Quantitative Precipitation Forecast
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FOUS30 KWBC 031435
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1035 AM EDT MON AUG 03 2015

...VALID 15Z MON AUG 03 2015 - 12Z TUE AUG 04 2015...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
40 SSE HHF 20 W AMA 25 SSW DHT 25 SE SPD 40 E LBL 15 NW AVK
30 W JWG 40 SSE HHF.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 N OCF 10 WSW ORL 15 ESE BOW 35 N PGD 20 SSW SRQ 55 WSW SRQ
90 W SRQ 90 W PIE 95 WNW PIE 70 SW CTY 20 N OCF.


1500 UTC UPDATE

THE PRIMARY CHANGE TO THE PREVIOUS EXCESSIVE RAINFALL POTENTIAL
OUTLOOK WAS TO REMOVE THE SLIGHT RISK AREA OVER WEST CENTRAL
FLORIDA IN THE VICINITY OF TAMPA.  CLOUD TOPS CONTINUE TO WARM
QUICKLY WITH THE AREA OF PRECIPITATION---WITH RADAR ESTIMATES OF
HOURLY PRECIPITATION CONTINUING TO DECREASE ALONG COASTAL
SECTIONS--SIGNALING A DECREASING THREAT OF RUNOFF ISSUES.
RAINFALL AMOUNTS AND INTENSITIES MAY INCREASE INLAND LATER THIS
MORNING INTO THIS AFTERNOON---WITH A CONTINUED DECREASE IN
INTENSITIES CLOSER TO THE COAST.   PLEASE SEE WPC`S MESOSCALE
PRECIPITATION DISCUSSION # 0413 VALID UNTIL 1615 UTC FOR FURTHER
DETAILS ACROSS THIS REGION.  ELSEWHERE---NO CHANGES TO THE
PREVIOUS OUTLOOKS AREAS ACROSS THE COASTAL AREAS OF SC/NC---THE
SOUTHERN PLAINS OR INTO THE GREAT BASIN---CENTRAL TO NORTHERN
ROCKIES REGION.

ORAVEC


PREVIOUS DISCUSSION

...MID ATLANTIC/SOUTHEAST/FL...

THE DEEP MOISTURE ASSOCIATED WITH A SURFACE WAVE THAT CROSSES
NORTHERN FL AND MOVES ALONG THE SOUTHEAST COAST TO THE COASTAL MID
ATLANTIC FEEDS CONVECTION ALONG A SURFACE TROUGH...AND THE
RESULTANT CONVECTION COULD PRODUCE HEAVY TO LOCALLY EXCESSIVE
RAINFALL FROM NORTH FL THROUGH THE COASTAL CAROLINAS.

THE NEXT SURGE OF CONVECTION TIED TO DEEP TROPICAL MOISTURE (WITH
PRECIPITABLE WATER VALUES BETWEEN 2.25 AND 2.50 INCHES) IS
APPROACHING THE CENTRAL PORTION OF THE WEST OF FL EARLY THIS
MORNING. THE BULK OF THE HIGH RESOLUTION GUIDANCE SUGGESTS THAT
THE BEST THRUST OF MOISTURE OCCURS IN THE 03/12Z TO 03/18Z TIME
FRAME...DURING WHICH AN ADDITIONAL COUPLE OF INCHES OF RAINFALL IS
POSSIBLE. FOR THIS REASON...A SLIGHT RISK AREA WILL NEED TO BE
RETAINED UNTIL THE DEEPER MOISTURE IS SCOURED BY A DEVELOPING
SURFACE WAVE OFF THE SC COAST DURING THE AFTERNOON.

