Scientific Forecaster Discussion
fxak67 pajk 192300
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
300 PM akdt Sat Aug 19 2017
Short term.../tonight through Monday/as of 1430 this afternoon, a
surface ridge has built in out of the southeastern Gulf. From what
we can see from radar (supplemented with infrared satellite and surface
obs), some locations throughout the Panhandle are experiencing
some scattered showers, with generally mostly cloudy conditions.
However, some areas in and around Cape Yakataga are experiencing
some slight clearing. In addition, winds are moderate throughout
many areas across the Panhandle, save for areas in northern Lynn
Canal and some zones along the eastern Gulf waters, where we
currently have small craft advisories issued.
For the rest of the weekend, the main story is the wet weather
expected beginning Sunday and going into the early part of next
week. A shortwave containing the remains of Typhoon Banyan is
currently located over the southwestern Gulf of Alaska, according
to WV satellite. This system will continue to dig into the Gulf,
and multiple jet features will enhance the circulation, assisting
with additional moisture advection over the area and increasing
lift needed for heavier amounts of precipitation. Model runs
lately have indicated most of the moisture impacts may be focused
over the southern Panhandle, but many areas can expect fairly wet
weather beginning Sunday night into Monday.
For the forecast, the NAM was used for changes to the pop, and a
combination of models (including the NAM dng, rfc, and ecmwf) were
utilized for changes to the quantitative precipitation forecast. Temps were also adjusted upwards
toward the early part of next week to account for warm air moving
Long term.../Monday through Saturday as of 10 PM Friday/ the
extended forecast begins wet, with a weather front moving across
the Panhandle Monday. The associated low over the central Gulf
will slowly track southeast toward British Columbia through Wednesday. Rainfall
will be heavy at times especially over the southern panhandle;
this is due to large quantities of moisture being advected toward
the area from the remnants of Tropical Storm Banyan. Conditions
will gradually improve Tuesday and Wednesday with a break in
precipitation possible between Thursday and Friday. Then another
front is forecasted to move across the Panhandle next weekend.
Temperatures throught the extended forecast have little variation
between daytime and overnight hours, with a slight cooling trend
in the forecast through the week.
Monday's quantitative precipitation forecast values were adjusted using the 3 hourly run of the
00z GFS as a base with a 0.75 ratio applied and then blended at a
50/50 ratio with the existing forecast. This lead to an over all
increase in precipitation over the southern Panhandle consistent
with the enhanced moisture arriving from Banyan. Models have
shifted this band of moisture slightly further north from
yesterday. Tuesday and beyond a blend of the existing forecast and
rfcs values were used. This puts total rainfall between Monday
and Wednesday between 1 and 4 inches over the Panhandle with the
highest values over the southern Panhandle especially near
Ketchikan and Annette Island.
Most of the changes to the extended forecast were made between
Monday and Wednesday, due to above average model agreement. Since
the low was strengthened, winds were increased Monday to small
craft levels within the eastern Gulf. A blend of the NAM and GFS
was used Monday, with a European model (ecmwf) and GFS blend used through
Wednesday. Wpc guidance was used beyond that timeframe. Overall
forecast confidence is above average in the long term although
precipitation totals could vary.
Aviation...a trough aloft will be moving east across seak for the
remainder of Saturday evening and early Sunday morning. The back
edge of the rain in nearing the Yakutat area late this afternoon.
Clearing conditions in the northern part of the Gulf evident on
satellite and the Cape Yakataga webcam imagery. MVFR to local IFR
flight conditions ongoing through the evening though should see
some night time deterioration. Potential for clearing over the
Yakutat region could see some patchy fog developing there
overnight, which could affect nighttime visibilities.
For Sunday afternoon will begin to see the impacts of former
Typhoon Banyan advance towards the Panhandle. Increasing winds may
increase the lower level turbulence near the terrain.
Hydrology...former Typhoon Banyan and developing Arkansas out of the
north central Pacific will be dropping 1-2 inches into the
Panhandle through Sunday afternoon... best focus for rains will
be to the southern Panhandle. The Arkansas will favoring the southern
Panhandle from Sunday night into Tuesday..so 4-7 inches expected.
With saturated soils, this rain should be directed into run off.
Increased water levels may threaten bankful, however presently we
are not expecting any flooding.
Marine...Small Craft Advisory for pkz012-043-051-052.
Small Craft Advisory due to seas for pkz041.