Scientific Forecaster Discussion
fxus66 ksew 170334
Area forecast discussion
National Weather Service Seattle Washington
834 PM PDT Mon Sep 16 2019
Synopsis...showers will diminish this evening. A frontal system
will bring rain and locally windy conditions later tonight and
Tuesday. Showery weather will follow the front into Wednesday with
an upper trough over the area. Drier weather is likely Thursday
through Saturday with high pressure aloft in control. Wet weather
will return Saturday night as a another front reaches the area.
Short term /tonight through Thursday/...radar shows showers
becoming more isolated this evening as the air mass stabilizes
with the loss of daytime heating. The lightning threat appears to
be about over as well.
A front will spread rain onto the coast after midnight tonight
and then into the interior Tuesday morning. There will also be
some windy conditions with the front but not enough for any
headlines. Rain will change to showers Tuesday behind the front.
An upper trough will maintain cool and showery weather into
Wednesday. It will be a bit unstable each afternoon and an
isolated thunderstorm cannot be ruled out.
Weak high pressure aloft will build in from the west on Thursday.
This will bring drier and more stable air with perhaps some
afternoon sunbreaks. Schneider
Long term /Friday through Monday/...previous discussion...despite
the weak ridge in place going into the long term, there appears
to be a subtle shortwave traverses thru its flow. The energy
associated with this shortwave appears to be sufficient for the
development of a few showers thru the day. This event will be
nowhere near as widespread as the previous several systems, but
could be enough to be a nuisance for outdoor plans. Guidance then
suggests another semi-organized frontal system late Saturday into
Sunday which could yield another widespread stratiform rain event.
A ridge may then build into the region in the wake of this system
for Monday, which may allow for a dry day.
Aviation...an upper trough over the area will shift inland
tonight. A frontal system will move through the area on Tuesday.
The flow aloft will be west to southwest. The air mass is moist
and unstable. It will become stable later tonight and then
unstable again Tuesday afternoon.
Mid and high clouds this evening. Ceilings will gradually lower
tonight but remain mainly VFR. Ceilings 2000-3000 feet should
become more widespread Tuesday morning as a front moves onshore.
Ceilings will probably be lower near the coast. Visibilities will
also lower to 3-5 miles at times in rain. Gusty southerly winds
are expected for parts of the area on Tuesday.
Ksea...mid clouds will gradually lower tonight. Several hours of
ceilings 1500-2500 feet are likely on Tuesday in rain as a front
moves through. South wind 4-8 knots will increase to 10-20 knots
and become gusty around midday Tuesday. Schneider
Marine...a frontal system will bring southerly gales to the
coastal waters, East Entrance Strait of Juan de Fuca, and
northern inland waters later tonight and Tuesday morning. Small
Craft Advisory strength southerly winds are likely most other
waters with the front.
Winds will subside later Tuesday and Tuesday night behind the
front. Onshore flow will prevail Wednesday through Friday with
high pressure offshore and lower pressure inland. A second frontal
system will approach from the west on Saturday. Schneider
Hydrology...the daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
Pz...Gale Warning from midnight tonight to 5 PM PDT Tuesday for
coastal waters from Cape Flattery to James Island 10 to 60
nm-coastal waters from Cape Flattery to James Island out 10
nm-coastal waters from James Island to Point Grenville 10
to 60 nm-coastal waters from James Island to Point
Grenville out 10 nm-coastal waters from Point Grenville to
Cape Shoalwater 10 to 60 nm-coastal waters from Point
Grenville to Cape Shoalwater out 10 nm.
Gale Warning from 5 am to 5 PM PDT Tuesday for East Entrance
U.S. Waters Strait of Juan de Fuca-northern inland waters
including the San Juan Islands.
Small Craft Advisory from 5 am to 5 PM PDT Tuesday for Admiralty
Inlet-central U.S. Waters Strait of Juan de Fuca-Puget
Sound and Hood Canal-West Entrance U.S. Waters Strait of
Juan de Fuca.