new Met Watch discussion
Posted by cypresstx on 9/17/2019, 4:59 pm
https://www.wpc.ncep.noaa.gov/metwatch/metwatch_mpd_multi.php?md=860&yr=2019



Mesoscale Precipitation Discussion 0860
NWS Weather Prediction Center College Park MD
442 PM EDT Tue Sep 17 2019

Areas affected...Upper Texas Coast

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

Valid 172041Z - 180241Z

Summary...As Tropical Storm Imelda continues to slowly drift
further inland, there will be an increasing threat of flash
flooding into the evening hours on the Upper Texas Coast. This may
include portions of the Houston Metro Area.

Discussion...There are two primary areas of concern for flash
flooding associated with Tropical Storm Imelda as it continues to
move inland late this afternoon and into the evening. The first
area would be associated with tropical rain bands to the east of
the track of the center, embedded within a ribbon of persistent,
enhanced moisture flux and stronger low-level flow in the eastern
semi-circle of the storm. Places around Galveston Bay would be the
most likely to be impacted by these rain bands. The second area
would be associated with increasing coastal convergence in the
coastal counties of Matagorda, Brazoria and Galveston. Winds
offshore would be likely to back slightly (increasing southerly
component) and remain stronger relative to over-land winds as
Imelda's center moves inland. This could increase convergence
closer to the coast, and coincide with a slight increase in
instability as more unstable air over the Gulf gets advected just
onshore. The combination of low-level convergence and instability
may support increasingly organized and stronger rain bands.
Overall, these expectations align with numerical model predictions
of the heaviest rain falling to the east and south of the center.
Given the slow motion of Imelda (likely to move only about 50
miles in 9 hours based on the official NHC forecast), organized
tropical rain bands may persist in these two areas for an extended
period of time, which would lead to an increasing flash flood
threat.

Rain rates in the stronger rain bands would be likely to reach the
2-3 in/hr range, and could exceed that if stronger instability can
develop onshore and coincide with low-level forcing mechanisms
mentioned above. If that occurs, rain rates into the 3-5 in/hr
range could occur on an isolated basis in the strongest, most
organized rain bands. Several gages in southern Harris County near
the Sam Houston Tollway and I-45 have measured 1.16 inches of rain
in 30 minutes (20:00 to 20:30Z), and 5-minute rain rates have been
as high as 0.36 inches. This is an early indicator of very high
instantaneous rain rates in the tropical rain bands, but thus far
the rain bands have been relatively narrow and transient relative
to stationary locations. However, if larger rain bands can become
established, more substantial hourly rain rates would be likely,
and this would be more likely to have notable flash flood impacts.

The concern for the overnight hours is if several inches of rain
from early rain bands pre-condition the ground along the Upper
Texas Coast this afternoon and early this evening, and then
another burst of convection develops closer to Imelda's center
overnight. Convection tends to concentrate closer to the center of
circulation in the overnight period of the diurnal cycle, and this
could enable additional heavy rain over areas that already
received heavy rainfall earlier in the day. The NHC forecast also
calls for the center to move very close to Houston by 06Z, which
would place a potentially favored area for nocturnal convection
directly over a major metro area. This will continue to be
monitored into the evening hours.

Lamers

ATTN...WFO...CRP...HGX...LCH...

ATTN...RFC...WGRFC...
46
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NHC Director Ken Graham will provide a special Facebook Live broadcast at about 3 p.m today (Tuesday) - Chris in Tampa, 9/17/2019, 2:32 pm
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