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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1456Z Oct 20, 2017)
 
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Abbreviations and acronyms used in this product
 
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1056 AM EDT FRI OCT 20 2017

...VALID 15Z FRI OCT 20 2017 - 12Z SAT OCT 21 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 N KGVX 15 NE LBX IAH 40 E CXO 25 NNE BPT KVBS 25 WNW KCRH.


15Z UPDATE...
A FAIRLY NARROWLY FOCUSED THREAT OF HEAVY CONVECTIVE RAINS REMAINS
ALONG THE UPPER TX COAST.  SATL/RADAR IMAGERY CONTINUES TO SHOW A
NW/SE AXIS OF STORMS FROM THE UPPER TX COAST NEAR GALVESTON BAY
SEWD INTO THE WRN GULF THAT HAVE BEEN SLOWLY PROPAGATING NNWWD.  A
COUPLE OF WEAK S/WV IMPULSES WERE NOTED IN WV IMAGERY ACRS TX
WHICH WAS HELPING TO AID LIFT..WHILE ALSO HELPING TO NUDGE THE
MAIN AXIS VERY SLOWLY EWD OVER THE PAST FEW HOURS.  BEST
INSTABILITY REMAINS NEAR AND JUST OFF THE COAST PER RECENT
MESOANALYSIS AND THUS THE HEAVIER RAIN THREAT IS NOT EXPECTED TO
PROGRESS TOO FAR INLAND FROM THE COAST.   THIS IS GENLY SUPPORTED
BY RECENT HRRR RUNS THAT SUGGEST SOME SEMI ORGANIZED TRAINING
BANDS WILL CONTINUE TO IMPACT PORTIONS OF THE UPPER TX COAST
THROUGH THE AFTERNOON WITH A SLOW EWD PROGRESSION OF THE AXIS. 
WHILE NOT EXPECTING RAINFALL AMOUNTS TO BE QUITE AS HEAVY AS WHAT
WAS SEEN EARLY THIS MORNING ACRS THE GALVESTON BAY AREA...STILL
CAN NOT RULE OUT SHORT TERM RAIN AMOUNTS OF 2 TO 3 INCHES WHICH
MAY CAUSE SOME LOCALIZED RUNOFF ISSUES.  SEE MPD #894 VALID UNTIL
18Z FOR ADDITIONAL DETAILS.  SULLIVAN



...MID TO UPPER TEXAS COAST...

MODELS CONTINUE TO SHOW A CONCENTRATED AREA OF SOUTHEASTERLY TO
SOUTHERLY LOW LEVEL FLOW OFF THE WESTERN GULF AND INTO COASTAL TX.
LOW LEVEL FLOW WILL TURN MORE SOUTHERLY INTO THE MORNING AS A
HEIGHT FALLS MOVE IN FROM THE WEST...WHICH MAY ALLOW FOR A NARROW
AXIS OF TRAINING CONVECTION ALONG THE COASTLINE NEAR HOUSTON. AT
THE CURRENT TIME THE HEAVIEST RAINFALL AMOUNTS HAVE GENERALLY
REMAINED OFFSHORE ALIGNED WITH THE BETTER INSTABILITY AXIS.
HOWEVER THERE ARE INDICATIONS THAT THE INSTABILITY AXIS MAY TRY TO
SHIFT ASHORE AS WE HEAD THROUGH THE MORNING HOURS...WHICH MAY
ALLOW FOR SOME OF THE HEAVIER CONVECTIVE CORES TO GET ONSHORE. LOW
LEVEL WINDS INCREASING TO AROUND 20 KTS AND ALIGNED WITH THE MEAN
FLOW DOES SUPPORT THE POTENTIAL FOR SOME BACKBUILDING AND TRAINING
ALONG THE COAST. PWAT VALUES ARE AROUND 1.8" IN A NARROW CORRIDOR
AND ARE SUPPORTIVE OF LOCALIZED HEAVY RATES.

OVERALL THERE IS A GOOD SIGNAL AMONGST THE HIGH RES GUIDANCE FOR
ISOLATED 3" AMOUNTS SOMEWHERE BETWEEN PALACIOS AND PORT ARTHUR. IN
GENERAL THE HIGH RES RUNS APPEAR TO BE A BIT TO FAR SOUTHWEST WITH
THE CONVECTIVE AXIS...AND THUS AM FAVORING A SOLUTION ON THE RIGHT
SIDE OF THE MODEL ENVELOPE. BASED ON RECENT RADAR AND MODEL
TRENDS...INCREASINGLY CONFIDENT IN ISOLATED 1-3" AMOUNTS ALONG
PORTIONS OF THE MID/UPPER TEXAS COAST. IN GENERAL AMOUNTS OF THIS
MAGNITUDE SHOULD NOT POSE MUCH OF A FLOOD THREAT. HOWEVER RECENT
HRRR RUNS HAVE SHOWN LOCALIZED AMOUNTS AS HIGH AS 3-7" THROUGH
18Z. NOT CONFIDENT TOTALS WILL INDEED GET THIS HIGH...AS WOULD
NEED CONVECTION TO ORGANIZE MORE THAN IS CURRENTLY SEEN...AND THEN
HAVE THIS ORGANIZED CONVECTION TRAIN FOR A FEW HOURS. THIS SEEMS
LIKE A LOWER PROBABILITY EVENT AND WOULD BE ISOLATED IN NATURE IF
IT OCCURS. HOWEVER CAN NOT RULE IT OUT COMPLETELY...AS THE SETUP
COULD POTENTIALLY SUPPORT SUCH A LOCALIZED EVENT...AND FOR THAT
REASON HAVE OPTED FOR A SMALL MARGINAL RISK CENTERED AROUND
HOUSTON. WILL NEED TO CONTINUE TO MONITOR THE SITUATION THROUGH
THE MORNING HOURS.

CHENARD