Scientific Forecaster Discussion
fxak68 pafc 230057
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
357 PM akst Wed Feb 22 2017
Analysis and upper levels...
satellite water vapor imagery shows the upper trough moving east
into Canada. The surface low over the northern Gulf continues to
move east-southeast and weaken. Radar imagery showed snow
diminishing across much of the northern Gulf.
A strong vertically stacked low over the western Bering Sea produced
storm force southwesterly winds across the western Aleutians and
gales across much of the Bering Sea and the remainder of the
Aleutians and western Alaska pen. The frontal boundary continued
south to near 30n with satellite imagery showing a deep moisture
fetch associated with a strong southerly subtropical jet. Some
areas of light snow were evident across the southern Mainland
with locally gusty winds along the coasts.
models are in fairly good agreement. However, there are some
differences in 850 mb temperatures, especially on Friday across
the Anchorage bowl/mat valley. The NAM is the cooler of the models
and would suggest any precipitation across the above areas would
remain as snow. For now will continue to lean that way.
Otherwise, the NAM will be used through the short term (through
Friday afternoon) in the east and the GFS in the west.
panc...a building ridge and warm advection will likely keep the
threat of MVFR and possibly IFR conditions across Anchorage
through Thursday morning.
Short term forecast southcentral Alaska (days 1 and 2)...
as the low in the Gulf weakens and moves away from the area most
areas will see some degree of clearing skies tonight as shortwave
ridging moves quickly into the area. The one exception will likely
be locations around Cook Inlet where persistent temperature
inversions and mountain barriers could result in low
clouds/stratus hanging around most of the night and perhaps may
even see some localized fog development. Places that do clear out,
though, will cool off fairly substantially, although even this
will be limited by the approach of clouds ahead of the next Bering
Sea storm that is heading our way. Clouds from this storm are
already thickening near Kodiak Island as of early Wednesday
The next storm coming into the area Thursday night and Friday will
bring a surge of warmer temperatures and good chances for
widespread precipitation to most areas. At this point it looks
like the cold air will hang on just long enough to keep things
mostly snow from Anchorage northward, with a mix of rain and snow
on the Kenai Peninsula and rain along the Gulf Coast. One fly in
the ointment is the potential for light freezing rain to fall out
of the low clouds stuck in Cook Inlet between now and Friday
morning (which is when the stronger precipitation should be able
to tap into some colder temperatures higher in the atmosphere to
produce snowflakes). With warm and moist air beginning to surge
straight up Cook Inlet only a few thousand feet aloft, most places
from Kenai north to the Anchorage area are at risk for this light
freezing rain, though confidence in timing or location are low.
Have opted to include some mention of this in the afternoon
forecast, and future upper air soundings will need to be closely
studied to diagnose risk.
Short term forecast southwest Alaska (days 1 and 2)...
the trough has moved across southwest Alaska and we are now
seeing the next front move across the Bering Sea through the
Pribilof Islands toward the Bering Sea coast. This will push
moisture and a warm tongue aloft over southwest Alaska by
Thursday morning. This introduces a challenge to the precipitation
type for the Kuskokwim Delta and Bristol Bay regions as the warm
air aloft initially spreads over the region. However, it does look
as though the warm air will rapidly mix to the surface bringing
the temperatures above freezing. Any freezing rain will be light
and short lived overnight and toward the morning over southwest
Alaska. South to southwest flow into the southwest Mainland will
allow for more widespread rain and snow over the Kuskokwim Delta
and northern Bristol Bay Thursday. The rain and snow will spread
east to the Iliamna area to the upslope regions of the Aleutian
mountains and Alaska Range late Thursday into Friday. Colder air
wraps back into the southwest Mainland by Friday, thus the
precipitation type will return to snow. At this point, a broad
southwest flow is established from the Bering Sea into southwest
Alaska, therefore expect a continuation of cloudy conditions with
periods of precipitation.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
the parent low of the gale to storm force front sweeping across
the Aleutians and Bering Sea to the southwest Mainland will move
to the Bering Strait as it dissipates through Friday afternoon. On
the heels of this storm is another vigorous storm that is moving
into the extreme western Bering Sea Friday afternoon. This will
push a gale to storm force front across the western Aleutians
Friday to the pribilofs by Saturday.
Long term forecast (days 3 through 7)...
the active weather pattern over the Bering Sea will begin to move
into the Mainland this weekend as high pressure slides southward
into the North Pacific. This will allow for a gale force front
over the Bering Sea to spread gusty winds and rain/snow into the
southwest Mainland with deep onshore flow for much of the weekend.
Then by early next week the models continue to be in good
agreement that an Arctic trough will deepen over the Mainland Mon-
Tue, ushering in colder air and gusty offshore flow to much of the
Gulf Coast. What is still unclear however is the exact track that
this trough will take through the Mainland, which is having a
significant impact in the possibility of snow to the south
Mainland into the middle of next week. The ec and Gem are more
aggressive in bringing Bering Sea moisture into the Mainland ahead
of the Arctic trough, and a result are more aggressive in the
potential for a snow event Mon-Tue. The GFS on the other hand has
been very consistently more aggressive in bringing cold/dry
interior air, leaving most of the south Mainland dry into the
middle of next week. For now the forecast favors the drier GFS
solution as it has been much more consistent. Forecast confidence
is below average.
Marine...Storm Warning 178.
Gale Warning 155 165 170 172>175 177 179 185.