Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
518 am akdt Tuesday Mar 31 2015
Analysis and upper levels...
long-wave trough remains firmly entrenched across Alaska and the
North Pacific...with high pressure ridges remaining over the far
western Bering and also over the northeastern interior of the
state. The subtropical jet is nearly zonal south of the Aleutian
chain then dives southeastward into the North Pacific. This is
resulting in a persistent storm track along the southern tier of
the forecast area. There are two relatively weak lows affecting
the forecast area...one just south of the Gulf and another just
south of the eastern Aleutians. As has been the case over the
past few days a series of upper level waves are tracking
northwestward across the Gulf and southcentral. Isolated
convective showers developed off of the Talkeetna Mountains and
moved across the Susitna Valley last (sun) evening as the 00z
Anchorage sounding depicted lifted indices near or slightly below
zero (more typical of late may or early june)...but these showers
have dissipated with the loss of daytime heating. The bulk of
precipitation has been over the northern Gulf with a weak vorticity
maximum moving moving slowly west to northwest toward the eastern
Kenai Peninsula. Isolated to scattered showers also continue over
southwest Alaska east of Bethel with the remnants of a weak
deformation band that has been nearly stationary in weak upper
flow across the area.
models remain in very good agreement with handling of large scale
features over the next couple days. Thus forecast confidence is
generally high. With weak flow outside of the main low pressure
systems the biggest forecast challenges are on the local
scale...with elements such as fog and diurnal clouds and shower
activity. Models do begin to diverge a bit as the system tracking
south of the akpen moves to the south of Kodiak Island Tuesday
night into Wednesday. They all hint at some development of a
triple point low...but right now almost all models depict the
initial parent low remaining the primary low over the far southern
Gulf. In any case these differences only affect the area in the
direct vicinity of the system. The GFS was largely preferred in
the short term...with some deference to the NAM early for local
Short term forecast southcentral Alaska (through Wednesday night)...
unsettled weather will continue across southcentral through Wednesday.
An upper trough coupled with a weak surface trough moves north
and west across southcentral today. The upper trough moves into
the interior tonight and will be followed by a secondary trough
pushing north across the Gulf of Wednesday. This secondary through will
be somewhat weaker then its predecessor. Available sounding and
model guidance indicate that showers will be the dominate
character of precipitation with these features. Freezing levels
through Wednesday will be high enough that of the precipitation will
fall as rain during the afternoon hours...then becoming more mixed
or snow during the overnight and early morning hours. Drier
conditions are expected overnight Wednesday as the low moving into the
Gulf creates northerly offshore flow.
Kodiak and Gulf
an occluded front moving into the SW Gulf early Wednesday will bring
snow and rain to Kodiak Island beginning early Wednesday morning.
Thickness values and 850mb wind directions (se) would indicate
that precipitation will start initially as snow or mix and rapidly
transition toward rain. Gales are expected to accompany the front
across the Gulf.
Short term forecast southwest Alaska (days 1 and 2)...
the Bristol Bay through the lower Kuskokwim valley will have
numerous snow showers with a weak disturbance pushing through the
area this morning. While...the Kuskokwim Delta Region will have
low stratus/patchy fog with scattered snow showers along the coast
in the morning today. The winds will shift to offshore flow and
will bring drier conditions by Tuesday evening. This will result
in the diminishing of the stratus and fog going into Wednesday.
There is a gale force North Pacific low just south the eastern
Aleutians. This system will track south of the Kodiak Island by
Wednesday. This will give wrap-around moisture spilling over the
Alaska Peninsula into the Bristol Bay area with continued warmer
temperatures through Wednesday.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
there is a gale force North Pacific low just south of central
Aleutians near Nikolski this morning. This system will bring
warmer temperatures along the central to eastern Aleutians as the
low tracks eastward along the chain...before tracking into the
Gulf of Alaska by Thursday morning. This will bring wet and
southerly windy conditions and northeasterly winds with snow
showers on the backside of the low as this system tracks eastward
through midweek. An upper level ridge will build into the western
Bering by Tuesday afternoon. By Wednesday...the akpen and eastern
Aleutians will have gusty northwesterly gap flow as the low
pressure system continues to track into the western Gulf. The
western aleutains will have a gale force low dropping down from
Kamchatka peninsula with an associated weather front bringing the
next round of precipitation by Wednesday. This will result in
strong southerly winds bringing in warmer temperatures from the
northern Pacific...which will cause the precipitation type to
transition over to rain as the warm front passes through the
area by Wednesday afternoon.
Long term forecast (days 3 through 7)...
the latter part of the week will be characterized fairly quiet
weather across the southern Mainland. There will be a near gale
force front moving up toward the northern Gulf Thursday before
weakening late Thursday into Friday. This will keep rain and rain
showers...mainly over the water but western Prince William Sound
could see numerous showers with this features. Colder upper level
temperatures aloft could support instability showers over the
coastal mountains with increasing cyclonic flow. This system will
weaken in the Gulf and move toward the Panhandle Friday. At this
point the pattern turns more amplified out west...but it is
unclear how much this will affect the pattern over the Mainland.
Expect a continuation of above normal temperatures and drier
conditions through early next week.
Marine...gales 132 138 155 172 173 174 178
heavy freezing spray 179 180 185.
Synopsis and model discussion...cirrocumulus
southcentral Alaska...British Columbia
southwest Alaska/Bering Sea/Aleutians...period