Graphic for MPD #0103
Mesoscale Precipitation Discussion 0103
NWS Weather Prediction Center College Park MD
1002 PM EDT Fri Mar 15 2024

Areas affected...Lower Rio Grande Valley of Texas...

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

Valid 160200Z - 160730Z

SUMMARY...Broad downdraft supercells with large hail and intense
rainfall rates to cross the Rio Grande with left-splits and
upscale development into a broader complex into the overnight
period.  Highly focused and slow cell motions may result in
2-3"/hr totals and possible flash flooding.

DISCUSSION...GOES-E and regional RADAR mosaic denotes a pair of
broad updraft supercells along the Mexican side of the Rio Grande
continuing to translate slowly along the instability axis.  Slow
southward sagging cold front is aiding moisture flux convergence
into the theta-E axis that is banked up along the Sierra Madre
Oriental/Serranias del Burro, with Tds of low to mid-70s and
return southeasterly flow parallel to the axis along the Rio
Grande at 20-25kts resulting in total PWats of 1.25 to 1.6 from
south to north.  Elevated mixed layer at 700-500mb from the
south-southwest has allowed for very steep lapse rates and
therefore continued very high instability of 2500-3500 J/kg of
CAPE for cells to utilize.   While this profile provides for hail
production, the ample deeper layer moisture (generally up to
800mb) and solid flux will allow for accompanying intense rainfall
production as well with 2-3"/hr rates possible.

The uncertainty resides in a couple of factors; (1) Duration,
though broad updrafts and slow right moving cell motions may allow
for 30-45 minutes of intense rainfall production overall totals
may still be a the lower threshold of the higher capacity soil
conditions in the region per FFG values of 3"/hr and 4"+/3hr.  (2)
Location, current trends are for continued strong easterly flow
through the theta-E axis into the upslope region of the Sierra
Madre keeping upstream redevelopment along the terrain and with
deep layer effective bulk shear over 60kts...cells are more likely
to turn right along the axis resulting in training more so on the
Mexico side of the boarder. However, left splitting updrafts are
likely throughout the cycle and likely limited in duration moving
away from the best moisture flux, still may remain potent enough. 
 (3) Upscale growth,  while models are less aggressive on this
scenario, there are suggestions in the WV/EIR loops and RADAR
mosaic for development to be filling in allowing for a larger
mesoscale complex to develop over the next few hours.   The RAP
hints as some increased divergence along the lower Rio Grande in
the next few hours with shortwave ridging developing.  This may be
manifest out of the strength of the updrafts and mesoscale outflow
jet or some combination with the upper-jet/closed orientation to
the NW across S NM/W TX.  Either way, any upscale growth would
favor an eastward migration resulting in intense rates into South
Texas.

Bottom line, strong cells with rainfall intensity to rapidly
overwhelm even dry soils may result in rapid runoff and possible
flash flooding in and around the Rio Grande Valley through the
early overnight period with low confidence in precise placement.

Gallina

ATTN...WFO...BRO...CRP...

ATTN...RFC...WGRFC...NWC...

LAT...LON   28169931 27969888 27519846 26629816 26099788
            26039833 26389913 26869945 27309960 27649980
            27869998 28030005 28119992
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Last Updated: 1002 PM EDT Fri Mar 15 2024