WPC Met Watch

WPC Met Watch


Mesoscale Precipitation Discussion: #0209 (Issued at 1017 AM EDT Thu May 02 2024
) MPD Selection


Graphic for MPD #0209

Mesoscale Precipitation Discussion 0209
NWS Weather Prediction Center College Park MD
1017 AM EDT Thu May 02 2024

Areas affected...Central to Upper TX Coast...Southwest LA...

Concerning...Heavy rainfall...Flash flooding likely

Valid 021415Z - 022000Z

SUMMARY...Significant flash flooding ongoing across southeast
Texas. While rates and areal coverage is expected to decrease
through the morning, high rain rates of 3"/hr and training over
the next few hours will pose additional areas of new considerable
flooding through the early afternoon.

Leading edge...Louisiana...
GOES-E Visible/10.3um IR loop along with RADAR mosaic, depicts a
mature/intense MCS with well defined MCV over the Piney Woods of E
Texas moving into west-central LA with well defined bow extended
southward to the Sabine Pass/BPT.  These bows have solid
southeasterly inflow with surface Tds in the mid 70s, while
boundary layer to 700mb moisture is pooled through the central LA
coast feeding northwest into the complex. Fairly saturated deep
profile from LCH still retains over 2000 J/kg of CAPE, to maintain
strong updrafts.  However, forward propagation may limit overall
rainfall totals, but given 2-3"/hr rates, initially, post frontal
shield precipitation should maintain modest rain-rates (.5"/hr) to
allow for streaks of 2-4" totals across Southwest LA, favoring
further south where moisture convergence will maximize in
proximity to the higher Theta-E/more unstable Gulf air. 

Southeast Texas including Houston...
While MCS cold pool is pressing the leading edge eastward, there
is currently a solid balance between weaker southward push of the
cold pool and southerly over-running ascent pattern that lays more
perpendicular to the outflow boundary.  As such, an east-west
linear feature with embedded elements tracking more eastward have
resulted in greatest risk of considerable/significant flash
flooding over the last few hours.  This is likely to maintain with
2-3"/hr rates over the next few hours with very slow southward
drift across Liberty/E Harris into Chambers county with spots of
additional 3-6" probable through to late morning/early afternoon.

GOES-Visible imagery shows the westward edge of the outflow
boundary is starting to become more shallow.  However, the old
rear-inflow jet convergence axis developed a secondary convective
area, that generated a second cold pool that is dropping south to
reinforce and steepen the depth of the cold pool and likely
increase boundary layer to 850mb moisture convergence. 
Additionally, VWP and GOES low level AMVs suggest weak veering and
confluence along this upwind edge as well.   This strongly hints
at upstream new development over the next hour or so, as the area
has ample untapped/unstable and highly saturated low level
moisture (with sfc/850mb PW over 1").   This should allow for
rates of 2.5-3"+/hr rates but there is uncertainty of duration.

This uncertainty is driven by the strength of the upstream cold
pool undercutting and pressing best convergence southward into the
Gulf, reducing overall duration and therefore totals.  This is the
ideal scenario and may result in localized 2-4" totals and spotty
flash flooding; however, there are model and observational trends
in RADAR/satellite imagery that may be much worst cast  scenarios.
 Worst case evolution would result from the development of a
meso-low generation along the initial outflow boundary in
proximity to the western Houston suburbs or points westward in
Austin/Colorado county that may stall southward cell motions
allowing for efficient 3"/hr rate cells to remain relatively
stationary for a few hours with spot totals of 6-10" by early
afternoon...which would result in significant/ considerable flash




LAT...LON   31749268 30959188 29619192 29659368 29189476
            28539630 29769715 30359703 30529645 30699461
            30959392 31479351 31739310

Download in GIS format:    Shapefile
 | KML

Last Updated: 1017 AM EDT Thu May 02 2024

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