Scientific Forecaster Discussion
fxak68 pafc 240128
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
428 PM akst Tue Jan 23 2018
Analysis and upper levels...
the upper level trough remains over Mainland Alaska with the
ridge over the Bering Sea. Very cold temperatures as associated
with the upper level low and that will be tracking into southwest
and southcentral Alaska later this week. The deformation band
did set up over southcentral Alaska late last night and has been
snowing itself out over the course of the day in the northern
Cook Inlet region and Susitna Valley. A tight pressure gradient
is set up along the north Gulf Coast allowing for some gusty
outflow winds but they are limited by a lack of upper level
support. There is upper level support behind the upper trough
axis from near Kodiak Island to the eastern Aleutians with the
tight northwest gradient aloft between the low and ridge.
the most significant model change to address is the way the 500 mb
low center is digging into southern Alaska Wednesday night into
Thursday night. Instead of the low center moving into Bristol Bay,
the models are showing the low moving to Kodiak Island Thursday
night. The sensible change from all this is that there is a
greater chance of clouds and precipitation in southcentral Alaska
with this low track and less chance across the southwest Mainland.
panc...generally MVFR conditions will occur through the evening as
snow lingers in the area. It should end by late evening and allow
dry northerly winds to clear out the area by morning. While there
is always a chance of fog after a snowfall, the northerly winds
and dry airmass moving into the area should keep the fog away
leading to VFR conditions in a northerly breeze for Wednesday.
Short term forecast southcentral Alaska (days 1 and 2)...
the radar and the models indicate that snow flurries will likely
linger into the overnight hours until the longwave trough moves
through. That said, some of the snow bands are probably below the
radar beam and it is not out of the question for a few locations
in Anchorage and the mat-su valleys to receive another inch or
two, but given the limitations of the models it is difficult to
pinpoint. Generally speaking, snow will taper off for the region.
Tomorrow cold air advection and drier conditions will impact
southcentral. Brutally cold temperatures are projected for
Wednesday and Thursday. In the Anchorage bowl and the mat-su
valleys single digit high temps are forecast. Additionally,
outflow winds continue to impact the region this forecast package
as the thermal gradient remains in place (most pronounced along
the Aleutian range). At 925 mb north of the Aleutian Range the
temperature ranges from -20c to -28c and south of the Aleutian
Range the temperatures range from -2c to -12c. These outflow winds
will result in rough seas and strong gap flows with gale to storm
gusts for the western Gulf marine areas through Thursday.
Short term forecast southwest Alaska (days 1 and 2: Wednesday and
the advance of a strong upper level low just east of the area and
its associated bitterly cold air continues across southwest Alaska
this evening. This will leave the area with more of the same
weather: clear and cold conditions. Temperatures are running
anywhere from 15 to 25 degrees below normal area-wide. Fortunately
the winds will remain light in most areas away from the coast,
so once again any wind chill concerns should remain localized
to interior windier locations north and west of Iliamna. Towards
the coast, temperatures are slightly warmer, and so the slightly
stronger winds should still keep wind chills shy of advisory
criteria. The only precipitation concerns will be confined to
the immediate Kuskokwim Delta coast and Nunivak Island Wednesday
into Wednesday evening with a fast-moving impulse of moisture,
though any snow that falls will be light.
On Thursday, the upper level low tracks to either side of the
Alaska Range. Regardless of its final track, behind the upper
low passage a warming trend in temperatures back toward normal
is expected as we head through late week.
Short term forecast Bering Sea/Aleutians (days 1 and 2: Wednesday
a broad warm ridge and associated surface high pressure centered
over the central Aleutians continues to dominate most of the
bering's weather, except near the coast and the far western
Bering. The coast is the transition zone to deep polar air over
the state. Meanwhile out west, another very deep low out near the
Kuril Islands will build the ridge northward. The building ridge
over the Bering will very effectively block the active weather
over the far western Bering from impacting any more of the area.
Besides the occasional weak impulse of energy rapidly tracking
southbound across the eastern Bering into the Alaska Peninsula,
the remainder of the area will remain under a mix of sun and
clouds as much of the stratus erodes. Conditions will deteriorate
Thursday night into Friday over the western Bering and Aleutians
as a front moves over the area and stalls.
Long term forecast (days 3 through 7)...
beginning the extended period forecast Thursday night, a deep
Arctic vortex will be moving south over Kodiak Island and then
into the western Gulf of Alaska by Friday morning. A very cold
air mass will be in place, especially over southwest Alaska as
the cold air takes the path of least resistance around the Alaska
Range into the Bering Sea. The tricky forecast at this juncture
will be over southcentral as the eastern periphery of the
aforementioned upper vortex will be advecting a warmer air mass
toward the coast (although still cold enough for all snow). This
will bring snow chances to all of the north Gulf Coast (and
potentially into some inland locales near the coast) and Kodiak
Island. The low quickly digs south with snow chances moving south
with the low so that by Sunday all of southern Alaska will be dry
as cold high pressure settles in.
The high latitude blocking high is still expected to develop
somewhere over eastern Siberia and the northwest Bering Sea which
will continue a northerly flow pattern aloft over southern Alaska
along with continued dry and cold through Tuesday. At this
juncture, the numerical model guidance falls apart as the high
latitude block shears the Mainland trough and upper level energy
splits over the North Pacific. However, this model discrepancy
really does not significantly impact the sensible weather over
the region, so expect continued cold and largely dry weather into
the midweek period.
Marine...heavy freezing spray warning 121 126 127 129 130 131 132
136 137 138 139 141 160 180.
Storm Warning 130.
Gale Warning 120 127 131 132 138 150 178.