Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
257 PM EST Tue Mar 05 2019
Valid 12Z Sat Mar 09 2019 - 12Z Wed Mar 13 2019
Today's guidance continues to show better than average agreement
and continuity in principle as the pattern evolves toward a mean
trough aloft becoming aligned near the western coast of the
mainland. A shortwave embedded within progressive North Pacific
flow may phase with this trough by around next Wed, leading to the
possibility the overall trough could take on a negative tilt.
This pattern should lead to one or more periods of focused
precipitation along the southern coast and Panhandle along with
episodes of strong winds/high waves from the Aleutians eastward.
The portion of the forecast spanning day 4 Sat into day 6 Mon
incorporates ideas from the 00-06Z GFS and 00Z ECMWF/CMC. There
was a robust cluster of 00-06Z operational models showing eastern
Bering triple point development tracking toward the western
mainland while the deep parent low over the western Aleutians as
of early Sat (950's mb, similar to yesterday's forecast) tracks
eastward along the Aleutians while weakening. The ensemble means
merged the two separate lows. Interestingly the new 12Z GFS has
come in with a merged scenario but on a track a little south of
the means. Thus confidence in exact details is not exceptionally
high. Other 12Z runs vary in specifics but do agree on an overall
low track to the north of the 12Z GFS. By day 6 Mon the
operational runs show better definition than the means with
another wave tracking near the Aleutians, favoring continued
operational model emphasis through mid-period.
By days 7-8 Tue-Wed the forecast rapidly adjusts toward a more
even blend of model and 06Z GEFS/00Z ECMWF mean solutions.
Guidance shows a general theme of surface low pressure emphasis
transitioning from the western mainland into the northeastern
Pacific as a reflection of potential phasing of North Pacific
shortwave energy with the upper trough near the western coast of
the mainland. The most likely scenario has low pressure tracking
within an area to the south/southeast/east of Kodiak Island by day
8 Wed. Especially by late next Wed the ECMWF mean and operational
runs all display deeper/eastward low pressure than the 06Z GEFS
mean to varying degrees with the ECMWF mean more stable over the
past day and GEFS means showing an eastward trend over recent
runs. As a result the preferred blend leans more toward the
majority cluster. Meanwhile operational models are generally
faster than the means with another North Pacific/Aleutians wave
that comes into the picture by next Wed. The model/mean blend
provides a good middle ground between the two clusters.
Across the Arctic there is decent agreement that during the
weekend into early next week flow aloft will gradually amplify to
the south of an upper low drifting along near 80N latitude, with
the preferred blend resolving differences in amplitude--GFS/GEFS
guidance generally deeper than other solutions. By Tue-Wed the
upper low/trough should drift far enough eastward to allow for
expansion of surface high pressure north of the mainland.
Rausch
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Sun-Tue, Mar 10-Mar 12.
- High winds across portions of the Alaska Panhandle and mainland
Alaska, Sat-Tue, Mar 9-Mar 12.
- High winds across portions of mainland Alaska, Sat-Sun, Mar
9-Mar 10, and for the Aleutians, Fri-Tue, Mar 8-Mar 12.
- High significant wave heights for coastal portions of the
Aleutians, Fri-Sun, Mar 8-Mar 10.
WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can be found at:
https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html