Area Forecast Discussion
Issued by NWS Portland, OR
FXUS66 KPQR 240337
Area Forecast Discussion
National Weather Service Portland OR
837 PM PDT Thu Mar 23 2017
.SYNOPSIS...An occluding Pacific frontal system will bring
continued rain into southwest Washington and northwest Oregon
tonight, with showers and a few thunderstorms lingering into Friday.
High pressure will cause showers to taper off late Saturday, but the
next frontal system will likely bring more rain Sunday followed by
showers Monday. Occasionally wet weather is expected to linger
through the middle of next week.
.SHORT TERM...Tonight through Sunday...Stratiform rain across the
area has dropped around 0.05" per hour for lower elevations and close
to 0.10" per hour in higher elevations in the Coast Range. This does
not warrant any adjustment to ongoing QPF forecast which looks pretty
good based on those rates. Latest satellite representation supports
HRRR which shows renewed development continuing overnight and into
tomorrow morning. The back edge of the precip will slowly slide east
of the area sometime tomorrow morning.
This is when conditions will need to be closely monitored for thunder
potential. Some dry air will come in behind this precip and may be
sufficient enough to scatter out clouds and get some sun. If this
does occur, at least some surface based instability will develop. Any
instability will be sufficient for thunder given the falling heights
and colder air aloft. In fact, given freezing levels are expected to
be around 4000 feet, it won`t take much of an updraft to form small
hail. If surface instability strengthens enough for updrafts to
extend beyond 10 kft (would guess around 400 to 500 J/kg), would
expect a few hail showers which may even lead to minor accumulation.
This all depends on the amount of clearing we get tomorrow
morning/afternoon, but the likelihood of clearing is high enough to
warrant a mention of the hail potential. As a result, expanded the
area of thunder slightly eastward and added a slight chance of small
hail for all areas north of Salem. These storms will be pretty
shallow. The effective shear will be significant enough, (25-30 kts)
to organize any stronger updrafts for hail formation, but these
shallow storms will lack the depth for any significant wind creation
either from evaporative cooling or water loading in the updraft. In
addition, extended thunder mention through 06Z as latest 3km NAM does
not bring the trough/cold front through the area until that time and
still has some convective elements along it as it moves through.
.LONG TERM...Sunday night through Thursday...12z global models are
in general agreement with evolution of the long wave pattern,
however, details are far too sketchy to pin down specific dry
periods. This is somewhat of a shift from earlier runs where the
early part of next week showed signs of a brief dry period. COld
front Sunday night will push east through early Monday. Plenty of
orographic showers will follow through Monday evening. There, the
GFS and ECMWF start to show differences among the operational runs.
Have thereafter decided to stick fairly close to a blend of the
ensemble means as the ECMWF is slightly more progressive with the
westerly flow while the GFS is continuing more of the amplified
pattern it`s shown the last few days. The 12z runs do come back
together on Wednesday/Wednesday night with a trough sweeping east
across the region. For now, that looks like the wettest day although
given the run to run inconsistency, have kept lower end likely pops
for now. Regardless of the details, overall conditions will close to
seasonal temperature norms which keeps snow levels bouncing between
4000 and 6000 feet. /JBonk
.AVIATION...Conditions have remained VFR so far this evening both
at the coast and inland as rain has spread over the area with the
incoming front. Still think MVFR is possible before the front
moves onshore along the coast 08z-10z and through the inland by
10z-12z. Conditions will then try to lift back to VFR behind the
front though showers in the onshore flow behind the front may
drop conditions at times to MVFR. In addition, there is a chance
of thunderstorms at the inland TAF sites Friday afternoon as
cooler air aloft spreads in behind tonight`s front.
KPDX AND APPROACHES...VFR conditions with rain through much of
tonight, with a chance of MVFR conditions. VFR conditions will
then tend to prevail on Friday with a chance of MVFR in any
heavier showers, and with a chance of thunder in the afternoon.
.MARINE...A strong cold front is over the coastal waters this
evening, with a peak gust at buoy 50 to 43 kt so far. The models
continue to say the front will move onshore a little after
midnight. The south winds will drop off to Small Craft Advisory
values late tonight and Friday morning, then turn more
southwesterly Friday afternoon, then turning even more westerly
and finally dropping below 20 kt overnight Friday night. The
next significant winds are expected with another rather strong
front Sunday. Look for high end Small Craft Advisory winds or low
end gales with the Sunday front.
Seas continue to be forecast to peak around 16 ft tonight, and
with the way our wave models perform it could be a couple of feet
higher. Seas are then forecast to drop to around 10 or 11 ft
during the day Friday and continue near those values through much
of Saturday, dropping below 10 ft late Saturday and Saturday
night. Seas will likely exceed 10 ft Sunday with the next front.
PZ...Gale Warning until 1 AM PDT Friday for Waters from Cape
Shoalwater WA to Florence OR from 10 to 60 nm.
Gale Warning until 3 AM PDT Friday for Coastal Waters from Cape
Shoalwater WA to Florence OR out 10 nm.
Small Craft Advisory for Rough Columbia River Bar until 6 AM
Small Craft Advisory for Rough Columbia River Bar from 1 PM to
6 PM PDT Friday.
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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. The area is
commonly referred to as the forecast area.