Mesoscale Precipitation Discussion 1020
NWS Weather Prediction Center College Park MD
1036 AM EST Thu Nov 22 2018

Areas affected...Southwest Oregon; Northern California

Concerning...Heavy rainfall...Flash flooding possible

Valid 221533Z - 230300Z

Summary...Heavy rain bands are likely to arrive on the coast of
southern Oregon and northern California late this morning or
around midday, and then affect the region into the afternoon. Rain
rates in excess of 0.5 in/hr are expected in the heaviest rain
bands, and much of that rain may fall in short bursts of only
15-30 minutes in duration. This could lead to flash flooding and
debris flows in the vicinity of burn scars, with localized runoff
issues elsewhere.

Discussion...GOES-17 water vapor channels and Air Mass RGB
composite show a digging PV anomaly approaching the West Coast,
evident in water vapor imagery as a dry stratospheric intrusion
around 43N/128W. This PV anomaly was situated in the left exit
region of a strong upper level jet streak and there was associated
strong upper level divergence just downstream. Evidence of strong
divergence existed in the satellite imagery with an arcing band of
cold cloud tops, and GOES derived motion winds highlighted this
well. Within the region of strong divergence, recent
polar-orbiting satellite passes have detected an increase in heavy
rain offshore with microwave instruments. The blended rain rate
product has shown offshore rain rates as high as 0.45 in/hr, with
numerous instances of 0.25 in/hr. These rain rates could easily be
doubled once the rain bands reach the coast and orographic
enhancement begins. Therefore, it seems reasonable to expect 0.5
in/hr rain rates with localized rates up to 1.0 in/hr in southwest
Oregon and northern California in favored upslope regions.

RAP forecast 850mb winds are fairly strong (55-65 knots), with a
narrow low-level jet impinging on the coast between 18-21Z.
However, the orientation of the LLJ will have a significant
southerly component, so orographic ascent may be focused more
substantially on NW-SE oriented terrain features. The strong LLJ
will be combined with PWs perhaps approaching 1 inch along the
immediate coast, and reaching 0.8 inches in the upslope regions
slightly further inland. There may be small amounts of instability
available for some convective banding, as the 12Z MFR sounding
showed 850-500mb lapse rates on the order of 6.8 C/km. This is
likely to lead to some narrow bands of heavier rainfall, and may
focus 0.5+ inch amounts into small time windows of 15-30 minutes.
Indeed, the HRRR shows this potential with narrow but fairly
progressive rain bands with high reflectivity and over 0.5 inches
of QPF. This will lead to concern for flash flooding and
significant debris flows in the vicinity of burn scars across the
region. The greatest concern initially (18-23Z) will be for the
burn scars in the Klamath Mountains near the CA-OR border region,
and the timing from the HRRR has become progressively earlier in
recent runs. Late in the afternoon and into the evening (22Z or
later), the concern would spread further down into the coastal
ranges of Northern California, and into the northern Sierras.

Lamers

ATTN...WFO...EKA...MFR...MTR...STO...

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

LAT...LON   43042463 42992397 42592368 42082360 41542338
            41082321 40932290 41042254 41232217 41242186
            40822186 40322166 39852114 39572076 39202049
            38952056 39052113 39352152 39752182 40162204
            40442219 40432259 40162270 39682259 39312253
            39002246 38652237 38402248 38292294 38532348
            38972390 39642399 39992430 40462462 40992437
            41482431 42002445 42592472