Area Forecast Discussion
Issued by NWS
FXAK67 PAJK 262338
Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
338 PM AKDT Sun Mar 26 2017
.SHORT TERM...A low W of Haida Gwaii will move NNW and reach the
far NE gulf by Mon afternoon. There were some model differences on
track and strength of this low, but based on satellite and obs at
18z, the 12z ECMWF and GEM were handling this low the best, so
used them through 18z Mon, then transitioned to previous forecast
by 00z Tue (for pressures/winds).
Main forecast concerns will be precip potential/ptype/amounts and
winds. As of 23z, rain was still just S of the far southern area.
Am expecting this precip to move N into the southern area this
evening. Looking like precip will reach the central area overnight
tonight, then the N Mon morning. The precip may be spotty
initially over the N half due to offshore flow, but later Mon this
offshore flow will weaken and allow more widespread precip to
occur. The precip will reach the NE gulf coast the latest,
probably late Mon morning. As far as ptype goes, much of the N and
central area will see at least some snow due to evaporative
cooling and some diurnal cooling of lower levels late tonight and
early Mon morning. Do not think this snow will last much past
mid-morning Mon through as boundary layer warms. Exception to this
would be over the far N where snow could mix in most of the
morning into the afternoon before the boundary layer warms.
Snowfall over the N third could reach an inch or 2 by mid-morning
Mon with highest amounts over the far N. Over the remainder of the
area, little or no snow accumulation is expected.
Strongest winds will be on N and E side of low moving through the
E gulf tonight and Mon. Should be some gales early tonight over
the far SE gulf, with SCA level winds expected over the remaining
gulf area. Winds on inner channels will pick up some tonight with
many areas reaching SCA levels especially over the more E-W
channels. N Lynn Canal will also pick up to SCA levels this
evening as northerly pressure gradient increases ahead of the low.
Looking like peak winds on most inner channels will occur Mon
morning, with a diminishing trend in the afternoon as low reaches
the far NE gulf and weakens.
.LONG TERM...Difficult forecast for Monday night with another
potential low moving up from the N Pacific into the SE gulf. The
mid level low centered over the AK peninsula continues to rotate
waves over the AK gulf. There is a wave crossing up on Monday
evening which would help to push up a surface low to the north.
At the surface operational models have a large spread with this
feature. GFS/Canadian show a deeper gale force low moving further
north compared to the NAM and especially the ECMWF. However no
model shows agreement on the exact position and track of the low.
Decided to use a blend of GFS/NAM as a compromise with a broad
brush approach to wind coverage, not going as high as the GFS
winds but stronger than ECMWF/NAM.
Once this system passes through models fall more in line depicting
the next series of waves and lows tracking into the region. There
will be a brief drier period Tuesday into Wednesday, especially
for the southern regions as the next wave moves into the central
gulf with ridging over the southern panhandle. Southwesterly
onshore flow keeping chance pops over most of the area with a weak
front crossing over the northern regions. Late Thursday into
Friday will be the next significant system. This low originates
off the coast of Japan, tracking across the Pacific and into the
AK gulf by the end of the week. Expecting at minimum gale force
winds from this low over the gulf and wide spread heavy precip.
Warm air advection will likely produce mostly rain from this
system. Both 850 temp and precipitable water anomalies show this
system to be rather warm with deep layer moisture.
Besides to early GFS/NAM blend not much change to extended
forecast as models were still mostly in line. A few nudges for
winds to a ECMWF/NAM blend then update with new WPC. Forecast
confidence is very low initially but rises after the Monday night
MARINE...Gale Warning for PKZ041-042.
Small Craft Advisory for PKZ012-013-021-022-031-033>036-043-051-
Small Craft Advisory due to Seas for PKZ053.
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