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
