Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
302 PM akst sun Dec 28 2014
Short term...strong high pressure in excess of 1050mb over the
Yukon is causing a cold and clear/northerly outflow weather
pattern. This strong of a high would normally cause very strong
outflow winds over the Panhandle...but observations confirm that
they are a bit weaker than one would expect for this time of year.
Think that the limited snow pack could be a contributing factor.
Despite this there are still gale force winds in northern Lynn
Canal and 40-50kt winds expected out of interior passes.
The Yukon high will gradually shift south into central b.C.
Through tonight and Monday and cause the strongest pressure
packing to shift along the coastal mountains over the southern half
of the Panhandle. For this reason have included a strong wind
headline for wind gusts increasing to 40mph near Portland Canal
tonight through Monday. The northern Panhandle will see winds
gradually decrease with the change in gradient orientation.
Outflow winds and a ridge of high pressure aloft are causing dry
conditions and mostly clear skies. Some high clouds are streaming
over the ridge top from a front moving into the western Gulf...but
do not expect those clouds to make much progress through the short
term period. This will allow for good radiational cooling
tonight...especially for wind sheltered locations. Therefore
expect low temperatures to be a few degrees colder than they were
last night. Locations that have the gusty outflow winds will be
about 10 degrees warmer than the calm areas. Tomorrow will be
mostly sunny again but temperatures wont be able to warm quite as
high after a colder start. Those locations over the northern
Panhandle that lose the wind will also be a little cooler without
the mixing/downslope affects.
Updated the pressure fields with the 12z NAM for its good
initialization. This shifted the high in Canada south slightly
faster and increased the pressure gradient through Portland Canal a
bit. Forecaster confidence is high for the dry and mostly clear
conditions. Confidence is slightly lower for temperatures due to the vast
difference that can occur between calm and breezy locations.
Long term...strong upper ridge will reach peak amplitude over Southeast
Alaska Tuesday morning...then be slowly flattened by shortwave energy
moving through the ridge top for later Tuesday into midweek. This will
allow weakening occluded front to move slowly east into the
area...eventually moving through the area by late Wednesday. Things get
complicated for late week as models struggle handling energy in
split streams. After coordinate with weather forecast office pafc...ended up using latest
wpc for Thursday on out.
For Monday night...did slow down the increase in clouds as upper
ridge will hold off all but the highest level clouds...which will
be on the thin side. Did not change temperatures though as radiative
cooling might be slowed enough by what high clouds do make it in.
For Tue-Wed...clouds should come in then precipitation will follow by Tuesday
night. Currently think precipitation may be snow at the start over the
northern and eastern Inner Channels where some cold air will remain...but
precipitation should become mainly rain Wednesday. Precipitation amounts should be
highest over the northwest third of the area...but not expecting it to be
too heavy. Precipitation should diminish from the northwest Wednesday into Wednesday evening
as front moves past the area.
The Thursday-sun period gets very tricky due to complex split flow and
interactions between shortwaves in both streams. Wpc was a middle
ground compromise on this so went with it at this point. Basically
have chance probability of precipitation for much of the area during this time. Needless
to say...forecast confidence in this time frame is very low.
public...strong wind until 6 PM akst this evening for akz025.
Strong wind from 6 PM akst this evening through Monday afternoon
Marine...Gale Warning for pkz012-052.
Small Craft Advisory for pkz011-013-021-022-031-032-034>036-
Visit US at http://www.Weather.Gov/Juneau