Quantitative Precipitation Forecast
Issued by NWS

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FOUS30 KWBC 211459
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
958 AM EST SAT JAN 21 2017

...VALID 15Z SAT JAN 21 2017 - 12Z SUN JAN 22 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 ENE CYD 30 SSE KMIS JKA 25 E GZH 30 ENE GZH 10 SSE ANB
15 SE GAD 20 ENE GAD 10 NNW RMG 25 NNE 47A 20 NW GMU 20 SW FQD
FQD 10 S RUQ 10 S MEB 25 S FLO 25 ESE OGB 10 ENE NBC 35 E HXD
75 ESE HXD 95 SE HXD 85 ESE SSI 10 SSE SGJ 45 S CTY 130 ESE KIPN.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
180 W ACV 130 W CEC 65 WNW CEC 10 WNW CEC 40 SE CEC 30 W O54
30 WSW O54 45 N UKI 25 ENE STS 10 ESE LVK 45 NNW PRB 20 SE PRB
40 SW BFL 10 WSW SDB NTD 40 NNW NSI 70 SSW LPC 110 SW VBG.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
KIKT 30 NNW KVOA 10 NNW CEW 15 N AUO 15 W CCO AHN 20 WSW UZA
15 W OGB 10 W TBR 15 N LHW 35 SSE HXD 70 E SSI 10 SSW JAX
60 SSW AAF.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
90 SW O87 60 SW O87 30 S O87 25 NNW UKI 10 ESE UKI APC
10 ENE PAO 15 N WVI 20 SE SNS 25 NNE 87Q SBP 15 ESE SMX
10 SSE LPC 30 SW LPC 50 SW XVW 85 WSW VBG.


1500 UTC UPDATE

CHANGES TO THE PREVIOUS EXCESSIVE RAINFALL POTENTIAL OUTLOOK WERE
TO TRIM THE WESTERN EDGE OF THE MARGINAL AND SLIGHT RISK AREAS
FROM SOUTH LA---SOUTHERN MS AND SOUTHWEST AL TO REFLECT THE
CURRENT EASTWARD PROGRESSION OF THE ORGANIZED CONVECTION.  WITH
THE INCREASED PROGRESSION---THE RISK OF RAINFALL AMOUNTS EXCEEDING
FFG HAS LIKELY REDUCED---BUT DID NOT WANT TO REMOVE THE SLIGHT
RISK AREA AS RADAR ESTIMATES CONTINUE TO SHOW AMOUNTS OF UP TO 2"
IN AN HOUR.  NO CHANGES MADE TO THE RISK AREAS ALONG THE CENTRAL
TO NORTHERN CALIFORNIA COASTS.

ORAVEC


...GULF COAST STATES...

AN ACTIVE PERIOD IS IN STORE FOR THE GULF COAST STATES. WEST TO
SOUTHWEST UPPER FLOW HAD ESTABLISHED A MID LEVEL BAROCLINIC ZONE
NEAR THE GULF COAST...WITH RICH MOISTURE AND MODERATE INSTABILITY
POSITIONED ACROSS THE FULL LENGTH OF THE NORTHERN GULF. AT 08Z A
LARGE AREA OF ORGANIZED THUNDERSTORMS...INCLUDING SUPER CELL
MODES...WAS EVOLVING OVER LOUISIANA AND ADJACENT SOUTHERN
MISSISSIPPI...WHERE LOW LEVEL FLOW HAD ACCELERATED IN ADVANCE OF A
TROUGH OVER THE PLAINS. THIS ACCELERATED INFLOW WAS TAPPING INTO
THE WELL OF STRONGER INSTABILITY OVER THE WESTERN GULF. THE PERIOD
THROUGH BOTH SATURDAY AND SUNDAY SHOULD PROVE PARTICULARLY
ACTIVE...WITH THE FIRST TROUGH BRUSHING ACROSS THIS REGION WITH
SEASONABLY STRONG DEEP LAYER FLOW...AND THEN A 3.5 SIGMA BOWLING
BALL OF AN UPPER LOW PUNCHING OUT AT LOW LATITUDE ON SUNDAY. ADD
TO THIS SETUP THE MORE SPRING-LIKE MOISTURE AND WARMTH...AND WE
HAVE A RECIPE FOR BOTH SEVERE WEATHER AND FLASH FLOODING. PLEASE
REFER TO STORM PREDICTION CENTER PRODUCTS FOR SEVERE WEATHER
FORECASTS.

