MESOSCALE PRECIPITATION DISCUSSION 0081
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1014 PM EDT WED MAR 28 2018

AREAS AFFECTED...COASTAL AND SERN TX...NRN AND CTRL LA...CTRL MS

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 290213Z - 290800Z

SUMMARY...FLASH FLOODING WILL BECOME INCREASINGLY LIKELY ALONG
PORTIONS OF THE WESTERN GULF COAST AS HEAVY RAIN SHOWERS AND
THUNDERSTORMS BECOME MORE NUMEROUS OVER THE NEXT SEVERAL HOURS.
RAIN RATES SHOULD AROUND OR JUST ABOVE 2 IN/HR. PERSISTENT HEAVY
RAIN RATES SUCH AS THOSE IN URBAN AREAS COULD LEAD TO MORE
SIGNIFICANT FLASH FLOODING.

DISCUSSION...NUMERICAL MODELS AND SATELLITE AND RADAR TRENDS
INDICATE THAT GREATER CONVECTIVE COVERAGE IS LIKELY TO EMERGE IN
THE 02-05Z TIME FRAME...PARTICULARLY FROM SOUTHEAST TEXAS INTO
SOUTHWEST LOUISIANA. GOES-EAST WATER VAPOR SHOWED A STRONG
SHORTWAVE IN WEST TEXAS...GRADUALLY MOVING EAST AND WITH
BLOSSOMING CONVECTION OUT AHEAD OF IT. IR SATELLITE CHANNELS
SHOWED COOLING CLOUD TOPS ACROSS MUCH OF THE REGION...EVEN AS FAR
WEST AS SAN ANTONIO...WITH A CORRESPONDING INCREASE IN OBSERVED
LIGHTNING. THIS FITS WITH THE OBJECTIVE RAP FORECASTS OF
INCREASING UPPER LEVEL DIVERGENCE (IN THE RIGHT ENTRANCE REGION OF
AN UPPER LEVEL JET STREAK) AND PVA AHEAD OF THE APPROACHING
SHORTWAVE. AS CONVECTION BECOMES MORE WIDESPREAD...THE POTENTIAL
FOR GREATER RESIDENCE TIME OF HEAVY RAINFALL AT ANY ONE LOCATION
WILL INCREASE...PARTICULARLY WHERE LOW-LEVEL CONVERGENCE IS
MAXIMIZED AND CAN FOCUS RENEWED CELL GROWTH IN MOIST UNSTABLE
INFLOW.

MORE SUBSTANTIAL BOUNDARY LAYER CONVECTIVE FOCI APPEAR MOST LIKELY
IN COASTAL SOUTHEAST TEXAS. RECENT RAP FORECASTS SHOW A STRONG
BOUNDARY LAYER MOISTURE CONVERGENCE SIGNATURE THAT PERSISTS FOR
SEVERAL HOURS. THIS WOULD FIT WITH A SLOWLY ADVANCING COLD FRONT
INTO INCREASING LOW-LEVEL INFLOW AS A NOCTURNAL INCREASE OF THE
LLJ COMMENCES ALONG THE TEXAS COAST. FURTHERMORE...THERE IS A
WELL-ESTABLISHED COLD POOL WITH A SOUTHWARD EXTENT (UPSHEAR FLANK)
SITUATED FROM NEAR HOUSTON METRO INTO SOUTHWEST LOUISIANA. SURFACE
TEMPERATURES NORTH OF THIS BOUNDARY WERE IN THE LOW 60S AT
01Z...WHILE AREAS TO THE SOUTH WERE IN THE MID-70S WITH DEWPOINTS
AROUND 70 DEGREES. AS THE LOW-LEVEL JET INCREASES ALONG THE TEXAS
COAST...THIS MAY FAVOR RENEWED CELL GROWTH NEAR AND JUST INTO THE
COLD POOL. GREATER CONCERN FOR TRAINING CONVECTION WILL EXIST
AFTER 03-04Z WHEN THE COMBINATION OF THE ADVANCING SURFACE FRONT
AND AN EXISTING COASTAL FRONT SHOULD ESTABLISH A CONVERGENCE AXIS
ROUGHLY PARALLEL WITH THE MEAN FLOW. THIS COULD ALLOW FOR NEW
CONVECTIVE CELLS AND CLUSTERS TO FEED INTO SIMILAR AREAS...AND
PERHAPS CONGEAL WITH ONGOING CONVECTIVE ENHANCEMENT NEAR THE COLD
POOL BOUNDARY. IT MAY BE WORTH MONITORING WHERE THE EFFECTIVE
TRIPLE POINT BETWEEN THE COLD POOL AND COLD FRONT EXISTS...AS
LOW-LEVEL CONVERGENCE COULD BE PARTICULARLY ENHANCED. AS OF
02Z...THIS APPEARED TO BE NEAR K11R (BRENHAM TX)...AND THERE WAS A
CORRESPONDING CLUSTER OF VIGOROUS CONVECTION NEARBY. EXTRAPOLATION
OF CURRENT TRENDS AND RAP FORECASTS COULD PLACE THIS NEAR HOUSTON
METRO IN A FEW HOURS. THIS IS YET ANOTHER FACTOR THAT COULD
INCREASE THE RESIDENCE TIME OF HEAVY RAINFALL.

