Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 11/12/20

Southeast Alaska Forecast Discussion

National Weather Service Juneau AK

 336 PM AKST Wed Nov 11 2020

.SHORT TERM...Mixing is devil`s work to a snow forecast. Models
indicated a stable regime Wednesday across the central Panhandle,
and we allowed for this fact with cooler temperatures, and at
least a weak overrunning event possible. But MOS numbers for
winds at least hinted at something far less stable, somewhat
countering soundings. This morning, as we began to analyze AMDAR
Soundings from flights coming and going to Juneau Airport, we
noticed a much more unstable regime within the boundary layer. But
the tight easterly gradient confounded us even more by drying the
boundary layer and thus delaying precip onset. And finally, the
same drying of the column would serve to chill the atmosphere
sufficiently and potentially at the surface to make accumulating
snow possible--should the winds ease as was forecast by models.
Thus, we delayed the Juneau winter weather advisory`s
cancellation until this afternoon.

However, looking north, overrunning did occur with the Haines
Highway receiving light snow throughout the day--3 inches had been
recorded from 7 AM to 1 PM, even as amounts were much lower due to
an easterly wind mid-morning at sea level. For this reason, the
advisories for the Klondike and the Haines Highways continue
through 6 AM. However, rates will be greatest this evening. The
front pushes east through the night and thus shuts off all but
lighter showery precipitation.

After a couple of updates to bump up winds, we faced facts and
posted gusts to 40 mph in both Skagway and Juneau early this
evening, and even gale force winds in Stephens Passage. But
guidance supports easing of winds as gradients relax across the
gulf and a wave robs energy to the south near Haida Gwaii, at
least temporarily.

The rest of the forecast mainly lies unchanged. However, looking
ahead, marginal thickness values and 850 mb temperatures are
casting increasing doubt on any organized snow accumulations until
at least Friday. But we certainly cannot rule out some snow mixed
in with rain showers during that time.

.LONG TERM.../Fri through Wed as of 10 pm Tue/ The general upper
level pattern for the long range has not changed all that much
from yesterday. It still features a broad upper level trough
sitting over the Gulf and Alaska through the period. This trough
will be deepening into early next week as a ridge starts to
intrude from the west.

At the surface, not to much has changed overall from yesterday. A
weak low in the northern gulf will start out the extended period
spreading showers across the panhandle (Rain for the south, snow
and rain for the north). No significant QPF expected so any snow
accumulation should be light unless several showers start to train
over the same area. This general dampness will gradually shift to
being more in the southern panhandle by late weekend and early
next week as the low gradually sides into the SE gulf. At the same
time, another shot of cold air will be heading south from the
Yukon Sun into Mon. North/south pressure gradients increase late
Sun and continue into Tue with max gradients reaching around 4 to
6 mb. Forecast confidence on this is increasing as more
operational models are agreeing on this scenario so the forecast
was trended drier in the north with some winds increased in the
usual outflow areas (up to 20 to 25 kt) for Sun night and beyond.

Kept mostly with the WPC guidance for long range edits.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Strong Wind until 6 PM AKST this evening for AKZ025.
     Winter Weather Advisory until 6 AM AKST Thursday for AKZ018-019.
     Strong Wind until 3 PM AKST this afternoon for AKZ017-020.
MARINE...Gale Warning for PKZ031-042.
     Small Craft Advisory for PKZ012-013-021-022-032>036-041-043-
     051>053.

&&

$$

JWA/EAL

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