Debris Flow in Washington, April 10, 2023

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1013 AM PDT Mon Apr 10 2023

Updated aviation and marine discussions.

.SYNOPSIS...Expect steady rain to continue through Monday as a
moderate atmospheric river impacts the area. Cool and showery
conditions are expected mid-week as an offshore upper level
trough moves over the Pacific Northwest. Trending drier and
relatively warmer Friday into next weekend, but uncertainty
remains high in regards to exact high temperatures.

&&

.SHORT TERM...Today through Wednesday night: The atmospheric river
continues to bring steady rain to much of the area. However, the
peak has occurred late yesterday afternoon and early evening with
amounts moving forward more akin to a weak AR versus the Moderate
spike which occurred earlier. 24 hour rain totals have been less
than forecast and rather unimpressive except along the exact core of
the AR covering the north central Oregon Coast and extending
northeast to the southern slopes of Mt St Helens. These areas have
recorded 1.50 inches to nearly 3 inches over the past 24 hours, and
most of that during the Sunday afternoon/evening timeframe. It
appears the bulk of the transported moisture stayed closer to sea
level than most ARs since there continued to be significant rain
shadowing across the Willamette Valley/Portland Metro and the
interior lower Columbia. Accumulations have been half or less than
what was expected.

Models show the precip focus will waiver a little north to south
today. This is an unfortunate time for KLGX to have gone down, as it
makes the northern extent of the incoming precip harder to determine.

Ground obs do show that rainfall rates have dramatically slowed, and
nearly stopped, north of a line extending from about Tillamook to
Kelso. This has allowed the Grays River to stop rising and at least
hold steady, if not begin to fall, over the last few hours. The
Wilson River has also shown a steep rise over the past several hours
but also seems to have peaked for the time being. This is also the
case for the rest of the monitored coastal rivers with all of them
remaining well below flood stage for now.

Don't expect any significant movement of the AR track for the next
18 hours, although a shortwave moving northward along the flow will
enhance rates again by late morning through early evening. Current
forecast has about one half inch of rain for every 6 hours today
through this evening. In general, this should not be enough to push
most rivers to flood stage with only the Tualatin River near Dilley
remaining in bankfull/action stage and peaking about 1/2 foot below
flood after midnight Monday night/Tuesday morning. This all remains
consistent with the CW3 assessment of a weak AR from this point
forward. Don't see a need to update the Hydrologic outlook at this
point. Will ask day shift to reassess after the morning package is
produced by the Northwest River Forecast Center plus continue to
monitor current river reading in the meantime.

This evening will see the parent upper trough over the NE Pac move
closer and deepen which should cut off the moisture stream and bring
a cold front across the region for lower snow levels. Will see the
current freezing level at 7500-8500 feet quickly drop to 2300-3300
feet Tuesday morning. In addition to precipitation becoming
convective and showery in nature, that should also put the brakes on
general air mass snowmelt for a spell as well. Thus, expect rivers
will start to recede from whatever peak they reach today/tonight.

As the upper trough further moves over the area Tuesday, breaks in
the clouds will allow for spotty surface heating and a low (less
than 25%) chance a thunderstorm will affect any given point. This
threat is largely limited to the afternoon and early evening hours
with the main impact being brief small hail and heavy rain.
Basically, more of what has already occurred previously this
spring. Once the sun lowers in the sky, it will be harder to trigger
the deeper convection.

Late Tuesday night and early daylight Wednesday brings an upper low
center moving eastward across southwest Washington. Cannot rule out
accumulating skiffs of snow between 500-1000 feet with maybe an inch
to the foothills above 1000 feet. The current track of the upper
low, however, maintains weak southerly flow along the length of the
Willamette Valley into the Portland Metro which, historically, is
not conducive to accumulating snow at any time of the year, let
alone in the middle of meteorological spring. Areas further north,
such as the higher elevations around Kelso/Longview/Rainer, will
have a better chance of seeing those very light accumulations given
their proximity to the upper low and distance from the southerly
warming winds. Whatever does fall will quickly disappear by mid-day
as daylight heating occurs. It is not lost, however, that this is
eerily close to the April snowfall anniversary which occurred last
year and brought an inch of snow to PDX proper. The difference this
time is that the upper low is not as cold as that one was (500
heights of 536 dm vs 526 dm) and there is less available moisture to
work with. Regardless, Wednesday and Wednesday night will remain on
the cooler side with weak showers. /JBonk

