Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
243 PM EST Sun Feb 11 2018

Valid 12Z Thu Feb 15 2018 - 12Z Mon Feb 19 2018

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

The start of the forecast period will feature a series of short
wavelength systems accompanied by a split in the jet streams. On
Thursday, ejecting height falls will be in the process of pushing
across the Aleutians with an embedded shortwave accelerating
through the Gulf of Alaska on Friday morning/afternoon. The 00Z
ECMWF has tended to be quicker than other solutions but all do
agree on the existence of this feature. Farther upstream within a
persistent area of lower heights across the north-central Pacific,
multiple lobes of vorticity should lift northward across the
Bering Sea. Each perturbation in the flow will lead to ample
surface cyclogenesis with multiple such cyclones traversing the
Bering Sea waters. In previous days, run-to-run model
inconsistencies coupled with intermodel differences led to very
low confidence forecasts. More recently, there is a better sense
of clustering as shown by the operational/ensemble solutions.
Decided to keep a sufficient amount of operational model guidance
in the mix through at least Friday to capture additional details
of this wave train. While not juxtaposed on top of one another,
the guidance cluster reasonably well on Friday morning with the
deep cyclone entering the western Bering Sea. Like preceding
waves, it will also lift northward and become embedded within the
broader-scale circulation. By next weekend, solutions come into
marked agreement with a pattern shift favoring well above average
mid-level heights across Alaska. The 00Z ECMWF ensemble mean has
shown a large increase in heights relative to the cycle 24 hours
previous. While some operational models attempt to erode sections
of this positive height anomaly during the Day 7/8, February
18/19, time period, the forecast is too uncertain to incorporate
such details. However, there is agreement favoring some fairly
potent upstream cyclones, albeit with spatial-temporal
differences. Overall, the preferred solution through Saturday
morning featured a combination of the 12Z/06Z GFS and 00Z ECMWF
with their respective ensemble means. Thereafter, building
uncertainties in the pattern led to an equal blend of the 06Z
GEFS/00Z ECMWF ensemble means.


...Weather Highlights/Threats...

Relative to the forecast created yesterday, some of the
temperatures have come down a bit, most notably on the minima.
Friday and Saturday morning could be quite chilly over sections of
north-central to northeastern Alaska as lows plunge into the -20s.
This would particularly be the case in areas with stagnant
low-level thermal inversions as is common in favored valley
locales. As usual, the mildest air will cling to the southern
Alaskan coast where maritime influences should dominate the
diurnal temperature shifts. Regarding precipitation threats, it
will likely be fairly wet over the Alaskan panhandle in advance of
the mentioned system crossing the Gulf of Alaska. The 12Z/06Z GFS
are notably wetter with up to 1.50 to 2.00" 12-hour amounts ending
16/1200Z. A lighter band of precipitation, all wintry in nature,
is expected farther north across interior Alaska on Thursday into
early Friday. Conditions should dry out considerably with the
building ridge as precipitation begins to focus farther west
toward the Aleutians. Strong poleward moisture advection ahead of
the attendant cold front will spread abundant precipitation from
west to east across the Aleutians this weekend. Winds may also
become quite gusty given some of the robust operational solutions.
The 00Z ECMWF suggests surface wind gusts may approach the 50 to
60 knot range although plenty of model uncertainty exists.


Rubin-Oster