Scientific Forecaster Discussion
fxak67 pajk 260732 cca
Southeast Alaska forecast discussion...corrected
National Weather Service Juneau Alaska
1132 PM akdt Sat Mar 25 2017
Short term...an upper trof will remain over the Gulf through sun.
A developing low SW of haida gwaii will move north to the far southeast Gulf
by late Sun afternoon. An occluded front will move north into the southeast
Gulf and southern Panhandle sun.
Main forecast concerns will be precip potential/ptype/amounts, and
winds. Scattered to locally numerous showers are over the eastern
Gulf and much of Southeast Alaska late this afternoon. The showers will
decrease from the S this evening, with the far north area getting out
of the showers by Sun morning as low level flow becomes more
offshore ahead of approaching low and front. However, steadier
precip with the front will move into the S late tonight then try
to move further north sun, but the offshore flow will tend to break
up the precip area as it reaches the north half. East mtn slopes will be
favored for precip over the S and central areas especially closer
to the outer coast. The north should dry out Sun afternoon although
there could be a few sprinkles around the Icy Strait area.
Ptype will be mainly diurnally driven. Warm boundary layer temps
in the afternoon will generally mean rain or a mix, except in the
heaviest showers where more snow may occur. As temps cool
tonight, precip will become more snow especially over the north half
of the area. Then on sun, any snow should be mostly during the
early morning before temps warm well above freezing. Snowfall will
be highest around Yakutat where more frequent snow showers could
lead to an inch or 2 of snow by late tonight. Elsewhere over the
N, snowfall should be an inch or less, with most places getting
Winds will be increasing over the southeast Gulf later tonight and sun
as occluded front moves in. Expect strongest winds over the far southeast
Gulf Sun afternoon where gales can be expected. Much of the
remaining East Gulf from Cross Sound southward will see winds pick
up to 25-30 knots by Sun morning. Over the Inner Channels, winds will
become more offshore and increase later tonight, with Small Craft Advisory level
winds likely out of Cross Sound and parts of Clarence Strait. Rest
of the area should see winds pick up to the 15-20 knots range by
late tonight and continue into sun.
Long term...at upper levels southwesterly flow will last through
mid week with a low extending down from the Alaska interior and
ridging off the western Continental U.S.. surface low mentioned in the short
term will continue its northwesterly track across the Alaska Gulf with
associated occluded front moving over the Panhandle. The low will
weaken by the time it reaches Prince William Sound Monday
evening. Onshore flow will keep chance of precip over the
Panhandle, mainly for the northern half as ridging moves over the
southern Panhandle resulting in drier more Spring like conditions.
Next waves begin to move into the Gulf mid week with first low
tracking northeastward toward Prince William Sound followed by a
more significant low and front moving over the Gulf Thursday
evening into Friday. Better chance of more significant rainfall
with this later front. Began a trend to raise low temps due to
more southerly flow later in the week which will likely begin a
transition to more rain than snow.
Still fair amount of model spread for the Sunday night into
Monday low. Used a blend of GFS/NAM which were in better
alignment but did shift the low center further to the west and
north. For later time periods ensembles were still in line with
previous forecast so little change was made. Positive precipitable
water anomaly and 850 mb temps with the late week system add to
likelihood of a more significant rain event. Forecast confidence
Marine...Gale Warning for pkz041-042.
Small Craft Advisory for pkz021-022-036-043-051.