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Excessive Rainfall Discussion
 
(Latest Discussion - Issued 0106Z Dec 04, 2022)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
805 PM EST Sat Dec 03 2022

Day 1
Valid 01Z Sun Dec 04 2022 - 12Z Sun Dec 04 2022

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL CALIFORNIA COAST...

...01z Update...
Little to no changes required to the inherited outlook this
evening, as much of the expected rainfall has yet to occur across
coastal and inland portions of central CA. The main anticipated
shortwave impulse has yet to approach the shore near the Santa
Lucia Range (see MPD #1142 and subsequent MPDs for more info), so
the Slight Risk upgrade for portions of the central CA coast
remains on track. Additional embedded convection is also expected
over some of the same areas of southeastern AZ, with only minor
adjustments (trimming) needed for this Marginal Risk area.

Churchill


...16Z Update...

The main change to the Day 1 ERO is an upgrade to a Slight Risk in
northern San Luis and southern Monterey counties in California.
While the overall lack of instability will keep hourly rainfall
rates generally below 0.50"/hr most of the day, after 21Z,
southeasterly 850-700mb layer winds will increase and aid in a
surge in 850-700mb moisture flux over the terrain. The
strengthening wind field oriented orthogonally to the terrain will
also support stronger ascent via enhanced upslope flow. Recent
runs of the HRRR/3kmNAM have shown the potential for elevated
MUCAPE by 00Z that could be ~100-200 J/kg just off the coast,
which would be more supportive of hourly rainfall rates >0.5"/hr
later this afternoon and evening. Coming into Saturday morning,
soil moisture percentiles in the 0-40cm layer were already up to
80% in some portions of these counties according to NASA
SPoRT-LIS, and will only continue to moisten up as rain continues
to fall over the area this morning. The latest 12Z HREF depicted
3-hr FFG exceedance probabilities of 15-20% between 21-00Z this
afternoon and 00-03Z this evening, which aligns closely with when
the heaviest rainfall rates are likely to transpire. Localized
rainfall totals between 16Z today and 12Z Sunday morning could
surpass 5", with the latest 12Z HREF's 24hr QPF >5" probabilities
as high as 50% within the newly added Slight Risk area. In
collaboration with WFOs San Francisco/Bay Area and Los Angeles, a
Slight Risk was added along the higher terrain of northern San
Luis and southern Monterey counties where the combination of
moistening soils and sufficiently heavy rainfall rates could lead
to rapid runoff along complex terrain and localized water rises in
poor drainage areas.

For the other two Marginal Risk areas, made minor tweaks to
account for minor adjustments in QPF trends. The forecast
reasoning from overnight still appears on track as of this
issuance.

Mullinax


...Previous Discussion...

...California...

An upper level cutoff low centered off the coast of OR will drift
southeastward towards the northern CA coast today. This forcing
will team up with a subtropical disturbance and associated Pacific
moisture to drive it into central CA today. The front moving into
the Point Conception area will continue drifting northward through
the morning, with some intensification in the rainfall rates as
the upper levels increasingly contribute to the heavier rainfall
rates. That said, instability is very low in this area, so the
mountains will be the primary driver of heavy rain. This area of
rain will stall between Monterey Bay and Point Conception today,
which should allow for several (2-4) inches of rain to fall into
the coastal mountains. The only real change from inherited was to
nudge the southern end of the Marginal a bit north towards Point
Conception, since there is a well defined back edge to the rain
moving north and is not expected to move back south once that back
edge moves through. Hourly rainfall rates have been one quarter
inch or less so far, which is well below anything that would cause
flooding.

Further inland, the unidirectional southwest flow will run up
nearly perpendicular to the Sierras of eastern California today.
These much higher mountains will then wring out even more
precipitation than the coastal mountains. While the highest peaks
can expect snow, the high snow levels should allow much of the
precipitation to fall as rain. The inherited Marginal Risk area
remains unchanged. Expect 3 to locally 5 inches of rain near or
over the mountains that will runoff quickly, causing isolated
flash flooding in the foothills.  

...Southeastern Arizona...

