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Southeast Alaska forecast discussion 
National Weather Service Juneau Alaska
542 am akdt Wednesday Jul 30 2014

Short term...satellite imagery this morning showing weakening low
in the Alaska Gulf extending from a broad low pressure system in the
southwestern Gulf. The entire system is retrograding westward
today as a high pressure ridge over the southern Panhandle
continues to build northward. This ridge will then extend into the
eastern Gulf tonight into Thursday. Shower activity from the
frontal boundary extending from the Gulf low has been diminishing
from south to north as the front exits the region and the ridge
builds. Low clouds, patchy fog and scud will lift through the
morning as the atmosphere begins to dry. With the decreasing cloud
cover and the ample moisture from the previous day expecting
radiational fog development late tonight with a stronger nocturnal
inversion. Day time highs will be warmer today with middle to upper
60s to the south and low 60s for the central and northern regions.
MOS guidance a few degrees warmer but with the lingering cloud
cover and scattered shower activity will not see the warm up
expected over the next few days quite yet. Lower temperatures overnight
due to the radiational cooling and colder air moving in aloft.
Generally light winds over the Panhandle except for the northern
areas where the gradient between the building ridge and a low over
the Yukon is producing southerly winds in the 20 knots range. Winds
for northern Lynn Canal are close to but should be just below
small craft level. As the ridge axis shifts to the west and the
Yukon low moves east the gradient will weaken with the southerly
winds subsiding tonight.

Good model agreement with European model (ecmwf)/NAM/Gem showing best fit. GFS
matched well for the Gulf low but was off with the ridge axis over
the Panhandle. The 00z model runs were very similar to previous
forecast for winds and pressure so little change was needed. Main
question will be extent of fog formation later tonight and if
visibility will be significantly reduced. Forecast confidence is
above average.

Long term...retrograded upper level low mentioned above will be
over the far SW Gulf by Thursday with weak upper level flow over the
Panhandle. Then a weak upper level ridge of high pressure settles
over the area and these features persist through Saturday night.
This pattern will allow for several days of dry weather and
sunshine. One area of concern that could impact the cloud cover
forecast is the onshore flow aloft. This could cause some more
high clouds to pass overhead.

At the surface...high pressure will settle over the northern Gulf
with a weak inverted thermal trough over the southern Panhandle.
This will cause winds through the inside waters to have have a
northwesterly direction. The pressure gradient will be light enough for
afternoon sea breezes to counter this north flow each afternoon in
Lynn Canal. Other locations the northwesterly sea breeze will strengthen
the winds slightly each afternoon then become light again
overnight. The surface high pressure will likely cause nocturnal
inversions...this could be an issue with trapping in moisture each
morning. But think that the inversion will break each day with the
sea breeze developing. Along the outer coast will need to watch
for a marine layer to develop under the high.

The low in the SW Gulf starts to move back toward the central
Gulf on Sunday. The European model (ecmwf) moves the low to the north central Gulf
by Monday night while the GFS keeps the low closer to Kodiak. Both
would bring precipitation back into the forecast for the start of next
week...but the European model (ecmwf) solution would be earlier and a wetter one. Onshore
flow becomes stronger with both models starting late Saturday night...so
more confident with increasing clouds for Sunday and increasing
chances of rain Sunday night into Monday. This onset of precipitation is
delayed compared to earlier forecasts with Saturday looking to
have more sun and Sunday dryer. Friday and Saturday look to be the
warmest days of the week with highs easily reaching the low 70s
and Sunday/Monday still warm but with more clouds. Increased
temperatures by a few degrees...but if for some reason a stronger
inversion develops and does not break...then temperatures will be
colder.

Models were in good agreement through the Middle Range forecast and
were closer to the dryer GFS solution of the past couple days.
Models diverge with the low moving back into the central Gulf for
next week. Still kept with a blend that resulted in a weaker low
center and limited probability of precipitation to chance due to the uncertainty.

&&

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

&&

$$

Prb/Ferrin

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