Tornado in Oregon, June 13, 2020

Area Forecast Discussion
National Weather Service Portland OR
250 PM PDT Sat Jun 13 2020

.SYNOPSIS...Showers will decrease tonight and be mostly limited to
the higher terrain on Sunday. A cold front will produce rain Monday
with a couple of shortwave troughs maintaining showers through Tuesday.
Isolated thunderstorms are possible Monday and Tuesday afternoons. An
upper level ridge builds Wednesday through Friday for dry weather and
gradually warming temperatures.
&&

.SHORT TERM...Tonight through Monday night...Water vapor imagery
shows an upper low around 150 miles offshore of the Washington coast
this afternoon. This feature is the main contributer to the showers
being observed across Southwest Washington and Northwest Oregon.
Instability will increase late in the afternoon into the early
evening as the upper low moves inland near the Washington/Oregon
border.

Skies have remained overcast this afternoon limiting the surface
heating, and the threat for surface based  instability to get
thunderstorms going. Am therefore less confident in the possibility
of thunderstorms for this afternoon, and have scaled back the areal
coverage of the slight chance for thunderstorms to SW Washington
where the greatest atmospheric instability is.

Showers will decrease tonight as the upper trough axis moves further
east, and be limited on Sunday as a shortwave upper ridge moves over
the pacific NW. Onshore flow will favor orographic showers on SUnday,
and think that the showers will be fairly limited to the higher
terrain. Partial sunshine and a slightly warmer airmass will allow
for warmer temperatures on Sunday, but do not think many if any
afternoon temperatures will make it into the 70s. The best chance for
temperatures peaking in the lower 70s are the interior valleys of
Lane County.

A front will return rain Sunday night and Monday. Modeled
precipitable water shows plenty of moisture available with this
system and is forecasting precipitable water values near an inch
Monday morning. The position of an upper level jet streak favors lift
across NW Oregon Sunday night into Monday morning. The combination of
the available moisture and the lift increases confidence in
widespread measurable rain for Monday where some areas in the
Cascades and coast range may measure up to an inch or slightly more
than an inch by Monday night. Mondays temperatures will be cooler
with afternoon temperatures likely in the lower 60s.

Model instability indices have backed off a bit for Monday afternoon
but an isolated thunderstorm is not out of the question as an upper
level short wave moves over the region Monday afternoon and evening.
This shortwave will maintain showers through Monday night. ~TJ

.LONG TERM...Tuesday through Friday night...An upper level trough
moves over the region Tuesday maintaining showers. Models suggest that
the trough axis will move inland in the afternoon increasing
instability. This instability will enhance showers and possibly
trigger isolated thunderstorms Tuesday afternoon. A change to a
warmer and drier pattern is expected late in the week as an upper
ridge is expected to build Wednesday through Friday. This will keep
the storm track well north of the region. A thermally induced surface
trough will build into NW Oregon and SW Washington on Friday for
temperatures possibly reaching into the 80s. ~TJ

&&

.AVIATION...Low pressure centered near Vancouver Island will
continue moving southeast across western Washington and
northwest Oregon over the next 12 to 18 hours. Current radar
observations this afternoon show numerous showers across the
area, especially for locations north of KSLE. Some of these
showers are resulting in brief heavy downpours. Although
lightning has not been observed with these showers as of 230 PM
PDT, an isolated thunderstorm or two cannot be ruled out later
this afternoon and evening. With that said, the chances are very
slim and confidence is not high enough to include a mention of
thunder early in the 00z TAF cycle. Regardless of the
thunderstorm potential, strong showers with heavy downpours and
small hail are possible will be fair game through 03z. Brief
visibility reductions are expected with stronger showers or
thunderstorms, along with erratic changes in the wind direction.

Stronger convection will ease by 04Z, with leftover showers
gradually decreasing from west to east tonight. MVFR cigs will
become increasingly likely this evening into tonight with onshore
flow. Cigs should begin to improve back to VFR Sunday afternoon
as some breaks of sun develop ahead of an approaching warm front
that is set to move through the area late Sunday night through Monday
morning. -TK

KPDX AND APPROACHES...Primarily VFR, aside from brief periods of
MVFR cigs and reduced visibilities under heavier showers or
thunderstorms for the remainder of this afternoon and evening.
The most favorable time for heavier showers or thunderstorms will
be between 22z and 03z. -TK

&&

.MARINE...Expect seas to remain in the 4 to 5 ft range tonight
through Sunday afternoon, before a warm front moves across the
waters Sunday night into Monday morning and brings increasing
winds. The 12z GFS/NAM/EURO deterministic runs agree well in
regards to the timing of this front, which should make it to the
coast by 15z Monday morning. A brief period of gusty winds is
expected with this frontal passage, with 15 to 20 kt southerly
winds gusting to 25 kt Sunday night into Monday morning. Although
combined seas will only rise to 6 or 7 ft during this time, it
appears low-end small craft winds will occur for at least a
brief period of time.

Winds will then decrease Monday night and Tuesday with seas
remaining around 7 ft. On Wednesday, current guidance depicts a
thermally induced surface pressure trough building up the
California coast into southern Oregon. This pressure trough will
then continue building northward towards the southern Washington
coast on Thursday and Friday. This will result in a tightening
pressure gradient over the waters with increasing northerly
winds, especially over the southern waters. Expect the gusty
northerly winds to be most prominent during the afternoon and
evening hours on Wednesday, Thursday, and Friday. Although the
latest blend of model guidance keeps sustained winds around 15 kt
during that time, BUFR soundings and the overall surface weather
pattern suggests winds closer to 20 kt. Even stronger winds may
be possible over the southern waters, where small craft winds
will be most likely to occur. -TK

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...None.
&&

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