Mesoscale Precipitation Discussion 1054...Corrected
NWS Weather Prediction Center College Park MD
142 PM EST Sun Dec 16 2018

Corrected for addition of "...in an hour" in second paragraph

Areas affected...portions of northern CA

Concerning...Heavy rainfall

Valid 161835Z - 170635Z

Summary...Periods of heavy rain ahead of a progressive cold front
will impact the Klamath, Cascade and northern Sierra Nevada
Mountains late this evening and overnight. Rainfall rates in
excess of 0.5 in/hr for an hour or two when combined with
pre-frontal rainfall is expected to allow for 12 hour rainfall of
2-4 inches.

Discussion...Surface observations and satellite data helped place
a cold front about 60 miles west of Cape Mendocino at 18Z with a
pre-frontal axis of rainfall making landfall across far northern
California. A Wunderground.com observation in Honeydew measured
.77 inches in an hour ending 1809Z on Cape Mendocino, similar to
the 12Z nmmb and 12Z nam_nest forecasts through 18Z. 850 mb winds
ahead of the cold front as measured by KBHX VAD wind data were 60
kt with decreasing magnitude to the south. Precipitable water
values varied across the region but were near 1 inch in the
northern Sacramento valley and ranged from roughly .90 to 1.2
inches northward from Point Arena along the coast.

The cold front is expected to continue a steady movement off
toward the east out ahead of a large scale upper trough moving
toward the West Coast. Low level moisture flux values are forecast
by the RAP to gradually decrease as the cold front moves inland as
south-southwesterly 850-700 mb winds diminish in strength a little
through late evening. This will likely have the effect of capping
max rainfall rates below 0.75 in/hr. However, given there will be
a window of time for pre-frontal rains to impact interior
locations of northern California combined with a one or two hour
period of rain rates between 0.5 and 0.75 in/hr as the cold front
moves through with winds orthogonal to pieces of the terrain
stretching from the Klamath, Cascade and Sierra Nevada ranges,
peak 12 hour rainfall totals of 2-4 inches are expected through
06Z. The expected rainfall intensity may cause localized runoff
problems for the Hirz/Delta and Camp Fire burn scar regions.

Otto

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

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

LAT...LON   41472203 41412190 41252180 41082171 40932167
            40872163 40692143 40502125 40282111 39722097
            39512094 39392103 39302121 39562167 39752179
            39992194 40192193 40332199 40482214 40522234
            40512274 40802290 41102288 41282268 41402244
            41462223