Quantitative Precipitation Forecast
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FOUS30 KWBC 271851
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
250 PM EDT SAT SEP 27 2014

...VALID 18Z SAT SEP 27 2014 - 00Z MON SEP 29 2014...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 E LLJ 20 WNW DLN 10 SSW BTM 10 E BTM 30 E BTM 20 WSW BZN
30 SSW BZN 30 NE RXE 40 SE U78 25 NNE FIR 30 ESE FIR 45 WNW VEL
45 W VEL 35 ENE PUC 25 NNE U28 U28 25 SW U28 4HV 25 SW 4HV
30 E BCE 40 S BCE 35 NNW GCN 10 E FLG 30 WNW SOW 35 SSE SOW
45 N SAD 25 N SAD 45 NNE DMA 20 SE CGZ 20 SSW GYR 70 WNW LUF
40 ESE EED 15 N IFP 15 ENE LSV 70 WNW SGU 80 S ELY 30 ESE ELY
20 SSW ENV 40 NNW ENV 20 SW 77M 15 ENE BYI 35 E SUN 30 E LLJ.


...WESTERN U.S...

AT MIDDAY SATURDAY THE HEAVY RAIN RATES AND FLASH FLOODING IN THIS
REGION HAD BEEN CONFINED TO SOUTHERN UTAH AND SOUTWHARD THROUGH
ARIZONA. ALTHOUGH SURFACE BASED INSTABILITY WAS LACKING FARTHER
NORTH...THE HIGH RESOLUTION MODELS STILL SHOW SOME SIGNAL FOR
POCKETS OF HEAVIER RAIN THIS AFTERNOON AS FAR NORTH AS EASTERN
IDAHO AND ADJACENT SECTIONS OF MT/WY. THEREFORE...WE MADE LITTLE
CHANGE TO THE SLIGHT RISK AREA...BUT WOULD EXPECT THE RISK OF
FLASH FLOODING TO DIMINISH BY TONIGHT AND SUNDAY WITHIN THE MORE
STABLE REGIME NORTH OF I-70.

TO THE SOUTH...A CONVEYOR BELT CHARACTERIZED BY GREATER
INSTABILITY AND DEEPER MOISTURE WILL CONTINUE TO POSE A
CONCENTRATED RISK OF FLASH FLOODING. CONVECTION HERE WILL BE BASED
NEARER TO THE SURFACE...AND IS CAPABLE OF ANCHORING ITSELF ALONG
COLD POOLS...AIDED FURTHER BY WEST-EAST MOUNTAIN RANGES
INTERCEPTING THE MOIST LOW LEVEL FLOW. LARGELY UNIDIRECTIONAL AND
UNCHANGING WIND PROFILES WILL ALLOW FOR TRAINING AND POTENTIAL
FLASH FLOODING.

FROM THE DESERT SOUTHWEST INTO THE SOUTHERN GREAT BASIN..ISOLATED
1+ INCH RAINFALL AMOUNTS IN AN HOUR OR TWO AND LOCAL 2-3 INCH
EVENT TOTALS ARE LIKELY...WHILE THE LONGER DURATION EVENT FARTHER
NORTH IS LIKELY TO LEAD TO AREAL AVERAGE 0.50-1.50+ INCH AMOUNTS
OVER A RATHER LARGE AREA. HEADING INTO SUNDAY...HOWEVER...THE
FAVORABLE PW PLUME OVER THE SOUTWHEST MAY BECOME PINCHED OFF AND
SEPARATED FROM THE FORCING. THE FORCING ITSELF WILL BECOME LESS
PRONOUNCED AS THE LOW BECOMES FULLY CLOSED. MODELS DO SHOW SOME
INDICATION OF HEAVY RAIN POTENTIAL OVER WESTERN NM/CO ON
SUNDAY...BUT THE LIMITING FACTORS CAUSED US TO STOP SHORT OF
EXPANDING THE RISK AREA THAT FAR EAST.


...SOUTH TEXAS...

