Area Forecast Discussion
Issued by NWS
FXAK67 PAJK 281358
Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
558 AM AKDT SUN AUG 28 2016
.SHORT TERM...Fairly broad upper trof will remain over the area
through tonight. Several shortwaves within the trof will affect
the area through tonight. A strong shortwave will move north to
the northern BC coast this afternoon...then weaken as it moves
inland over the far southern panhandle this evening. Another
shortwave will lift north into the southern area late tonight. A
couple of shortwaves will move south into the northern gulf
through tonight...with one dropping into the north-central and
northeast gulf this morning...and another moving in a little
further to the east late tonight. At the surface...low pressure
will slowly deepen to the west of the Haida Gwaii area and wobble
around there through tonight. High pressure will generally remain
over northwest Canada through tonight. Used mainly a blend of the
06z NAM and 00z GFS for the forecast through tonight.
Given how complicated the flow will be across the area...most
forecast parameters will be tricky. Cloud cover will be highly
variable with clouds moving around the area from several
directions. Think the thinnest/highest clouds will be over the
north with most shortwave energy staying to their west...so did
keep less cloud cover there as sun should be able to get through
it fairly easily. The northeast gulf coast will also see some
improvements once shortwave moves by them to their south...and
will have clouds decreasing there late in the day.
Elsewhere...expect it to be mostly cloudy to cloudy for much of
the time but cannot rule out larger breaks occasionally.
Precip chances will be highest over the south...near the southern
shortwaves and surface low. One area of precip is lifting north
across the central area and this will weaken and drift offshore
this afternoon. More precip will move into the far south toward
late afternoon or early evening as first shortwave moves in. The
precip will increase during the night as second shortwave
approaches the southern area later in the night and surface low
moves a bit closer to them. There will be precip associated with
the shortwaves moving in from the north but expect it to remain on
the Canadian side of the border due to downslope on Alaska side.
Temps will ultimately depend on how much sunshine can get through
the higher level clouds and winds. Some areas are still fairly
warm early this morning due to persistent downslope winds...with
temps in the 65-70 range in downtown Juneau. Given that thinnest
and lowest amount of cloud cover over the north...temps will be
able to get into the 70s over much of the northern third today.
Central and southern area could get near 70 as well especially if
they see longer breaks in cloud cover this afternoon. For now
keeping much of the south half of the area in the mid to upper
60s. Will probably have some big localized differences in temps
tonight. Where offshore and downslope wind remains all
night...temps could remain in the 60s. In areas where winds die
off and larger breaks in clouds remain...temps could fall well
into the 40s.
Winds will be strongest near the low west of Haida Gwaii. Expect
25-30 KT winds to reach the far southeast gulf by this afternoon
and continue tonight. Some of these stronger winds will affect the
far southern panhandle as well. Over the north...winds will have a
decent diurnal component...with strongest winds during the night
when drainage flows will align with pressure gradient flow...while
daytime sees sea breeze forcing opposing the pressure gradient
somewhat so winds will be lightest during the late afternoon. Will
likely see winds increase to SCA levels over northern Lynn Canal
late tonight due to effects of deepening low to the south. Other
areas will see outflow winds in a northerly/easterly favored
direction. These winds should remain 20 KT or less however.
.LONG TERM...Low pressure to the south off the coast of Haida
Gwaii looks to persist through Wednesday. Latest model runs have
also increased the high pressure to the north which resulted in
stronger northerlies through the inside waters. NELY outflow winds
will have some gusts up to 35kt, namely during the overnight
hours, but if this were the winter time those would be quite a bit
stronger with this type of pattern. Expect bands of showers to
wrap around the low each day. Placement of the low will cause the
bulk of showers to affect the southern panhandle and east facing
slopes while the northern areas will have fewer/weaker showers
due to downslope effects.
The next front looks to cross the gulf from the west next weekend.
There are timing/strength differences between models, as usual 7
days out, but the general consensus is rain increasing Saturday
night and becoming likely for next Sunday.
Used a blend of available models, including some of the previous
runs to limit pressure packing somewhat over the northern inner
channels. Forecast confidence is average.
.HYDROLOGY...Suicide Basin glacial dam release into Mendenhall
Lake peaked around 8.3 feet around 08z last night...and has
fallen to 7.5 feet as of 1215z. Expect it to continue falling
quickly this morning. Updated SPS earlier to mention the lake
peaking and now falling.
MARINE...Small Craft Advisory for PKZ012-036-041.
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