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000 FXAK67 PAJK 222244 AFDAJK Southeast Alaska Forecast Discussion National Weather Service Juneau AK 244 PM AKDT Tue Aug 22 2017 .SHORT TERM...
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/through Thursday night/ A slow moving cold front will be east of the area by early this evening. Low pressure over the far southern gulf will drift ESE to near Haida Gwaii by Wed evening. An occluded frontal system will move into the eastern gulf by Thu evening. Used mainly the 12z NAMnest to handle adjustments through Thu night, but did use some of the 15z SREF to help with POPs. Heaviest rain will be exiting east of the area early this evening as cold front moves out. More showery precip will move in as low moves toward the area later this evening and overnight. Still could be a few TSTMS near and east of the low center over the gulf tonight into Wed with forecast CAPE values around 300-400 J/KG. Think they should remain offshore though, so no mention for land zones. Most of the showers will be over the southern half of the area later tonight into Wed with low approaching, while areas further N see less shower activity. Strongest winds will be in band to east and north of low center with 25-30 kt likely over the southeast gulf and far southern inner channels tonight into Wed. The showers and winds should diminish Wed night as low moves over Haida Gwaii. Next frontal system should spread rain into the area Thu afternoon and evening. Further ahead of the front, showers will likely increase in coverage ahead of the main rain band as low level convergence increases over the area and shallow instability continues. This system looks fairly wet, but nowhere close to the rainfall amounts that occurred with the current system is expected with the Thu night front at this point. .LONG TERM.../Friday through Tuesday/ Broad upper low tracks into the AK gulf from the west moving high pressure ridge over the panhandle eastward Friday into the weekend. Center of the low will swing over Prince William Sound by Sunday with a second wave just the west rotating into the south central gulf. Sunday night into Monday expect the entire upper level low to push east with a ridge building in behind, but some timing uncertainty. Another low will move into the western gulf behind the ridge next week. At the surface, occluded frontal boundary associated with the parent low moving over the AK peninsula will cross the AK gulf then over the panhandle through the day Thursday. This will end any brief dry period for SE AK. As more surface short waves rotate around the main low into the gulf bands of precip will move over the area before finally beginning to diminish Sunday into Monday. Potential for some areas of heavy rain with this system as indicated by GFS integrated water vapor transport and NAEFS mean precipitable water standardized anomaly of 1 to 1.5. However, highest QPF depicted from these is mostly over the far southern panhandle. Gulf winds around the waves increasing to at least small craft to possible gale force due to barrier jet formation along the front. Still getting fair model agreement at least through early Sunday, so used a blend of GFS/ECMWF through then. Model spread rapidly increases with respect to the timing of the exiting low and the system moving in next week, so kept with WPC guidance for rest of the extended range. Forecast confidence is initially above average for these next systems but drops significantly by the end of the weekend.
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&& .HYDROLOGY...
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With rain diminishing across the area, water levels on area streams should drop off tonight. Some of the larger rivers will continue to rise due to their slower reaction to the rainfall, but none are expected to reach bankfull at this point. Will most likely allow the Flood Advisory for the Ketchikan area to expire at 4 PM this afternoon.
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&& .AJK Watches/Warnings/Advisories...
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PUBLIC...Areal Flood Advisory until 4 PM AKDT this afternoon for AKZ028. MARINE...Small Craft Advisory for PKZ036-041-042. Small Craft Advisory due to Seas for PKZ043.
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&& $$ RWT/PRB Visit us at http://www.weather.gov/Juneau

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