THE MORNING RUNS OF SEVERAL HIGH-RESOLUTION MODELS...INCLUDING
CONSECUTIVE HRRR RUNS AROUND 13-15Z...HAD PREDICTED SIZEABLE AREAL
COVERAGE OF INTENSE RAINFALL THIS AFTERNOON IN DEEP SOUTH TEXAS.
CERTAINLY PROFILES OF DEEP MOISTURE GIVE THIS REGION SOME
POTENTIAL FOR ORGANIZED HEAVY RAIN...BUT BASED ON SATELLITE/RADAR
TRENDS AND PERFORMANCE OF THIS WEATHER SYSTEM OVER THE PAST TWO
DAYS...THE MODELS ARE LIKELY OVER-FORECASTING THE RISK HERE.
ONGOING WIDESPREAD OVERTURNING WILL LIMIT HEATING...AND CELL
MOTIONS WERE NOT PARTICULARLY FAVORABLE FOR PERSISTENT HEAVY RAIN
AT A GIVEN LOCATION...WITH SHALLOW SEA-BREEZE CONVECTION BEING
SWEPT WESTWARD...AND PROPAGATION VECTORS FOR LARGER SCALE
CONVECTION DIRECTED EASTWARD INTO THE AIR ALREADY OVERTURNED.
DURING THE DIURNAL PEAK...SOME ISOLATED 2"/HR RAIN RATES ARE
POSSIBLE...BUT THE RISK OF EXCEEDING FLASH FLOOD GUIDANCE AT A
POINT IS JUDGED TO BE LESS THAN THE 5 PERCENT CRITERIA FOR A
SLIGHT RISK.


...FLORIDA...

A BROAD REGION OF DEEP AND RICH MOISTURE HAD CONTRIBUTED TO
LOCALLY HEAVY RAINFALL AND SOME FLASH FLOODING IN RECENT DAYS. THE
CENTRAL AXIS OF THIS MOISTURE PLUME AND ASSOCIATED 850-700 MB
CONVERGENCE WILL DRIFT WEST TODAY INTO THE GULF OF MEXICO AND WILL
BRUSH ACROSS THE FLORIDA PANHANDLE. THIS WESTWARD SHIFT MAY REDUCE
AREAL COVERAGE...BUT COULD CAUSE EVENTS TO BECOME EVEN MORE
FOCUSED ALONG THE IMMEDIATE WEST COAST OF THE PENINSULA AND NEAR
THE COAST OF THE PANHANDLE. AT MIDDAY PSEUDO-ORGANIZED CONVECTION
WAS BLOSSOMING NEAR AND NORTH OF ORLANDO...CLOSE TO AN AXIS OF 925
MB CONVERGENCE THAT IS FORECAST TO PERSIST ACROSS AT THE NORTH
SIDE OF GRADIENTS ASSOCIATED WITH A REMNANT LOW LEVEL BOUNDARY.
EASTERLY LOW LEVEL WINDS OF 10-15 KNOTS WILL ALSO ENHANCE
CONVERGENCE ALONG THE WEST COAST SEA-BREEZE...WHILE PROVIDING
INFLOW FROM THE WARM LAND-BASED CAPE SOURCE REGION. INTENSE LOCAL
RAINFALL MAY AGAIN BE CAPABLE OF EXCEEDING 3 TO 4 INCHES IN 2 TO 3
HOURS. SSEO TOOLS INDICATE THAT THE PROBABILITY OF RAINFALL
EXCEEDING 3-HOUR FLASH FLOOD GUIDANCE WITHIN 40 KM OF A POINT WILL
PEAK ABOVE 50 PERCENT BETWEEN 22-02Z ALONG THE WEST COAST OF THE
CENTRAL AND NORTHERN PENINSULA. THE NEXT DIURNAL CYCLE ON SUNDAY
SHOULD NOT PRODUCE QUITE SO FOCUSED A RISK...AS EASTERLY INFLOW
SLACKENS DURING THE AFTERNOON.

BURKE
$$




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