Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
532 am akdt Tuesday Jul 22 2014
Analysis and upper levels...
a progressive split-stream pattern remains in place across the
alaskan domain this morning. This pattern has been generally in
place for almost 2 weeks...and features a progression of upper
lows/troughs moving through interior Alaska and another separate
progression of disturbances across the Aleutians and into the
southern Gulf. Currently...a disturbance is moving onshore over
northwest Alaska...with a southern stream upper low near Dutch
Harbor. The axis of the ridge that made for such a nice day
across southcentral yesterday has slid eastward...with deep-layer
(but weak) southwest flow spreading into all of southern Alaska.
A much larger ridge is beginning to take shape over the western
Bering...where marine fog/stratus remain widespread. A few showers
have moved onshore over southwest Alaska as both the northern and
southern stream disturbances slowly approach the coast...though
none are looking particularly impressive at this time.
the models are in excellent overall agreement in the movement of
the main weather features over the next several days. The ridge in
the Bering Sea will continue to strengthen and will become the
dominant weather-maker across the area. In the last 24 hours the
models have struggled a bit with weather the ridge axis will be
over the northwest Bering or the northeast Bering. This will have
most significant impacts downstream across Mainland Alaska
concerning amplification of shortwaves dropping down the front of
this ridge. With the models trending a bit further westward with
the ridge axis in the last 24 hours...not surprisingly a northern
stream trough coming over the ridge now looks as if it will
amplify and bring a round of steady rain to much of south central
Alaska sometime in the Thursday night or Friday timeframe. Thus
will trend the forecast this direction. Otherwise...with good
overall agreement...the high resolution models were leaned on for
Short term forecast...
midlevel clouds will be on the increase today...but will not
be a major detriment to another nice day overall. Increasing
southerly flow behind the departing ridge will provide an
additional kick to typical diurnal sea breezes around the region
today...though not extreme by any means. Another impact of this
broad southerly flow will be the possible movement of some low
cloudiness from near Kodiak Island toward the north Gulf Coast
later today. Meanwhile...with this high pressure/ridging nosing
into the northern Gulf there is a chance southerly winds along
the Copper River may join with a surging thermal trough and low relative humidity
values...but with generally weak and slow-moving surface features
it is most likely that both fire weather criteria (wind and rh)
will not quite reach significant levels. By this evening cloud-
cover will increase for most areas...with the presence of the
upper low near Kodiak Island likely keeping some scattered shower
activity along the coastlines and mountain ranges through the
overnight hours and continuing into Wednesday.
moist onshore flow will intensify today as showers/rain become
increasingly widespread. Marine fog/stratus is also likely to
surge onto the coastlines with limited spread inland as well. As
the upper low currently near Dutch Harbor moves toward Kodiak this
afternoon there will likely be an increasingly convective look and
feel to some of the showers along and west of the Aleutian
Range...leading to locally heavier downpours. The main trough
clears to the east by midday Wednesday...but models now indicate
an additional shortwave dropping down the front of the ridge will
bring another quick shot of rain through the area Wednesday night
before the Bering ridge dries out the atmosphere more
substantially for the end of the week.
high pressure in the region will only get stronger through the end
of the week. This means less and less shower activity and more
and more fog/stratus problems as we head into the weekend.
Winds across much of the Bering will keep a northerly component
over the coming days...which will allow the Pacific side of the
Aleutian chain to see better conditions overall than the Bering
side. This pattern looks to be locked in for the better part of
the next week.
Long term forecast from previous discussion...
the long range pattern (thursday night through monday) will
generally consist of a ridge over the central Bering Sea/Aleutians
and a trough over the Gulf of Alaska and over northern Alaska.
This will result in northwesterly flow aloft for the Mainland. A
few systems will push through southern Alaska...bringing
considerable clouds and some precipitation. Due to the direction
of the upper flow and the fact that the main lows will be to our
west and south...precipitation will generally be showery and
accumulations are not expected to be very high.
The wpc (weather prediction center) forecasts are generally a
blend of the 00z GFS ensemble mean...the 12z GFS deterministic
model (less influence as time increases until by day 8 it is not
used at all)...and the 00z European model (ecmwf) ensemble mean. The GFS is
weighted slightly higher than the European model (ecmwf). The afternoon forecast
strongly leans toward the wpc guidance.
Amend Jul 14