AS MENTIONED EARLIER...THERE IS SOME SPREAD CONCERNING HOW THE
SURFACE WAVE EVOLVES...AND HOW THE DEEP MOISTURE BECOMES INVOLVED
IN THE DEVELOPMENT OF ADDITIONAL CONVECTION NEAR THE COAST.
FOLLOWING THE SEQUENCE OF EVENTS FROM THE 00Z GFS (WHICH APPEARED
TO HAVE THE BEST SUPPORT FROM THE 12Z ENSEMBLE
SOLUTIONS)...CONVECTION DEVELOPS ON A FRONTAL
BOUNDARY/SURFACE TROUGH AHEAD OF THE SURFACE WAVE...STRETCHING
OVER COASTAL SC/NC. IT IS UNCLEAR...BASED ON THE 00Z GFS (AND THE
PREPONDERANCE OF OTHER 00Z GUIDANCE SOLUTIONS)...WHETHER THE
HEAVIEST RAINFALL OCCURS ALONG THE COAST OR JUST OFFSHORE. THE
BEST INSTABILITY SHOULD GET ONSHORE AHEAD OF THE SURFACE
WAVE...AND THE CONVECTION SHOULD TAP THE DEEP MOISTURE TO PRODUCE
AN AXIS OF 1.50 TO 2.25 INCHES OF QPF OVER THE COASTAL CAROLINAS.

THREE HOUR FLASH FLOOD VALUES ALONG THE COASTAL CAROLINAS ARE HIGH
(GENERALLY ABOVE 3.00 INCHES)...SO AT FIRST LOOK IT WOULD APPEAR
THAT THE THREAT FOR FLASH FLOODING IS LOW. HOWEVER...MOST OF THE
HIGH RESOLUTION GUIDANCE (FROM THE 00Z WRF ARW/NMMB TO THE 00Z
REGIONAL GEM) SUGGEST THE POTENTIAL FOR 2-4 INCHES OF
QPF...ESPECIALLY IF A CONVECTIVE COMPLEX FORMS IN THE INCREASING
SOUTHERLY INFLOW NEAR THE NC COAST. THE AIRMASS PRODUCED LARGE
RAINFALL AMOUNTS ACROSS NORTHERN FL THE PAST FEW DAYS...SO IT IS
CAPABLE OF SUPPORTING RAINFALL AMOUNTS NEAR 4 INCHES. GIVEN THIS
INFORMATION...A SEE TEXT WAS PLACED OVER  PORTIONS OF THE SC/NC
COAST.


...SOUTHERN PLAINS...

AN INCREASING LOW LEVEL INFLOW FEEDS MOISTURE AND INSTABILITY
ACROSS NORTH TX/WESTERN OK/WESTERN KS DURING THE SECOND HALF OF
DAY 1...AND THERE IS A MULTI MODEL INDICATION THAT AN MCS COULD
DEVELOP AFTER 04/00Z. HIGH RAINFALL RATES WITH THE DEVELOPING MCS
COULD RESULT IN EXCESSIVE RAINFALL...MAINLY AFTER 04/06Z.

AN EARLY MORNING COMPLEX OVER NORTH TX SHOULD FADE AND MOVE ITO
SOUTHWEST OK AS THE LOW LEVEL INFLOW WEAKENS AND INSTABILITY
BECOMES ELEVATED. AN MCV FROM THE COMPLEX COULD RESULT IN LIFT FOR
CONVECTION ACROSS SOUTHWEST OK THROUGH THE MORNING HOURS...BUT
HEAVY RAIN WITH THIS SYSTEM IS UNLIKELY AFTER 03/12Z.

THE PATTERN APPEARS TO REPEAT ITSELF TONIGHT...AS A 20 TO 30 KNOT
INFLOW OVER NORTH TX/WESTERN OK/WESTERN KS TRANSPORTS MODERATE TO
STRONG INSTABILITY AND PRECIPITABLE WATER VALUES IN EXCESS OF 1.75
INCHES INTO NORTH TX. THE 00Z NAM SIMULATED SATELLITE IMAGERY
SHOWED THE MCS DEVELOPING OVER NORTH TX/WESTERNMOST OK BETWEEN
04/00Z AND 04/06Z...THEN TURNING SOUTHEAST AS IT FOLLOWS THE BEST
LOW LEVEL INSTABILITY. MOST OF THE REMAINDER OF THE 00Z GUIDANCE
FORMS A CONVECTIVE COMPLEX OR MCS FURTHER NORTH INTO WESTERN
OK/SOUTHWEST KS. GIVEN THE FACT THAT RECENT CONVECTIVE COMPLEXES
THAT DEVELOPED IN NORTHEAST NM TRACKED ACROSS NORTH TX...THE
EXCESSIVE RAINFALL APPEARS MORE LIKELY OVER NORTH TX AND NEARBY
WESTERN/SOUTHWESTERN OK.