.LONG TERM...Thursday through Sunday: No significant changes with
the bulk of the previous discussion following. Still trending drier
and relatively warmer late in the week into next weekend as the
upper flow pattern becomes more zonal and then more amplified with a
potential ridge somewhere over the area. However, a good deal of
model uncertainty is still evident in regards to the exact placement
and amplitude of the ridge. This will determine how warm we get.
Right now, Saturday and Sunday have the best chance for warm
conditions. This is when all four clusters show a ridge to some
degree, however cluster one depicts a weak ridge centered over far
eastern WA/OR with near normal heights over northwest OR and western
WA (this cluster makes up 30% of the CMC ensemble, 44% of the EURO
ensemble, and 0% of the GFS ensemble). The GEFS favors the strongest
ridge and warmest solution, which is depicted well by cluster 2. If
this solution were to verify, then high temperatures would likely
warm well into the 70s (which matches the latest NBM 75th-90th
percentile guidance). If the cooler members from the EPS/CMC verify,
then temperatures would likely wind up right around normal for mid
april with highs in the lower 60s. Either way, it will be relatively
warmer when compared to earlier in the week. -TK/CB

&&

.AVIATION...At 17Z Monday, radar and surface weather observations
showed a band of light to moderate rain associated with an
atmospheric river stretched across northwest Oregon into
southwest Washington, roughly from the south WA Cascades to the
central Oregon coast. MVFR to IFR conditions were continuing to be
observed under this band of rain, with MVFR to VFR conditions on
either side of the rain band. The overall trend through tonight
will be a slow eastward progression of the aforementioned rain
band. However, small scale disturbances will likely shift lower
cigs west of the rain band at times. For example, later this
morning and afternoon a disturbance should pull lower cigs back
to the KAST area. Also expect visibility reductions to 2-5 SM
with rain.

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

KPDX AND APPROACHES...MVFR conditions expected to prevail today
with steady light to moderate rain and cigs in the 1500-3000 ft
range. Rain should end around 07-08Z Tuesday before another band
of rain arrives around 12Z Tuesday. IFR cigs cannot be ruled out
as there is a 15-25% chance for cigs below 1000 feet through 03Z
Tuesday, increasing to a 25-45% chance between 04-15z Tuesday. -TK

&&

.MARINE...A surface front remains offshore with a weak surface
ridge diagonally across the coastal waters resulting in southwest
winds for the north and west zones, and northerly winds for the
south and east zones. As the front shifts towards the coast
through tonight will see southwest winds spread across the waters
as well as increasing winds as gusts ramp up to around 25 kt
for all marine zones Monday night into Tuesday.

Significant wave heights have decreased to around 8 to 10 ft and
will remain within or near that range through the remainder of
the week aside from brief increases to 11-13 ft with the stronger
westerly winds on Tuesday. -TK

&&

.HYDROLOGY...No significant changes. Previous discussion follows. A
moderate atmospheric river will continue to bring widespread rain
and mountain snowmelt to the area through Monday, resulting in
rising water levels on creeks and rivers in southwest WA and
northwest OR early in the week. The primary concerns are along some
creeks and rivers draining the Coast Range and Willapa Hills, along
with creeks and smaller rivers in the Willamette Valley. The Grays
River near Rosburg, the Wilson River near Tillamook, the Tualatin
River near Dilley, and the Luckiamute River near Suver have the best
chance of reaching minor flood stage, but only at 10%. A few other
rivers and creeks have a 5-10% chance of reaching minor flood stage,
including: the Nehalem River near Foss, the Trask River above Cedar
Creek, and Johnson Creek at Sycamore. The aforementioned
probabilities are based on the latest HEFS guidance, which accounts
for both hydrological and meteorological uncertainty. Overall,
widespread river flooding is not expected with this event
but the chance of minor flooding is just high enough to warrant
messaging.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory until 11 AM PDT this morning for Columbia
     River Bar.

     Small Craft Advisory from 7 AM to 5 PM PDT Tuesday for Columbia
     River Bar.

     Small Craft Advisory until 3 PM PDT this afternoon for Coastal
     waters from Cape Shoalwater WA to Florence OR out 10
     NM.Coastal waters from Cape Foulweather OR to Florence OR
     from 10 to 60 NM.

     Small Craft Advisory from 2 AM to 8 PM PDT Tuesday for Coastal
     waters from Cape Shoalwater WA to Florence OR out 10
     NM.Coastal waters from Cape Foulweather OR to Florence OR
     from 10 to 60 NM.

     Small Craft Advisory until 8 PM PDT Tuesday for Waters from Cape

     Shoalwater WA to Cape Falcon OR from 10 to 60 NM.

&&

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PORTLAND OR
1010 AM PDT MON APR 10 2023

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

1000 AM     HEAVY RAIN       COUGAR                  46.05N 122.30W
04/10/2023  M2.36 INCH       COWLITZ            WA   TRAINED SPOTTER

            STILL RAINING AT THE TIME OF OBSERVATION.

&&

TK