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Southcentral and southwest Alaska forecast discussion 
National Weather Service Anchorage Alaska
426 PM akdt Friday Oct 21 2016

Analysis and upper levels...
a weakening deformation zone continues from the Gulf of
Alaska, across the central Gulf, to over the Copper River basin.
Snow was continuing to fall over the Copper River basin, as well
as, the Valdez and Thompson Pass area. Warmer air kept
precipitationas rain along the Gulf Coast. Flurries continued
across the Anchorage bowl and matsu valleys this afternoon. Out
west, ridging over the eastern Bering Sea combined with lower
pressure over the Gulf produced brisk northerly winds across much
of southwest Alaska and Bristol Bay. Except for a few showers
associated with open cell cu over the eastern Bering Sea,
conditions were relatively dry.


Model discussion...

Models in general were in good agreement through the short term.
The high res WRF and NAM with used in the east with a combination
of the NAM and GFS out west.


Short term forecast southcentral Alaska (days 1 and 2)...

The Arctic trough is now digging into the northern Gulf of Alaska
this evening. The deformation zone has shifted east, and the
associated snow band with it. Accumulating snow threats will
continue in parts of the Copper Basin and Valdez areas into
tomorrow before the offshore pressure gradient strengthens and
dries southcentral out.

The main story then becomes strong offshore winds as the gradient
along the coast strengthens tonight. Peak winds will occur after
midnight tonight through Saturday morning before slowly tapering
off Saturday night. However, northerly flow aloft and another
shortwave diving down will keep enough of a gradient for outflow
winds to continue through Sunday. Due to the amount of
accumulating snow in Thompson Pass, a Blizzard Warning has been
issued from mile Post 23 to 33 where the coldest temperatures and
best blowing snow conditions are expected.


Short term forecast southwest Alaska...
ridging will continue to move in over southwest Alaska where it
will help to clear out cloud cover and allow for overnight
temperatures to drop into the teens for inland locations. The
ridging will hold strong into Monday and will only allow an
approaching front from the Bering to make it over coastal areas of
the Kuskokwim Delta Monday morning. The other concern from the
ridge for southwest Alaska is overnight fog formation around the
Kuskokwim Delta tonight and Saturday night. The uncertainty comes
with how much winds in the area will die down overnight.


Short term forecast Bering Sea/Aleutians...
the Bering and Aleutians will be characterized by several lows and
fronts streaming through the area from the North Pacific for the
next several days. The strong southerly push from each of these
systems will be enough to keep precipitation rain for the Bering
and Aleutians. The strongest system, which looks to bring storm
force winds to the central Aleutians Monday, will track into the
southern Bering late Monday.


Long term forecast (days 3 through 7)...
the southcentral region has an upper level ridge moving into the
region. This results in northerly offshore flow accompanied by
colder air filtering into the region through the upcoming midweek.
By Wednesday the ridge moves eastward allowing southwesterly flow
with warmer temperatures to advect into the southcentral region.
Meanwhile, the Bering Sea has several low pressure systems
tracking from the south to north direction before exiting into the
eastern siberian region. These systems progressively become more
stronger with each passing storm. By Monday afternoon, the GFS
has a 971 mb low tracking across the western Aleutians before
rapidly intensifying to a 959 mb storm force low located in the
central Bering by Monday evening. As this system moves farther
north look for hurricane force southwesterly winds to develop just
southwest of Saint Matthew Island by Tuesday morning. This system
brings widespread rain/stratus with warmer temperatures to the
Bering through the upcoming midweek time-frame. By Wednesday this
systems exits the region across the eastern siberian region into
the Chukchi Sea. Therefore, the Bering/Aleutians will be under a
showery weather regime through rest of next week.

The models have been struggling with this major Bering storm with
the European model (ecmwf) moving this system faster. While the GFS has been
stronger with this system, and slower. The intensity of these
systems between models has ranged from 20 to 30 millibars but has
become closer in there differences with each model run. The GFS
was the preferred model of choice in the long term time-period.
This system remains under close scrutiny for the possible weather
impacts as it tracks through the Bering.


Afc watches/warnings/advisories...
public...Blizzard Warning zone 131
marine...gales 125 126 127 130 131 132 138 139 140 173 175 177 178
185 411 412 413 414.



Synopsis and model discussion...en
southcentral Alaska...ja
southwest Alaska/Bering Sea/
long term...period

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