Storm Warning 26, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
305 PM AKST Tue Dec 24 2019

.SHORT TERM...The tail end of the front that affected most of the
panhandle yesterday is finally leaving the southern part of the
area. As the front departs, the focus shifts to two specific
features coming across the gulf. The first is a trough coming in
late Tuesday night into Wednesday. Not much in the way of wind or
precip for most of the panhandle with this feature, but it might
be a different story for the Haines and Klondike Highways. As the
trough comes in, overall southerly flow will increase in the
Haines and Skagway areas, helping to maintain warmer temperatures
in Haines and Skagway proper, and providing the moisture support
and instability to maintain active weather. Heavier precip will
start up late Tue night or early Wed and continue throughout the
day on Wednesday. The main concern remains temperatures in the
area for what ptypes will be seen. It is looking increasingly
likely that the actual cities of Haines and Skagway won't see all
that much in the way of snow, and given the strong southerly flow,
rain is probably going to be the primary concern for these
cities. The highways are a different story. The Haines Highway
will initially see snowfall Tuesday evening into Wednesday, before
gradually transitioning to a rain snow mix as southerly flow
continues to overrun the cooler temperatures in the Highway area.
There is also the possibility of some terrain shadowing of the
Chilkat valley at times possibly limiting snow amounts further,
but for the moment temps are expected to remain cold enough and
enough qpf will be in the area for some significant snow to be a
possibility earlier on. The Klondike Highway during this time
period looks like a more sure bet for snowfall, and the winter
storm watch which had been been issued starting early Wed morning
for both highways has been upgraded over the Klondike Highway into
a full-fledged winter storm warning for heavy snow.

Feature number two will be much more noticeable with heavier
precip and strong winds accompanying it as it pushes into the
panhandle tomorrow afternoon and evening. Precipitation will be
in abundant supply throughout the entirety of SE Alaska, with rain
dominating across much of the southern and central panhandle. Expected
qpf will be around 1 inch from Wed afternoon through Wed night
for most areas.

In the northern area, the winter weather for the
Klondike highway will likely continue with little break in between
this system and the first trough although temperature concerns
remain. There is a chance over Wednesday night that winds may flip
from the south to the north, which could bring in some cooler air
into Skagway. The Haines highway will continue to pose a forecast
concern as there remains some uncertainty as to whether or not it
will maintain warm air advection from the E during Wednesday
night. Should the winds flip, and outflow dominate for a portion
of the forecast period along the Highway, the area could see a
renewed chance of snow. In addition, it does look like Hyder could
see substantial snow amounts Wednesday night, and a winter storm
watch has been issued for Hyder in acknowledgement of these
concerns. Elsewhere, high winds will be a major concern, and most
areas will experience gusty winds Wed night. Wind gusts of 50 mph
or more are possible especially along the outer coast and in some
of the inner channels, and winds may still go higher than this.
Given these concerns, have issued a high wind watch to reflect
these concerns. Gale force winds are also likely to be wide spread
in both the gulf and inner channels during frontal passage.
Overall forecast confidence is average.

.LONG TERM.../Thursday through Tuesday night/...Models continue
to be in good agreement on the location and track of a gale force
low and associated front that will already be impacting Southeast
Alaska at the start of this extended range forecast period. Pre-
frontal winds out of the southeast will have already peaked by
Thursday morning, but moderate to heavy rain will still be in
progress, as will heavy snow for the far northern inner channels.
Precipitation from this system will be winding down as the next
front approaches from the south....

The location and track of the low associated with this second
front was problematic yesterday, but ECMWF and GFS are now in
better agreement. Given one more model cycle of better agreement
on the location and track of the low and significant changes to
the wind field for Friday and Friday night will be warranted.
Regarding the precipitation associated with the front itself,
modes were in good agreement yesterday and continue to be so
today. Therefore, have made significant changes to the PoP grids
with substantial increases to the QPF grids. As a result, another
round of heavy snow for the far northern inner channels is looking
likely beginning Friday afternoon and lasting through Saturday
night.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Storm Watch from late tonight through Thursday morning
     for AKZ018-019.
MARINE...Small Craft Advisory for PKZ022-041>043-051>053.

&&

GFS/Fritsch

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