DUE TO HEAVIER RAINFALL ACROSS NORTH TX AND WESTERNMOST OK THE
PAST COUPLE OF NIGHTS...ONE AND THREE HOUR FLASH FLOOD GUIDANCE
VALUES ARE AS LOW AS 1.50 INCHES. SOME OF THE HIGH RESOLUTION
GUIDANCE PLACES AS MUCH AS 3 INCHES OF RAINFALL WITH THE MCS
(INCLUDING THE 00Z WRF ARW/NMMB AND 00Z REGIONAL GEM). CONSIDERING
THAT PRECIPITABLE WATER VALUES TOP OUT BETWEEN 1.75 AND 2.0O
INCHES...THIS IS PLAUSIBLE. A SLIGHT RISK AREA WAS PLACED OVER
PORTIONS OF NORTH TX/WESTERN OK FOR THE FLASH FLOOD POTENTIAL.


...NORTHERN AND CENTRAL ROCKIES...

MOISTURE AND INSTABILITY STRETCHED OUT ALONG A FRONTAL BOUNDARY
EXTENDING FROM THE NORTHERN ROCKIES INTO THE CENTRAL PLAINS FEED
CONVECTION THAT COULD PRODUCE HEAVY TO LOCALLY EXCESSIVE RAINFALL
DURING DAY 1.

A LOW LEVEL UPSLOPE FLOW ACROSS THE NORTHERN AND CENTRAL ROCKIES
INCREASES DURING THE DAYLIGHT HOURS...REACHING 15 TO 30 KNOTS
ACROSS WESTERN WY INTO EASTERN ID AND WESTERN CO BEFORE 03/21Z.
THE LOW LEVEL INFLOW TRANSPORTS 0.75 TO 1.00 INCH PRECIPITABLE
WATER VALUES INTO THESE AREAS (WHICH IS ABOUT TWO STANDARD
DEVIATIONS ABOVE THE MEAN). SYNOPTIC SCALE LIFT IS SUPPLIED BY A
SHORT WAVE RIDING NORTH OUT OF CENTRAL UT EARLY THIS MORNING TO A
POSITION OVER CENTRAL WY BY THE END OF THE PERIOD. THE MAIN
INGREDIENT MISSING FOR THE POTENTIAL FOR EXCESSIVE RAIN FALL COULD
BE INSTABILITY. 00Z GFS MODEL SOUNDINGS SHOWED MARGINAL
INSTABILITY ACROSS WESTERN WY (GENERALLY AROUND 500 J/KG OF
SBCAPE)...PEAKING AROUND 04/00Z.

MUCH OF THE 00Z GUIDANCE SHOWS THE POTENTIAL FOR AREAS OF AN INCH
OR SO OF BASIN AVERAGE QPF...AND SOME OF THE HIGH RESOLUTION
GUIDANCE (SUCH AS THE 00Z NSSL WRF AND 00Z REGIONAL GEM) SHOWED
MAXIMUM QPF AMOUNTS OVER 2.00 INCHES. ONE AND THREE HOUR FLASH
FLOOD GUIDANCE VALUES ARE AS LOW AS AN INCH OR LESS OVER PORTIONS
OF WESTERN WY (MAINLY IN THE TERRAIN)... AND GIVEN HIGHLY
ANOMALOUSLY MOISTURE IN PLACE...HEAVY RAINFALL IS POSSIBLE. A SEE
TEXT WAS PLACED OVER PORTIONS OF EASTERN ID/WESTERN WY INTO
NORTHERN CO TO ADDRESS THE FLASH FLOOD POTENTIAL. IF INSTABILITY
WERE FORECAST TO BE MORE ROBUST...A SLIGHT RISK AREA MAY HAVE BEEN
WARRANTED.


HAYES

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