Scientific Forecaster Discussion
fxak68 pafc 222358
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
358 PM akdt Mon may 22 2017
Analysis and upper levels...
an upper wave and associated surface front and low are quickly
lifting north across southcentral Alaska. Satellite and radar data
shows cloud breaks developing across many areas over southcentral
with a few showers moving north across the mat-su valleys. Gusty
winds on the back side of system are producing gusty south winds
along the Copper River and gusty southeast winds across Turnagain
Arm and Knik River Valley. A weakening front extending across
southwest Alaska to Kodiak Island is producing areas of light
rain/showers, especially over Bristol Bay.
Further west, a 140kt+ upper level jet continues to drive a
trough and surface cold front eastward across the Bering Sea and
toward the southwest Alaska coast. Fairly cool air behind the cold
front is leading to instability off the surface. Thus, winds
across the region are quite gusty in nature. It's a fairly cold
rain along the front with temperatures in the 40s.
numerical forecast guidance is in very good agreement with the
significant features over the next few days and forecast
confidence is generally high. The primary forecast challenge
is capturing all of the local detail in a progressive pattern.
Specifically, there will be a lot of fairly strong and gusty
winds and areas of moderate rain potential over southcentral
Wednesday. Will generally make adjustments out west using the GFS
and the NAM/namnest across southcentral.
panc...gusty southeast winds are expected across the Airport
complex through late this evening. VFR conditions will persist
Short term forecast southcentral Alaska (days 1 and 2)...
The active weather pattern will continue over southcentral Alaska
through Wednesday. Some breaks in the clouds have allowed some
instability to build over the Cook Inlet region this
afternoon...and weak convergence along the Kenai Peninsula was
enough to tap into the instability and spark some convective
showers. Widespread gusty winds are being observed due to a well
mixed boundary layer and a strengthening pressure gradient as a
ridge of high pressure builds along the northern Gulf Coast. This
brief ridge of high pressure will continue to build into the Gulf
tonight through Tuesday, allowing for a slightly drier day
Attention then turns to what is setting up to be a wet
and windy Tuesday night through Wednesday. The ridge will quickly
exit to the east Tuesday night and be replaced by deep, moist
southwesterly flow moving up the Cook Inlet Tuesday night. A
potent upper-level southwesterly jet coupled with strong
southwesterly flow in the lower levels will setup a deep fetch of
subtropical moisture moving into our area. This will fuel periods
of moderate to heavy rainfall starting Tuesday night and lasting
through Wednesday evening. The heaviest rainfall will be in areas
where orographic ascent will enhance lift, including the northern
and eastern Susitna and Matanuska valleys and east Anchorage. In
addition to the rain, expect gusty winds Wednesday through
channeled terrain in the Kenai and Chugach Mountains and the
Alaska Range. These winds will be in response to a deepening
pressure gradient between low pressure over southwest Alaska and a
ridge of high pressure building along the northeastern Gulf
Short term forecast southwest Alaska (days 1 and 2)...
an unseasonably cool, cloudy, and wet next few days are in store
for much of southwest Alaska. Weak ridging (high pressure) moving
into the area today is keeping conditions fairly dry, except for
showers along the Alaska and Aleutian ranges this evening, but
this will give way to stronger onshore flow and more significant
rainfall by Tuesday morning around the Bristol Bay zone. This
widespread rain will continue in the Bristol Bay area and spread
across the rest of southwest Alaska Tuesday night into Wednesday
morning as a low pressure system tracks through the Bering Sea and
toward the Bering Strait by midweek. Onshore winds will get
gusty, especially along the coast, Tuesday night and Wednesday but
will not be strong enough or long-lived enough to cause any
coastal issues despite the unseasonably active weather.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
the 990 mb low in the northwest Bering Sea will slowly track
eastward through midweek. Conditions will remain gusty and showery
with west winds continuing through midweek, but the bigger deal
will be the continuation and even exaggeration of the relatively
cold air mass already present in the western Bering. By midweek,
this cold air will have spread eastward and temperatures will be
cold enough to see snow mixing in with the rain for much of the
Bering Sea and Aleutian island locations.
Long term forecast (days 3 through 7)...
the mid to long term forecast starting Wednesday evening seems to
be that of a tale of non Summer like weather. The current model
depiction is that Alaska will be stuck in troughing pattern where
the weather will be active through the weekend. Unfortunately,
this is a dramatic flip from guidance just a few days ago. The
first system will be exiting south central as we move into
Wednesday night. Weak low pressure will hang around the Gulf
through the end of the week, leaving coastal and upslope regions
moist. By Saturday afternoon the next front moves past Kodiak and
brings another round of unsettled weather through Monday.
Marine...Gale Warning 150 155 172 174.