Hail in Oregon, January 18, 2018

Area Forecast Discussion
National Weather Service Portland OR
852 AM PST Thu Jan 18 2018

.SYNOPSIS...A deep low pressure system off the central BC coast this
morning will weaken today and tonight, while keeping a moist and
unstable onshore flow over NW Oregon and SW Washington. This system
is sending large surf toward the coast, which will combine with high
tides for some coastal flooding in low lying areas today. Snow
levels will lower below the Cascade passes today, then likely remain
below pass level for the next several days. Cool onshore flow will
cause showers to continue through Saturday, then the next front will
bring steadier rain and mountain snow Sunday. Active weather looks
to persist well into next week.

&&

.SHORT TERM...Today through Saturday...The strong surface low that
has brought the high seas to the coast, was stalled off the central
BC coast this morning, and will weaken slowy today and tonight. The
parent upper level trough axis off the coast will slowly eadge
towards the coast, bringing cooler temps aloft which will act to
further destabilize the atmosphere. as the trough approaches though,
the best dynamics will tend to shift south of the forecast area. The
cooler air aloft will result in continued showers today and tonight,
but also a small chance for thunderstorms due to increased
instability with the cooler air aloft.

Remainder of short term discussion unchanged...With freezing levels
lowering, any heavy showers or thunderstorms have the potential to
produce small hail; we have already had spotter reports of BB to pea
sized hail from the thunderstorms along the coast early this
morning. If enough sunbreaks occur to trigger strong updrafts at the
surface, the low-level shear profile appears favorable for some
longevity to any cells that develop, and a couple stronger cells
with slightly larger hail, gusty winds, and perhaps even a funnel
cloud cannot be ruled out today.

Cool onshore flow will cause showers to persist through Saturday,
with a shortwave providing some enhancement Fri night/Sat morning.
The risk of thunder looks less after tonight as temps warm slightly
aloft. Onshore flow will eventually weaken Saturday as our next
system approaches from the Pacific. This should result in a break in
the showers sometime late Saturday, but any break will probably be
brief with the next round of steady rain and Cascade snow arriving by
Sunday morning.  Weagle

.LONG TERM...Saturday night through Wednesday...No changes. Previous
discussion follows. Fairly zonal flow will be in place over the
central and eastern North Pacific, likely keeping our weather active
well into next week. Models show decent agreement that a frontal
system will move through the forecast area Sunday, though there are
some disagreements regarding the strength of the front and whether
or not split flow tries to develop off the West Coast. The 00z ECMWF
took an interesting turn as it develops a 993 mb low off the Oregon
Coast Tuesday, eventually curling it north and moving it onshore
near the Olympic Peninsula. The 00z/06z GFS were more splitty with
the Pacific jet energy, not showing as much phasing between the
northern and subtropical jets. If the 00z ECMWF pans out, we will
probably need to watch the Gorge for some sort of frozen/freezing
precip. Both the 00z ECMWF and the 06z GFS then come back into
agreement that much colder air comes in behind this system Wednesday
and Thursday. For now, went with fairly high PoPs in general for the
extended forecast period, with temps near to perhaps a couple
degrees below normal.	Weagle

&&

.AVIATION...Generally VFR across at all TAF sites this morning.
Post-frontal unstable air mass today and tonight will result in
periodic bouts of MVFR. There is also a slight chance of
thunderstorms from about 18Z today through 02Z Fri from the coast
to the Cascade foothills. Any thunderstorm that develops will
have the potential to produce brief periods of MVFR cig/vis and
wind gusts 30 to 40 kt. Another surface trough is expected to
move inland late tonight and Fri morning for a higher probability
of MVFR conditions.

PDX AND APPROACHES...VFR to be the predominant category through
Fri morning. However, there will be a chance of brief MVFR in
heavier showers this afternoon through early evening. There is
also a 10 to 20 percent chance of thunderstorms at the terminal
and vicinity 20Z through 02Z Fri. Weishaar

&&

.MARINE...Post-frontal low-end gales to continue through 00Z for
PZZ255 and PZZ275 and into the evening for the north zones.
Wind speeds will slowly diminish this evening and tonight, but
will remain at solid small craft advisory levels through at least
Fri afternoon. Wind speeds fall below small craft advisory levels
Fri night. The next system is forecast to reach the outer waters
late Sat night and move onshore Sun morning. The GFS suggests
40-45 kt boundary layer wind speeds 09Z Sun through about 15Z.
There is the possibility for another round of storm force wind
with this system, but models have been varying on timing and
strength. Will maintain the high-end gale force wind speeds for
now.

Weak high pres moves into the waters late Sun into Mon, but
additional weather systems are expected the rest of the week.
This includes another period of potential gale force wind Tue.

Impressive wave heights noted at all buoy locations. At 16Z buoy
46029 was still at 32 feet. The 46243 buoy, just west of Astoria
reported 37 feet at 1130Z with a period of 18 seconds. This buoy
apparently has drifted to near the jetty, with the top one-tenth
wave heights up to 45 ft. The ENP model ended up verifying better
than the ECMWF. The latest ENP holds seas near 30 ft through
mid-afternoon. Seas lower to around 25 ft by 06Z Fri and to 20 ft
by 12Z Fri. The weather system late Sat night and Sun will likely
boost seas in the 20 to 25 ft range. Weishaar

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Coastal Flood Warning until midnight PST tonight for Central
     Oregon Coast-North Oregon Coast.

WA...Coastal Flood Warning until midnight PST tonight for South
     Washington Coast.

PZ...Gale Warning until 7 PM PST this evening for Coastal Waters
     from Cape Shoalwater WA to Cascade Head OR out 60 NM.

     Gale Warning until 4 PM PST this afternoon for Coastal Waters
     from Cascade Head OR to Florence OR out 60 NM.

     Small Craft Advisory for Rough Columbia River Bar until 10 AM
     PST Friday.

&&

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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 CWA or forecast area.