastal sections of the Kenai
Peninsula with rain and snow...and Prince William Sound with rain
and snow as the front moves to the Copper River basin on Sat. The
forecast utilizes the ec with high resolution models influencing
details in the east...in particular rain versus snow over
Short term forecast southcentral Alaska (through Saturday evening)...
Light winds and a shallow low-level temperature inversion will
allow fog to continue well into the morning hours. Scattered
showers will continue as well with decent lapse rates between
2500 and 10000 feet.
A pattern change begins late this afternoon into this evening as a
sharp upper level trough moves across the Cook Inlet region
tonight and across the rest of southcentral Friday. Southwest
up-inlet winds at both the 850- and 700-mb levels will increase
over the southern and eastern Susitna Valley this evening. These
low-level winds along with decent upper level jet speed
divergence ahead of the middle-level trough will result in snow
developing from the Parks Highway east by early evening. The main
inhibitor to significant snow accumulations will the short
durations that the most favorable conditions occur over any one
area. The system will transition during the overnight hours to
begin to impact Anchorage and the mat-su valley...with the
heaviest accumulations of snow expected toward Hatcher Pass and
along the Anchorage hillside. Accumulations should generally add up
to 2 inches...though amounts toward 4 inches are possible in the
more favorable areas.
Snow will end as a brief period of drizzle or freezing drizzle and
possibly fog Friday morning as only incremental drying will occur
from the upper to lower layers of the atmosphere...with the lower
levels remaining fully saturated with continued southwest up-inlet
A strong upper level jet pushes across the Bering Sea and onto the
Mainland Friday evening into Saturday bringing a warm front/cold
front combo to the area. A decent period of frontal upglide with
jet support combined with a moist southwest flow aloft will bring
a good period of precipitation Friday evening through Saturday
afternoon/evening. The biggest question mark with the system will
be precipitation type. The thermal profile aloft looks to support
snow...however turnagain wind could bring some warmer air at the
surface and keep temperatures in the middle to upper 30s which would
make for a cold rain. Several areas have the opportunity to get
significant accumulating snow with this system including the
Susitna Valley and Valdez area...the latter will also be dependenton
Short term forecast southwest Alaska (today through Sunday morning)...
Numerous snow showers continue across much of southwest Alaska
this morning as the upper level trough continues to push through
the area. Recent radar images from the Bethel 88-d show the back
edge of the precipitation about halfway between Toksook Bay and
Bethel. The back edge will continue to shift east during the day
as the upper trough pushes through with much of the Bristol Bay
area and Kuskokwim Delta seeing an end of snowfall by this
evening. A potent front associated with a low in the western
Bering Sea will bring precipitation chances back to southwest
Alaska late tonight and on Friday. Precipitation type becomes a
bit of an issue across the Bristol Bay area as surface temperatures
and surface to 850 mb thicknesses continue to warm...which should
transition any snow that falls from King Salmon south to rain on
Friday. North of King Salmon and into the lower Kuskokwim valley
and Kuskokwim Delta should predominately stay snow (except for a
chance of rain mixing with snow in the Delta Friday afternoon).
Snowfall totals will remain below advisory thresholds...with 2 to
4 inches expected for the Kuskokwim Delta...3 to 5 inches in the
lower Kuskokwim valley and up to 2 inches in the Bristol Bay area
north and east of King Salmon. The western convective available potential energy of the Bristol
Bay area could see upwards of 4 to 6 inches of snowfall should
precipitation not briefly transition to rain Friday afternoon.
Once the front pushes eastward into southcentral Alaska...steep
northwesterly flow aloft will reside and a shortwave will push through
the area late Saturday and into Sunday. This will keep scattered
to numerous snow showers in the forecast through the weekend.
Short term forecast Bering Sea/Aleutians (today through Sunday morning)...
It appears more likely that the low pushing off the Kamchatka coast
this morning will bring storm force winds to the north and western
Bering offshore zones on Friday. While the low itself will not be
as deep as many systems seen this winter (981 mb)...it will be
clashing with a ridge over the central and eastern Bering Sea to
create a strong pressure gradient. Gusty winds will be seen across
the central and western Aleutians as the front associated with
the Kamchatka low pushes quickly across the Bering Sea. A very
potent warm front will traverse the southern half of the Bering
Sea during the day today and keep precipitation initially as rain
until colder air filters in on Friday. The warm front will also
limit the mixing of stronger winds down to the surface.
Long term forecast (days 4 through 7)...
confidence continues to increase that significantly colder
temperatures are in store for the Mainland by early next week as
zonal flow gives way to a more amplified pattern with an upper
trough digging towards the eastern Gulf. This will also usher in a
period of drier weather across the southern Mainland...with gusty
outflow winds developing along the north Gulf Coast.
The exception to the overall strong model agreement in this time
period is over the western Bering...where there are significant
discrepancies with the placement of a surface low in the vicinity
of the western Aleutians. Most of the guidance keeps this feature
south of the chain...whereas the GFS brings it into the central
Bering by Monday afternoon. Sided with the more southern solution
for now which would keep the bulk of the impacts south of the
Bering. Models are in better agreement by the middle of the week
as high pressure builds over the Bering...which would help to re-
enforce the trough over the Mainland.
Marine...gale 170 173 175 176 177 178 179 185
heavy freezing spray 185.
Synopsis and model discussion...ds
southwest Alaska/Bering Sea/Aleutians...mc