Quantitative Precipitation Forecast
Issued by NWS

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711
FOUS30 KWBC 210811
QPFERD

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

...VALID 12Z 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
40 WSW KIPN 10 SW CYD 25 W KMDJ 7R3 30 ENE MCB 30 WSW TCL
20 SE 3A1 15 SE DNN 20 SE AVL 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
20 S KVKY 10 S BVE 20 NNE AXO 15 N NEW 10 SSW HBG 30 E NMM EET
15 SSW WDR 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.


...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 SUPERCELL
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 / SHASTAS. THE EVENT
WILL WORK DOWN INTO SOUTHERN CALIFORNIA TOWARD DAY 2.

BURKE
$$





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