Mesoscale Precipitation Discussion 0226
NWS Weather Prediction Center College Park MD
512 AM EDT Mon May 24 2021
Areas affected...Middle and Upper Texas Coast into the Hill
Country
Concerning...Heavy rainfall...Flash flooding possible
Valid 240910Z - 241400Z
Summary...Showers and thunderstorms developing within a pronounced
moisture plume advecting from the Gulf of Mexico will increase in
coverage through morning. Rainfall rates of 2"/hr are likely, and
as these storms move slowly and train to the northwest, 1-3" of
rainfall is likely with locally higher amounts. Flash flooding is
possible.
Discussion...Mid-level trough axis which has plagued Texas for
several days now is slowly beginning to weaken, but will maintain
its position across western portions of the state through this
morning. East of this trough, a shortwave evident on GOES-E WV
imagery is lifting slowly northward along the I-35 corridor near
Austin. Beneath this shortwave, mid-level lapse rates are becoming
subtly steeper, which is combining with increasing onshore flow
from the Gulf of Mexico to enhance instability across eastern and
southeastern Texas. A tropical moisture plume of PWs above 1.8"
according to GPS observations is also lifting onshore, and the
combination of these features has led to slowly expanding coverage
of showers and thunderstorms early this morning on KHGX and KCRP
WSR-88D reflectivity.
As the shortwave continues to lift northward through the morning,
model fields indicate that both instability and moisture will be
drawn more efficiently northward. MUCape is forecast to rise
towards 2000 J/kg, while PWs may approach 2", well above the 90th
percentile for the date. This should produce increasingly
efficient rain rates, and the HREF probabilities indicate the
potential for 2"/hr rates across the region. In the vicinity of
the shortwave aloft, mean winds weaken to 5-10 kts, suggesting
storm motions inland will be quite slow. Nearer to the coast
850-300mb mean winds may be as high as 20 kts, but Corfidi vectors
angled against this mean wind suggests backbuilding into the
greater instability over the Gulf is likely, with regeneration and
training leading to heavy rainfall accumulations.
The high res guidance this morning agrees that an axis of rainfall
exceeding 2" is likely, but the longitudinal placement varies
considerably. However, recent rainfall has been excessive,
reaching 600% of normal the past 7 days, and this is reflected by
soil moisture that is above the 98th percentile to a depth of
100cm according to NASA sport, leading to 3-hr FFG as low as 2".
These efficient rain rates could quickly turn to runoff as these
soils cannot handle additional rainfall, so any training or
backbuilding could lead to pockets of flash flooding anywhere
within the discussion area.
Weiss
ATTN...WFO...CRP...EWX...FWD...HGX..