Quantitative Precipitation Forecast
Issued by NWS

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FOUS30 KWBC 200822
QPFERD

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

...VALID 12Z 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 SSE LBX 10 N LBX SGR 35 ENE IAH 15 N BPT KVBS 25 WNW KCRH.


...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
$$




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