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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0817Z Oct 20, 2018)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
417 AM EDT Sat Oct 20 2018

Day 1
Valid 01Z Sat Oct 20 2018 - 12Z Sat Oct 20 2018

...A MARGINAL RISK OF EXCESSIVE RAINFALL CONTINUES OVER PORTIONS
OF EASTERN TEXAS...

...Portions of Eastern Texas to the Upper-Middle Texas Coast...
Deep-layer instability remains an inhibitor to high short term
rainfall rates north of the surface front (north of CLL-UTS-LKF).
However, persistent frontogenetic right-entrance region forcing
along with southwesterly low-level flow veering more parallel to
the mean 850-300 mb flow will make for repetitive elevated
convection clusters overnight over the northern portion of the
Marginal Risk area. The latest HREF and SSEO QPF>FFG exceedance
probabilities support the Marginal.

Farther south toward the coast -- deep layer instability will be a
bit more robust early (MUCAPES 1000-2000 j/kg through 06Z),
supportive of higher 1-3 hour rainfall rates along the surface
boundary. The lack of low-level inflow however (15 kts early, then
decreasing to ~10 kts) along with the persistent directional shear
in the low-mid layers would continue to promote progressive QLCS
motion, as evidenced by the w-nw Corfidi vectors at 10-15 kts.
Thus have maintained the Marginal Risk across this region as well.

Hurley


Day 2
Valid 12Z Sun Oct 21 2018 - 12Z Mon Oct 22 2018

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER SOUTH TEXAS
AND PORTIONS OF NEVADA, SOUTHWEST UTAH AND NORTHWEST ARIZONA...

...South Texas...
Tropical moisture and marginal instability focused along a surface
trough near the South TX coast feeds low topped convection that
produces heavy to locally excessive rainfall during Day 2. The
best moisture and instability remain near the far South TX coast,
especially before 22/00z, and eventually becomes focused on the
inverted surface trough, which appears as though it aligns itself
with the Rio Grande River near KBRO. Much of the 00z guidance
showed the axis of highest rainfall along and south of the river,
with the 00z ECMWF keeping the axis north of the river.

Assuming that the best instability remains near and south of the
inverted trough, the southward shift in the guidance seems
plausible. This lowers the flash flood threat across far south TX,
and the Slight Risk no longer appears to be supported by the WPC
QPF and the majority of the 00z guidance. With this in mind, a
Marginal Risk was placed along the Rio Grande River to cover the
flash flood threat.

...Southwest/Great Basin...
The closed mid level low crossing CA will provide steepening lapse
rates in conjunction with deep moisture to feed low topped
convection capable of producing heavy to locally excessive
rainfall during Day 2. Model soundings showed generally less than
500 J/KG in place over portions of far southern NV into northwest
AZ and far southwest UT, peaking near 22/00z. However, a
moistening column in the presence of 0.75 inch precipitable water
air (which is between two and three standard deviations above the
mean) could at least partially offset the lack of deeper moisture
to produce low topped convection. Weak 850-300 mb mean winds and
propagation vectors foster an environment with cell mergers and
short term training. 
 
While the 00z GFS may be overdone with its maximum rainfall
amounts over northwest AZ and far southern NV (with amounts
approaching 2.00 inches), local 1.00 inch amounts are possible
where cells merge or train ahead of the closed mid level system.
These rainfall amounts may be sufficient for local runoff issues,
and possible isolated flash flooding. With this in mind, a
Marginal Risk was placed here for Day 2. 

Hayes

Day 3
Valid 12Z Mon Oct 22 2018 - 12Z Tue Oct 23 2018

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER THE WESTERN
AND CENTRAL GULF COAST...

...Western and Central Gulf Coast...
Two areas could experience heavy to locally excessive rainfall
during Day 3 across the Western and Central Gulf coast. The first
is over portions of interior south TX (generally south of 30
degrees north), where deep moisture (with precipitable water
values between 1.50/1.75 inches) and elevated instability
(generated rooted near 850 mb) could support the development of
low topped convection. Short wave energy rotating around a
weakening mid level ridge over the western Gulf of Mexico crosses
central TX about 22/18z, and could produce enough synoptic scale
lift to organize any elevated convection.

Most of the 00z guidance shows a swath of 1.00/1.50 inches of
rainfall across south central TX (though both the 00z NAM/ECMWF
are higher with their rainfall amounts here). The placement of the
rainfall is expected to remain south of the area that has received
more than 500 percent of normal rainfall in the past week. Since
the three hour flash flood guidance is generally above 2.50 inches
here, a Marginal Risk was placed over interior sections of South
TX to cover the flash flood threat. If model guidance comes into
better agreement with the placement of the axis of heaviest
rainfall in this area, a Slight Risk could be needed in subsequent
forecasts.

The second area of concern is along the Gulf coast from south TX
to southeast LA. A wave of low pressure forming on a frontal
boundary extending across central Gulf of Mexico is expected to
remain south of the LA coast during Day 3. Drier air associated
with surface high pressure over the TN Valley and Mid Atlantic
states is expected to displace the best instability away from the
coastal sections, keeping the highest rainfall amounts just
offshore (with the exception of southeast LA, where there seems to
be a better signal for heavy rainfall). The 00z ECMWF brings
rainfall amounts over 2.00 inches to portions of the middle and
upper TX coast (including the Houston metro area), but this
solution is further north than the consensus.

For now, a large Marginal Risk area was stretched from the south
TX coast through the upper TX coast to coastal LA (which was
collaborated with the affected WFOs). Should there be better model
agreement bringing the heavy rain threat closer to the coast from
Houston to New Orleans, a Slight Risk could be needed in later
forecasts.

...Utah/western Colorado...
As a closed mid level low over western NV early on Day 3 opens up
and tracks across the Great Basin, steepening lapse rates could
result in marginal instability across UT and western CO,
especially after 22/18z. Model soundings generally showed 500 J/KG
or less of MUCAPE during this time, which should be sufficient for
at least scattered convection. The convection could to tap into
the 0.50/0.75 inch precipitable water air in place (which is
between two and three standard deviations above the mean),
resulting in locally heavy rainfall across the higher terrain.

The 850-300 mb mean winds become better aligned with the
propagation vectors in the afternoon and evening, which could
result in short term training, especially over central and eastern
UT. Right now, it is unclear whether the convection becomes
widespread enough to produce a low level flash flood threat.
Because of this, no risk areas were assigned to UT/western CO.
However, if there is better model agreement on increased
convective coverage on Day 3, a Marginal Risk could be needed here
in later forecasts.

Hayes


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