Hail in Oregon, March 7, 2021

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
852 AM PST Sun Mar 7 2021

...Updated aviation and marine discussions...

.SYNOPSIS...Low pressure dropping south offshore will keep a threat
of showers over the region through the early part of the week. A
cold front moving east will bring a few hours of stratiform rain
with mountain snow early this morning. Upper level high pressure is
expected to bring drier weather for Wednesday through early
Friday, and possibly through the weekend.

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.SHORT TERM...Today through Tuesday Night...Not much has changed in
the large scale of the pattern over the last 48-72 hours. A longwave
trough has settled over the NW Pacific with an embedded closed low
centered a few hundred miles off the northern tip of Vancouver
Island. The low is sending a cold front across the PacNW at this
hour which is roughly aligned with I-5. Have received a brief period
with gusts 25-30 mph across the board, including the coast.
Precipitation amounts are hardly noteworthy as well. Generally 0.20-
0.30" over the last 6 hours along the coast where most of the rain
has fallen behind the front. Expect a similar story to play out as
the front continues moving east this morning. The Cascades may pick
up a couple inches of new snow or so ending around 9 AM or so.

Behind the front, satellite and radar show plenty of shower
potential to come onshore given an approaching cooler air mass. The
upper low track is still slated to drop south over the next few days
with the low center staying beyond 130W until Tuesday morning. That
will place the CWA under varying amounts of diffluent flow aloft
atop unstable lower levels until then. Today will also see the
addition of the jet stream aloft to provide a little extra lift such
that thunderstorms seem plausible, especially for portions of the
region north of a McMinnville to Portland Metro area line. WOuld
really like to see a couple hours of decent solar heating to hit the
ground to really trigger the deeper convection required for
thunderstorms today and that may be a little hard to sustain given
the overall unstable environment from the advancing trough plus
ample low level moisture. Still have minimal concern for funnel
cloud development, especially along the lower Columbia where terrain
forced SE surface winds will provide a little more rotational
potential against the SW winds aloft. Elsewhere, there will still be
potential to see some stronger showers, possibly embedded with small
hail. In both cases, the stronger convection will ease close to
sunset around 6 PM this evening leaving residual showers to continue
this evening before they pull west toward the coast overnight.

Monday, a shortwave rounding the base of the trough will provide
additional instability to our area. Expect showers will develop with
daytime heating as the trigger. Meanwhile, the best instability will
then have moved into SW Oregon to keep the thunderstorm threat to
our south. Showers should again decrease in coverage near sunset.
Tuesday, will effectively be a near repeat of Monday, but now with
the upper low center coming inside of 130W putting us under less
diffluent flow aloft but offset by being closer to the cold core
aloft.

Snow levels through this period remain relatively low. Some snow may
be mixed down to around 1000 feet or so under heavier showers, but
should not accumulate for very long, if at all. The primary
accumulation potential will stay closer to 2000 feet and above. Each
day could locally bring a couple of inches of snow to the Cascades
and higher Coast Range but do not see widespread impacts. /JBonk

.LONG TERM...Wednesday through Saturday...Still have very good model
agreement through early Friday showing the region remaining mild and
dry as an upper ridge noses east and north of the upper low
departing to our south. Near normal temperatures will likely prevail.

Friday, there is general model agreement that another long wave
trough will drop south from Alaska. There is notable disagreement,
however, about how far south and east this trough will spread. About
2/3rds of possible model and ensemble solutions push the energy well
south and west of the PacNW to form a cut off low well west of
Northern California. This solution would keep the PacNW dry through
the weekend. The other plausible solution shown by many of the
remaining model outputs, retain most of the energy within the parent
trough and begin bringing it across the PacNW with rain arriving at
the coast Friday afternoon. Will maintain PoPs in the forecast given
the non-trivial number of scenarios that bear this out in some form.
/JBonk

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.AVIATION...In the post frontal environment, radar is showing
the developing showers over most of the area. Most locations will
at lease see rain showers inland and snow showers over the
Cascades, but some pea sized hail is also being reported. Mainly
VFR flight conditions should prevail through the evening, aside
from brief periods of MVFR cigs possible with passing showers.
Showers will increase in coverage into the afternoon, with a
slight chance of weak thunderstorms. Any shower or storm that
does develop this afternoon will have the potential to produce
reduced visibilities, small hail, and wind gusts between 20-30
kt.

Weak high pressure moves late tonight after 05Z Monday, bringing
nearly calm winds and clearing skies. This will bring the
potential for some patchy fog after 10Z Monday - especially for
the interior lowlands.

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

KPDX AND APPROACHES...Mainly VFR through through this evening
aside from brief periods of MVFR cigs with passing showers or
thunderstorms. Any shower or storm this afternoon will have the
potential to bring small hail, brief gusty winds between 20-30
kt, and lowered visibilities. Confidence regarding timing of
thunderstorm activity is low so did not include in TAFs, however
the most prone time will be between 20Z Sunday through 04Z
Monday. There is also the potential for fog to develop at the
terminal after 10Z Monday.   -Muessle/TK

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.MARINE...Westerly swell be on the rise today, with model
guidance still suggesting a peak around 18 ft this evening and
dominant wave periods increasing to 14 to 16 seconds. This falls
just shy of meeting the thresholds for a High Surf Advisory.
Admittedly, there is a small chance that seas could end up closer
to 19-20 ft, which would just barely meet high surf criteria.
Since the most likely outcome is seas closer to 17-18 ft, will
not be issuing a High Surf Advisory for this event. Regardless of
the outcome, there will be a high threat for sneaker waves along
the coastline. Will note that there is a slight chance for
thunderstorms this afternoon through the evening over the
northern waters. Small hail and erratic gusty winds are possible
with any forming showers or storms.

Expect seas to decrease on Monday, falling to 10-13 ft by Monday
evening. Southerly winds increase a bit Monday night, possibly
gusting between 30-35 kt over the central waters and 25-30 kt
over the northern waters. Winds should begin to ease again on
Tuesday and eventually turn offshore Tuesday night. Rather
benign conditions expected over the coastal waters Wednesday
through Friday as high pressure settles in and brings dry
weather, lighter winds, and calmer seas. In fact, the current
forecast during that time has winds and seas that are below small
craft criteria.   -Muessle/TK

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.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until 4 PM PST Monday for coastal waters
     from Cape Shoalwater WA to Florence OR out 60 NM.

     Small Craft Advisory from 9 AM this morning to 4 PM PST Monday
     for Columbia River Bar.

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PORTLAND OR
1033 AM PST SUN MAR 7 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1030 AM     HAIL             2 WSW ASTORIA           46.18N 123.85W
03/07/2021  M0.50 INCH       CLATSOP            OR   NWS EMPLOYEE

            NEAR ASTORIA HIGH SCHOOL.

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