Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 230022
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
422 PM AKDT MON AUG 22 2016
.ANALYSIS AND UPPER LEVELS...
There is an upper level trough over western Alaska that extends
out through the central Bering Sea. There is an upper level ridge
over western Canada. Southerly flow exists between these features
over much of southern Alaska. Northerly flow is over the western
Bering and western Aleutians on the back side of the trough. A
fairly strong and moist front associated with the trough is slowly
pushing through southern Alaska, bringing plenty of rain to the
area. The moisture plume associated with this front stretches from
about 40 degrees north latitude northward to the Gulf coast.
The models are in good agreement through 30 hours (late Tuesday
morning) then begin to diverge. The Canadian GEM and the ECMWF
then take a surface low northward across the Alaska Peninsula
Wednesday morning, while the GFS and NAM are slower in taking it
across the peninsula in the afternoon. The ECMWF and GEM also
bring considerably more precipitation further westward into
southwest Alaska on Wednesday than the American models, which only
bring significant rain into the south central portion of the
state. The ECMWF and GEM solution was preferred. Forecast
confidence is a little below normal today due to these
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Rain, moderate at times, will continue to impact the region
through Wednesday, with a series of upper level jet streaks and
deep moisture flow into Southcentral. Heaviest rainfall amounts
will continue to fall along south- and east-facing slopes of the
Alaska and Chugach Ranges. The rainfall will continue to result in
rises on area streams, with the potential for minor flooding
especially along the North Gulf Coast/Prince William Sound and the
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
With the surface low and associated upper level trough currently
centered over Southwest Alaska, rain will continue overnight across
the greater Bristol Bay area and Kuskokwim River Valley with
scattered showers west of the trough axis over the Kuskokwim Delta
and Alaska Peninsula. The upper level trough will elongate and
shear somewhat on Tuesday as an upper level low digs south of the
Eastern Aleutians and Alaska Peninsula allowing precipitation to
take on a more showery characteristic across southwest Alaska.
With an unstable air mass in place and some sun breaks possible
on Tuesday, isolated thunderstorms are possible in inland areas in
the afternoon and early evening. A developing frontal will lift
north across the Alaska Peninsula Tuesday night stabilizing
conditions and pushing additional widespread rain across Southwest
Alaska Wednesday morning.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Upper level ridging will continue to sharpen over the western
Bering and spread the northerly flow aloft further east tonight
and Tuesday. A frontal system coming in from the west will push
into the western Aleutians Tuesday night and Wednesday pushing
the ridge east and flattening it somewhat.
.LONG TERM FORECAST (Days 3 through 7)...
A pattern shift will begin to unfold Thursday evening as the
southwesterly flow aloft that has continued to usher in a very
moist air mass begins to shut off. A more zonal upper level flow
pattern will develop Friday into Saturday which should cut off
much of the rainfall except for the immediate northern gulf coast
and parts of Southwest Alaska. By Saturday, surface high pressure
will begin to build across the Alaska mainland which should be
more than enough to inhibit widespread rainfall. If ensemble
guidances are right (and they have all been trending in this
direction the past 24 hours), then this upcoming Friday, Saturday
and Sunday time frame should be quite dry with breaks in the
clouds observed and temperatures in the mid to upper 60s across
much of the state. Model guidance begins to struggle quite
significantly on Monday and Tuesday as models diverge on whether
the high pressure gets cut off across the state or whether a
series of shortwave ridges and troughs are observed as wave
propagate eastward along the zonal upper level flow. One thing
that looks likely for Monday and Tuesday is that rainfall chances
look to increase but to what extent is unknown at this time.
SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR