Scientific Forecaster Discussion
fxak67 pajk 201356
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
456 am akst Sat Jan 20 2018
Short term...a low to the south of haida gwaii will travel
northwest and dissipate by Saturday evening. Another low is
tracking across the Gulf along 50 north. Behind this system, a third
low begins to drop down from the Mainland into the western Gulf
late tomorrow into early Monday. Overall, we are looking at a
very active pattern into next week.
Run-to-run model consistency has not changed much with regards to
the location and strength of the next two systems, in which minor
modifications to the pressure pattern were based on the current
NAM. With the tightening gradient over the northern Panhandle,
winds will be gusty in Skagway throughout the day and northern
Lynn increasing from Small Craft Advisory to gale force status. As
a result, some light freezing spray may exist along the northern
Inner Channels into tonight. Light winds and partial clearing
overnight has allowed for some patchy fog to develop across the
central and north panhanlde this morning.
Pop's were adjusted using a blend of the NAM/GFS early on and some
sref/NAM into the tonight and tomorrow periods, which increased
the pop coverage over the northern Panhandle into tomorrow. Pop's
will decrease throughout the day today as the strengthening outflow
pattern will keep most precipitation farther south. As winds
switch over to more of an onshore pattern, this will increase rain
and snow showers over the southern Panhandle tomorrow night. Snow
totals over the Panhandle by tomorrow night range from 1-2", with
higher snowfall amounts into Monday.
Local edits were made to temperature, either increasing or
decreasing maximum temperatures in certain areas. Most maximum and
minimum temperatures were well within the model spectrum spreads
with a few exceptions. For one, temperatures in Skagway were
needing to be increased due to downsloping and temps over the
Inner Channels increased as well. Although it will be a colder
night, minimum temperatures for tomorrow night were increased due
to cloud cover. Forecast confidence remains average to above
average, although some uncertainty remains with the strength and
timing of the next system.
Long term.../Monday through Saturday as of 10 PM Friday/
still some important model differences to resolve with handling of
energy in base of upper trof over the Gulf and epac for next week.
Ended up using wpc for the most part as it was based on ensemble
means and operational model blend. This shifted some of the
systems a bit further S especially for around Thu. Does appear
that a couple of fronts will move NE across the area for Mon-Wed,
so kept in higher pops for those days. Pops were nudged down a
bit for Wed-Thu especially north area due to southward shift of main
low S of the area. Another trof will dig into the west Gulf for late
week and the weekend and this would help increase precip threat
again as flow aloft becomes more S-SW.
Used GFS/ec blend to handle the 24 hr pre-wpc period for system
moving northwest through the East Gulf during Mon. Still looks like mainly
snow over the north half of the area, but ptype will be trickier over
the S as some milder air may come in ahead of the system. Will
likely be rain over the far S and outer coast from pasi
southward for Mon. Once this system moves past, colder air
wrapping around the upper trof will move in, and much of the S area
will become cold enough for at least a rain/snow mix, with the north
still seeing snow. Still a decent amount of uncertainty on how
much snow would fall over the north as this will likely depend on how
much cold air is in place and the direction of the 850-700 mb
flow. Too much of an east wind component in that layer would
diminish precip on west side of coast mtns, but would enhance precip
along east side of Baranof and chichagof islands.
For Tue into late week, next front will likely bring more precip
to the area Tue, with mostly snow north and mix or snow S. Latest
models hint at a low moving into the far East Gulf for Wed. This
system looks like it would try to warm things some especially over
the S so that precip there would trend more toward rain. After
Wed the model differences increase so forecast confidence
Did not change temps for long range. Still some model differences
on how much cold air moves in from the north and from the SW. Some
operational model MOS guidance suggested temps might be a few
degrees warmer, but ensemble MOS spread remains on the high side
so felt staying with current forecast was best thing to do.
Still looks like some outflow winds will occur but not expecting
more than gale force winds due to lack of a strong high over the
Yukon. The various fronts/lows moving in will probably have 25-35
knots winds with them over the Gulf as well.
public...strong wind until 6 am akst for akz018.
Marine...Gale Warning for pkz012.
Small Craft Advisory for pkz013-022-031-035-041>043.
Small Craft Advisory due to seas for pkz051-052.