Gale Warning 181, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
236 PM AKDT Mon Aug 6 2018

.SYNOPSIS...A fall like weather system will affect the Panhandle
over the next 72 hours. A stream of moisture tropical in origin
has set the stage for an atmospheric river and heavy rainfall to
impact Southeast Alaska beginning with Yakutat and slowly drifting
southeast through Thursday. Rainfall during this timeframe has
potential to be record breaking.

&&

.SHORT TERM...Expect heavy rainfall associated with an atmospheric
river to fall over the Panhandle over the next 72 hours. A shift
from the dry summer weather to a wet fall like pattern is
currently unfolding over southeast Alaska. Current water vapor
satellite imagery shows a strong moisture signature extending
from the far western Aleutians all the way to the northeast Gulf
Coast. This band will slowly track southeast across the Panhandle
producing long lasting periods of heavy rainfall.

Precipitation enhancement is expected as strong southwesterly
flow at mid levels (i.e. 700mb) becomes more prevalent late
Tuesday and persists through the remainder of the event. This
wind direction favors upslope enhancement of precipitation which
would allow larger storm totals.

As far as timing for the system, heavy rainfall has already begun
along the northeast Gulf Coast and will continue through Tuesday
night. Tuesday night will orient the primary stream of moisture
from Fairweather to Sitka spanning as far inland as Juneau. These
areas will see heavy rainfall through Wednesday before the system
finally shifts off the to the southeast producing further rainfall
over the southern Panhandle between late Wednesday night and
Thursday afternoon. Precipitation totals will range between 4 and
6 inches over the northern Panhandle with lesser amounts near
Haines and Skagway. 3 to 4 inches of rain are expected near
Sitka. 1 to 2 inches are expected further south.

Careful detail was added to the precipitation forecast as well as
the winds. Detail was added not only to the timing for the
heaviest rainfall rates, but also the amounts and progression of
the system. Additionally some of the far northern Panhandle was
shadowed out where the terrain acts to limit precipitation totals.
Forecast confidence is above average through Wednesday with
slightly less confidence on Thursday for the timing of the
system's exit.

.LONG TERM... /Thursday through Monday as of 10 PM Sunday/... A
parade of shortwaves this week will bring a period of wet and
windy conditions to the panhandle. A mid-week system with the
remnants of Typhoon Shanshan will kick off the start of the long
term. A small ridge will build up by Thursday, sticking its
vespine-like nose into the panhandle. By Friday, another diffluent
pattern aloft will impact the panhandle with another round of
high QPF amounts.

The pressure field didn't require many changes, with the
exception of a few time blocks that were nudged towards WPC. The
main concern being the amount of uncertainty later on in the week
with this nebulous web of systems, as several models (including
the ECMWF) are out to lunch. Most wind edits were local edits,
based mostly on wind direction. It could get pretty windy mid-
week with the tightening pressure gradient, then dropping off
towards the late week.

Both the 6-10 and 8-14 day CPC outlooks showing a split pattern
for temperature/precipitation- below normal temps, above normal
precipitation for the northern panhandle; above normal temps,
below normal precipitation for the southern panhandle. POP's were
updated using SREF/ GFS early on, showing nice form of the system
mid- week, followed by WPC to update the overall pattern. QPF used
a combination of GFS and RFC, not wanting to go full on QPF for
the mid-week system. It will remain wet of course, but just how
wet will be the question. But, there still may be some pretty
hefty rainfall totals along the Yakutat and northern coastal
areas, with generally 2-5" spread across Tuesday morning through
late Wednesday/early Thursday morning.

Temperature updates used the GFS, showing some cooler temperatures
in the interim and nudging towards NBM later in the week. Models
spectrums are showing greater spread, but 850 mb temperatures
remain largely unchanged.

Forecaster confidence is about average mid-week and below to
average for the remainder of the week. The finer details should
present themselves through the uncertainty later on in the week.

&&

.AVIATION...Expect low ceilings reduced visibilities associated
with heavy rainfall beginning along the northeast gulf coast and
progressing to the southeast over the next few days.

&&

.MARINE...While the primary impact over the next 72 hours will be
heavy rainfall, small craft level winds are expected from Cross
Sound through the Icy Strait Corridor and up Lynn Canal. Fredrick
Sound, Stephens Passage, and Chatham Strait will also increase
gradually Tuesday and reaching small craft levels by early
Wednesday morning.

&&

.HYDROLOGY...Heavy rainfall will occur over the northern Panhandle
through Wednesday night as an atmospheric river slowly drifts
southeast. Rainfall totals will range from 3 to 6 inches and
likely break single day records. Due to the prolonged dry spell
significant impacts are not anticipated; however, small streams
and rivers will see considerable rises.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind through late Tuesday night for AKZ018.
MARINE...Gale Warning for PKZ012.
     Small Craft Advisory for PKZ013-022-043-051>053.

&&

JB/SS

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