Tornado in Washington, March 19, 2021

Area Forecast Discussion
National Weather Service Portland OR
352 PM PDT Fri Mar 19 2021

.SYNOPSIS...The large low pressure system that has been quasi-
stationary off the Oregon coast for a few days will finally come
ashore tonight, bringing more rain, mountain snow, and at least a
few brief thunderstorms. Showers should gradually end by Saturday
night, with a brief break in precipitation Saturday night and most
of Sunday.  Thereafter, the first in a series of northwest flow
waves and associated fronts will impact the Pacific NW beginning
late Sunday through the middle of next week, bringing a return to
cool and showery weather.

&&

.SHORT TERM...Tonight through Sunday night...
Multi-spectral satellite imagery this afternoon shows the large
upper level low that had taken up residence off the Oregon coast for
the past few days is finally on the move eastward.  The main band of
forcing for ascent, accompanied by a strong southerly coastal jet is
currently coming ashore, and this will gradually weaken while
continuing to push inland overnight. This will keep showers and at
least low chances for thunderstorms going through the evening as
colder air aloft works eastward. We haven't seen too much lightning
activity in the past couple of hours (especially compared to
offshore this morning), but as the front, main area of forcing for
ascent, and frontal convergence axis moves inland, the environment
still warrants at least a slight chance mention eastward to the
Cascade foothills through this evening.

Snow levels around 3000 feet currently, will lower further to the
2200-2600 foot range overnight as the 700-500mb cold pool moves
overhead. Snow accumulations are a bit tricky as in most areas we
believe that we will have a hard time reaching advisory thresholds,
but some of the more favored orographic locations are likely to see
a 6-9 hour period of frequent moderate to heavy snow showers, and
this could result in the snow adding up rather quickly later tonight
and Saturday morning in a few localized mountain areas.  For now, we
will hold off, but the evening/mid shifts will need to monitor short
term trends closely for the need for a Winter Weather Advisory
across the parts of the Cascades.

Shortwave ridging then builds briefly over the area beginning late
Saturday into most of Sunday, which should bring a brief break in
the precipitation.  However, the next wave/front will be rapidly
diving southeastward along the British Columbia coast on Sunday, and
will spread increasing chances for rain from northwest to southeast
beginning Sunday afternoon, with PoPs ramping up into likely or
greater by Sunday night across at least the northern half of the
Portland CWA.
 Miller

.LONG TERM...Monday through Thursday...
Medium range deterministic and ensemble guidance suggests that much
of next week will have the Pacific NW under cool, moist, and
conditionally unstable onshore west-northwest flow, along with a
series of waves riding southeastward (the first being the
aforementioned one Sunday night/Monday). This will likely keep at
least some chance of precip going Monday-Thursday, especially in the
mountains (both Coast Range and Cascades), but it is likely that
there will be some short relative minimums in precip coverage in
between individual waves/fronts.  If the current guidance is to be
believed, one of those periods may be later Monday night/Tuesday,
but it should also be noted that the confidence on specific timing
this far out in such a flow regime should be interpreted with some
caution. Finally, there is some indication in the longer range
ensemble guidance that ridging over the eastern Pacific may shift a
bit eastward over the Pacific Northwest late next week right near or
just after the current 7-day official forecast window. If this
verifies, the results could be a more sustained period of drier and
eventually warmer weather beginning toward the end of next week.
 Miller

&&

.AVIATION...Covering the 00Z TAF issuance...
Low pressure offshore will move onshore near Vancouver Island late
tonight. Moist unstable flow aloft will continue to bring showers
and chances of thunderstorms across the region until late this
evening. Hail and gusts near 35 kt are likely with passing cells.
Expect predominantly VFR conditions with pockets of MVFR with
passing showers. As the upper low shifts inland over the Olympic
Peninsula tonight, will see upper flow turn more westerly. As the
upper level trough moves onshore this should provide enough lift
produce widespread MVFR ceilings and light rain across the region.

For detailed regional Pac NW aviation weather information, go online
to: https://weather.gov/zse

KPDX AND APPROACHES...VFR conditions will continue into tonight with
chances of thunderstorms increasing this afternoon, to early evening
period. Wind gusts near 35 kt with small hail are likely with
passing cells. MVFR conditions are likely early Saturday morning
with light rain. -BPhillips

&&

.MARINE...Broad area of low pressure will make its way onshore
near Vancouver Island late this evening. Buoy observations as of 3
PM this afternoon continue to show wind gusts between 30 to 35 kt
with localized areas reaching 40 to 45 kt. Gale gusts are expected
to drop below 34 kt this evening and the Gale Warning will be
downgraded to a SCA across all coastal waters.  Thunderstorms across
the waters are expected to dissipate around midnight. Wind waves
will drop off this evening and the dominant driver will be the
freshly generated swell that will continue to trend around 12 to 16
ft. Winds will turn westerly overnight with the cold front arriving
early Saturday morning. Wave heights will be slower to respond and
will likely stay slightly above 10 ft through most of the day
Saturday.

The next front will drop across the waters from the NW Sunday
afternoon. The parent low is well to the north of the area and winds
will likely stay below Gale Warning criteria. Active weather will
continue into next week, with a front expected later Mon and another
on Wed. -BPhillips

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Gale Warning until 6 PM PDT this evening for coastal waters
from Cape Shoalwater WA to Florence OR out 60 NM.

     Small Craft Advisory until noon PDT Saturday for Columbia River
     Bar.

&&

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