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Next 12 Hours

Forecast data from the National Digital Forecast Database.

5 Day Forecast

• Thursday
• 
• High: 70 °
• Low: 55 °
• Mostly Cloudy

• Friday
• 
• High: 63 °
• Low: 54 °
• Rain

• Saturday
• 
• High: 63 °
• Low: 54 °
• Mostly Cloudy

• Sunday
• 
• High: 72 °
• Low: 54 °
• Partly Cloudy

• Monday
• 
• High: 64 °
• Low: 54 °
• Mostly Cloudy

Southeast Alaska forecast discussion 
National Weather Service Juneau Alaska 
408 PM akdt Wednesday Jun 19 2013 


Short term...tricky forecast as we initiate a pattern change 
for the weekend. Aloft we have easterly flow over the Panhandle 
becoming southeasterly tonight, still spreading pulses of 
vorticity from the southeast. A ridge will develop over northern 
British Columbia tonight and strengthen Thursday. This will begin reorienting 
flow aloft over southeast from offshore southeast to more onshore 
south by Thursday. 


At the surface, a ridge developed last night over the central 
Gulf and spread onshore flow onto the shores of southeast 
overnight...and in many cases, lasting in the form of low clouds 
through early afternoon. Visible satellite imagery shows a 
Brilliant white mass of stratus over the outside waters, with 
only a few breaks showing up this afternoon offshore of Yakutat. 
This ridge should remain while low pressure over the Coast 
Mountains will also remain thus, we should see a diurnal 
encroachment of low clouds and fog late tonight and into tomorrow 
morning as well. 


At the midlevels, we still have some Alto cumulus and Alto 
stratus moving east into the southern Panhandle and northwest 
into the central Panhandle. Streamline analysis as well as 
profilers in the Juneau area confirm offshore flow may be holding 
this back due to some downsloping. Most models has trended probability of precipitation 
downward over the last 24 hours and still further today. But with 
cape still an issue over much of the south and even moving into 
the north central Panhandle late this evening, I still kept 
isolated showers with a slight risk of a thunderstorm, even as 
models trended away from precipitation. These areas of cape seem to 
correspond to waves of positive vorticity passing northwest into 
the Panhandle this evening. NAM in particular is taking this 
precipitation and wrapping northward against the St. Elias mountains. 
This should increase probability of precipitation over the northern mountains on 
Thursday, at least to isolated and scattered levels. While NAM is 
still producing cape in this region, think any thunderstorm 
activity will lie across the border into Canada Thursday 
afternoon. 


Sky cover seems to always present problems in easterly flow and 
transitional situations. While most models seemingly have smeared 
clouds all over the region beginning tonight, worded skies as 
partly sunny even as observation will carry mostly cloudy. My feeling is 
that most folks will see some unfiltered sunshine tomorrow for 
large periods of the day, even as healthy layers of altocumulus 
and altostratus inland will add texture. Also with onshore flow 
developing aloft, clouds seem a safe bet, even as I hedge on the 
sunshine. Temperatures were largely left alone, although did 
increase some temperatures a little today inland as temperatures were 
10 degrees warmer than yesterday on the whole this morning. 


Model of choice for precipitation was the NAM and NAM dmg, for 
pressure/wind, did not change forecast as most models carried 
subtle differences that would amount to very little change. 
Forecast confidence good, but falls on thunderstorm guidance, 
since thunderstorms have yet to appear over British Columbia. 


Long term.../thurs night through Wednesday/ return to typical Juneau 
weather at the end of the work week. For the weekend, clouds, 
showers, and seasonable temperatures across the Panhandle. 
Forecast confidence lowers to much below average for the start of 
next week as weather models are struggling to figure out the 
solutions of the breakdown of the blocking pattern over the 
interior of Alaska and northwestern Canada. 


A short wave disturbance is going to move northwestward across 
the Panhandle on Friday, and this will Sparks some scattered and 
numerous showers across the area. Showers will diminish on 
Saturday as the short wave exits the area. 


The forecast becomes very tricky for the rest of the long range 
period, especially beyond this weekend. There are multiple weather 
features that will influence the upcoming pattern change. 


The first feature is the strong upper low that is located over 
Bristol Bay. This upper low was the same system that produced 
scattered thunderstorms in the northern Panhandle on Monday. This 
upper level disturbance is expected to track northwestward, around 
the eastern periphery of the blocking high over interior Alaska, 
towards east siberian sea via the berings. Some of the energy 
associated with this upper low will get left behind over the 
Aleutians and the Bering Sea. The second feature is a recurving 
typhoon transitioning into an extratropical system. This weather 
feature will inject subtropical moisture and energy into the 
Aleutians, and merge with the remnant of the upper low (first 
feature). As a result, a strong upper low will develop over 
eastern Aleutians this weekend, and this upper low will be strong 
enough to nudge weather systems further east. The current blocking 
high over interior Alaska will be pushed eastward with its ridge 
axis set up across the British Columbia/Alberta border. Some energy associated 
with this strong upper low will split off southeastward towards 
the Pacific northwest next week. 


Models are struggling to determine how far south this upper low 
will go next week. If the upper low remains in a northerly track, 
the Panhandle will be under onshore flow with clouds, rain, and 
cooler conditions for most of next week. If the upper low GOES 
further south, upper ridging will build over Yukon and interior 
Alaska. This solution will lead to return of above-normal 
temperatures and drier conditions back to the Panhandle next week. 


Due to high uncertainty in the forecast in the extended period, I 
decided to use an ensemble approach of European model (ecmwf)/GFS/wpc guidance. 
Right now, I think the southern solution is more probable. Thus, 
kept the high temperatures slightly above normal, about middle 60s to 
upper 60s, for most of the Panhandle next week. Mostly cloudy 
skies and scattered showers will be dominant through the end of 
the forecast period. 


In summary, a tricky long range forecast as we are undergoing a 
weather pattern change. 


Ajk watches/warnings/advisories... 
public...none. 


&& 


$$ 


Jwa/rcl