MESOSCALE PRECIPITATION DISCUSSION 0007
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
409 AM EST SAT JAN 27 2018

AREAS AFFECTED...PORTIONS OF SOUTHERN LA

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

VALID 270908Z - 271508Z

SUMMARY...TRAINING CONVECTION COULD LEAD TO HOURLY RAIN TOTALS TO
2" WITH LOCAL AMOUNTS TO 5" THROUGH 15Z.

DISCUSSION...A MESOSCALE CIRCULATION WHICH ORIGINATED OFFSHORE THE
TX COAST ALONG A THERMAL GRADIENT HAS MOVED ASHORE SOUTHWEST LA
LEADING TO HEAVY RAINFALL ACROSS WESTERN CAMERON PARISH. THIS IS
OCCURRING SOUTHEAST OF A SHORTWAVE AND ASSOCIATED JET STREAK
MOVING THROUGH EASTERN TX.  GOES-16 IMAGERY REVEALS SOME
BROADENING OF THE COLD CLOUD TOPS ASSOCIATED WITH THIS ORGANIZED
CONVECTION.  PRECIPITABLE WATER VALUES OF 1.4-1.6" EXIST HERE PER
GPS VALUES. 

MUCAPE VALUES OF 500-1000 J/KG EXIST HERE PER SPC MESOANALYSES,
WHICH IS SUFFICIENT FOR ISSUES WITHIN A SATURATING ATMOSPHERE. 
INFLOW AT 850 HPA IS BECOMING CONVERGENT ACROSS WESTERN LA, WITH
THE FLOW BECOMING INCREASINGLY UNIDIRECTIONAL WITH HEIGHT.  AS THE
SHORTWAVE MOVES MORE TO THE NORTH OF THIS REGION, DIVERGENCE ALOFT
SHOULD MAXIMIZE IN THE NEXT FEW HOURS. 

THE MESOSCALE GUIDANCE SHOWS LOCAL AMOUNTS UP TO 5", WHICH IS
POSSIBLE SHOULD CONVECTIVE COVERAGE CONTINUE TO INCREASE (WHICH IS
FORECAST) AND LEAD TO MORE INSTANCES OF ROUGHLY WEST TO EAST CELL
TRAINING AS ACTIVITY ATTEMPTS TO MOVE PARALLEL TO THE MUCAPE
GRADIENT.  SO FAR, THE MESOSCALE GUIDANCE RAIN PATTERNS APPEAR TOO
FAR NORTHWEST AND THEIR CONVECTIVE EVOLUTION IS A FEW HOURS
DELAYED.  THIS MEANS THAT DESPITE THE SIGNAL OF 50% EXCEEDANCE OF
THE THREE HOURLY FLASH FLOOD GUIDANCE DURING THE 13-16Z PERIOD, IT
COULD OCCUR A FEW HOURS SOONER.  HOURLY RAIN TOTALS TO 2" SHOULD
REMAIN POSSIBLE.  DESPITE RECENT DRYNESS, THESE TOTALS COULD LEAD
TO FLASH FLOODING ACROSS PORTIONS OF SOUTHERN LA, PARTICULARLY IN
URBAN AREAS.

ROTH

ATTN...WFO...LCH...LIX...

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

LAT...LON   30959172 30919060 29939063 29399158 29569277
            29669351 29749359 30629261