Mesoscale Precipitation Discussion 1021
NWS Weather Prediction Center College Park MD
1230 AM EST Fri Nov 23 2018

Areas affected...southern OR coast into northern CA

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

Valid 230530Z - 231730Z

Summary...Heavy rain is likely to move into southwestern OR and
northern CA after 12Z with 12 hour totals peaking in the 2-4 inch
range through 18Z. Rainfall rates of 0.5+ in/hr are expected for
consecutive hours allowing for heavy rainfall through the day on
Friday. These rains could lead to flash flooding and debris flows
in the vicinity of burn scars. with localized runoff issues
elsewhere.

Discussion...GOES water vapor imagery depicted a well defined
shortwave near 41.5N 131.2W at 05Z, moving steadily toward the
ESE. A combination of surface observations and a 0329Z pass of
AMSU's 89 GHz imagery placed a weak surface low near 42.7N 136.9W
along a frontal boundary with blended TPW imagery identifying
precipitable water values above 1.25 inches to the south of the
surface low and warm front.

12-hr extrapolation of the shortwave's track and RAP forecasts are
in agreement with the arrival of the shortwave to just south of
Cape Mendocino just before 12Z. There have been relatively
consistent forecasts from the RAP in line with the 00Z NAM and GFS
to take the surface low to roughly 150 miles west of the OR/CA
border at 12Z. This track will take warmer temperatures and deeper
moisture in the 850-700 mb layer toward the OR/CA coasts with
slower rises in the 500 mb temperature, allowing for steepening
lapse rates and at least weak instability to reach the coastline
by late morning (15-18Z). In addition, 850 mb wind speeds are
expected to peak in the 50-60 kt range initially from the
southwest, then veering toward the west after the warm frontal
passage around 15Z. Quasi-zonal flow in the low to mid-levels
along the OR/CA coastline will help to support a period of
persistent westerly flow at 850 mb into the northern CA coast and
potentially support rainfall rates of over 0.5 in/hr for a number
of consecutive hours at a few locations. The greatest threat of
heavy rain appears to be across Curry and Josephine counties in
southwestern OR and Del Norte, Humboldt and western Siskiyou
counties in northern California where 2-4 inches of rain is
anticipated through 18Z. The expectation of locally heavy rainfall
rates could produce flash flooding and debris flows in burn scar
regions across the region.

Otto

ATTN...WFO...EKA...MFR...REV...STO...

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

LAT...LON   42692406 42652373 42572358 42312331 42152319
            42012302 41862289 41722285 41532282 41372268
            41272221 41092188 40912157 40752136 40602121
            40422095 40192075 39812077 39672081 39562090
            39462102 39412118 39472135 39612148 39802167
            39972177 40142188 40302195 40422205 40462216
            40452228 40442241 40372254 40222270 39962264
            39792262 39582259 39342251 39212253 39062264
            38982306 38942365 39772441 40452469 41222454
            41732466 42252466 42412461 42552453 42652427