Subj : HVYRAIN: Excessive Rainfa
To   : All
From : Mike Powell
Date : Sat Nov 11 2023 08:27:00

FOUS30 KWBC 110747
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
247 AM EST Sat Nov 11 2023

Day 1
Valid 12Z Sat Nov 11 2023 - 12Z Sun Nov 12 2023

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Pereira


Day 2
Valid 12Z Sun Nov 12 2023 - 12Z Mon Nov 13 2023

....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN TEXAS...

A mid-to-upper level trough will drift east across northwestern
Mexico and the southwestern U.S. this period.  Moist southwesterly
flow aloft along with strengthening low level easterly winds will
fortify an already moist airmass in place across the region. 
Forecast soundings show a deeply saturated column with PWs near 2
inches (more than 2 standard deviations above normal) along the
lower Texas Coast back into the Rio Grande Valley.  This moisture
along with large-scale ascent afforded in part by right-entrance
region upper jet forcing will support widespread precipitation
across the region.  While moisture and lift will be ample,
instability will not -- limiting the threat for heavy rainfall
rates.  However, a deep warm cloud layer and high low-to-mid level
RHs will likely support efficient rainfall rates that may cause
localized flooding in poor drainage and urbanized areas.  While
both the Euro Ensemble and the GEFS agree that accumulations of 2
inches or more are likely, the Euro has been farther north than
the GEFS, indicating a greater threat for heavy rainfall extending
into south-central Texas.  The current Marginal Risk accounts for
both camps but may require adjusting pending model trends.

Pereira


Day 3
Valid 12Z Sun Nov 12 2023 - 12Z Mon Nov 13 2023

....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTHERN LOUISIANA...

As a mid-to-upper level trough continues to drift east into Texas
and northeastern Mexico, a surface low is forecast to develop
along a lingering frontal boundary and track east from the
northwestern to the north-central Gulf.  Deep southwesterly flow
ahead of the wave will direct increasing moisture into an area of
large-scale ascent supported by favorable upper jet forcing.  The
deepest moisture is forecast center along the central Gulf Coast,
with PWs increasing to around 2 inches across southern Louisiana
this period.  The lack of instability will remain a limiting
factor for heavy rainfall rates, however deep warm cloud layers
and high low-to-mid level RHs will support efficient rates,
raising the threat for localized runoff concerns in poor drainage
and urbanized areas.  Both the Euro Ensemble and the GEFS show
high probabilities for accumulations exceeding 2 inches within the
Marginal Risk area.

Pereira

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