Quantitative Precipitation Forecast
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FOUS30 KWBC 280151
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
950 PM EDT MON APR 27 2015

...VALID 03Z TUE APR 28 2015 - 00Z WED APR 29 2015...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
105 ESE KVOA 15 WSW KIKT 15 N GSM 15 W KCMB 25 NW BPT
15 WNW DRI 15 W PIB 25 NNW NSE PAM 40 SSW AAF 105 ESE KVOA.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 NNE 1B7 10 SE 7R3 10 SW ARA 30 N LFT 20 NE BTR 30 SSE GPT
20 NNE 1B7.


...CENTRAL GULF COAST...
...UPGRADE TO MODERATE RISK IN SOUTHERN LOUISIANA...

REPEATING HEAVY RAIN HAD GRADUALLY LED TO A GREATER RISK OF FLASH
FLOODING OVER SOUTHERN LOUISIANA. THE REGION WAS SWEPT BY 1-2 INCH
AREAL AVERAGE RAINFALL EARLY MONDAY...AND A SECOND WAVE WITH HEAVY
DOWNPOURS WAS PASSING THROUGH MONDAY EVENING...WITH MULTIPLE
MESONET OBSERVATIONS INDICATING CLOSE TO 1 INCH OF RAIN IN 1 HOUR.
THIS BATCH WILL LIKELY CONTINUE TO LOWER FLASH FLOOD GUIDANCE
VALUES DOWN BELOW 1.5 TO 2 INCHES ON 1 TO 3 HOUR TIMES SCALES.
LATER TONIGHT...A THIRD ROUND OF LOCALLY HEAVY RAINFALL IS
FORECAST TO DEVELOP OVER LOUISIANA LATE TONIGHT AND THEN PROGRESS
ALONG THE GULF COAST TO THE FAR WESTERN PART OF THE FL PANHANDLE.
HERE...AHEAD OF A SURFACE LOW THAT WILL BECOME BETTER DEFINED FROM
28/06Z TO 29/00Z...SUSTAINED H85 INFLOW OF 20 KNOTS WILL INCREASE
TO 30-35 KNOTS WHILE SLOWLY MIGRATING EAST. NORTHWARD ADVECTION OF
DEEPER MOISTURE WILL RAISE PRECIPITABLE WATER TO NEAR 1.75 INCHES
FROM SOUTHERN LA TO FL. MAIN LIMITING FACTOR WITH RESPECT TO
ORGANIZED AND SUSTAINABLE CONVECTION INLAND WOULD BE THE POTENTIAL
FOR CLUSTERS OF THUNDERSTORMS OR ORGANIZED CONVECTIVE SYSTEMS TO
TRACK ALONG AN OUTFLOW-ENHANCED BAROCLINIC ZONE OFFSHORE IN THE
GULF...THUS LIMITING THE NORTHWARD TRANSPORT OF MOISTURE AND
INSTABILITY. A TROUBLING TREND IS THAT THE HRRR RUNS INITIALIZED
BETWEEN 21-00Z WERE ESPECIALLY AGGRESSIVE WITH THIS
NOTION...SWEEPING THE GULF WITH AN ORGANIZED MCS AND FAILING TO
INITIATE HEAVY RAIN INLAND. ALTHOUGH A COMPLEX COULD GROW FROM
STORMS OCCURRING IN AND NEAR SOUTHERN LOUISIANA MONDAY
EVENING...IT WOULD LIKELY WEAKEN UPON MOVING DOWNSTREAM AHEAD OF
THE BETTER LOW LEVEL INFLOW AND INTO AN AREA RECENTLY OVERTURNED.
BACK INLAND...WE ARE IN FAVOR OF THE RAP...18Z NAM...AND 12Z NSSL
WRF...WHICH FORECAST ADDITIONAL ORGANIZED RAINFALL OVER LA AND
PERHAPS FAR SE TX WHERE THE COMBINATION OF SUSTAINED INFLOW AND
INCREASING LIFT IN THE 850-500 MB LAYER AHEAD OF THE SOUTHERN
PLAINS TROUGH SHOULD SUPPORT REDEVELOPMENT. MOIST VERTICAL
PROFILES COULD LEAD TO INTENSE RAIN RATES AND AT LEAST 1-2 INCH
AREAL AVERAGE RAINFALL AMOUNTS...CLOSER TO 2-3 INCHES WITH HIGHER
LOCAL AMOUNTS OVER FAR SOUTHEAST LA.


...CENTRAL AND SOUTH FLORIDA...

THE AIRMASS WILL BECOME INCREASINGLY MOIST ON TUESDAY...WITH PW
APPROACHING 2 INCHES...AND ANOMALIES BASED ON THE ECMWF REACHING 3
STANDARD DEVIATIONS FROM CLIMATOLOGY...OR IN THE 90TH TO 95TH
PERCENTILE FOR LATE APRIL. FLASH FLOOD GUIDANCE VALUES ARE
LARGE...PER USUAL...BUT THIS TYPE OF AIRMASS WOULD OBVIOUSLY
SUPPORT INTENSE HOURLY RATES. THE MAIN CONCERN WOULD BE IF THE
RELEASE OF INSTABILITY COULD BECOME FOCUSED AHEAD OF A WELL TIMED
MCS ARRIVING FROM THE EASTERN GULF. SIMULATED REFLECTIVITY FIELDS
FROM THE 12Z NSSL WRF AND 18Z NAM CONEST SUGGEST SUCH AN EVENT IS
POSSIBLE IF AN MCS FORMS OVER SOUTHEAST LOUISIANA BY MORNING.
QUESTIONS ABOUT TIMING...MODEL SPREAD...AND EVEN THE LATITUDE AT
WHICH SUCH AN EVENT WOULD TRACK...PRECLUDE US FROM INTRODUCING A
RISK AREA AT THIS TIME...BUT A SLIGHT RISK MAY BE INTRODUCED IN
LATER OUTLOOKS.

BURKE/TERRY
$$





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