AWUS01 KWNH 130750
FFGMPD
CAZ000-ORZ000-WAZ000-131800-

Mesoscale Precipitation Discussion 1152
NWS Weather Prediction Center College Park MD
250 AM EST Wed Nov 13 2024

Areas affected...Coastal and Cascades Ranges of E WA & E
OR...Northwest CA...

Concerning...Heavy rainfall

Valid 130800Z - 131800Z

SUMMARY...Progressive but potent slug of enhanced moisture flux
along Coastal and Cascade Ranges producing 2-4" totals with
occasional hourly rates to .5"/hr with greatest totals in SW
facing orography increasingly so in SW OR/NW CA.

DISCUSSION...GOES-W WV shows main core of the upper-low just south
of 47N near 132.5W starting to slide northeast with upstream
energy starting to dig the upper-trough, expecting to go more
neutral tilt over the next 6-12hrs as it approaches the West
coast.  The 150kt jet core just rounding the base of the trough is
broadening with large cirrus shield.  GOES-AMV and RAP analysis
note a split in the jet downstream northeast of the 40N130W
benchmark; this is enhancing broader and more intense upper-level
divergence along the Oregon coastline. Additionally, a weak
inflection is developing in the low levels with upstream
height-falls leading to sharper boundary layer moisture
convergence along and downstream backing the progressive frontal
zone a bit more flat to the mean steering flow and increasing
deeper layer moisture convergence.   CIRA LPW notes, 1-1.25" total
PWat in this area where surface and slug of 850-700mb moisture
overlap.  Given 60-70kts of 850mb flow (~700-900 kg/m/s IVT),
convergence is going to result in enhanced showers capable of
.5-.6"/hr rates likely to intersect the northwest to central OR
Coastal Range over the next few hours.  So while the strength of
orographic ascent will be consistent along the front, this
slightly backed flow ahead of the inflection will have increased
totals compared to the average 2-3" totals, some localized 3-4"
values may occur before midday.  Strength of AR and moisture flux
may be supportive enough to bleed through the coastal range with
some enhanced rainfall totals across the western slopes of the
northern and central Oregon Cascades.

To the north western WA...
Post cold front showers will remain widely scattered, and with
steeper lapse rates will support higher but much shorter duration
rainfall across areas with modest 2-3" totals so far through the
event, while NASA SPoRT soil saturation values are higher than
normal, the burst nature is still not likely to result in any
rapid inundation/flashy conditions but maintain above normal
run-off.

South of the inflection across SW OR into NW CA...
The core of the AR moisture stream will remain along/ahead of the
cold front, but will be narrowing as the mid-level moisture slug
slides ashore and surface moisture band narrows/stretches in
width.  Height-falls will swing through later in the morning/early
afternoon, but traditionally favorable southwesterly flow into
generally steeper terrain and confluent 45-50kts of 850 flow
around the Cape, prolonged moderate rainfall with streaks of
enhanced showers up to .5"/hr are probable.  This is still fairly
gentle/typical for the boreal rain forests to handle much of the
rainfall, and while run-off will be enhanced is not likely to
result in any significant concerns that would not be for an
ordinary AR.  IVT values will be decreasing with reduced
moisture/low level flow from 800 kg/m/s across the central OR
coast toward 500-600 kg/m/s.  Still, traditional orographic ascent
will support spots of 2-3" by 18z.

Gallina

ATTN...WFO...EKA...MFR...PQR...SEW...

ATTN...RFC...CNRFC...NWRFC...NWC...

LAT...LON   49052223 49052156 48412141 46912178 45782182 
            44322201 43042201 42212223 41802250 41082314 
            39922370 39922426 40262453 40672460 41272434 
            41832441 42292464 42812479 43492460 44162434 
            45292434 46312415 47362445 47962475 48452475 
            48192381 47962344 47252329 46952317 46832291 
            47102235 47622223 

$$
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