IN TERMS OF THE FLASH FLOOD POTENTIAL...THE REGION FROM CENTRAL
LOUISIANA TO CENTRAL MISSISSIPPI TO NORTHERN ALABAMA HAD
EXPERIENCED RECENT RAINFALL AND WILL BE MORE PRONE TO RAPID
RUNOFF...WHEREAS THE REGION CLOSER TO THE COAST FROM FAR SOUTHEAST
LOUISIANA TO SOUTHERN GEORGIA AND NORTHERN FLORIDA WILL LIKELY
BECOME SUSCEPTIBLE ON LARGER SCALES ONLY ONCE MULTIPLE ROUNDS OF
THUNDERSTORMS HAVE MOVED THROUGH A GIVEN AREA. EACH INDIVIDUAL
ROUND...HOWEVER...WILL HAVE POTENTIAL TO PRODUCE 1.5 TO 2.5 INCHES
OF RAIN IN AN HOUR BASED ON THE QUALITY OF INFLOW / MOISTURE...AND
EARLY OBSERVED RATES OVER LOUISIANA THIS MORNING. IN THE LONG
RUN...MUCH OF THE HEAVIER PRECIPITATION IN THE WPC DETERMINISTIC
FORECAST IS PLACED CLOSER TO THE COAST IN MS/AL AND SOUTH OF
ATLANTA IN GA...WHERE FLASH FLOOD GUIDANCE VALUES ARE HIGHER. THE
00Z HI-RES GUIDANCE HAD TRENDED TOWARD DEPICTING HEAVIER RAIN IN A
SWATH FARTHER NORTH...WHICH WOULD BE OF CONCERN...BUT
OBSERVATIONAL TRENDS SUGGEST THOSE SOLUTIONS MAY BE TOO AGGRESSIVE
AT THAT LATITUDE...WITH THE ONGOING EVOLUTION OF A QLCS OVER LA/MS
THIS MORNING LIKELY TO PROPAGATE EAST AND EVENTUALLY SUPPRESS THE
LOCATION OF PERSISTENT CONVERGENCE / UPGLIDE LATER TODAY.
THEREFORE...CONFIDENCE IN THE DETAILS IS FAIRLY MODEST...RESULTING
IN A RATHER BROAD SLIGHT RISK OF EXCESSIVE RAINFALL TO COVER THE
RANGE OF SCENARIOS...BUT WITH NO APPARENT NEED FOR A MODERATE RISK
GIVEN THAT THE HEAVIER RAIN IS EXPECTED WHERE FFG IS HIGHER AND
THE MOST RECENT GUIDANCE IS DRIER. THE NEED FOR A MODERATE RISK
AREA...HOWEVER...WILL BE RE-EVALUATED LATER TODAY...AS IT IS
LIKELY THAT ANOTHER WELL ORGANIZED ROUND OF PRECIPITATION WILL
OCCUR SATURDAY NIGHT...POSSIBLY OVERLAPPING WITH AREAS OF FRESHLY
COMPROMISED SOILS.


...CALIFORNIA...

THE NEXT ATMOSPHERIC RIVER QUICKLY ORGANIZES BACK OFFSHORE AND
APPROACHES SATURDAY NIGHT...WITH THE INITIAL ONSLAUGHT OF HEAVIER
RAINFALL AIMED AGAIN AT NORTHERN CALIFORNIA. ONCE AGAIN A DEEP AND
AMPLIFYING TROUGH WILL BRING THE RIVER ONSHORE...ALTHOUGH WITH
STEADY PROGRESSION THERE WILL NOT BE TERRIBLY LONG RESIDENCE TIME
OF THE ENHANCED MOISTURE FLUX AT ANY ONE LOCATION. CERTAINLY THERE
WILL BE CAUSE FOR CONCERN IN FLOOD-PRONE
AREAS...HOWEVER...ESPECIALLY GIVEN THE RECENT EXTENDED STRETCH OF
WET WEATHER...AS ONCE AGAIN MODERATE PW AND MOISTURE FLUX
ANOMALIES WILL DEVELOP...SUPPORTING HEAVY 1-HOUR AND 3-HOUR RAIN
RATES. A CONSENSUS QPF APPROACH MATCHES REMARKABLY WELL TO THE
GEFS REFORECAST DATA BASED ON ANALOG CASES...INDICATING AREAL
AVERAGE AMOUNTS 1.5 TO 2.5 INCHES THROUGH SUNDAY MORNING IN THE
FAVORED COASTAL TERRAIN AND NORTHERN SIERRAS / SHASTA`S. THE EVENT
WILL WORK DOWN INTO SOUTHERN CALIFORNIA TOWARD DAY 2.

BURKE
$$




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