Mesoscale Precipitation Discussion 0647
NWS Weather Prediction Center College Park MD
459 AM EDT Mon Aug 13 2018

Areas affected...portions of southwestern to northern TX

Concerning...Heavy rainfall...Flash flooding possible

Valid 130858Z - 131458Z

Summary...A steady increase in training cores of heavy rain is
expected through 15Z. Flash flooding will be possible across
portions of southwestern to northern TX with localized 2-4 inch
rainfall totals.

Discussion...Over the past one to two hours, radar has shown an
increase in shower/thunderstorm activity on either side of I-20
from near SWW to MWL. This has been co-located with a 400 to 600+
J/kg increase in MLCAPE over the past 3 hours (ending 08Z)
according to the SPC mesoanalysis page. A rather strong mid-upper
level trough was observed on water vapor imagery over the southern
High Plains, moving east into western TX/OK. On the southeast side
of this trough axis was a RAP analyzed 50-70 kt jet streak at 250
mb along with a region of pronounced upper diffluence. The region
of largely stratiform rain with embedded heavier elements has been
progressing northward across TX since 03Z, following the leading
edge of a 20-30 kt 850 mb low level jet. 850-300 mb flow is from
the south to southwest at 20 kt and so Corfidi Vectors are zero or
oriented into the inflow at up to 10 kt. There is plenty of
tropical-like moisture in place with precipitable water values
near 2 inches across south-central to north-central TX, but
instability and stronger forcing have been missing ingredients up
until this point for a more widespread precipitation event.

As lift ahead of the approaching upper trough continues to
overspread the region through 15Z, weak instability (less than
~1000 J/kg) is forecast to move into the Red River Valley. Given
favorable thermodynamic and kinematic profiles in place, periods
of training rainfall is expected to develop with rainfall rates of
1-2 in/hr. Instability is the greatest limiting factor in the
forecast, but pockets of locally higher instability are likely to
support areas of training heavy rain across portions of north TX
into southern OK.

Farther to the southwest, forecast 850 mb convergence should set
up from near the Big Bend region of TX (6R6) to I-20 (SWW, DYS)
beneath an area of upper divergence tied to the right entrance
region of the aforementioned upper level jet streak. Again, weak
instability is a limiting factor, but the degree of moisture and
forcing may be enough to overcome lower instability to produce an
axis of repeating cells within a region of low Flash Flood
Guidance.

Otto

ATTN...WFO...EWX...FWD...LUB...MAF...OUN...SHV...SJT...TSA...

ATTN...RFC...ABRFC...LMRFC...WGRFC...

LAT...LON   34449622 34189476 33689462 33279490 32979561
            32559681 32299804 31979891 31439989 30860049
            29940118 29640159 29650183 29710222 30130231
            30800209 32080150 33170050 33929904 34369766