Scientific Forecaster Discussion
fxak68 pafc 261257
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
457 am akdt Tue Sep 26 2017
Analysis and upper levels...
a vertically stacked upper level low is currently located over
Cold Bay and moving southwest while slowly dissipating. A
northward moving portion of the sub-tropical jet stream is
advecting a large plume of moisture and warm air north into the
Gulf of Alaska. A surface front associated with the leading edge
of this warm moist air is spreading into southcentral, with rain
and gusty winds beginning to develop in Kodiak and the northern
Gulf Coast. Out in the Bering, an area of high pressure is nudging
east over the western Aleutians leading to large scale northerly
flow across most of the Bering Sea and Aleutian Islands.
models have come into better agreement regarding the overall
synoptic pattern over the state including the surface low and
warm front moving north into the Gulf of Alaska today. Models
begin to diverge on Wednesday as some solutions form an embedded
shortwave trough within the main low and swing it up into the
northern Gulf Coast. The NAM is more aggressive and forms a
separate low center along the coast leading to a storm force
barrier jet. The GFS and ec are less bullish with this feature and
have shown more consistency over the past couple of days, so the
forecast package leaned more towards those solutions.
panc...VFR conditions and light winds will persist through the day
Short term forecast southcentral Alaska (days 1 and 2)...
the weakening gale force low moving into the southern Gulf will be
the focus for most of the weather in the area over the next few
days. The models have trended further east with the position of
this low, and as a result winds/seas in the Shelikof Strait- Cook
Inlet areas were dropped with the morning forecast package as a
much weaker pressure gradient will develop than initially
expected. Another impact of this shift in the low track was to
delay the heaviest of the rainfall along the Gulf Coast until late
this evening. There is some uncertainty in the exact timing in
how quickly rainfall will spread westward back into the region,
but model consensus would indicate that rain should spread
throughout the Gulf Coast by late tonight, and then into
southcentral for Wednesday. This could occur slightly
faster/slower depending on which model solution you look at, but
there is good agreement that rain will spread into most areas at
some point by Wednesday. Afterwards, conditions should begin to
improve over southcentral from east-west late Wednesday. However,
with a slowly approaching Gulf low showers will likely continue
with increasing winds into Thursday.
Short term forecast southwest Alaska (days 1 and 2)...
Northerly offshore flow is increasing this morning across the
coastal regions. However, winds have been weak enough for some fog
development across parts of the Kuskokwim Delta this morning.
This should burn off quickly by late morning with dry conditions
then persisting until a front moving toward Kodiak Island spreads
light rain over the Alaska Range Wednesday into Thursday. Dry
conditions will remain over the Delta as cooler northerly winds
continue through Thursday.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
A period of unusual quiescent weather looks to continue through
Thursday as broad northerly flow encompasses the entire Bering
Sea. Besides sporadic showers, winds will remain below 30 knots.
Long term forecast (days 3 through 7)...
Day 3 (thursday) will start off with a surface ridge centered
over Shemya that extends across the vast majority of the Bering
Sea and an area of low pressure over Kodiak Island. This low
pressure system will bring decent rainfall to the Gulf Coast and
Kodiak Island before it weakens on Friday and begins to drift
eastward. The sfc ridge over the Bering will quickly retreat into
the North Pacific over the weekend as a pattern shift begins to
take hold across Alaska and alaskan waters. The pattern change
comes in the form of the upper level pattern shifting from a more
subtropical (meridional) influence to a more Arctic (zonal)
influence. This will keep the subtropical jet well south of Alaska
while the Arctic jet begins to strengthen as cold air begins to
build up over Russia. It's that time of the year where the cold
air over Russia helps low pressure systems develop/strengthen over
Kamchatka and Siberia and the models are hinting at the potential
for several robust system to push off the russian coast and into
the Bering Sea next weekend and through the first half of next
week. It's way too soon to give any details on any potential
system next week but the pattern change is on the way.
public...Flood Warning 121 125.
Marine...Gale Warning 132.