MESOSCALE PRECIPITATION DISCUSSION 0420
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1203 PM EDT SUN JUL 08 2018

AREAS AFFECTED...SC AND SE TEXAS...SW LOUISIANA

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

VALID 081602Z - 082200Z

SUMMARY...VERY SLOW-MOVING STORMS ARE EXPECTED TODAY. THEY WILL
BECOME MORE NUMEROUS AROUND MIDDAY AND MEANDER AROUND THE REGION
THROUGH THE AFTERNOON. RAIN RATES MAY REACH 2-3 IN/HR WHICH COULD
PRODUCE LOCALIZED FLASH FLOODING... ESPECIALLY IN URBANIZED AREAS.

DISCUSSION...CIRA BLENDED TOTAL PW PRODUCT SHOWS PW VALUES AROUND
1.9 TO 2.3 INCHES OVER MOST OF THE OUTLINED REGION FROM
SOUTH-CENTRAL TEXAS INTO SOUTHWEST LOUISIANA. THIS CLOSELY MATCHES
BOTH 12Z SOUNDINGS FROM THE AREA...AND NUMEROUS GPS-PW
OBSERVATIONS WITH THE HIGHEST OBSERVATIONS FROM HOUSTON METRO TO
NEAR LAKE CHARLES LOUISIANA (AROUND 2.2 INCHES). DESPITE THE
TYPICAL HUMID CLIMATE ALONG THE GULF COAST IN THE SUMMER
MONTHS...2.2 INCH PW VALUES WOULD BE AROUND OR JUST ABOVE THE 90TH
PERCENTILE FOR MID JULY ON THE LAKE CHARLES SOUNDING CLIMATOLOGY.
THUS ANOMALOUSLY HIGH LEVELS OF ATMOSPHERIC MOISTURE ARE IN
PLACE...AND SHOULD PROMOTE EFFICIENT RAINFALL PRODUCTION IN
STORMS.

FURTHER EXACERBATING THE THREAT OF LOCALLY HEAVY RAINFALL ARE THE
VERY WEAK WINDS THROUGH MUCH OF THE DEPTH OF THE TROPOSPHERE. A
LOW-LEVEL TROUGH (SFC-850MB) STRETCHED IN A BROAD ARC FROM NEAR
LAREDO TO NEAR BATON ROUGE...AND THIS WAS COINCIDENT WITH A
WEAKNESS IN THE MID-UPPER LEVEL FLOW. FORECAST MEAN WIND
(850-200MB) ACROSS THE REGION WAS GENERALLY AROUND OR LESS THAN 5
KNOTS. FLOW THIS WEAK SHOULD ALLOW STORMS TO BE NEARLY STATIONARY
UPON INITIAL DEVELOPMENT...WITH PERHAPS SOME SLOW PROPAGATION AS
CELL MERGERS FORCE DEVELOPMENT OF SMALL CLUSTERS AND LOCALIZED
COLD POOLS. THERE IS NOT A STRONG LOW-LEVEL BOUNDARY FOR
CONVECTION TO FOCUS ON...SO THE CONVECTION MAY NOT PERSIST IN THE
SAME AREA FOR MANY HOURS. HOWEVER...THE SLOW DRIFT AND POTENTIAL
FOR CELL MERGERS AND INTERACTIONS SUGGESTS THAT HEAVY RAIN CAN
PROBABLY BE SUSTAINED AT SOME LOCATIONS FOR A COUPLE HOURS...WHICH
MAY LEAD TO A FLASH FLOOD THREAT. RAIN RATES OF 2-3 IN/HR SEEM
ACHIEVABLE GIVEN THE DEEP SATURATED PROFILES...MODERATE TO STRONG
INSTABILITY...AND VERY SLOW STORM MOTIONS.

ONE MESOSCALE FEATURE OF INTEREST IS A NNE PROPAGATING OUTFLOW
BOUNDARY...EVIDENT ON GOES-16 VISIBLE SATELLITE LOOPS AND KHGX
RADAR. AT 16Z IT STRETCHED FROM ABOUT 40W OF VCT TO NEAR BYY TO
NEAR 25ESE OF SGNT2. EXTRAPOLATING CURRENT TRENDS WOULD PUSH THE
OUTFLOW BOUNDARY INTO THE VICINITY OF HARRIS COUNTY AND HOUSTON
METRO BETWEEN 19Z AND 21Z. THIS MAY PROVIDE ADDITIONAL FOCUS FOR
CONVECTIVE ORGANIZATION AND DEVELOPMENT JUST DOWNSTREAM FOR AN
HOUR OR TWO...AND THUS BRIEFLY ENHANCE RAIN RATES. SOME SCATTERED
CONVECTION WAS ALREADY DEVELOPING ACROSS HOUSTON METRO...BUT THIS
MAY ENHANCE RAIN RATES ALONG THE I-10 CORRIDOR EARLY THIS
AFTERNOON.

LAMERS

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

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

LAT...LON   31639362 30939249 30079276 29599375 29359448
            28929505 28709574 28699678 28059832 28569913
            29519936 30319808 31029622