Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
343 PM akdt Sat Apr 18 2015
Short term...current satellite imagery shows deep clouds moving
over the Panhandle as front continues its slow push
eastward...primary dynamics are to the west of the area moving
northeast. This is consistent with model-anticipated winds not
increasing significantly over most land areas with this system.
However plume of moisture from the south is deep and extended and as
expected Yakutat area has been the focus of rainfall so far in the
progression of the slow-moving gale-force front. Model updates
have 1.5 to 2 inches of rainfall there by 03z Monday and this is
consistent with previous runs. Other areas at sea level in the
central/northern Panhandle expect 0.75 to 1 inch mostly tomorrow
as the primary frontal impact occurs after mostly light rains
today and tonight. Meanwhile...winds are gale-force as predicted
over the northwestern offshore marine zones enhanced by barrier
jet processes...and gales should reach into zones 43 and 42
tonight. Slow- moving nature of front will allow strong offshore
winds to persist through tomorrow morning...slowly weakening
after that. Small Craft Advisory-level winds anticipated for the exposed Inner
Channels as well as those favored for north- south isobar
orientation into tomorrow afternoon. Local northern Lynn and
Skagway vicinity winds have persisted gusting to 40 kts at the
former this afternoon. Anticipate that these will slowly decrease after
00z as gradient does the same.
Should be no precipitation type issues outside of the upper Klondike
Highway region tonight as temperatures should be a bit warmer
tonight. Border temperatures at The Klondike remain just below
freezing this afternoon and an inch or two is expected tonight up
high. Only updates to the inherited pressure/wind grids were
tweaks in the winds as pressure field was in good shape.
Thermodynamic fields updated with an ec/NAM/inherited combo with
no major changes. Temperatures updated per MOS and again the changes were
minor. Went with stratiform precipitation region-wide per current visible
satellite and radar imagery.
Long term...model agreement aloft on the synoptic scale is good.
Both European model (ecmwf) and GFS are painting the same overall pictures with
differences in the details. A deep upper low is situated over the
eastern Bering Sea and an upper ridge extends south from the
Beaufort Sea over western Canada, then out to sea over Vancouver
Island at the start of the long range forecast Sunday afternoon.
The ridge shifts to the east as an upper trough pushes east across
By Tuesday morning, the Bering Sea upper low will move
north to near the Bering Strait and a new upper low will develop
in the base of the trough over the Aleutians. This new feature
will dig east-southeast to near haida gwaii by Thursday morning, but then
retrograde back to the west and become absorbed by a stronger
upper low that will dig even further to the southeast through the
middle of next week.
At the surface, the new Aleutian upper low will phase with a
developing surface low that will enter the western Gulf Tuesday
night or Wednesday morning. This is where model differences really
begin to be noticeable with respect to weather over the eastern Gulf and
Southeast Alaska. All models deepen the surface feature rapidly
once it phases with the upper low. GFS tracks it north to near
Kodiak, occludes it sooner, and then develops a triple point low
over the southwestern Gulf by Wednesday morning. Better agreement
exists between European model (ecmwf) and Gem, neither of which track the surface
low any farther north than 54n. Both Gem and European model (ecmwf) then occlude
the system and track it off to the southeast. Some residual energy
in European model (ecmwf) then moves northwest along the outer coast coast on
Friday while GFS maintains a significant surface circulation over
the eastern Gulf through Friday night.
Opted to use wpc for updates to pressure and wind beginning
Wednesday morning with no changes Sunday night through Tuesday
night. Did update pop and quantitative precipitation forecast through Sunday night using a blend
of Gem and GFS which maintained good continuity with the previous
forecast. Updates to temperatures limited to overnight lows Monday
night through Thursday night to better reflect colder low level
air advecting in behind a frontal feature currently located over
the western Gulf. Overall forecast confidence is average.
Marine...Gale Warning for pkz042-043-051-052.
Small Craft Advisory for pkz012-021-022-033>036-041-053.
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