Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
247 PM EST Fri Feb 09 2018

Valid 12Z Tue Feb 13 2018 - 12Z Sat Feb 17 2018

...Pattern Overview, Guidance, and Predictability Assessment...

The focus of the forecast across the high latitudes will be a
negative height anomaly which is seemingly anchored to the
north-central Pacific. Within this region of lower heights, a
myriad of smaller perturbations in the flow will be foci for a
multitude of surface waves which take aim on the far western
Aleutians and Bering Sea. Given the small wavelength between each
system, as expected, the predictability of the position and
strength of each individual wave is rather uncertain. Looking at
the state of the flow at the start of the period, Tuesday,
February 13, a longwave trough will be exiting toward the British
Columbia coast while shortwave ridging briefly sets up over
mainland Alaska by mid-week. Farther upstream, the mentioned
series of shortwaves will enter the picture with a bit more
confidence in placement relative to yesterday's guidance. By early
Wednesday morning, the 06Z/00Z GFS and 00Z ECMWF/CMC all place a
low center over the western Aleutians although this cluster has
bucked a bit as the 12Z GFS trended to the north. At this point,
they further diverge as the GFS/CMC solutions favor a deeper
surface reflection into the Bering Sea while the 00Z ECMWF shears
out the system. Eventually, the 00Z ECMWF shifts focus to a
trailing wave which rapidly strengthens over the south-central
Bering Sea on Thursday morning. However, previous forecasts show
no merit for such a solution which adds more uncertainty to the
mix. Toward the Day 7/8, February 16/17 realm, operational and
ensemble guidance favor a rather deep cyclone emerging south of
the Kamchatka Peninsula with placement and intensity variable. As
a whole, was able to incorporate some of the 00Z GFS/ECMWF into
the forecast in conjunction with ensemble means through Day
5/Wednesday while shifting toward a more ensemble-based approach
to conclude the forecast into early next weekend.


...Weather Highlights/Threats...

A gradual cooling period is expected over interior sections of
Alaska and up across northern sections of the state. This will
draw high temperatures down into the single digits with possibly
some embedded below zero readings. Overnight lows should plunge
into the negative teens, particularly in areas where sufficient
low-level thermal inversions set up. Regarding precipitation
threats, the active wave train lurking upstream should keep
conditions wet for a majority of the forecast period. There should
be plenty of cloud cover accompanied by these series of
precipitation threats given appreciable moisture transport. Given
the robust nature of some of these low centers, there could be
substantial poleward moisture fluxes taking aim at sections of
southern Alaska. The progressive nature of each system should
mitigate any major heavy precipitation threats with 12-hour
amounts likely capping out around 1 inch or so. Much of the
Alaska's interior can expect periods of snow as vorticity centers
slide eastward from the Bering Sea. And there may be a decent
threat for windy conditions over the Aleutians depending on the
strength of the waves of low pressure. If the 12Z ECMWF were to
verify, its forecast would favor near hurricane force wind gusts
across the far western Aleutians on Wednesday.


Rubin-Oster