Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
548 am akdt Tuesday may 21 2013
Short term...early morning satellite image shows a high
pressure area continues building across the Gulf of Alaska and
Yukon with an inverted trough low pressure extending
northwestward from British Columbia across Southeast Alaska.
Increasing offshore flow across the forecast domain and modified
air mass by the inverted trough have maintained overnight
temperatures mostly in the lower to middle 30s over the northeastern
Gulf Coast and north central Panhandle. None of the observing
locations reported freezing temperatures lasting more than
6 hours. Therefore...the freeze warning and frost advisory were
canceled at 4 am. Temperatures aloft have been warmed up and rest
of the Panhandle remained in the upper 30s to the middle 40s. Near
gale force winds prevail over the northern Lynn Canal and small
craft level winds blow across the remaining coastal waters.
Models remain fairly consistent for high pressure continues
building across the Gulf of Alaska through late tonight with a
weak inverted trough of low pressure remaining across the
Panhandle. Refreshed the inherited forecast package with an
ensemble approach for today and tonight. Did not invite MOS
guidance for temperatures. A plenty of sunshine across Southeast
Alaska will cause afternoon sea breezes along the immediate
coastal areas. A slow warming trend will continue through
tonight...but mostly cloud free skies will again enhance
radiation cooling after sunset. Not widespread...but patchy frost
may form during the late night through early morning hours over
the northeastern Gulf Coast and wind sheltered locations in the
north central Panhandle. Clouds wrapping around the low will
bring some clouds over the southern Panhandle along the inverted
trough axis.
Long term...surface high pressure over the Gulf of Alaska begins
to weaken or flatten as a weak shortwave well west of US in the
western Gulf begins to push eastward. The inverted trough over the
Panhandle does remain; however, a vorticity low spinning aloft over
the northern Panhandle moves north and in turn weakens surface high
pressure over northwest Canada. Thus gradients will slacken
across the board...outflow winds and even winds offshore across
the marine zones beginning Wednesday afternoon and continuing into
Wednesday night.
But even as the overall trend supports winds weakening, they will
be quite strong in the usual Micro-regions...northern Lynn Canal
and southern Lynn will both begin Wednesday morning as small
craft, with northern Lynn being on the stronger side. In addition,
downtown Juneau is still at risk of a mountain wave event,
supported by the NAM. However, gradients between Juneau and Dease
Lake have been prognosticated as slightly less than they were advertised
as yesterday, and the lack of wind development overnight limit my
confidence for Wednesday morning. Even the Juneau Tram arguably
never experienced any significant northeast winds. But the needed
element of light cross-barrier wind component between 400 and 600
mb was never promised for this night's wind no-show. And at least
the NAM argues Wednesday morning's potential event does. Thus kept
a northeast direction for downtown...limiting gusts to 35 mph,
even though a NAM inspired mountain wave model suggests gusts to
45. Thermal mixing after sunrise may help direct winds to the
surface. The event should be short-lived as ingredients begin to
fall apart late morning into the afternoon.
NAM, ECMWF, and UK all trying to bring some moisture over the
Coast Mountains Wednesday afternoon into Wednesday evening, but
have yet to introduce even slight chance probability of precipitation to anywhere but
Hyder for the mid-range.
Inherited sky grids still seem to represent view that Gulf winds
begin to turn more onshore beginning late Wednesday and thus we
should have a steady encroachment of cloud cover towards the
coast. Also in light of overnight temperatures not falling that
much, feel that Thursday morning temperatures may be a tad on the
low side despite low humidity...especially given warming air mass
and corresponding higher daytime highs on Wednesday well into the
middle and upper 60s for locations across the central and southern
Panhandle. Less time to cool as well. However, feel that lighter
winds may contrarily support the forecast...as is. Whatever the
case, may frost is over. Let Summer begin.
By Friday, upper level ridge rebuilds over the Gulf and ahead of
it surface ridge rebuilds northward again and shunts marine layer
once again to the southeast rather than onshore. But as we
approach the weekend, some moisture seeps over the mountains in
the form of scattered showers and middle-level clouds to spread from
the southern Panhandle into the interior north. In addition the
Gulf showing signs of more activity as the Memorial weekend
climaxes toward Monday.
Made no changes to pressure field and confined changes to local
areas for sea-breezes, outflow winds, and other mesoscale effects.
Kept overall pattern of ec/NAM inspired forecast with wpc
influence for the extended range. Confidence
Ajk watches/warnings/advisories...
public...none.
Marine...Gale Warning for pkz012.
Small Craft Advisory for pkz011-013-021-022-031>034-036-041>043-
051.
&&
$$
Ahn/jwa
