900 
fxak68 pafc 192215 
afdafc 


Area forecast discussion 
southcentral and southwest Alaska 
115 PM akst Wednesday Nov 19 2008 


Analysis and upper levels...large low complex embedded within high 
amplitude low wavelength through is centered south of the Gulf. This 
position is farther north than on Tuesday and systems are now 
affecting southcentral. There are currently four surface lows 
embedded within the large scale upper level circulation. Of most 
concern today is a 995 mb low southeast of Kodiak Island and the 
occluded front which extends northeast from the low center. There 
is a band of storm force winds ahead of the front along the Gulf 
Coast as seen in Quickscat winds. Gap winds have increased across 
southcentral overnight through this morning while nearby areas 
remain under the influence of a temperature inversion with light 
winds. This has created large temperature differences between windy 
areas and non-windy areas across very small distances. Radar returns 
in the Prince William Sound region have looked impressive...but 
strong offshore flow is helping to maintain low level dry air so 
very little precipitation reaching the ground. Snow is falling over 
Kodiak Island under strong northwest surface flow...which is helping 
maintain temperatures cold enough for snow and reducing visibilities 
to around one half mile. A tight pressure gradient combined with 
cold advection is also producing gusty northerlies across southwest 
Alaska and through the Alaska Peninsula...although much of the 
Kuskokwim Delta and valley remain under an inversion with 
temperatures well below zero. 


Another large upper low is moving out of eastern Asia with upper jet 
driing northeastward toward the western Bering Sea. This is helping 
to push on and amplify the ridge sitting over the Bering. Winds are 
generally light across the central Bering while winds are rapidly 
increasing across the western Bering with the approaching system. 


Model discussion...models initialized well with synoptic features 
and show fair to poor run-to-run consistency through the first three 
days of the forecast. The general evolution of the pattern is the 
same...but the details of individual low tracks and strength are not 
very consistent. There is a little more confidence in the 
Gulf/southcentral forecast where the models are in better agreement. 


The model preference in the east is toward the 12z GFS while out 
west will trend slightly toward this solution. Also of note...all 
models are overdoing precipitation across Prince William Sound...and 
this will be reflected in the new forecast. 


Short term forecast... 


Southcentral Alaska...occluded front will dissipate offshore tonight 
as the low over the southwest Gulf rotates southward. There is an 
upper level vorticity-maximum that will rotate a little farther 
westward across the Kenai Peninsula and Kodiak Island before 
weakening and rotating southwestward. This will help push snow 
onshore over western Prince William Sound and maintain snow over 
Kodiak Island through much of tonight. Due to continues strong 
offshore flow...amounts will be light over Prince William Sound. 
Models hint at a little bit of snow making across the coastal range 
to Anchorage and the western Kenai Peninsula...but think it will 
be very difficult for snow to reach the ground due to steady supply 
of dry air under northerly surface flow. A developing low on the 
east side of the upper circulation will rotate into the southern 
Gulf later tonight and become the new primary center of the upper 
circulation. The low will then track eastward Thursday through 
Friday as the systems out west gradually force the pattern to 
progress. Thus the front associated with this second low will not 
make it very far north and west across the Gulf so there will be 
just a chance of light precipitation along the immediate Gulf Coast. 


Winds will diminish across the region Friday as the gradient weakens. 
The latest models indicate the Bering low tracking to southcentral 
Saturday brining a shot of snow...but there is low confidence in 
this solution. 


Southwest Alaska/Alaska Peninsula...as the Bering ridge continues 
to amplify an upper trough currently over the North Slope of Alaska 
will be forced southward to southwest Alaska on Thursday. This will 
advect Arctic air southward which will combine with an alreeady 
tight pressure gradient to produce strong and gusty northerlies 
across southwest Alaska and the Alaska Peninsula. It appears that 
there is enough cold advection aloft to eliminate the surface-based 
inversion and allow winds to mix down to the surface across the 
entire region. The strongest advection and winds look to extend 
down The Heart of the Kuskokwim valley to Bristol Bay and across 
the Alaska Peninsula south of King Salmon. The cold advection will 
end late Thursday night and the surface gradient will weaken as the 
Gulf low tracks eastward...so winds will diminish quickly. Bering 
low looks like it will track along the Alaska Peninsula Friday night 
and Saturday and bring a chance of precipitation to these areas...but 
there is low confidence in this track. 


Bering Sea/Aleutians...a strong low currently over the south end 
of Kamchatka will track northeastward today pushing a strong front 
eastward across the western Bering and Aleutians. A strong jet over 
the northwest Pacific will aid development of a low along the front 
which will intensify as it crosses the western Aleutians into the 
Bering Sea on Thursday. There is still quite a bit of model 
disagreement on the strength and track of this low...so have stuck 
fairly close to current forecast which maxes winds out at 45kt. All 
models do indicate Bering Sea ridge will break down completely 
Friday...so expect storm system to make eastward progress Friday 
and Saturday as another storm moves to the western Aleutians. 


Long term forecast...mean trough will encompass the forecast 
areas through the extended period with a strong jet just south 
of the Aleutians. Thus...expect a very active pattern with fast 
moving storm systems. 


Aer/alu...watch/warning summary 
public...none. 
Marine...Gale Warning...120 130 132 138 140 150 155 160 165 170 172 
175 176 185 179. 
Heavy freezing spray warning...160 180. 
Fire weather...none. 


Seb Nov 08 

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