Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
235 PM akdt Sat Apr 25 2015
Short term... / Saturday night and Sunday / the new surface low
will continue to deepen slightly to the upper 970s mb tonight as
it moves north into the central Gulf as a vertically stacked
feature with much of the upper level support moving from the
eastern Bering Sea over the last day or so. As such there is a
fair bit of cool air aloft to support the system and moderately
strong pair of jets of 120 to 140 knots about the upper level trough.
The surface front is spreading northeast over the eastern half of the
Gulf of Alaska through Saturday evening with gale force winds. The
winds are expected to pick up to 40 to 45 knots with the front and
slack off to small craft or less. As the winds start reaching the
coastal areas tonight have kept the West Coast of Prince of Wales
Island under the strong wind headline tonight and early Sunday
Believe the rains will start over the southwest coastal areas
after midnight and more towards sunrise. The rains should spread
north to the north central Panhandle by midday and to the
northeast Gulf Coast and northern Panhandle by Sunday afternoon.
The winds for the rest of the Inner Channels as the front moves
over the area weakening will be 20 to 30 knots.
Larger rainfall amounts will be over the southern Panhandle with
up to an inches expected while I expect about a quarter inch in
the Juneau area and less over the northern Panhandle.
Long term...upper low is forecast to be centered immediately S of
the Gulf by Sun night. Associated gale force surface low will be
located in a similar position...while occluded frontal boundary
extends northward into the northern Gulf...then southeastward along the coast of the
Panhandle...where it will become diffuse and weaken by midday Monday.
Meanwhile...500 mb temperatures at or below -34 c are forecast to become
injected into the eastern-northestern periphery of the upper low Monday morning
through the afternoon. This will aid in destabilizing the
atmosphere resulting in a transition to showers across the region.
In addition...a relatively warm boundary layer combined with
steepening lapse rates will support MUCAPE values from 200-500
j/kg over southern and coastal portions of the Panhandle...as well as
the adjacent outside waters. This thermodynamic environment
combined with several Mobile midlevel vorticity lobes will aid in a
slight chance of thunderstorms during Monday.
European model (ecmwf) and Gem show a small-scale surface low pressure center
forming near haida gwaii by 00z Tuesday in advance of an embedded
upper-level shortwave trough. This low then moves north across the
southern/central Panhandle before dissipating over the northern Panhandle by
18z Tuesday. Coordination with previous long-term shifts indicates
that this feature has shown run to run consistency...thus decided
to introduce a surface wave into the inherited pressure grids
using 12z ECMWF/Gem. These grids will need to be nudged further in
order to capture the full magnitude of the low...and winds over
the southern Panhandle do not currently reflect the full strength
associated with the deeper low depicted in European model (ecmwf)/Gem solutions.
Future shifts will need to continue to evaluate model trends.
A wet pattern continues into the middle of next week due to upper
troughing over the Gulf and continued onshore flow over the
Panhandle. Model agreement decreases rapidly by 00z Sat through
the weekend...with the European model (ecmwf) showing an amplified ridge and dry
subsident environment developing over the Gulf shifting east to the
Panhandle...and the GFS depicting moist southwesterly flow spreading across
the Gulf to the Panhandle. Handled these differences by using wpc
guidance...which was based mostly on European model (ecmwf)/European model (ecmwf) ensemble data.
Otherwise...pressure grids were based mainly on a
Gem/ECMWF/inherited blend through 12z Wednesday...with a transition to
wpc thereafter. Pop and quantitative precipitation forecast were based mainly on GFS/European model (ecmwf)
guidance before previously mentioned wpc was incorporated during
the later periods of the long term.
public...strong wind from 1 am akdt Sunday through Sunday morning for
Marine...Gale Warning for pkz036-041>043-051-052.
Small Craft Advisory for pkz011-021-022-031-033>035.
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