Area Forecast Discussion
Issued by NWS Juneau, AK
Issued by NWS Juneau, AK
741 FXAK67 PAJK 181304 AFDAJK Southeast Alaska Forecast Discussion National Weather Service Juneau AK 504 AM AKDT Wed Sep 18 2024 .SHORT TERM...Windy and wet conditions are beginning to move into the panhandle as a front associated with a parent low near the northern gulf moves eastward. Minor changes to the overall forecast package mainly for marine. For an excellent description of QPF and winds that holds true for this forecasting package, refer to the prior short term AFD. Still expecting 20-25 knot winds in the inner channels as the front progresses eastward, but increased areal extent of 25-30 knot winds in the gulf for the post frontal period. Mid to upper level cold air advection will provide a punch of westerly winds behind the front. Current ASCAT passes show 25-30 knot W-SWerlies around the central gulf, which further supports this idea. Additionally, due to the stout westerly component to the winds behind the front, winds in the inner channels will likely take an increasingly SW direction with the frontal passage. Finally, while not enough confidence to force a change to the PoP forecast, mid level drying along the southern panhandle in the post frontal environment may lead to only periods of rain, with breaks in between. Regardless much of the southern panhandle is still expecting rain, at least measurable amounts every three hours. .LONG TERM...The long-term forecast is characterized by a plethora of storms moving across the Gulf into the Panhandle bringing wind and rain concerns from Thursday into the weekend. Touching on rain, an atmospheric river will impact Queen Charlotte Sound, with the southern Panhandle on the northern edge of the higher moisture, potentially limiting more significant impacts. For mariners operating in our area, especially in the inner channels, expect changes on wind speeds in some inner channels as guidance shows better agreement through the week. More details: At 500mb, a longwave trough will eject into Canada Thursday, with the accompanying surface low in the Gulf beginning to weaken as it loses support aloft, allowing surface winds speeds and rainfall to diminish. Thursday afternoon into early Friday are now looking like the only window for a break past the weekend. Early Friday ensembles highlight another system moving into the southeast Gulf, deterministic runs show decent divergence from the mean, with the GFS and ECMWF trending towards gale force southerly winds for Clarence. Furthermore, the trends of both deterministic are moving the low further north with a drop in surface pressure and agree in location. For now, the forecast reflects near-gale force southerly winds impacting the southern coast, Dixon, and Clarence Strait by Friday, with elevated easterly outflow winds shooting east out of Cross Sound, and north winds in Lynn Canal. For now, the highest winds Friday to Saturday look to impact the central and southern Panhandle. By late Sunday or Monday, a broad low is expected in the Gulf. Switching to rainfall, the jet will begin to push a moderate atmospheric river into Haida Gwaii and Queen Charlotte Sound with the southern Panhandle on the northern edge of the AR. Expect rainfall through the weekend into next week, resulting in about 1 to 2 inches per day, heaviest rainfall in the southern Panhandle. With that said long range guidance is struggling with the location, magnitude, and extent of the atmospheric river which could swing more rainfall than anticipated further north. && .AVIATION...Conditions are starting to deteriorate across the northern panhandle this morning. Yakutat is already seeing IFR ceilings down to 600 ft. Rain is beginning in the rest of the northern panhandle and outer coast early this morning, but VFR conditions are still hanging on. Expect ceilings and vis to drop into MVFR or lower criteria through the morning with the southern panhandle getting in on the act by afternoon. Ceilings will likely stay low into Wed night, but vis may improve as rain starts to diminish late. Winds are the other issue with gusty winds in northern Lynn Canal already this morning. The rest of the northern panhandle will see some increased winds as well by midday though the strongest will be around Skagway. Some light turbulence and low level wind shear are possible mainly today with frontal passage. && .MARINE...The next front is moving in from the west, causing wind speeds to increase. 20 to 30 knots in the outer coastal waters with 20 to 25 knots in the inner channels. Northern Lynn Canal will be the highlight of this system, with winds increasing to 30 knots this morning and continuing to remain elevated through Wednesday night. Seas will get up to 3 to 5 feet in the inner channels with gulf seas getting up to 10 to 16 feet for this morning and begin diminishing in the evening. && .AJK WATCHES/WARNINGS/ADVISORIES... PUBLIC...Strong Wind until 10 AM AKDT this morning for AKZ318. MARINE...Small Craft Advisory for PKZ012-013-022-031>033-053-641>644-651- 652-661>664-671-672. && $$ SHORT TERM/MARINE...NTC LONG TERM....AP AVIATION...EAL Visit us at http://www.weather.gov/Juneau