FXUS66 KSEW 152157

Area Forecast Discussion
National Weather Service Seattle WA
257 PM PDT Mon Jul 15 2024

.SYNOPSIS...High pressure is expected to remain in the region for
the majority of the week. A thermal trough will build along the
coast, and will cause temperatures to increase Tuesday and
Wednesday into the upper 80s and low 90s. With instability in the
Cascades via a small upper level trough Tuesday into Wednesday,
there will be a slight chance of an isolated thunderstorm for any
elevated convection that takes place.


.SHORT TERM /TODAY THROUGH WEDNESDAY/...Satellite imagery this
afternoon shows the majority of the marine push retreating to
along the coast (with some stratus remaining west of the
Washington coastline). A small upper level ridge remains over
Washington this afternoon. High temperatures today took a penalty
due to this marine push, with most areas just now getting into the
70s this afternoon. Most places will see highs top out into the
mid and upper 70s (could see an 80 in the Southwest Interior but
the chances have decreased with the early morning marine push).

By Tuesday, a thermal trough will begin to build along the coast.
This will help bring warm air down to the surface from aloft with
southwest flow behind the ridge sliding into Idaho/Montana
Tuesday. With relatively light winds, and no marine push for
Tuesday morning, this will cause high temperatures to sear into
the upper 80s and low 90s across the region. The highest
temperatures will be in the Southwest Interior. Heat risk will
remain moderate (orange) for the majority of lowland areas, with a
couple isolated areas in the Seattle metro seeing major (red)
level heatrisk. Given the short duration of this heatwave
(compared to the one a week ago) and low temperatures expected to
drop into the 50s (with Tuesday night only being the best chance
of lows in the low 60s) - will not be issuing any heat headlines
at this time (although all persons outdoors should continue to
practice heat safety, such as taking breaks in air conditioning,
and staying hydrated).

There`s an additional shortwave trough/mid- level low tracking
into Oregon and Washington Tuesday night into Wednesday. This will
create some instability over the Cascades late Tuesday night into
Wednesday. There`s a conditional 15% chance of an isolated
thunderstorm for any convection that is able to initiate along
this corridor. Coverage again is expected to remain isolated, and
precipitation is expected to be light with any pulse storms that
do develop.

.LONG TERM /THURSDAY THROUGH SUNDAY/...The pattern largely remains
warm and dry with high pressure through the weekend (although some
ensembles show the potential for troughing northwest of the state
over B.C. Canada). Nevertheless the region will continue to see
high temperatures in the 70s and 80s, with another shot of 90 by
Saturday with heights building overhead via upper level ridging.
Low temperatures will still drop into the 50s during this dry



.AVIATION...High pressure over the NW brings dry and stable
weather through Tuesday. The flow aloft is light and variable. The
low level air mass remains moist and stratus clouds will impact
the coast overnight for IFR to MVFR conditions. The strait and
south sound may see patchy low clouds by 12z for brief IFR to MVFR
conditions as well. Low clouds will burn back to the coast by the
afternoon for VFR conditions. 33

KSEA...Northerly winds around 5 kt with VFR conditions. 33


.MARINE...High pressure over the NE Pacific will maintain onshore
flow across western WA this week. A stronger onshore push may
bring Small Craft Advisory winds to the strait on Wednesday and
again on Thursday. 33


.FIRE WEATHER...High pressure will persist over the Pacific
Northwest for continued warm and dry conditions, with low level onshore
flow providing good relative humidity recoveries through majority
of the week.

There is a main period of concern, Tuesday night into Wednesday,
for some southerly flow to provide ample instability for
thunderstorm development across the Cascades. Given the dry nature
of the fuels and the threat for lightning being present, went
ahead and issued a Fire Weather Watch for zones 658 and 659.
Relative humidity values will be in the mid 30s through the day,
but cannot rule out extremely localized pockets of 25-30.

It is worth noting that especially with a pattern like this, with
our fuels already being so receptive, all it takes is a rogue
lightning strike from a thunderstorm to light these fuels.





NWS SEW Office Area Forecast Discussion

Discussion Script by Ken True at Saratoga Weather - WFO Script by SE Lincoln Weather