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Next 12 Hours

Forecast data from the National Digital Forecast Database.

5 Day Forecast

• Wednesday
• 
• High: 77 °
• Low: 54 °
• Partly Cloudy

• Thursday
• 
• High: 77 °
• Low: 55 °
• Mostly Cloudy

• Friday
• 
• High: 77 °
• Low: 52 °
• Chance of Rain

• Saturday
• 
• High: 68 °
• Low: 50 °
• Chance of Rain

• Sunday
• 
• High: 77 °
• Low: 52 °
• Mostly Cloudy

Southeast Alaska forecast discussion 
National Weather Service Juneau Alaska 
339 PM akdt Tuesday Jun 18 2013 


Short term...afternoon satellite image shows a weakening trough 
of low pressure across the central and eastern Gulf...slowly 
drifting westward. Numerous convective cells developing over 
British Columbia slowly propagate westward cross the border along 
coast mountain range...increasing thunderstorm threats over the 
southern Panhandle areas. A cut-off upper level low responsible 
for last night thunderstorms has moved over the northern Gulf and 
weakened. Daytime highs sore to the lower 70s at Skagway and 
Angoon. Sea breezes developing many locations. 


Models remain fairly consistent for the weakening trough of low 
pressure across the Gulf of Alaska continues to move westward 
through Wednesday morning before dissipating by Wednesday 
afternoon. Models also show a good agreement for a ridge of high 
pressure building over the southern Gulf tonight. The ridge will 
continue to expand northward across the central and eastern Gulf 
through Wednesday. Used the NAM to update forecast package for 
tonight and Wednesday. 


Pressure gradient will remain relatively weak and thus expecting 
no significant winds. As the ridge of high pressure starts 
building across the Gulf...a weak inverted trough of low pressure 
will remain along coast mountain range. No significant surface 
features but upper level low over British Columbia coast will 
continue to play a major role bringing upper level vorticity 
fields with increasing instabilities for potential thunderstorms 
into the southern Panhandle then expand northward up to 
Petersburg through the afternoon. 


Long term...as advertised, a pattern change will emerge beginning 
late in the work week, just in time for the weekend as the 
easterly flow that has wrought the warm temperatures and the 
convective weather Southeast Alaska has been experiencing over the 
last few days will yield to more southerly flow...especially by 
Friday. An upper low will scoot eastward into the western Gulf 
while the low to the southeast of the Panhandle will move east as 
well. Convective precipitation will become more stratiform and 
temperatures will cool just in time for the weekend. At the 
surface, a ridge will develop beyond the aforementioned weak 
trough mentioned in the short-term Wednesday night into Thursday. 
In addition, slight warm air advection from British Columbia over Southeast Alaska Thursday 
will once again establish a thermal trough over southeast. While think 
areas of fog and low clouds will begin appearing over the offshore 
and northern outside waters, the thermal trough should shunt any 
encroachment onshore off to the southeast...at least this is the 
thinking for now. Both NAM and GFS indicated cape values are still 
high enough for an isolated risk of thunderstorms for the southern 
third of the Panhandle, zone 26, 28, and 29 for Wednesday evening. 
As previous shift has mentioned, this likely is associated with a 
lobe of positive vorticity migrating westward across the area. 
While the low migrates into the middle northern half of the 
Panhandle early Thursday morning, not confident enough to hold 
together convection that late in the night, for such a poorly 
defined feature. For Thursday, temperatures may be slightly 
warmer, but cape values not as impressive and thus have decided 
not to continue thunderstorms for now. 


While minor shortwaves will rotate around a main upper level low 
feature from Friday throughout the weekend giving Southeast Alaska 
chances of rain, even making rain likely for Friday into Friday 
night, models are indicating the main low will migrate eastward 
early next week with more chances of rain. While temperatures 
indicated now are more fitting for southeast convective flow, 
later shifts will likely need to cool temperatures for the weekend 
even further and especially for next week as well, as trend looks 
more likely to be a more marine influenced air mass. 


Used a blend of NAM, ECMWF, and GFS to handle the building thermal 
trough Wednesday night and Thursday. For precipitation, decided 
existing forecast looked good for Wednesday night and Thursday, 
but instead expanded likely probability of precipitation for Friday and Friday night with 
shortwave moving north becoming more probable. Made few changes to 
long term, did employ some wpc to smooth pattern for Monday and 
Tuesday. Confidence in forecast just average since pattern changes 
sometimes are difficult to nail. 


Ajk watches/warnings/advisories... 
public...none. 
Marine...Small Craft Advisory for pkz012. 


&& 


$$ 


Ahn/jwa