Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
409 AM EDT Fri Apr 19 2024
Day 1
Valid 12Z Fri Apr 19 2024 - 12Z Sat Apr 20 2024

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

...Texas into southern Oklahoma...

The inherited Marginal Risk area from the Big Bend of Texas north
and east to the Red River Valley has been downgraded with this
update.

Starting south and working north, model guidance is in very good
agreement that any convection Friday afternoon and evening near the
Rio Grande will largely stay on the Mexico side of the river. While
it is probable that some remnant dying convection may make it over
the Rio Grande to the Texas side from Eagle Pass north, it appears
likely that much like today, the loss of terrain support should
ensure any convection that tries to move off the mountains fizzles
out quickly. However, these localized areas of convection can have
a mind of their own, which is to say may develop embedded
mesolows, which can allow the convection to persist for far longer
than expected. Since at the moment this appears to be a low
possibility, the combination of high FFGs and highly localized
convection should preclude flash flooding on the Texas side, so the
Marginal was removed.

Along the Red River, the area will be at the head of a rather
modest LLJ that will advect some Gulf moisture northward. Much of
the guidance shows an area of showers developing in response, but
lack of instability largely holds the shower activity to that
level, with relatively little embedded convection expected
overnight Friday night. Any showers and the embedded thunderstorm
or 2 should both move off to the east, and the limited instability
will cap the strength of any storms. FFGs in this area are also
quite high, so the flash flooding potential is low enough to not
meet Marginal criteria.

...Portions of the Southern Appalachians...

The remnants of the convection ongoing across portions of the Ohio
Valley will make their way into the southern Appalachians Friday,
particularly into the Virginias. Associated westerly flow will
upslope when the storms reach there, allowing them to maintain
their strength up until the crest of the Appalachians. Despite the
upslope, there will be little else supporting the showers...with
limited atmospheric moisture and instability. The one factor
causing just a hint of concern are the antecedent conditions, with
low FFGs into the southern half of WV and southwest VA. Since the
rain will really struggle to develop vertically into thunderstorms,
much less be strong enough to support heavy rain, the threat here
is also sub-Marginal. The area will continue to be monitored.

Wegman