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Southeast Alaska forecast discussion 
National Weather Service Juneau Alaska
539 am akdt sun Aug 2 2015

Short term...an upper-level low is in the process of developing
over the far eastern Gulf early Sun morning. Meanwhile...a
smaller/compact vorticity lobe is moving S along the western periphery of
the developing low. This vorticity lobe will be entrained within a
stronger zone of westerlies present to the S of the primary low. As
this takes place...the vorticity lobe will eject eastward toward a position
S of haida gwaii by 12z Monday...which will subsequently cause the
upper low to gradually drift eastward over the southern Panhandle.
Farther north...a midlevel vorticity streamer will be shunted southward
across the Yukon territory today into tonight. As this
occurs...high pressure will surge southward toward the northern
Panhandle...with a weak trough of surface low pressure forced by
a modest increase in downslope nlys becoming positioned from the
central Panhandle northwest to the northestern Gulf late Sun night.

Most significant short term hazard /relatively speaking/ is an
area of rapidly cooling cloud tops beneath the developing upper
low over outside marine zone 42. Model guidance indicates midlevel
temperatures around -23c will yield steep lapse rates and ample
cape for deep moist convection...thus have included slight chance
thunderstorm potential over marine zone 42 that shifts southeastward to
marine zone 41 by afternoon. Destabilization is also forecast to
occur over the southern half of the Panhandle this afternoon as the
cold upper low moves east-southeast...which will aid in additional slight
chance thunderstorm potential focused mainly over the elevated
terrain. Finally...band of showers will shift southward into akz018-019
this morning through evening. This activity will also have access
to sufficient cape for isolated thunderstorms...mainly north and west of
Haines/Skagway.

Otherwise...ridge of surface high pressure extending from the northestern
Pacific to the southwestern Gulf...combined with low pressure over western
Canada...will continue to favor predominantly northwesterly winds across
much of the region. Similar to Sat...the one exception will be
Lynn Canal...where southerly winds up to 25 knots are expected over the northern
portion of The Channel this afternoon due to a locally tight
pressure gradient. However...winds over Lynn Canal will also flip
to northerly late tonight as previously mentioned area of high pressure
over the Yukon surges southward. Elsewhere...relatively dry north-northwesterly
boundary layer flow combined with a convective environment will
generally preclude fog formation over the Panhandle. Possible
exception will be Yakutat due to decreasing midlevel clouds and
resultant strong decoupling at the surface.

Pressure grids were nudged toward the 00z NAM. Pop grids were
generally left untouched...and quantitative precipitation forecast was updated with a GFS/NAM
blend.

Long term...the upper low first discussed above will stick around
the Hyder area through Tuesday night before finally getting pushed
into British Columbia by another quasi-stationary low in the western Gulf that,
in turn, will be kicked by a short-wave digging south from
northern Alaska on Wednesday. The second upper low will likely
begin affecting our weather late in the week, and models are
speeding up the process.

The first upper low will keep scattered showers in the forecast
for much of the Panhandle with the only clear exception...Yakutat,
where both decent offshore flow and the influence of a Mainland
Alaska Ridge will keep skies clear of the convective clouds
farther to the southeast. A ridge between the two lows over the
Gulf may allow marine stratus to develop, and thus, the northeast
Gulf sky forecast is somewhat tenuous. Currently though it looks
partly cloudy at worst early in the week.

The conundrum for the showers across much of the northern interior
Panhandle is how widespread and intense they may not be given
strengthening downsloping winds over much of the north. MOS
guidance is indicating low to middle 70s for the northern interior
on Monday and only slightly cooler temperatures for Tuesday...but
still relatively warm. While we nudged things higher during this
period...we did keep Juneau out of 70s for now, given cloud cover
and shower chances. Skagway and Haines though could easily see
middle 70s...even under mainly cloudy skies. Temperatures across
the south will be much cooler...near 60 underneath the cold upper
low. Some guidance indicating showers will be more widespread
across Prince of Wales than we have indicated. But this is a
tricky pattern with a lot of hedging.

The next upper low reaches the eastern Gulf late in the week, but
as stated above recent guidance suggests it may make an earlier
entrance, and thus Thursday and Friday look increasingly wet. Thus
at some point...there will be a transition from showers to just
rain. Presumably there will be a brief break between systems. But
we do not think the break will be very significant in time. For
those tired of rain...the best hope is for the upper low and its
associated upper spin in the atmosphere to weaken mid-week.

Used NAM/European model (ecmwf) for edits Monday into Tuesday. After that few
changes...although we did expand the chance of showers a bit
through mid-week. Average confidence for the period.

&&

Ajk watches/warnings/advisories...
public...none.
Marine...Small Craft Advisory for pkz012.

&&

$$

Garner/Adkins

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