Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
425 PM akdt Friday Jul 29 2016
Analysis and upper levels...
a large upper level low over the Arctic is extending over most of
northern Alaska today. This is the source of the trough that will
dig southward and bring significant rainfall to southwest Alaska
There is a weaker upper level low south of the Alaska Peninsula
with a ridge extending over the Bering Sea into southcentral
Alaska which is situated in between these upper level lows. The
surface map shows higher pressure south of the region with a low
centered near St Matthew island and a stationary front extending
from this low through the interior of the state. This frontal
boundary is producing a lot of shower activity. The high pressure
to the south and pressure gradient over southern Alaska is
bringing persistent gusty south winds over channeled terrain and
that should remain the case over the next day or so.
the models continue to be in good agreement throughout the short
term and seem to also agree on the slower solution of the upper
level low passing through southwest Alaska and then into the Gulf.
Models remain in good agreement through the middle of next week
with the upper level pattern being almost identical on the GFS and
European model (ecmwf).
Short term forecast southcentral Alaska (days 1 and 2)...
Flat ridging has built into southcentral this evening as an Arctic
trough encroaches on the North Slope. An associated frontal zone
encompasses most of interior Alaska with some of that frontal
energy impacting the Susitna Valley north to the Alaska Range.
This front is also enhancing a rather tight pressure gradient from
the coast to the interior low, with rather strong southerly winds
blasting up Cook Inlet to near gale force and gusty southeast
Turnagain Arm winds. This general pattern will persist through
Saturday before the trough over the North Slope sags southward and
a upper pv anomaly at the base of the trough moves into southwest
Alaska. Rain and clouds will begin moving east and south of the
Alaska Range and eventually through Kodiak and to the Kenai
Peninsula through Sunday. The Copper Basin will see rather strong
downslope shadowing, however, with inland locations north of the
Kenai seeing widespread lighter rains but nothing significant as a
Short term forecast southwest Alaska (days 1 and 2)...
moist westerly flow will keep extensive cloud cover over the
region tonight, with a few showers possible across portions of the
Kuskokwim valley as a weak disturbance moves overhead. More
widespread rain will then move over the Kuskokwim Delta late
tonight as a deep upper trough approaches from the northwest, with
rain spreading across the rest of the southwest Mainland on
Saturday and persisting through the weekend as the trough
continues to dig along the coast. Precipitation will transition
to a more showery regime on Sunday as the trough cuts off and
evolves into a closed upper low over Bristol Bay. Colder air
aloft associated with the low will serve to steepen mid level
lapse rates and bring an increased chance of thunderstorms to much
of the area on Sunday afternoon.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
the advancing upper level trough will bring rain to much of the
eastern Bering through the weekend, with rain mainly confined to
the Alaska Peninsula by Sunday as the trough/upper low move into
Bristol Bay. High pressure will build in quickly behind the
departing trough, yielding a typical low stratus and fog regime
over much of the Bering. Elsewhere, a weak North Pacific front
will linger near the western Aleutians through the weekend,
bringing a chance of rain to those areas through Sunday.
Long term forecast (days 3 through 7)...
Monday through Wednesday a closed low is progged to track from
the southwest Mainland to the eastern Gulf with a ridge building
across the Bering. Models are in good agreement with the synoptic
change which stays progressive through the end of next week. The
next upper low develops east of the Kamchatka peninsula mid week
and picks up an easterly pace on Friday. There are some differences
between model solutions on this system...the less aggressive European model (ecmwf)
and it's mean was used to update the extended range.
Synopsis and model discussion...ez
southwest Alaska/Bering Sea/Aleutians...cumulonimbus