Scientific Forecaster Discussion
fxak67 pajk 211530
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
630 am akst Thu Nov 21 2019
Short term...sufficient shifts in guidance necessitated a few
substantial changes in the forecast through Friday night. But just
to be clear, the message of rain through much of the weekend
holds. Shortly after one front clears the region this morning, a
A frontal wave, indicated by satellite and ascat winds will press
northward to near Dixon Entrance later today. Model guidance is
varied with this fast-mover as they often are. Winds could be
gusty and strong with this system, but direction and strength will
largely depend on the track and strength once it arrives later
this morning. Strong winds associated with the front last night
have held up this morning, and so we have extended the duration of
these gusty conditions for Juneau, Haines, Skagway, and and even
Tenakee Springs. Although confidence is not great, we also added
some strong gusty winds for Prince of Wales Island this morning
associated with the lifting wave to around Dixon Entrance.
Moving forward in the future, we identified the need to raise wind
speeds with the second front, both before, during, and after.
Effectively every inner channel will Post a Small Craft Advisory
Friday or Friday night. In the meantime, northern Lynn Canal may
blow gale, but we have raised it significantly to 30 kt at this
stage. Confidence in winds at Skagway and Juneau is high also.
Regarding the rain itself, we made adjustments on the timing of
the two systems. For starters, the current front draped across the
bulk of the Panhandle will stall a bit this morning before moving
east into Canada. Likewise, we slowed the second just as much. Due
to brief period of cool temperatures aloft and increasing lapse
rates, we added showers for Yakutat between the two systems today,
but quickly change then back to rain by tonight as more heavier
precipitation spreads east with the second system. We still are
advertising amounts of 1 to 3 inches of rain across the Panhandle.
Another significant change was to increase the threat of
significant snow for White Pass and higher elevations. Model
guidance suggests that temperatures last night will be the warmest
period over the next several days, and White Pass temperatures at
this writing hover around 32 degrees with snow clearly depicted
in webcams. While three-hour precipitation amounts at sheep Camp
in the upper Lynn Canal region were a little concerning overnight,
accumulation looked minimal overnight. As a compromise of sorts,
we limited snow totals today to 3 inches by keeping snow ratios
low. We avoided Winter Weather Advisory criteria and instead
pushed for a Winter Storm Watch from midnight tonight to mid-
afternoon Friday. We kept temperatures at freezing or below for
the period, as the only solution above freezing was the nbm. All
told, we think 8 inches of snow are possible during this period.
Lastly cold air advection begins in earnest late Thursday night
into the weekend. Initially on Friday, shortwaves passing across
the Gulf should be able to incite enough vertical lift, already
enhanced from sharp lapse rates, for at least a slight risk of
We favored the GFS, NAM, and a suite of high res models for
changes. Confidence is good.
Long term.../Saturday through Thursday/ as of 10pm Wednesday. The
start of the long term continues with a wet pattern across the
Panhandle. By Saturday, a front will bring widespread rain, but
although guidance agrees on the wet pattern, there differences in
exact location of a weak surface low in the Gulf. Location of
low, will dictate which areas of the Panhandle will experience
heaviest rainfall rates and accumulations. After the main frontal
passage, the flow becomes more west-southwest by early Sunday. Cold air advection will begin
across the northern Gulf causing moderate showers through the
Guidance continues to suggest cooling by the start of the weekend
and into next week. 850mb temps fall from about -3c Saturday
morning to -11c late Monday night.These two factors should keep
showers in the forecast along with an increasing potential for
snow to be mixed in each day. Surface temperatures will be low
enough across the majority of Panhandle Sunday night into Monday
morning to support ocean effect snow showers.
Models are in agreement on a cooling trend in the forecast, but
have larger differences on waves or new lows passing through. The
GFS has been the most volatile between runs, placing lows in
different areas. This would drastically change where the precip
will/will not be. Again, the places without precip will likely see
colder temperatures. Main changes to forecast included increasing
pop for the Saturday time frame along with bumping temperatures
down another nudge from previous.
public...strong wind until noon akst today for akz027.
Strong wind until 6 am akst early this morning for akz025.
Strong wind late tonight for akz017-021.
Strong wind until 9 am akst this morning for akz019.
Winter Storm Watch from late tonight through Friday afternoon
Marine...Storm Warning for pkz052.
Gale Warning for pkz031-043-051.
Small Craft Advisory for pkz012-013-021-022-032>036-041-042-053.