Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
330 PM akdt Sat may 25 2013
Short term...satellite imagery this afternoon showing surface
high pressure ridge and stable air mass over the Alaska Gulf with
broad low pressure system over British Columbia moving an
unstable air mass over the Coast Mountains into the Alaska Panhandle.
A middle level low and rotation in the western Gulf can be seen in
visible and water vapor satellite imagery. The result of this is
a marine stratus layer with low clouds and fog over the Alaska Gulf
while weak bands of rain showers have been moving up from the
southwest over the Panhandle. While a high cloud deck remained
over most of the Panhandle along with lower clouds from the
showers there were a number of clear breaks developing and
tracking north.
Not much change in these surface features through Sunday other
than the high pressure ridge over the Gulf shifting more to the
east with the marine stratus layer moving back over the Panhandle
coast late tonight then pulling back tomorrow morning. The low
over b.C. Becomes less organized resulting in the scattered
showers that have moved over the Coast Mountains and into the
Panhandle diminishing through the evening with some lingering
showers in the morning. The upper low in the Gulf looks to track
to the northern Gulf later tonight and brings periods of rain
into the northwestern Panhandle. With only weak forcing precipitation
totals have been and will continue to be light. A developing low
in the North Pacific will just begin to move into the western
Gulf Sunday will increasing winds in the far western portions of
the Gulf.
Weak pressure gradient resulted in light winds over the region.
Cloud cover limited day time heating and kept temperatures in the middle to
upper 50s so far. Only a weak sea breeze developed this afternoon
due to the limited land -sea temperature gradient. With some
increased clearing and insolation for Sunday expecting some
warmer temperatures with highs getting into the middle to upper 60s. Models
not in great agreement on high temperatures for Sunday...so decided to
keep on the low side following persistence. With the increased
thermal gradient along with a tightening southerly gradient will
have higher winds Sunday afternoon than today over the Inner
Channels.
Good model consensus on synoptic scale features but the mesoscale
scale features are not being well depicted. Tracking down timing
of the shower bands has been tricky. Used a blend of 12z NAM/00z
European model (ecmwf) to refresh inherited grids as GFS was a bit of an outlier
depicting what looked like middle level features at the surface.
Overall only a few changes to some wind directions and minor
speed adjustments. Forecast confidence is above average with
general synoptic pattern but lower for tracking mesoscale scale
features.
Long term...the forecast models continue to be in fair agreement
through the midrange but there still remains some uncertainty to
how much clouds and showers will move over the mountains from the
east and over the Panhandle early next week. There was not much
changes done due to lower confidence with the flow from the east
and some upper level dynamics moving over the area. Updated the
middle term periods with the NAM and then wpc for the extended portions.
Overall there is about average forecaster confidence in the long
term portion of the forecast.
At the start of the long term period a upper level low over the
north central Gulf will pull to the northwest as a upper high over Canada
slowly drifts to the west into the Yukon. The offshore flow will
increase and clouds will decrease through Monday over the northern
areas as the seas stratus over the Gulf will push to the west and
away from the coastal areas of Southeast Alaska. An area of vorticity
embedded at the base of the upper high will move over the Coast
Mountains Sunday night as a weak upper trough under cuts the high
and moves over the southern areas Monday afternoon/evening. This
upper low will advect some cooler air aloft to increase the
instability near the Hyder area and may trigger some
thunderstorms Monday afternoon/evening. The rest of the southern
areas will see some isolated to scattered showers through Monday.
The flow from the east will persist into Tuesday night and
continue keep a threat of showers from the east over the forecast
area.
There will warm air advection and temperatures will be on the warmer side at
night and will remain in the upper 40s to lower 50s. During the
dat temperatures will get into the middle 60s with some areas away from the
water reaching the lower 70s early next week as the sun probability of precipitation
through the clouds.
This warmer air will enhance the thermal low in the Canada and
increase the wind in Lynn Canal to range from 15 to 20 knots Monday
afternoon and then again on Tuesday. The rest of the marine waters
will see relatively light winds from the south through most of the
week.
In the extended periods(wednesday-saturday)...there looks to be a
cold front that will move over the area from the south and that
front will produce some significant rain starting Wednesday evening.
A broad trough associated with the front over the North Pacific will
slowly lift north through the end of the week to keep the threat
of rain in the chance to likely categories. There is some time
differences in this but the overall trend is to see this front
move through the forecast area late week.
Ajk watches/warnings/advisories...
public...none.
Marine...none.
&&
$$
Prb/abj
