AWUS01 KWNH 192003
FFGMPD
CAZ000-ORZ000-WAZ000-200630-

Mesoscale Precipitation Discussion 1159
NWS Weather Prediction Center College Park MD
302 PM EST Tue Nov 19 2024

Areas affected...Coastal Ranges of WA, OR and Northwest CA...

Concerning...Heavy rainfall

Valid 192000Z - 200630Z

SUMMARY...Approach of occluded/cold front into WA/OR after 00z;
start of prolonged moderate rainfall with strong Atmospheric River
into SW OR/NW CA...

DISCUSSION...GOES-W shows a classic evolution of a rapidly
deepening 'bomb' cyclone just north of 45N132W, with impressive
comma-head baroclinic shield expanding along the NW and N side
while a clear slot/descending branch over the SW and southern
hemispheres.  Central pressures have gone from 1014mb to estimated
955mb from OPC in the last 24hrs.  A broad sub-tropical jet cirrus
canopy with subtle ridging at the apex of the cold front
42.7N129.5W indicates the anticyclonic rotor of the nose of the
140kt jet directed toward the central OR coast.  Clearing aloft,
has allowed a view of narrow convective elements along the
occluded portion of the front where steep lapse rates/CAA aloft
support a narrow ribbon of 500-750 J/kg of SBCAPE.  CIRA LPW shows
sfc-700mb layers remain very narrow stretched along/ahead of the
cold front, before broadening to a broader wedge of slightly above
average moisture values south of 38N and west of 134W,
approximately delineating a warm front.   Enhanced 700-500
moisture hugs the entire length of the mid to upper level jet and
portion of the cold front (WSW to ENE orientation) that parallels
(generally west of 130W).   As such, total PWats of 1-1.25"
southwest of the warm front are showing very high moisture flux on
60 to 90kts of 850mb southwesterly flow helping to build IVT
values of near 900-1000 kg/m/s.

This front will continue to advance quickly toward the coast over
the next few hours with similar orientation/northward expansion of
the warm front.  Further strengthening of the occluded low will
bottom out toward mid to upper 940mb range but start a cyclonic
loop reducing the surface to boundary layer impetus and expand the
occluded front in a northwesterly manner.  As such, elevated
convection will reduce in forward speed reaching the coast and
with further narrowing influences and upper-level jet expanding
further eastward into the central OR...convective elements will
become more fractured in nature only to increase orographic ascent
light rainfall from 1/20th to .1" per hour with scattered streaks
of .33-.5"/hr resulting in spotty 1-1.5" totals along favored
southwest facing coastal ranges from central OR northward.

Further south, the leading edge of heavy rainfall/WAA should be
arriving in the 03-06z time frame toward Southwest Oregon and
Northwest California.  IVT values in the 400-500 range will
increase toward 700 kg/m/s by 06z. Weakly unstable air near the
triple point and just upstream along the cold front in the warm
sector could see enhancement of rates from .5-.75"/hr along or
just offshore by 03z.  HREF 1"/hr probability reach 50% along the
coast by 06-09z, though 1"/3hr values are over 75% with spots
offshore near 100% nearing the NW CA coast.   At this point, there
is not likely to be sufficient coverage/intersection with coastal
locations to result in excessive rainfall/flash flooding...though
may become more of a risk after 06z.

As such, will leave the hazard tag as only Heavy Rainfall expected
for this MPD; but this rainfall south of central OR will only set
the stage for upcoming likely significant rainfall across the area
over the next few days.  Please keep attune to further MPDs, AHDs
from the National Water Center and flooding/hydrological
advisories and statements from local forecast offices over the
coming days.

Gallina

...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=
DZ3fjg!-aoHmH-pg9_tbypJXyH5IpgccCd57z5Fbcvw0YTle1efs_OIdqJmrTmfGQpuSwemuzuD=
E2CC-hYKQhjX9vo5iH--CfA$  for graphic product...

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

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

LAT...LON   48362483 47992415 47312359 46222336 44762359=20
            43682404 42632396 41762381 41132348 40532340=20
            40082363 39962415 40302456 40942483 41312460=20
            42102453 42852471 43562456 45212426 46672449=20
            47422475 47872490 48102498=20

=3D =3D =3D
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