Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
609 am akst Thursday Dec 12 2013
Short term...985 mb low in the Alaska Gulf 150 nm west of Cross
Sound will weaken though the day as it retrogrades northwestward
toward Prince William Sound. High pressure over the Yukon will
begin to shift to the east. The associated front is exiting the
Panhandle this morning with precipitation diminishing through the
afternoon from north to south. The interaction between this
systems warm moist air flow as seen on water vapor satellite with
the cold Arctic air pushing down from the Yukon produced
significant snow fall over the central and northern Panhandle
starting Wednesday morning. Snow fall amounts ranging from 4 to
18 inches of heavy wet snow were reported through Wednesday
night. As temperatures cooled overnight snow has become drier
with an additional 3 to 10 inches expected today with a second
band moving in behind the low. Highest snow amounts have been
over the northern Panhandle due to colder temperatures. The rain
snow line is currently near Frederick Sound and not expecting it
to shift too much as temperatures will remain steady or fall
slightly through the day. Tight northerly gradient between the
Yukon high and Gulf low will weak through the day as the Yukon
high shifts east and the low shifts northwest. Easterly outflow
winds and strong north winds over the northern Inner Channels and
Panhandle will subside overnight with the decreasing gradient.
Another low developing south of the Aleutians will track
eastward through Thursday night reaching the central Gulf. The
associated front is moving a bit faster and should bring another
round of precipitation to the Panhandle late Thursday night. This
low will again have a long fetch of subtropical moisture but the
difference will be the absence of the Arctic front. Thus while
precipitation will start out as snow or a rain snow mix for the central
Panhandle it may transition to mostly rain by Friday morning with
snow fall amounts significantly lower than the current system
produced. Winds over the Gulf will begin to increase again with
the advancing system.
Fair model agreement with the 00z GFS/ 06z NAM with the position
of the Gulf low this morning and were used to update forecast
grids. European model (ecmwf) more of an outlier having the low bit further north.
All models were moving in the next wave of precipitation faster on
Thursday night and Friday and this was the main change to the
forecast. NAM MOS guidance still on the cold side but not as
extreme as previous runs. Main forecast questions will be
additional snow fall amounts for the day as there is a warm
850 mb temperature ridge moving over the area which could support
a quick over running event with colder surface temperatures or if the
warmer temperatures mix down see lower snow totals as rain begins to mix
in. Forecast confidence is average.
Long term...with upper level low firmly anchored over
southwest Alaska...southwesterly flow aloft will begin ushering
another round of overrunning moisture over the Panhandle. An upper
low currently crossing the 42d parallel well south of the eastern
Aleutians will make bring a connection to subtropical moisture.
And usually with subtropical moisture it brings warmth. With these
overrunning events it becomes imperative to discern whether the
cold air hugging the inside passages will hold firm or whether it
will be supplanted by the warm air mass that begins moving into
the Panhandle at least aloft on Thursday night. At this point,
only the NAM seems to hold the cold air in place. So have begun
the transition to warmer air by raising temperatures for Friday.
Left some snow mixing into the central Panhandle for Friday
morning and Friday night...but feel increasingly confident that
once things turn to rain Friday afternoon, it may stay rain
through Saturday...which is bad news for snow-lovers. However, for
areas north of Juneau, the warm air will not arrive until well
after significant accumulations will fall once again. For Lynn
Canal including Skagway/Haines/White Pass and even for Yakutat
because confidence in warm air advection less confident here at
the surface, left temperatures the same as previous forecast.
Based on ECMWF/GFS...the canal will not get the southerly push
until late Friday night...however, newer runs suggest it may come
Friday evening. But Friday into Friday evening may carry another
punch at least for the northern interior portion of the Panhandle.
Since current winter storm still in force, have not gone with
watch...but next shift may need to seriously consider one.
Snowfall amounts may approach another foot for this region for
Friday and Friday night. However, an earlier southerly push could
temper amounts...especially near shores of Lynn Canal Friday
night. So confidence shakier for amounts Friday night. Based on
gradient have switched wind directions for Stephens, Lynn Canal,
Glacier Bay and northern Chatham earlier in line with this
southerly push. Stephens and northern Chatham should switch Friday
morning...Glacier Bay Friday afternoon...Lynn Canal Friday night.
With this have lightened winds through the transition for
With the tropical connection and warm air advection, many parts
of the Panhandle will see heavy precipitation...mainly in the
form of rain beginning Friday afternoon and lasting into Saturday
as a cold front pushes the precipitation into the Coast Mountains
late Saturday afternoon. Snow mixing in to precipitation for
central Panhandle likely overdone for Thursday night, but did not
want to erase it completely. Think low temperatures Friday night
will be very similar to highs on Friday afternoon. Make no
mistake this looks to be a messy period after so much fresh snow
is still falling with our current system. Behind cold front air
mass becomes unstable and becomes showery. Since Saturday night
late day 3...felt better about keeping chance/likelihood wording
rather than bolder coverage wording. Instability should drive
snow level to the surface even where temperatures are a few
degrees above freezing even into the southern
Panhandle...especially late Saturday night and Sunday.
Beyond the weekend...cold air behind saturday's front...looks to
stay with US through next week...with chances of snow showers for
the north and central and some rain shower chances for the south.
Confidence in forecast average through period. A little better
for southerly push on Friday. Model of choice was European model (ecmwf) with some
GFS. Did use sref along with these models to boost probability of precipitation through
public...Winter Weather Advisory until noon akst today for akz024.
Winter Storm Warning until 4 PM akst this afternoon for akz019-
Winter Storm Warning until noon akst today for akz017-021-022.
Blizzard Warning until 4 PM akst this afternoon for akz018.
Marine...Gale Warning for pkz012-013-022-032-034-036-043.
Small Craft Advisory for pkz011-021-031-033-035-051-053.
Small Craft Advisory due to seas for pkz041-042-052.