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...
Carla/Alicia/Jerry(In The Eye)/Michelle/Charley/Ivan/Dennis/Katrina/Rita/Wilma/Humberto/Ike/Harvey
Member: National Weather Association
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I'm in Rosharon and was driving through Angleton / Rosharon / Manvel areas when all of this was coming through and didnt notice very much wind. Maybe gusts in the 20 mph or so range if anything. Are all the winds displaced to the east side ?
NWS has issued an Urban Flood Advisory for SE Harris County and Galveston County until 515pm
Band of heavy rainfall on the NE flank of the circulation center extends along the I-45 corridor from Galveston to League City and then WNW toward Pearland.
Rainfall rates of 1-2 inches are likely with this band which may produce street flooding. This rainfall is falling in top of areas that have already experienced 1-3 inches in the last 24 hours and rises on area creeks in the SE part of Harris County along Clear Creek, Beamer Ditch, and Turkey Creek are likely.
24 hour storm total of 4.67 inches has been recorded at the HCFCD gage at Turkey Creek and FM 1959.
Additional bands of heavy rainfall are likely to develop and move across the area this evening into the overnight hours.
Carla/Alicia/Jerry(In The Eye)/Michelle/Charley/Ivan/Dennis/Katrina/Rita/Wilma/Humberto/Ike/Harvey
Member: National Weather Association
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Twitter @WeatherInfinity
Cromagnum wrote: ↑Tue Sep 17, 2019 4:19 pm
I'm in Rosharon and was driving through Angleton / Rosharon / Manvel areas when all of this was coming through and didnt notice very much wind. Maybe gusts in the 20 mph or so range if anything. Are all the winds displaced to the east side ?
I'll try to not be to critical of the nhc but I'm iffy about this being a ts. Quick run along the beach and maybe 15-20 kts sustained with maybe a few TS force gusts if I'm being real generous.
Either way has no real overall impacts with regards to the rain.
Cromagnum wrote: ↑Tue Sep 17, 2019 4:19 pm
I'm in Rosharon and was driving through Angleton / Rosharon / Manvel areas when all of this was coming through and didnt notice very much wind. Maybe gusts in the 20 mph or so range if anything. Are all the winds displaced to the east side ?
I'll try to not be to critical of the nhc but I'm iffy about this being a ts. Quick run along the beach and maybe 15-20 kts sustained with maybe a few TS force gusts if I'm being real generous.
Either way has no real overall impacts with regards to the rain.
It is a tropical storm... a minimal tropical storm is not going to have widespread winds as the windfield isn't organized enough. The rainfall remains the main threat. And I don't see how metro Houston misses the core of Imelda as its already effecting the southern half of Harris County, its pretty obvious at this point that the center of circulation will pass through the city.
I am getting increasingly concerned with the potential for a core rain event. The WPC basically said as much without using those exact words. Additionally they highlighted what I thought I saw on some of the earlier meso models - a convergence zone setting up a heavy band on the south side of the center some time tomorrow.
I know its the NAM and I like to make fun of it as much as anyone else, but it has been picking up on both of these things fairly consistently for the past few runs.
Watching the last few frames of radar it looks like the storm just took a hard jog to the west. Maybe I've been looking at a computer screen too long today. Anyone else see it?
sau27 wrote: ↑Tue Sep 17, 2019 4:51 pm
Watching the last few frames of radar it looks like the storm just took a hard jog to the west. Maybe I've been looking at a computer screen too long today. Anyone else see it?
Imelda seems to be a small storm in size which is probably allowing her to tighten up and become more defined as it moves inland instead of rapidly weakening after landfall,some of the models hinted at this.
