MESOSCALE PRECIPITATION DISCUSSION 0858
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
144 PM EDT TUE OCT 03 2017

AREAS AFFECTED...SOUTHEAST TEXAS...

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 031745Z - 032245Z

SUMMARY...HIGHLY EFFICIENT/RATES 2.5-3"/HR ACROSS HOUSTON METRO
POSES FLASH FLOODING RISK. POTENTIAL FOR BACKBUILDING/TRAINING MAY
EXACERBATE SITUATION IF IT SETS UP.

DISCUSSION...GOES-16 IMAGERY LOOP SUITE DEPICT A STRONG TROPICAL
UPPER LOW ACROSS THE SW GULF WITH A NORTHWARD EXTENDING TROPICAL
WAVE ACROSS SOUTH CENTRAL TX CURVING BACK INTO RETURN FLOW ACROSS
NE TX. STRONG INFLUENCE OF SOUTHWEST RETROGRADING STRONG RIDGE
INTO MS/AL ALLOWS FOR MEAN FLOW TO BE GENERALLY WEAK BUT ALSO
QUITE CONFLUENT ACROSS SE TX/SW LA AND ADJACENT GULF WATERS. 
WHILE THE MEAN LAPSE RATES ARE MOIST ADIABATIC/GENTLE THE VERY
WARM GULF AIR MASS IS PROVIDING AMPLE LOW LEVEL LATENT HEAT WITH
UPPER 70S TDS BEING ADVECTED ASHORE BY 20-25KTS INTO THE CONFLUENT
FLOW.  THIS PROVIDES STRONG MST FLUX CONVERGENCE TO SPARK DEEP
CONVECTION.  SURFACE TEMPS NEARING 90F WITH THE SEA BREEZE WASHING
IN...HAVE ALLOWED FOR CAPES IN EXCESS OF 3000 J/KG.  AS SUCH
RECENT 10.3UM SHOWS RAPID COOLING WITH THESE VIGOROUS UPDRAFTS
REACHING AS COLD AS -75C. WITH TROPICAL ENVIRONMENT/VERY HIGH
FREEZING LEVELS...WARM CLOUD PROCESSES TAPPING TPWS OVER
2.25"...ALLOWING FOR RAIN RATES UP TO 2.5-3"/HR. 

ADDITIONALLY...MEAN CLOUD BEARING FLOW AROUND THE SOUTHWEST SIDE
OF THE DEEP RIDGE ARE GENTLE ABOUT 5-10KTS...WHILE INFLOW IS
20+KTS...ALLOWING FOR UPSTREAM PROPAGATION. MEAN FLOW IS ALSO HAS
FAIRLY CLOSE  ORIENTATION TO THE LINE OF BEST MST CONFLUENCE
SUPPORTING THE POTENTIAL FOR TRAINING.  AS SUCH A FEW HOURS OF
TRAINING POSE THE RISK OF 3-5"/HR WHICH CURRENTLY APPEARS TO BE
NEAR/OVER HOUSTON METRO INCREASING THE RISK FOR HYDROPHOBIC RUNOFF
IN SHORT DURATION LEADING TO FLASH FLOOD THREAT.  

SLOW WESTWARD ADVANCEMENT OF DRIER MID-LEVEL FLOW AND FLATTENING
(EASTERLY) INFLOW SHOULD REDUCE CONVERGENCE ORIENTED FAVORABLY TO
THE MEAN FLOW AROUND 21-22Z.  HOWEVER...STRONG CONTINUED
FRICTIONAL CONVERGENCE ALONG THE COAST WITH BACKBUILDING HAS THE
POTENTIAL OF SHIFTING THE THREAT SW TOWARD BRAZORIA COUNTY AND
DOWNSTREAM COUNTIES.  

GALLINA

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

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

LAT...LON   30879532 30809503 30549485 30099459 29599432
            29609423 29519442 29269481 28999508 28859543
            28889571 29489619 30369610 30739583