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FXAK67 PAJK 232318

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
318 PM AKDT Sat Sep 23 2017

.SYNOPSIS...A front will move in off the gulf and spread heavy
rain across portions of the panhandle through Saturday night.
Another low will cross over the southern panhandle on Sunday


.SHORT TERM.../Saturday evening through Sunday night/...Satellite
imagery is indicating a front located over the central gulf.
Deformation in the frontal cloud shield is also indicating the
development of what appears to be a triple point low west of the
southern outer coast. Infrared cloud top temperatures of -60C and
satellite based microwave rain rate information are all suggestive
of heavy rain in the associated front. Models all support these
things elements as well. Lightning was observed late last night
and earlier today well to the west over the central gulf, but none
has been observed this afternoon. Modeled lifted index and CAPE
not conducive for more lightning this afternoon or tonight. Still,
very cold cloud tops warrant caution. And so, have opted to
retain a slight chance of thunderstorms over the eastern gulf in
marine zone 310.

Showers already have, or will soon, transitioned to rain with no
break in precipitation. Rain will be heavy at times, particularly
over the southern half of the panhandle. 36 hour forecast storm
totals ranging from around 1/2 inch to more than 2 inches with the
heaviest amounts over the southern inner channels. Some models
are suggesting that even more precipitation is possible. As such,
this represents the most critical element of the afternoon
forecast and there will be responses from area creeks and streams.
No flooding is expected at this time, as this front will move
through fairly quickly. Look for updates from this office as this
event unfolds.

Winds associate with this front will be limited to small craft
intensity. Have small craft advisories for portions of the outer
coast as well as Cross Sound, Clarence Strait, and Northern Lynn
Canal. Winds will increase and become gusty from south to north
and from outside to inside as the front moves onshore beginning
late this evening over the southern inner channels, late tonight
over the central inner channels, and finally tomorrow morning from
the Icy Strait corridor and northward. Post frontal winds and
gusts will be less.

A few tweaks to winds to highlight gusty conditions associate with
the front`s progress north. Updated PoP and QPF using some of the
NAM, GFS and SREF to capture the details associated with the
deformation associated with the developing triple point.
Otherwise, no significant changes made to the previous forecast.
Overall forecast confidence is average.

.LONG TERM...Monday through Saturday...An upper level shortwave,
developing from a parent circulation with its center positioned
near the Aleutians, will propagate across the northeastern gulf
and lift across the panhandle Sunday night into early Monday
morning. A subtropical jet feature will assist with advecting
additional moisture with this system, but most of the
precipitation increases will primarily be confined to the more
southern portions of the panhandle. A ridge of high pressure will
then begin to build over the eastern gulf Monday. We should start
to see most locations across the panhandle dry out for the day.
Fog development may be a possibility, especially during the
morning commute on Monday, so we`ll continue to monitor this
trend for subsequent forecasts. The ridge should then begin to
weaken and retreat east before another shortwave develops and
progresses northeast across the gulf, impacting the panhandle on
Tuesday. Models continue to show additional subtropical jet setups
to assist with pushing additional moisture over the panhandle.
This type of pattern appears poised to continue through the
remainder of next week, with additional shortwaves that develop
out of the parent trough over the gulf. Due to this tropical
moisture push over the area, we expect chances of precipitation to
remain high through the week and temperatures to maybe be
slightly above average, especially in southern areas.

For adjustments to the forecast, we updated PoPs with a blend of
the ECMWF and Canadian, relying more on the EC, to continue to
show the increasing precipitation trend through the week. We also
increased winds throughout the eastern gulf waters to account for
the tighter pressure gradient that develops with the subsequent
shortwaves that progress out of the parent circulation. A lot of
uncertainty will then remain regarding the synoptic pattern after
about Day 8.


.AVIATION...Phone line problems in Yakutat are preventing the ASOS
obs from being disseminated. Had to NIL the TAF there due to
deteriorating weather tonight and loss of useful webcam views by
sunset /0311Z/. Continued very low cigs/vsbys across panhandle in
our very moist air mass. Wind shear mentioned in southern
panhandle TAFs overnight tonight with vigorous short wave moving
quickly through southern areas overnight. Rain changes to showers
during the day Sun with gradual stabilization. Outlook for
Mon...possible dense fog impacting TAF sites.


.AJK Watches/Warnings/Advisories...
MARINE...Small Craft Advisory for PKZ012-022-036-041-043-052.
     Small Craft Advisory due to Seas for PKZ051.




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