THE ENVIRONMENT SEEMS TO BE SUPPORTIVE OF RAIN RATES THAT WILL
REACH MAXIMUM VALUES AROUND OR JUST OVER 2 IN/HR...AND WITH A
GREATER RISK OF PERSISTENCE OF THESE HEAVIER RATES...FLASH
FLOODING SEEMS TO BE MORE LIKELY. A REGION OF ENHANCED CONCERN
WOULD BE ALONG THE SOUTHEAST TEXAS COAST INTO SOUTHWEST
LOUISIANA...ROUGHLY FROM VICTORIA TX...TO HOUSTON METRO...TO LAKE
CHARLES. IN ADDITION TO THE FAVORABLE ENVIRONMENTAL FACTORS...THIS
ALSO INTERSECTS A LARGE METRO AREA AND SUSTAINED RAIN RATES IN
EXCESS OF 1 IN/HR WOULD BE LIKELY TO PRODUCE SOME FLOODING. MORE
SIGNIFICANT FLOODING COULD RESULT IN SOME OF THESE AREAS IF HOURLY
RAIN RATES CAN PUSH UP TO AROUND 2 IN/HR AND LINGER FOR AT LEAST
90 MINUTES...WITH AT LEAST 5 INCH RAINFALL TOTALS. ALL OF THIS
SEEMS ACHIEVABLE...BUT IT WILL COME DOWN TO MESOSCALE DETAILS.

FURTHER NORTHEAST...THE SQUALL LINE WAS SHOWING MORE EASTWARD
PROGRESS IN FAR NORTHEAST LOUISIANA AND CENTRAL MISSISSIPPI.
DESPITE THAT...INDIVIDUAL CELLS IN THE LINE WERE MOVING AT A
LIMITED ENOUGH ANGLE TO THE LINE ORIENTATION TO PRODUCE MRMS
ESTIMATED RAINFALL RATES OF 1-2 IN/HR. THEREFORE...FLASH FLOODING
WILL ALSO BE LIKELY IN AN ISOLATED FASHION FURTHER TO THE
NORTHEAST IN PORTIONS OF LA/MS AS A SQUALL LINE CONTINUES TO PUSH
INTO A MARGINALLY UNSTABLE AIR MASS AND PERSISTENT LOW-LEVEL JET.

LAMERS

ATTN...WFO...CRP...EWX...FWD...HGX...JAN...LCH...LIX...SHV...

ATTN...RFC...LMRFC...SERFC...WGRFC...

LAT...LON   33608949 33568846 32768842 31998926 31109082
            30239226 29349417 28159613 27789725 28199786
            28989843 29909802 31169569 32039351 32909110