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Southeast Alaska forecast discussion 
National Weather Service Juneau Alaska
351 PM akdt Tuesday Aug 19 2014

Short term...quasi-zonal flow aloft extends from the northern Pacific into
northern Canada this afternoon. Several low amplitude upper waves are
embedded within this flow regime...one of which is currently
entering the southwestern Gulf...and a second is moving east-southeast over northwestern
Canada. A scattered to broken layer of midlevel clouds is
occurring over the Alaska Panhandle SW of the Canadian wave.
Relatively stronger insolation over the far southern Panhandle is
yielding temperatures climbing through the 60s...with upper 50s to
low 60s observed elsewhere. Midlevel airmass is fairly dry across
the region...except for the Coast Mountains...which is where
isolated shower development will be most likely to occur through
Wednesday afternoon.

Moderately tight surface pressure gradient will exist in vicinity of northern
Lynn Canal late this afternoon and evening...which could
potentially produce a period of 20 knots slys. However...limited
heating inland of the canal has thus far precluded significant
strengthening of winds. Otherwise...20 knots northwesterly sea breeze will be
possible over Cross Sound Wednesday afternoon. Surface pressure gradient
is forecast to weaken significantly tonight resulting in light
wind speeds for most of Southeast Alaska. Resultant decrease in boundary
layer mixing combined with breaks in the midlevel cloud cover will
favor areas of fog late tonight into Wednesday morning. Visibility
restriction will likely be greatest for locations around
Petersburg.

Refreshed the pressure grids with a NAM/European model (ecmwf) blend. Remainder of
the short term is basically a continuation of the forecast
inherited from last night...with only minor modifications
necessary. Forecast confidence is generally above average.

Long term...while the relatively strong ridge popping up over the
Gulf and the Panhandle Thursday and Thursday night look good for
the forecast...changes for the weekend will be highlighted below.
Warm August sunshine along with the warm air pushing up with the
ridge will help temperatures largely rise to the lower 70s over
the southern and interior Panhandle with middle to upper 60s for
the northeast Gulf Coast from Baranof Island northward. These
temperatures and sunny conditions will follow some areas of
morning fog of course. Cannot ignore longer nights sneaking in the
calendar as we March towards September.

Let US talk change for the weekend. While we have monitored the
developing ridge for quite sometime, it has been unclear how
quickly the ridge will pass over US. We are entering a time of
year where systems tend to tilt more progressively to the positive
with strengthening westerlies. With the European model (ecmwf) as harbinger of
these changes from yesterday's runs...the GFS is now timidly
suggesting it was right with its 06z vote on this election day as
the most positive towards the ecmwf's position. Thus the 12z
European model (ecmwf) and the 06z GFS...in close coordination with our friends at
the Anchorage weather forecast office...were used to represent a developing short-
wave rounding a large cut-off low over the western Aleutians. The
conundrum is the 12z European model (ecmwf) pushing forward a more outward track of
the surface low around the upper low that would place it squarely
in the central Gulf by Friday afternoon. This was an advance over
the 00z European model (ecmwf) to the east. Whereas the 12z GFS went further
northwest and inland with the low carrying it up into the south
central part of the state. The 06z GFS offered a more gentle
compromise that agreed more with the 12z European model (ecmwf) and offered a nicer
transition from this volatile low-confidence forecast period
leading into the weekend to the higher confidence ensemble
approach of wpc used to keep the next week trending wetter with
southwest onshore flow aloft much more of a given.

What are the results of these changes? We have reintroduced the
chance of rain for Saturday for the Panhandle...even to the Point
of adding it in late Friday night for the northern areas. Still
think Sunday may be a drier day than Saturday, but could not
entirely dispense of the rain chances here. While the text public
forecasts may not indicate it...many areas without mention of rain
will actually see a slight chance of rain...less than 25%
chance...but so low it does not warrant mentioning at this point.
It should be understood that this is a low-confidence forecast,
but these changes needed to be made in order to allow a better
base to trend rain certainty upward or downward. Mariners should
take note that a wind direction change to the south is a better
bet with this change and that given any frontal passage, they may
be higher than indicated at any particular time.

Even while Sunday rain may once again arrive on southeast
shores...Friday still looks dry and sunny for earlier in the day
over the north and most of the day over the south. However...it
looks like some higher clouds may start to ever so slightly Grey
the sunshine a little in the afternoon. In addition...we shall
monitor the marine layer that is likely to show up somewhere over
the Gulf underneath the surface ridge middle to late week. Right
now...best guess is reaching the shore-line between Yakutat and
Sitka area late Thursday night/Friday morning...limiting the
warming in these zones.

No changes for Wednesday night and Thursday. Used 06z GFS and 12z European model (ecmwf) for
updates Thursday night and especially Friday. Friday night transitioned with
12z GFS to 12z wpc into Saturday. Forecast confidence good through
Thursday night...poor for weekend and increasing once again for
the rains looking likelier for next week.

&&

Ajk watches/warnings/advisories...
public...none.
Marine...none.

&&

$$

Garner/jwa

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