Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
645 am akdt Friday may 24 2013
Short term...mostly cloudy skies over the Panhandle this morning,
with rain developing in much of southern Southeast Alaska by this
afternoon. Relatively light winds will persist, and only Cross
Sound has been given a Small Craft Advisory. This short range
section concludes with an analysis of a warming event that took
place at Skagway early this morning.
The largest adjustments to the short range forecasts are for
raising the cloud cover amounts for most of the area of
responsibility. Note the cloud cover increases are raised in three
ways: first, the GOES vi Sable imagery this morning reveals an
extensive coverage of sea stratus and the onshore the Panhandle.
Second, patches of fog have been reported and patchy fog will
develop tonight. Third we have raised amounts of middle and upper
level deck with causes that are discussed in the next paragraph.
Confidence is average.
A 500h low over the Seattle vicinity has a synoptic-scale flow
that extends as far as central b.C. The GOES infrared loop depicts the
upper level east flow reaching southern Southeast Alaska with
cloud cover from the weakening of smaller-scale cyclonic features.
A 500h ridge extends west-southwest from over Great Bear Lake and over the
central Panhandle. We sometimes call a ridge aloft "dirty" if
short waves are able to travel through it, even if these waves are
weakening. In the present case, we have a dirty ridge that is
weakening the short waves enough to limit precipitation, however
the cloud mass at middle and upper levels will continue to be
ceilings.
Models: the GFS model run at 24.00 was easier to reconcile with
observable features aloft, and the model has had a higher pop
field farther west across the southern Panhandle. In this western
extent of the raised probability of precipitation as well as for cloud cover, the GFS was
an outlier, and correct.
The Yakutat and Annette Island radiosonde observations at 12z show strong subsidence
inversions. The Yakutat radiosonde observation inversion includes a measurement of
9c at 900h, and if such a value is adiabatically subsided to near
sea level, would be 18c, or 64f. A similar subsidence event raised
the temperatures at Skagway this morning. The Skagway Airport at
3 am reported a temperature of 50f with calm winds. By 4 am that
Airport had winds from the north at 7kt with gusts to 14kt and a
surface temperature of 60f. During the same two hour period the
dew point went down from 37f to 30f. The north winds at Skagway
provided the dry adiabatic process this morning.
.Long term...Area will remain under a generally east-southeast flow aloft
over the next 5-7 days...with upper ridge/high remaining to the NE
of the area and upper trough to the SW. This pattern is often not
handled well by the models as far as the details go. Will probably
have weak shortwave features move across the area from the east
during the period...but the GFS and European model (ecmwf) do suggest a sharper
lobe of the upper ridge/high to the NE will extend into the area
at least sun-Tue...which would tend to weaken the shortwaves.
Also...will have a small upper low loop around the eastern Gulf Sat
through sun then pull northwest and out of the area by early next week.
After coordinate with weather forecast office pafc went with a blend of the 00z GFS and
European model (ecmwf).
For Sat...weak shortwave will drift northwest across the area with bands
of middle-level clouds and possibly a little precipitation. Best chances for
precipitation appear to be along the Coast Mountains with a weak onshore flow
in lower levels and some middle-level warm advection over the top.
For Sat night into sun...upper low will drift north then northwest through the
eastern Gulf. This will interact with the leftover moisture from the
Sat shortwave and may organize precipitation over the NE Gulf and
adjacent coastal area. Kept in chance probability of precipitation there to handle this
system. Further inland and S...probability of precipitation should lower as the sharper
lobe of the upper ridge/high begins to affect the area. Have
lowered probability of precipitation some in these areas. Have kept in mostly cloudy skies
though due to weak onshore flow keeping a stratocu layer in.
For Sun night into Tuesday...sharper ridge lobe should dominate the
area. There may be some weak shortwave energy drifting across the
far southeastern area though...so will keep in low probability of precipitation there. Otherwise
should see more breaks in the clouds especially Monday. If the models
maintain the strength of the ridge lobe over the area...later
shifts will likely need to decrease cloud cover especially over
the northern area.
From midweek on...looks like upper trough to the SW will try to
drift closer to the area and the ridge lobe will weaken. This
would bring in a better threat for precipitation due to increased Ely
wave activity. Left the chance probability of precipitation in to cover this. Precipitation may
be more showery as well as convection that fires over British Columbia could
drift westward into the area. This will need to be watched closely.
Ajk watches/warnings/advisories...
public...none.
Marine...Small Craft Advisory for pkz022.
&&
$$
Jbt/rwt
