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Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
250 PM akst sun Nov 19 2017

Short term...tonight through Tuesday...currently, a weakening
shortwave is progressing east through British Columbia, Canada.
The deep ridge that extended over the western Gulf the last few
days has also weakened and is beginning to propagate east, due to
a strengthening shortwave digging southeast out of Mainland
Alaska. This synoptic setup has given US our clear skies today and
provided some unseasonably below normal temperatures this
morning. The current snow cover probably also assisted with the
radiational cooling process early this morning to provide single
digit temps for some locations, including Gustavus, Hoonah, and
the Mendenhall Valley area of Juneau. Some outflow winds have
persisted today due to a relatively tight pressure gradient that
remains over the Panhandle, so we have some small craft advisories
that continue for Lynn Canal and along the eastern Gulf water
zones.

From current infrared satellite imagery, we can see the developing low
near the surface over the Gulf that is associated with the
shortwave pushing out of Mainland Alaska. This low will continue
to strengthen overnight tonight. Upper level potential
temperature analysis still shows this low cutting off from the
main flow aloft. The associated weather front will then lift
across the eastern Gulf early Monday and stall along the
coastline of the Panhandle. Confidence is increasing for an
overrunning event with this feature concerning snow accumulation
in the 3 to 6 inches range, mainly for portions of the northern
Panhandle, with more significant accumulations expected through
the Icy Strait corridor and along eastern Baranof Island and
southeastern Admiralty Island. Terrain effects and upslope flow
will help with providing additional snow accumulation. A Winter
Weather Advisory has been issued for these locations and is in
effect from 9pm Monday to 9am Tuesday. In addition, as the
gradient tightens between this developing low and a strong ridge
over Canada, we'll expect the winds to increase throughout the
Inner Channels and along the front that will stall through the
offshore and eastern Gulf waters.

On Tuesday, a Rex block pattern will set up, with the ridge of
high pressure from Canada positioned over the cut off low over the
Gulf. This is another indication that this pattern will remain
steady for the next few days. Another feature will then lift
northeast out of the central Gulf using the same energy from the
cut off low beginning late Thursday. Due to the position of this
new system, we'll see warm air advection initiate through
southern areas of the Panhandle, and we'll have some higher
precipitable water amounts with this system. 925 mb temperature
analysis shows the freezing line shifting north with this warm
air push, so there will be a concern as to where the rain/snow
line will position itself during this event.

For model guidance, we chose predominantly a combination of the
NAM and GFS for any major adjustments. We also opted to use some
higher resolution models for the pops and the quantitative precipitation forecast. We stayed with
mostly bias corrected models to adjust the temperatures in the
short-term, mainly for increasing them for Tuesday as we get the
warm push from the next feature.

Long term.../Tuesday through Sunday/models have a reasonable
agreement Tuesday in regards to a low that drops down from the
western Gulf into the North Pacific. It will not be the last time
Southeast Alaska will see this low pressure, as it does a sweeping
concave turn towards haida gwaii. This is about the time that the
models start to diverge in their solutions. The GFS is the most
aggressive bringing a closed low to near haida gwaii, while the
NAM has the slowest approach with the ec taking the middle Road.
By Thanksgiving the GFS has brought the to near Yakutat, while the
European model (ecmwf) solutions dawdles in the southeast Gulf and eventually
fills the low. The European model (ecmwf) has also become the rogue model solution
with major solutions between the last two runs, this leaves the
GFS a reasonable model to do blending to as needed. Overall using
an ensemble approach will be the most prudent solution for the
later part of the upcoming week. Used wpc to extend out through
Sunday, with a small nudge using the GFS.

A slow trend to warmer weather by mid week for the southern
Panhandle and then spreading into the central Panhandle as far
north of Juneau over the weekend. Areas in the south and along
the outer coastal waters from Sitka south with be predominately
rain, while a mix of precipitation will be the norm for the
central areas. By Saturday the warmer air should reach Yakutat and
Haines areas making for a mix of rain and snow during the warmer
part of the day. Snow will continue in the Skagway.

Strong outflows will be diminishing as the southerly winds advance
northward, Lynn Canal area and northern areas of Glacier Bay will
be the hold outs with northerly winds persisting but diminishing
into the weekend.

Forecast confidence in the mid range /Tuesday-Wednesday/ is
average and then slips lower. With the meager model agreement and
model run disparities with the ECMWF, confidence remains low for
the details late week and into the weekend.

&&

Ajk watches/warnings/advisories...
public...Winter Weather Advisory from 9 PM Monday to 9 am akst Tuesday
for akz020-021-024-025.
Marine...Gale Warning for pkz011>013-021-022-031-034-042-043-051-052.
Small Craft Advisory for pkz032-033-035-036-041-053.

&&

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