Flood in Oregon, May 28 to 29, 2022

Area Forecast Discussion
National Weather Service Pendleton OR
409 PM PDT Sat May 28 2022

.Updated aviation discussion

.SHORT TERM...Latest water vapor imagery shows an impressive
upper low just off the Oregon Coast. This feature will move esewd
across Oregon tonight and early Sunday. Strong vertical motion and
orographic lift is setting the stage for a heavy rain event
beginning this evening and continuing into Sunday for the eastern
mountains and foothills. HREF/model ensembles and latest 18Z NAM
show the potential for a significant hydro event starting this
evening and continuing into Sunday for the eastern mountains and
central and eastern foothills. In fact the HREF and 18Z NAM are
showing the potential for 1 inch of Rain in just a three hour
period VCNTY of MEH between 03-06Z this evening. Overall...QPF
amounts of 1-1.5 inches in the foothills and 1.5-3 inches in the
mountains will be possible. With this in mind have issued a flood
watch for the aforementioned areas through Sunday afternoon. The
only mitigating factor for a significant flood event is limited
snow cover in the mountains...but with these high QPF amounts
flooding is still a significant concern. There may be two periods
of heavy rain, one tonight and another during the first half of
the day Sunday.

On Sunday afternoon westerly winds winds will pick up across the
lower elevations with winds of 20-30 mph and gusts 40-45 mph
especially Cascade gaps. It looks like winds will be just below
advisory levels but the situation will need to be monitored.
Another weaker upper low will drop sewd into the region behind
the initial one and will mostly keep a chance of rain and high
mountain snow showers over the eastern mountains through Monday.
This will be followed by drier conditions on Tuesday and warmer
but still below normal temperatures. 78

.LONG TERM...Tuesday through Saturday night...By Tuesday morning,
an upper-level trough will slide southeast into the Great Basin,
though a piece of energy is advertised to linger across the Pacific
Northwest and ride along the leading edge of a building upper-level
ridge. Near-normal temperatures for this time of year are forecast
Tuesday, and as a result of the shortwave, could see a few showers
across the eastern mountains during the afternoon. The ridge moves
more directly overhead on Wednesday with above-normal temperatures
expected. Ensemble clusters, and deterministic guidance, both
indicate some slight differences in the amplitude of the ridge, but
location and timing appear in very good agreement.

General concensus is that an upper-level low will swing towards the
Pacific Northwest by Thursday afternoon, but deterministic runs and
ensemble clusters both show some differences in the progression of
its track with the ECMWF and EPS favoring a more progressive pattern
with the low swinging closer to the Oregon coast. Meanwhile, the GFS
and GEFS are inclined to keep the low rotating further northwest
nearer to the Gulf of Alaska. What all guidance has in common is
height falls with southwest flow aloft ushering in anomalously high
PWATs of generally 130-180 percent of normal across the Pacific
Northwest by Thursday afternoon.

This leads to some uncertainty in the forecast for Thursday, Friday,
and Saturday as the low rotates offshore, though this general
pattern of a breakdown of an upper-level ridge, with south or
southwest flow aloft, anomalously high PWATs, and multiple
shortwaves passing through the flow, supports thunderstorms. NBM
thunder probabilities continue to suggest a slight chance of
thunderstorms Thursday afternoon and again on Friday afternoon and
perhaps Saturday afternoon. Plunkett/86

&&

.AVIATION...00Z TAFs...Periods of MVFR conditions will develop this
evening and continue overnight, with possible brief periods of IFR
due to lowering CIGs and VIS in heavier showers. Conditions will
worsen as the night moves on. Then on Sunday, there will be some
improvement by afternoon with mostly VFR conditions. However, CIGS
will be mostly between 035-060 through this evening. Winds will be
mostly light and under 10kts this evening and late at night, but
they could be from the southwest 08g15kt at times this evening. Then
on Sunday, winds will increase to mostly 15G25 Kts by late morning.
88

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  46  59  44  58 /  90  90  20  30
ALW  50  59  46  61 /  80  90  30  30
PSC  54  68  50  67 /  60  70  10  20
YKM  47  66  43  67 /  60  30  10  10
HRI  50  65  46  64 /  80  80  10  20
ELN  46  61  46  64 /  50  30  10  20
RDM  40  57  34  57 /  90  40  10  10
LGD  43  50  40  53 / 100  90  30  50
GCD  40  51  37  53 / 100  80  10  30
DLS  51  63  48  66 /  90  40  20  20

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Flood Watch through Sunday afternoon for ORZ049-050-502-503-507-
     508.

WA...Flood Watch through Sunday afternoon for WAZ029-030.

&&

SHORT TERM...78
LONG TERM....86
AVIATION...88