Severe Thunderstorm Warning 14, NWS Portland, OR

Area Forecast Discussion
National Weather Service Portland OR
941 AM PDT Thu Sep  7 2017

.SYNOPSIS...An upper level trough moving up from California will
continue to generate shower and thunderstorms across the area
today. A few showers may continue into Friday. Onshore flow will
bring cooler temps through the end of the week and into the weekend,
and also help to gradually clear out the smoke. A weak front will
bring another chance for some weak showers across the northern half
of our area on Saturday. An upper level ridge builds back up late
this weekend, likely bringing back warmer temps and smoke through
early next week.

&&

.SHORT TERM...Today through Saturday...A band of weak showers is
moving northward through the CWA this morning. The line is currently
extending from around Santiam Pass over through Lincoln City. Impacts
from this round of showers has been minimal, with precipitation
generally only totaling a trace to a couple hundredths of an inch at
most, and it should only weaken further as it continues to lift
through the northern half of the CWA during the late morning and
early afternoon hours.

The next, and likely much more significant round of showers and
thunderstorms will move into the region from the south during the
late afternoon and early evening hours. This will be associated with
forcing ahead of the shortwave off the northern California coast as
it lifts northward through the region. NWS doppler radar mosaic shows
some pretty vigorous convective activity right ahead of the shortwave
now in far SW Oregon right, which gives us confidence that things
will indeed pick up over our CWA later. Also boosting confidence is
the latest run of the SPC Storm-Scale Ensemble of Opportunity, in
which most of the models have an area of greater than 40 dBZ radar
echoes moving through the southeastern quadrant of our CWA, roughtly
along a line from Eugene up to just south of Mt Hood. These areas
look to have a pretty good chance of seeing thunderstorms, with
chances becoming less as you get north and west of this line. Given
the synoptic pattern, cannot rule out a thunderstorm at any location
at some point later today. Precipitable water values remain high,
with the 12Z KSLE sounding coming in with 1.43 inches. We are also
gradually moistening the lower levels of the atmosphere, so expect a
few heavier showers and thunderstorms, with the highest precip totals
over the Lane County Cascades and foothills.

Showers may linger into early Fri, but will be much more isolated.
PoPs have been removed all together after 00Z Sat. A dying front will
approach the area on Saturday which may bring some rain to the north
coast of Oregon and SW Coast of Washington, but will likely only drop
a few hundredths in a few areas inland. Pyle/Bentley

.LONG TERM...Saturday night through Wednesday...Confidence is high
that an upper ridge will return by the end of the weekend and into
early next week bringing a return to much above normal temperatures
and drier weather. However, uncertainty is much higher about whether
smoke returns to the area and how long the ridge persists. Current
thinking is that the ridge will stay much longer than currently
modeled considering the upper low off the coast of California
returns. Depending on the model you look at, this low will set up as
a rex block, or take an appearance more of an Omega Block combined
with the remnants of Irma in the east. Either way, a return to a
blocking pattern across the United States next week will likely keep
our area dry and warmer than normal for the foreseeable future.
/Bentley

&&

.AVIATION...No changes...Previous discussion follows: Smoke from
wildfires across the region will continue to bring periods of
reduced vsbys and occasional cigs. Conditions will slowly improve
tonight as onshore flow strengthens. Expect predominately VFR
cigs and periods of MVFR vsbys in the interior due to smoke.
Areas near the Cascades and Columbia River Gorge could see
periods of IFR conditions and mountain obscuration. Expect
IFR/LIFR conditions to persist along the coast due to fog/low
stratus, but brief improvements to MVFR, or even VFR, is possible
along the north coast this afternoon. Coastal stratus may filter
inland through the gaps and bring a period of MVFR cigs to the
interior this morning, but confidence is low. Showers will
continue to spread across the forecast area from the south, with
a slight chance of thunderstorms through this evening.

KPDX AND APPROACHES...Smoke will continue to bring periods of
MVFR vsbys, but conditions should improve tonight as onshore
flow strengthens. Cigs remain predominately VFR, but a brief
period of MVFR cigs are possible this morning as coastal stratus
filter inland. Showers will spread up from the south today and
may impact the terminal after 20Z. There is a slight chance of
thunderstorms through this evening. /64

&&

.MARINE...No changes...Previous discussion follows: A weak front
will move across the waters today, then dissipate tonight as it
moves onshore. Winds will generally be out of the south this
morning, but will become west to northwest behind the front this
afternoon. High pres will briefly build over the waters Fri and
maintain northwesterly winds, but gusts will remain below
advisory criteria. Another weak front will move across the waters
Sat and could bring some marginal southerly winds to 20 kt. High
pres rebuilds across the waters late in the weekend and early
next week, with thermal low pres strengthening over northern
California and southern Oregon. This will bring increasing
northerly winds, with an extended period of advisory winds likely
Sun through Tue.

Seas will generally be around 5 to 6 ft for the next several
days, but could build to around 7 to 8 ft early next week as
northerly winds increase. The good news is that a long period
westerly swell should prevent seas from becoming square. /64

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning until 11 PM PDT this evening for Central
     Oregon Cascade Foothills-Central Oregon Coast-East Slopes
     of the Central Oregon Coast Range-Mt. Hood National Forest
     West of Cascade Crest-North Oregon Cascade Foothills-North
     Oregon Coast-North Oregon Coast Range-Willamette National
     Forest-Willamette Valley.

WA...Red Flag Warning until 11 PM PDT this evening for Clark County
     Lowlands-East Willapa Hills-Eastern Gifford Pinchot
     National Forest Mt Adams Ranger District-Extreme South
     Washington Cascades and Foothills-South Washington Coast
     and West Willapa Hills.

PZ...None.
&&

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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. This area is
commonly referred to as the forecast area.

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