WPC Met Watch |
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Mesoscale Precipitation Discussion: #0019 (2021) |
(Issued at 1015 PM EST Thu Jan 28 2021
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MPD Selection |
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Mesoscale Precipitation Discussion 0019
NWS Weather Prediction Center College Park MD
1015 PM EST Thu Jan 28 2021
Areas affected...Southern California
Concerning...Heavy rainfall...Flash flooding possible
Valid 290315Z - 291515Z
SUMMARY...Long-lived atmospheric river making progress east across
southern California. Additional heavy rainfall is expected for the
Transverse Ranges overnight with the Peninsular Ranges seeing an
increase in rainfall later in the night and through Friday
morning. Some isolated concerns for flash flooding still exist for
the burn scar areas.
DISCUSSION...The slow-moving and deep upper trough that has been
responsible for the long-lived atmospheric river event the past
few days across a large part of California is beginning to
accelerate off to the east which is allowing the associated cold
front and axis of deeper Pacific moisture to make progress through
the coastal ranges of southern California.
Hourly rainfall totals in vicinity of the cold front continue to
locally be in the 0.50 to 0.75 inch range, with the dominant focus
over the last couple of hours on Ventura and to a lesser extent
Santa Barbara counties involving the Transverse Ranges. This is
being aided by a southwest low-level jet of about 30 kts which is
facilitating IVT values of 300 to 500 kg/m/s. These values have
been trending down through the afternoon hours with the low-level
jet continuing to gradually weaken, and as a result, the rainfall
rates are generally tapering down as well. However, the low and
mid-level Pacific flow remains generally oriented orthogonal to
the coastal ranges and this is helping to still promote a strong
orographic component to the forcing in addition to the larger
scale ascent associated with the warm-air advection and
upper-level trough.
The Transverse Ranges overnight from Santa Barbara county down the
coast through Los Angeles county, and inland to include parts of
San Bernadino, Riverside, and Kern counties will continue to see
heavy rainfall with the latest guidance still supporting some
rainfall amounts exceeding a 0.50 inch/hour. This will especially
be the case for portions of the San Bernadino and San Gabriel
mountains. However, as the front and associated upper trough
arrives, the snow levels will be falling, and most areas will see
snow levels falling to below 5000 feet by early Friday. Some of
the relatively lower, but sloped terrain is still expected to see
locally as much a 2 to 4 inches of additional rain going through
12Z (4AM PST).
By early Friday, the cold front and weakening axis of the
atmospheric river will be crossing the Peninsular Ranges farther
down to the south including the Santa Ana mountains. Snow levels
here will also be lowering with time, but some rainfall amounts of
as much as 1 to 2 inches cannot be ruled out going through 15Z
(7AM PST) on Friday.
Across all of the southern California coastal ranges, there
continues to be a concern for flash flooding around some of the
relatively fresh burn scars. However, with snow levels lowering
with time, this threat will be increasingly mitigated.
Conditions will be improving significantly across the entire
region by later Friday morning as the aforementioned upper trough
crosses the region, and this impressive multi-day atmospheric
river event comes to an end.
Orrison
ATTN...WFO...LOX...SGX...
ATTN...RFC...CNRFC...NWC...
LAT...LON 34792012 34761961 34751919 34731844 34411757
34321686 34081666 33741653 33401661 32791648
32481657 32501709 32401715 33021727 33681820
33771856 34041919 34331958 34392023
Last Updated: 1015 PM EST Thu Jan 28 2021
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