Lightning Strike in Oregon, May 19, 2022

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1018 AM PDT Thu May 19 2022

Updated aviation discussion and headlines

.SYNOPSIS...A cool and showery weather pattern will give way to
warmer and drier weather this weekend into early next week.

&&

.SHORT TERM...Today through Friday night...Radar observations from
early Thursday morning showed scattered light showers pushing across
western WA and far northwest OR. These showers were occurring ahead
of an incoming 500 mb shortwave trough and associated vort max,
within the favorable left-exit region of a ~165 kt jet max at 250 mb.
This jet will remain in place over the area today, while the
aforementioned shortwave trough dives southward across western OR.
This will result in increasing showers over northwest OR during the
mid to late morning hours. At the same time, showers should begin to
gradually decrease in coverage across southwest WA, ending for the
south WA coast and Willapa Hills first. Scattered showers are then
expected to continue through this afternoon for northwest OR and the
interior lowlands of southwest WA. The deterministic GFS/NAM are now
showing slightly higher surface-based CAPE (SBCAPE) values this
afternoon when compared to yesterday's model runs, peaking at around
250-500 J/kg (mainly over the interior lowlands of southwest WA and
locations to the east of I-5 in the Willamette Valley and Cascade
foothills). The NBM probability of thunder seems reasonable in this
geographic area, which often does see the highest chance of thunder
during westerly to northwesterly flow regimes. On the contrary, the
NBM thunder probabilities appear too high elsewhere where instability
looks to be more shallow; the calibrated SREF thunder probabilities
backs this statement up. Therefore, decided to maintain a slight
chance of thunder in the forecast for the aforementioned favorable
area, while leaving a mention of thunder out of the forecast
elsewhere.

Showers and/or thunderstorms will end from north to south this
evening as diurnal heating ceases and increasing high pressure brings
sinking motion and more stable conditions. The HREF ensemble mean for
cloud cover suggests cloud cover will scatter out quickly Thursday
evening across the interior lowlands while the Cascades remain
obscured by low clouds. Expecting winds to turn calm at the same time
as the surface pressure gradient relaxes over northwest OR and
southwest WA. Clear skies and calm winds will result in an ideal
setup for radiational cooling Thursday night, with most locations
likely dropping into the upper 30s to around 40 degrees. That said,
typical cool spots such as Hillsboro, Battleground, Vernonia,
McMinnville, and Corvallis have a considerable chance of dropping
into the mid 30s. The latest ECM MOS guidance backs this up as well.
Therefore, decided to manually lower overnight lows Thursday night to
better reflect expectations/MOS guidance. With temperatures most
likely dipping into the mid 30s for the aforementioned locations,
cannot rule out some patchy frost early Friday morning (most likely
between 3-8 AM PDT). While frost is not expected to be widespread
enough to warrant a Frost Advisory, protecting sensitive outdoor
vegetation may be worth considering Thursday night. This is
especially true if you do reside in a typical cool spot.

After a chilly start to the day on Friday, temperatures should
rebound nicely in the afternoon as high pressure remains over the
area and breaks of sun quickly heat up the surface. The NBM v4.0 and
v4.1 are both suggesting highs in the mid 60s across the interior
lowlands, with highs in the upper 50s along the coast. Similar to
Thursday night, clear skies and calm winds are likely yet again on
Friday night. While radiational cooling looks just as ideal Friday
night, overnight lows will likely end up a few degrees warmer due to
the relatively warmer temperatures to start the night. As a result,
frost is much less likely to occur Friday night when compared to
Thursday night. -TK

.LONG TERM...Saturday through Wednesday night...The latest model
guidance is still showing good agreement regarding the synoptic-scale
pattern on Saturday, with an upper ridge located over the northeast
Pacific and an upper level low over southern B.C. and far northern
WA. This upper low is now set to push a bit further to the west as it
dives southward today, at least when compared to previous runs of the
GFS/EURO. As a result, the probability of precipitation has increased
over the south WA/north OR Cascades but remains less than ~10% to the
west of the Cascades. In fact, forecast GFS soundings are showing
enough elevated instability to warrant a slight chance of thunder
Saturday afternoon for the south WA/north OR Cascades, which the NBM
is picking up on well. Aside from the potential showers and/or
thunderstorms over the Cascades, the remainder of the forecast area
is still looking dry with high temperatures in the lower 70s (except
along and near the coast where highs should top out around 60-65).

On Sunday, models and their ensembles are in excellent agreement that
the upper level ridge located over the northeast Pacific will push
eastward closer to the Pacific Northwest. This should keep the entire
area dry through the day, with temperatures once again rising into at
least the lower 70s. Highs in the mid 70s cannot be ruled out either,
as 850 temps look to warm to at least +8 degrees Celsius, which is a
bit warmer than temperatures aloft on Saturday.

Models and their ensembles continue to struggle with the overall
weather pattern early to mid next week, especially in regards to
precipitation chances and exact temperatures. That said, the NBM is
now trending noticeably drier for early next week. PoPs are now only
around 15-20%, mainly for locations to the north of Salem. While
several GEFS/EPS ensemble members do give measurable rain to northern
locations, the majority of the ensemble members keep the area dry.
Decided to stick with the NBM for PoPs given the remaining
uncertainty. In regards to temperatures, the NBM 25th to 75th
percentile guidance is suggesting highs may top out anywhere from the
mid 60s to mid 70s Monday through Wednesday. The current forecast
keeps highs generally in the lower 70s, which is right around normal
for this time of year. -TK

&&

.AVIATION...18z TAFs: Cool, slightly unstable onshore flow
continues this morning. KRTX radar shows bands of showers moving
across the forecast area towards the southeast with most of the
measurable precip remaining north of KSLE. Broken VFR ceilings
will continue with the potential for brief periods of MVFR with
heavier showers as the afternoon progresses. Instability
increases this afternoon for possible thunderstorms, roughly east
of I-5, from 18-00Z. But odds of thunder are too low (around 15
percent) for inclusion in TAFs at this time. Showers shut off this
evening with VFR conditions expected to continue through the
night. Patchy, shallow fog may develop towards 12Z Friday.

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

KPDX AND APPROACHES...Broken VFR with brief showers will continue
through the afternoon as cool, slightly unstable northwesterly
flow moves onshore. Afternoon heating will destabilize the low
levels leading to a slight potential for thunderstorm activity
between 18-00Z Friday across Clark and Multnomah County. West to
northwest winds continue through the day in the 5-10kt range, then
eases late this evening. -BMuhlestein/mh

&&

.MARINE...Moderate W-NW flow will persist across the coastal
waters today, with some gusts to 20 kt through the afternoon as a
weak disturbance brushes by to the north. Seas remain fairly steep
around 10 ft at 10 seconds. As usual the GFS wave model seems to
be eliminating the residual swell too quickly, largely due to a
wave data outage on Buoy 46005. So have adjusted heights up by
a foot through tonight. The Small Craft Advisory remains in
effect through this afternoon. The period is also rather short,
so seas will remain steep as well. Seas should gradually subside
to 5 ft or less over the weekend as high pressure establishes over
the eastern Pacific.

A very strong ebb cycle will produce rough conditions across bars
for the morning ebb today, and strong ebb for the Friday morning
ebb. Mariners moving in and out of harbor should make sure to
check tidal information for their area. /mh

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until 5 PM PDT this afternoon for coastal
     waters from Cape Shoalwater WA to Florence OR out 60 nm.

&&

&&

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