WPC Met Watch |
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Mesoscale Precipitation Discussion: #0008 (2019) |
(Issued at 1149 AM EST Mon Jan 14 2019
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MPD Selection |
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Mesoscale Precipitation Discussion 0008
NWS Weather Prediction Center College Park MD
1149 AM EST Mon Jan 14 2019
Areas affected...Southern California
Concerning...Heavy rainfall...Flash flooding possible
Valid 141648Z - 150300Z
Summary...Heavy rain will be overspreading the coastal ranges of
southern California throughout the day. Runoff problems and the
threat of flash flooding will exist over the area burn scars.
Discussion...A rather deep upper trough located offshore southern
California will be slowly lifting north today, but will be close
enough to allow an occluded front to approach and reach the
southern California coastal ranges later this afternoon and
evening. Already the latest radar imagery shows bands of moderate
to locally heavy showers beginning to cross into San Luis Obispo,
Santa Barbara, Ventura, Los Angeles and Orange counties, with some
of the heaviest rains currently impacting Los Angeles county. The
latest experimental GOES-17 10.3 um imagery is also showing some
modestly cooling cloud tops offshore near the Channel Islands,
suggesting stronger forcing which appears to be related to a
shortwave rounding the base of the upper trough. This should
result in some heavier shower activity that will arrive along the
coast over the next several hours.
A low level jet ahead of the front of 30 to 40 kts should provide
an area of relatively focused moisture convergence in across the
terrain through the afternoon and evening hours, and given a
favorable onshore component relative to the terrain, there will be
locally enhanced rainfall rates. In fact, the 850/700 mb moisture
flux anomalies reach 2 to 3 standard deviations above normal in
the 18Z to 00Z time frame which will foster more efficient
rainfall processes.
The latest consensus of hires models, led by the HRRR, and the 12Z
HREF suite of models support as much as 1 to 2 inches of rain with
locally heavier amounts to 3 inches today over the orographically
favored coastal ranges. Some of the rainfall rates will approach
and occasionally exceed a 0.50 inch/hr within the more organized
bands of showers, and the heaviest totals should tend to focus on
Santa Barbara, Ventura and Los Angeles counties where stronger
moisture flux, orographics and frontal convergence will be in
place while also being aided by large scale divergent flow aloft
ahead of the upper trough.
Expect there to be at least some concerns for runoff problems and
flash flooding, with again a focus on the area burn scars where
debris flows and mudslides will be possible. The most susceptible
areas will be the Thomas, Hill, Woolsey, and Holy fire burn scar
areas.
Orrison
ATTN...WFO...HNX...LOX...SGX...
ATTN...RFC...CNRFC...
LAT...LON 34971967 34901871 34631836 34471784 34241673
33751669 33381672 32801636 32571676 32611725
33211757 33481806 33881858 33781893 34251955
34582013
Last Updated: 1149 AM EST Mon Jan 14 2019
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