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Area forecast discussion 
southcentral and southwest Alaska 
145 PM akdt Tuesday Oct 7 2008 


Analysis and upper levels...showers continue to swirl around the 
upper low which is nearly stationary over the Kenai Peninsula. 
A weak surface low is centered just to the east over western Prince 
William Sound. The snow that fell overnight across much of 
southcentral has resulted in a very moist boundary layer. The 
combination of moisture and light winds has resulted in low ceilings 
and areas of fog this morning...especially over the western half of 
southcentral. Cold air advection on the back side of the low is 
producing gusty winds through the Alaska Peninsula and Kodiak Island 
region. 


A weak short-wave ridge is progressing eastward into the eastern 
Bering Sea helping to gradually shut off the showers that have 
dominated the region the past several days. A weak front behind 
the ridge is making slow progression eastward...bringing warm and 
stable air with it. 


Farther upstream...a trough is dropping down through the Kamchatka 
peninsula. A 160 knots subtropical jet south of this trough is aiding 
in development of a surface low over the northwest Pacific. The 
formation of this low has been anticipated with all of the 
ingredients coming together to produce a major storm. 


Model discussion...GFS/ECMWF/UKMET have finally all come into 
good agreement with handling of developing Pacific low and its 
movement into the Bering Sea. All three models also indicate 
development of triple point lows over the eastern Bering Thursday 
and Barren Islands region Thursday night. The GFS is a bit stronger 
with triple point low over Barren Islands...which greatly affects 
Turnagain Arm wind forecast. Based on strong dynamics associated 
with this system will go with the GFS solution for now. The NAM 
is an outlier with its low positions...but can still be used as 
guidance farther from the low centers where it solution matches 
up well with the other models. 


Short term forecast... 


Southcentral...the upper low will open up into a trough and slowly 
lift off to the northeast tonight through Wednesday. Upper levels 
will dry out...but low levels will remain moist so low clouds and 
fog are likely through Wednesday morning. There will be a brief dry 
spell (with clearing skies) Wednesday afternoon into Wednesday night 
as a short-wave ridge moves in. However...clouds winds and 
precipitation associated with the strong system out west will move 
to Kodiak Island Wednesday night and to the rest of southcentral 
Thursday. Precipitation may start as snow...particularly inland...but 
most places will quickly change to rain as very warm air is advected 
northward across the region. A strong front will approach the north 
Gulf Coast Thursday night and Friday with an orientation favorable 
for barrier jet formation. If a triple point low does develop in the 
Barren Island region this will help tighten the east-west gradient 
along the north Gulf Coast even further. Conditions look extremely 
favorable for High Wind Warning conditions through Portage Valley and 
Turnagain Arm. Of course a slight change in triple point development 
or track could change that. For now will issue a high wind watch for 
these areas for Thursday night. 


Southwest Alaska...southwest Alaska remains under influence of 
upper trough centered over southcentral. Weak waves embedded in 
the northwest flow will continue to produce clouds over much of 
the region as well as a few flurries or showers through Wednesday 
morning. The trough will completely exit later Wednesday as a 
short-wave ridge moves overhead. Conditions will quickly head 
downhill Wednesday night along the coast and spread inland Thursday. 
Strong warm air advection aloft will quickly change precipitation 
to rain most places...although it may take a little longer to scour 
out the cold air from the Kuskokwim valley. 


Bering Sea/Aleutians...with models in good agreement in handling of 
next system confidence level is much higher. Thus will begin to add 
more detail into latest forecast and increase wind forecast across 
the region. Expect storm force winds just ahead of the front along 
the eastern Aleutians and southern Alaska Peninsula Wednesday 
afternoon through Wednesday night. Models indicate an 850 mb jet of 
around 75 knots over this same area. Granted...this is a stable warm 
advection scenario...but affect of complex terrain will aid in 
acceleration of winds and this is a favorable direction for winds 
to blow in the Dutch Harbor area. Therefore...will issue a High 
Wind Warning for the eastern Aleutians Wednesday afternoon through 
Wednesday evening. Moderate cold advection on the back side of the 
low will produce storm force winds over the central Bering Wednesday 
night and Thursday. Winds may reach hurricane force...but will wait 
one more model cycle before upping winds that high. 


Long term forecast...the Bering Sea low complex will transition 
to the Gulf over the weekend...maintaining wet conditions over the 
southern Mainland. Meanwhile...another weaker and less amplified 
system will move into the Bering Sea. 


Aer/alu...watch/warning summary 
public...high wind watch 101 125. High Wind Warning 185. 
Marine...Storm Warning...155 170 171 172 176 179 180. 
Gale Warning 150 160 165 130 185. 
Fire weather...none. 


Seb Oct 08 

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