Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
629 am akdt Thursday may 23 2013
Short term...this was a tricky forecast defined by relatively
light flow and poorly defined features in the models and on other
tools like satellite. Let US start with what we know. We are in a
warm pattern with lots of southeasterly flow aloft and
temperatures will once again warm well into the 60s across the
region today. A few interior locations will likely see the 70s
like deeper parts of the Mendenhall Valley. Think in the far
north like Skagway and Haines, temperatures will recover now that
clouds have moved north. With weakening high pressure over the
Gulf of Alaska and a thermal trough of low pressure extending
northwest, most winds will be northwesterly across the Panhandle.
As interior parts warm rapidly this afternoon, we should see
another surge of westerlies into the ocean entrances, similar to
yesterday and thus have bumped up Cross Sound and southern
Chatham into Small Craft Advisory winds this afternoon, and emphasized some 25 knots
winds in Sumner without placing them in Small Craft Advisory. Sea
breezes will develop late morning into the afternoon before
shutting off in the evening. We also have a lot of sea stratus
offshore with some fog being reported by mariners. The sea
stratus reached Yakutat, Sitka, and Cross Sound around midnight.
This is something we expected and see no reason why the pattern
will vary much tonight. Fog was added across the offshore zones
using NAM boundary layer humidity fields and the fact that
confirmation has been reported.
Now for the tricky part: sky cover...a lot more high and
middle-level clouds arrived into the southern Panhandle yesterday
evening and overnight than previously thought. Cannot say that
mechanisms weren't revealed: there was some weak vorticity
advection riding over the ridge aloft overnight coming from the
southwest. But most of the models did not suggest cloud cover of
any significance, with the exception of the European model (ecmwf)...which seemed
to overcompensate for the lack of clouds in the other models.
While cloud cover on infrared satellite imagery overnight seems to
suggest weakening...clouds looked thicker than thought on webcams
and with coordination from Annette wso, elected to call things
mostly cloudy in the south. But feel as though they will have
some sunshine this afternoon. The important thing is dry weather
here. Cloud cover over the central Panhandle seemed thinner and
less shadow inhibiting, so called for mainly sunny conditions
here and to the north. For the northeast Gulf Coast, worried that
middle to high level clouds from the west may tamper with sunshine
this afternoon as marine layer breaks up...so went with partly
cloudy for this afternoon for Yakutat. Cloud cover tonight was
equally challenging. Models...especially GFS showing a surge of
moisture moving in from the southeast. MOS guidance cloud cover
has been unhelpful as it has cared little about the current
conditions over the south. Think some down-sloping will tend
toward clearer conditions than what models are indicating, and so
have left things mainly clear for much of the interior Panhandle
overnight. But partly cloudy skies may have been the safer
alternative.
This weak vorticity center over the southeast Gulf is forecast
to weaken and move north and as it does it will pump more warm
air over the region from the southeast, and with that inevitably
comes moisture as well. The moisture will begin to work through
the southern Coast Mountains this afternoon that may bring the
threat of an isolated or scattered shower near Hyder. Things
dissipate quickly over the evening, but a more significant lobe
of moisture arrives late that will spread scattered showers over
Hyder toward the early Friday morning hours. For more about this
onset of some wetter weather for the southern Panhandle see the
long term segment.
One final note about temperatures, cooled them a little for this
afternoon given our thinking at the time that skies would be a
little cloudier, so it could get warmer than advertised...especially
over the north. White Pass temperatures were warmed tonight
thinking an inversion may set in below keeping temperatures at
pass level relatively mild. Relied on NAM 925 mb temperatures for
guidance.
Models were having a difficult time with some of the positioning
and strength of the surface high offshore, and especially clouds
over the region. So instead chose not to change pressure field
and confined wind changes to local effects. Relied on satellite
interpretation for cloud cover and local knowledge of sea breezes
for help on winds. Forecast confidence average to good, but falls
for sky cover...especially over the southern and even the central
zones. Luckily, impacts of missed forecast will be minimal.
&&
Long term...the extended forecast period looks like it will be
dominated by high pressure aloft centered over western Canada and
eastern Alaska. The Pacific storm track will stay well south of the
Panhandle. A cold upper level low center producing rain and mountain snow
is stuck over the Pacific northwest. Cloudiness is curving around this low
and moving northwest over British Columbia toward Southeast Alaska. The Panhandle will
be under this southeast flow aloft through the middle of next week. The
main forecast challenge will be determining if and how much rain
will spill over the coastal mountains from British Columbia and
into Southeast Alaska. This weather pattern favors a bit higher chance of
rain over the southern Panhandle and drier but cloudy weather over
the northern Panhandle. There is better agreement in nearly all
models and National wpc guidance that maintain temperatures will remain
at or above normal through the extended range.
The GFS ensemble mean was used to update the grid forecasts. The
deterministic hemispheric models such as the GFS ec and Gem had
various solutions so the ensemble mean was is an effective
compromise for describing the general large scale features.
Forecast confidence regarding the location and timing of
precipitation is low.
The first shot of possible precipitation over Southeast Alaska appears to
be during the day on Friday over the southern zones. Deformation
aloft and an increasingly moist air mass could lead to light rain
especially north and east of Ketchikan. This will likely dissipate and
dry out as it moves north Friday night. Another batch of rain
showers could develop Sat and Sat night over the central and
northern coastal mountains. Related to this I lowered daytime maximum
temperatures a bit from previous forecasts through the weekend. The chance
of precipitation does appear to be higher and more widespread by Monday and
Tuesday but rain amounts are not significant...certainly less than
the last two very wet weekends in Southeast Alaska.
&&
Ajk watches/warnings/advisories...
public...none.
Marine...Small Craft Advisory for pkz022-033-041-042.
Small Craft Advisory due to seas for pkz043.
&&
$$
Jwa/ta