The inherited Marginal Risk area was trimmed a bit to align with
the forecast axis of heaviest precipitation, in coordination with
the EPZ/El Paso, TX, TWC/Tucson, AZ, and ABQ/Albuquerque, NM
forecast offices. The heaviest rainfall, which could exceed 2
inches in 24 hours in isolated spots, extends from the Mexican
border northeast through metro Tucson and then northeast to the 
New Mexico border. The Marginal Risk was removed from New Mexico
as the guidance becomes better aligned further north and west.
This separate plume of moisture from the one impacting the
California Coast will have some upper level support in the form of
a right entrance region of an upper level jet along with highly
anomalous moisture featuring PWATs of 1.25 inches, which is over 5
standard deviations above normal for this time of year. Thus, the
moisture has very tropical origins. With that said, the lack of
instability will be the primary factor reducing rainfall rates
over this area. Meanwhile soil moisture in this area is around
average for this time of year.

Wegman



Day 2
Valid 12Z Sun Dec 04 2022 - 12Z Mon Dec 05 2022

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHEAST ARIZONA AND SOUTHWEST NEW MEXICO...

Most of Arizona and New Mexico will remain beneath the
right-entrance region of a ~110 knot 250mb jet streak while,
simultaneously, an upper low off the Pacific Northwest coast
continues to funnel rich subtropical moisture overhead. NAEFS
climatological percentiles remain eceptionally high over southeast
Arizona and much of New Mexico Sunday morning and afternoon with
values above the 99th climatological percentiles in some cases.
The IVT also remains quite robust as the upepr low off the Pacific
Northwest coast and the ridge over the Gulf of Mexico work in
tandem to generate IVT values of 400-500 kg/m/s, which is also
above the 99th climatological percentile. Similar to Saturday,
however, instability will continue to be hard to come by, keeping
a lid on rainfall rates from becoming too heavy. That said, the
region will have picked up heavy rainfall from the previous day
and soil moisture will be more elevated than it was entering the
weekend. Chose to introduce a Marginal Risk for a similar area
that was under a Marginal Risk on Day 1, with the expectation that
soils will be more sensitive following Saturday's heavy rainfall.
Communities and roads that reside along complex/rugged terrain,
near dry washes, and in poor drainage areas are most prone to
localized flooding.

Mullinax


Day 3
Valid 12Z Mon Dec 05 2022 - 12Z Tue Dec 06 2022

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR NORTHERN
MISSISSIPPI & ALABAMA, NORTHERN GEORGIA, AND FAR SOUTHERN
TENNESSEE...

A strong cold front pushing through the Midwest on Monday will
occur at the same time as a warm front lifts north into the
Mid-South. High pressure over the East Coast will work in tandem
with lower pressure in lee of the southern Rockies to generate a
southerly 850mb moisture moisture fetch out of the western Gulf of
Mexico. This slug of moisture will coincidily join the subtropical
jet emerging from the Southwest and result in a robust IVT across
the Southern Plains and into the Mid-South. 00Z NAEFS suggests IVT
values of 400-750 kg/m/s are possible across the South Central
U.S. on Monday, which is >97.5% climatological percentile in many
cases. As the upper low off the California coast and deep upper
low in south-central Canada continue to help generate a strong jet
streak oriented WSW to ENE across the U.S., divergent flow will be
positioned over the South, setting the stage for a heavy
precitipation event along the warm front in the Mid-South on
Monday.

There do remain some differneces on global and ensemble guidance
regarding positioning of the heaviest QPF axis. Latest Day 3 QPF
resmblems more of a Euro/GFS/CMC blend, weighted a bit more
towards the Euro and its ensembles. This places the heaviest swath
of rainfall over northern Alabama, northern Mississippi, northern
Georgia, and southern Tennessee. Soil moisture percentiles are
right about average for the time of year, but the last 7-days have
featured some anomalous rainfall where some locations witnessed
300-400% of normal rainfall (northern Alabama and northern Georgia
in particular). Some ensemble guidance does show MLCAPE by Monday
evening could approach ~500 J/kg in the Lower Mississippi Valley
as the low level jet picks up in strength Monday night. Latest Day
3 QPF shows rainfall totals of 2.5-5" in northern Alabama,
southern Tennessee, and northern Georgia, which is where the bulk
of the Slight Risk lies. The Slight Risk also extends as far west
as northern Mississippi, which is currently forecats to lie
closest to the best available instability late Monday.

Mullinax


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt