We’re ramping up as we now have new very heavy rainfall setting up across western Harris County. A training band of heavy rainfall has set up from near Angleton to Sugar Land to Katy. Hourly rainfall rates are now above 3 inches and will likely continue increasing. Expect very rapid rises on local creeks/streams. This band may build in more and impact Houston proper later this evening. Some locations in Southeast Texas may receive up to 10 inches of rain by 1 AM – on top of the 15 to 25+ inches experienced over the last 36 hours. We’re going to see even more rain after that and into Monday, although there is some hope we’ll get a lull in precipitation again during the mid-morning hours Monday in parts of Houston and Southeast Texas. Once again, we’re quickly ramping back up on the rain across western parts of Harris County. This includes Angleton, Sugar Land, and Katy. Rainfall rates of 3 to 5 inches per hour are occurring and will lead to rapid on slot of flash flooding – and additional rises on the Barker Reservoir and Cincor Ranch areas. This rain is expected to build further east over the next few hours and impact more of Harris County and adjacent cities/counties. Track the rain with our interactive weather radar at texasstormchasers.com/radar. 

MESOSCALE PRECIPITATION DISCUSSION 0745
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
757 PM EDT SUN AUG 27 2017

AREAS AFFECTED…SOUTHEAST TX AND SOUTHERN LA

CONCERNING…HEAVY RAINFALL…FLASH FLOODING LIKELY

VALID 280000Z – 280600Z

SUMMARY…SIGNIFICANT TO CATASTROPHIC FLOODING IS POSSIBLE AGAIN
THROUGH THE OVERNIGHT HOURS WHERE BANDING FEATURES ASSOCIATED WITH
HARVEY SET UP.

DISCUSSION…THE GOES-16 SHORT WAVE IR LOOP WAS INDICATING THE
DEVELOPMENT OF NORTH/SOUTH ORIENTED BANDING FEATURES FORMING OVER
PORTIONS OF THE UPPER TX COAST…AND THE REGIONAL RADARS ARE
STARTING TO SHOW THE DEVELOPMENT OF BANDED FEATURES AS WELL. THE
BANDED FEATURES ARE FORMING IN THE 35 TO 45 KNOT CONVERGENT FLOW
EAST OF THE LOW LEVEL CIRCULATION… WHERE 2.50/2.75 INCH
PRECIPITABLE WATER AIR IS FEEDING THE DEVELOPING BANDED CONVECTION.

HOURLY RAINFALL RATES ARE ONCE AGAIN APPROACHING 2.00 INCHES ACROSS THE UPPER TX COAST OVER CHAMBERS…LIBERTY AND JEFFERSON COUNTIES IN TX. AS WAS THE CASE SATURDAY NIGHT INTO SUNDAY MORNING…BANDED CONVECTIVE FEATURES ARE EXPECTED TO DEVELOP AND TRACK SLOWLY EASTWARD…MAINLY ACROSS THE UPPER COAST OF TX AND GRADUALLY MOVE INTO SOUTHWEST LA.

THERE IS A STRONG HIGH RESOLUTION MODEL SIGNAL FOR BANDING TO
BUILD THROUGH 03Z ACROSS THE UPPER TX COAST…THEN START PEAKING
BETWEEN 03Z (10 PM) AND 06Z (1 AM) FROM THE HOUSTON METRO AREA INTO PORTIONS OF SOUTHWEST LA. WHILE THERE ARE SOME DIFFERENCES REGARDING THE PLACEMENT OF THE MAXIMUM RAINFALL BANDS…ALL MODELS SHOW THE
POTENTIAL FOR HOURLY RAINFALL RATES BETWEEN 2.00 AND 3.00 INCHES
OVER THE UPPER TX COAST…AND LOCAL 5 TO 10+ INCHES OF RAINFALL
THROUGH 06Z (1 AM).

THIS COULD INCLUDE THE HOUSTON METRO AREAS…AS SEVERAL HIGH
RESOLUTION MODEL RUNS (INCLUDING THE 12Z WRF ARW/NMMB AND 18Z NAM CONUS NEST) INDICATED THAT HOUSTON COULD ONCE AGAIN BE IN FOR
SIGNIFICANT TO CATASTROPHIC FLASH FLOODING. ADDITIONAL FLOODING IS
EXPECTED AFTER 06Z (1 AM) AS WELL.

FURTHER NORTH…A PERSISTENT EAST NORTHEAST FLOW OF MODERATE TO
HEAVY RAINFALL NORTH OF THE LOW LEVEL CIRCULATION WILL CONTINUE.
THE MOISTURE AND INSTABILITY BECOME ELEVATED OVER PORTIONS OF
CENTRAL TX…AND COULD RESULT IN AN ADDITIONAL 2.00 OR 3.00 INCH
RAINFALL AMOUNTS…EXACERBATING ONGOING FLOODING.

FINALLY…MOISTURE AND INSTABILITY FEEDING CONVECTION THAT RIDES
OVER A FRONTAL BOUNDARY OVER SOUTHERNMOST LA COULD PRODUCE
SCATTERED HOURLY RAINFALL RATES NEAR 2.00 INCHES AS IT REACHES THE
NEW ORLEANS METRO AREA. WHILE MUCH OF THE HIGH RESOLUTION GUIDANCE
IS NOT BULLISH WITH THIS ACTIVITY…LOCAL 3.00 TO 5.00 INCH
AMOUNTS HERE COULD CAUSE ISSUES IN THE METRO AREA.

HAYES