THE WEATHER PREDICTION CENTER

COLLEGE PARK, MD


EXCESSIVE RAINFALL DISCUSSION


[Abbreviations and acronyms used in this product]
Geographic boundaries:    Map 1- [Color] [B/W Print Version]      Map 2 - [Color] [B/W Print Version]



EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
148 PM EDT THU JUL 31 2014

...VALID 18Z THU JUL 31 2014 - 00Z SAT AUG 02 2014...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
55 SSE RTN 50 E ROW 40 SSE GDP 10 ENE E38 45 SSE MRF 65 SW MRF
60 WSW MRF 65 SE MMCS MMCS 45 SSW LRU 50 SSW DMN 75 ESE DUG
20 WSW OLS 80 W OLS 70 S GBN GBN 25 NNE SDL 25 NNE SJN
25 SSE GUP 35 WNW 4SL 25 W CPW 30 SE MYP 30 WSW FCS 25 NNW TAD
10 S TAD 55 SSE RTN.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 NNE RHP BCB 25 NNW LYH HBI 20 SSW FLO 30 E CHS 30 S CHS
35 NW NBC 15 SSE AGS 10 E 3J7 10 WNW LZU 10 S 47A 10 S DNN
20 NNE RHP.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 SSE GRD 15 W 27A 15 NW CEU 20 SW AVL 20 W MRN 10 NNE HKY
10 NNE AKH 25 SW UZA 10 SSE GRD.


SOUTHERN ROCKIES/ARIZONA
~~~~~~~~~~~~~~~~~~~~~~~~
MOIST UPSLOPE FLOW CONDITIONS ARE EXPECTED ACROSS NM AND WESTERN
TX WITH POTENTIAL FOR STRONG SLOW-MOVING CONVECTION FOCUSING ALONG
THE FAVORED ERN UPSLOPE AREAS OF SRN CO INTO NM/EXTREME WESTERN TX
UNDER A WEAK NORTHWESTERLY FLOW ALOFT.  PRECIPITABLE OF ~1.50
INCHES COMBINED WITH 850-700 HPA INFLOW OF 20+ KTS ALONG THE FRONT
RANGE OF THE ROCKIES AND CAPES EXPECTED TO EXCEED 1000 J/KG DUE TO
DAYTIME HEATING MEET THE CRITERIA FOR THE WET MULTI-CELL
ENVIRONMENT.   MODEL SOUNDINGS SHOULD AREAS OF TALL SKINNY CAPES
WITH NO LOW-LEVEL DRY INTRUSIONS ALONG WITH WEAK STEERING FLOW OF
LESS THEN 10 KTS ESPECIALLY ACRS PARTS OF SOUTHERN NM/EXTREME
WESTERN TX SUGGESTING THE POTENTIAL FOR SOME SLOW MOVING STORMS
CAPABLE OF PRODUCING SOME LOCALLY HEAVY RAINFALL AMOUNTS.  SOME
LOW-LEVEL INFLOW (10-20 KTS) ACROSS SOUTHERN AZ SHOULD FOCUS
CONVECTION WITHIN AND NEAR THEIR SOUTHEAST MOUNTAINS.  FLASH FLOOD
GUIDANCE VALUES ARE RATHER LOW ACROSS PORTIONS OF NM AND CO DUE TO
SOME RECENT RAINS WHICH GIVEN THE ABOVE SUPPORTS A SLIGHT RISK OF
EXCESSIVE RAINS.


SOUTHERN APPALACHIANS
~~~~~~~~~~~~~~~~~~~~~
ONGOING CONVECTION IN SC HAS LOWERED FLASH FLOOD GUIDANCE ACROSS
NORTHWEST SC, INCREASING THE OVERALL FLASH FLOOD THREAT FOR
TONIGHT.  A WEAK DISTURBANCE COULD CAUSE SOME CELL TRAINING FROM
THE SC COAST NEAR CHARLESTON INLAND TOWARDS THE TOPOGRAPHY THROUGH
THE REMAINDER OF THE AFTERNOON AND EVENING.  TONIGHT, MAINLY AFTER
06Z, 850 HPA INFLOW INTO THE MOUNTAINS OF NORTHERN GA INCREASES
OVER 25 KTS WHILE UPSTREAM MOST UNSTABLE CAPES REMAIN ~1000 J/KG
(PER RECENT RAP GUIDANCE), SETTING OFF A NEW ROUND OF ORGANIZED
CONVECTION.  FLASH FLOOD GUIDANCE VALUES ARE HIGH OUTSIDE OF
NORTHWEST SC AND PORTIONS OF WESTERN NC, WHICH LIMITS THE OVERALL
RISK.  HOWEVER, VEERING WINDS IN THE LOW-LEVELS SHOULD ALLOW FOR
SOME FORWARD PROPAGATION, AND THE FORWARD PROPAGATING CORFIDI
VECTORS SUGGEST 20 KTS OF NORTHEAST MOTION FROM NORTHERN GA INTO
THE WESTERN CAROLINAS.  AS AN 850 HPA LOW MOVES UP THE
APPALACHIANS, HEAVY RAINS SHOULD SHIFT UP INTO SOUTHWEST VA DURING
THE DAY TOMORROW.  PRECIPITABLE WATER VALUES ARE NEAR THE 90TH
PERCENTILE FOR LATE JULY/EARLY AUGUST, IN THE 1.75-2" RANGE.  THIS
SUGGESTS RAINFALL AMOUNTS OF 2" AN HOUR ARE POSSIBLE WHERE CELLS
BACKBUILD, OR MORE LIKELY, TRAIN.  LOCAL AMOUNTS IN THE 3-5" RANGE
ARE POSSIBLE, WHICH COULD BREACH THE HIGH FLASH FLOOD GUIDANCE. 
TOWARDS THE NC COAST, THE GUIDANCE IS INCONSISTENT ON A POSSIBLE
SIGNAL WITHIN OUR SECONDARY RAINFALL MAXIMUM WITHIN THE MARSHES OF
EASTERN NC, GENERALLY EAST OF I-95, SO LEFT THEM OUT OF THE
EXCESSIVE RAINFALL AREAS.


LOUISIANA/TEXAS BORDER
~~~~~~~~~~~~~~~~~~~~~~
MODEST LOW-LEVEL SOUTHWESTERLY FLOW DRAWING PRECIPITABLE WATER
VALUES OF ~2 INCHES INTO AND IN ADVANCE OF AN MCV/SHORTWAVE
DROPPING SOUTHEAST OVER OK ALONG AND AHEAD OF A SLOWLY PROGRESSIVE
COLD FRONT SHOULD SUPPORT ORGANIZED CONVECTION THROUGH THE PERIOD
WITH LOCALLY HEAVY RAINFALL RATES THRU PARTS OF EASTERN TX AND LA
AS 850 HPA INFLOW FROM THE GULF OF MEXICO INCREASES TO 25-30 KTS
WITHIN AN AREA OF 2-2.25" PRECIPITABLE WATER VALUES -- ABOVE THE
75TH PERCENTILE FOR LATE JULY/EARLY AUGUST.  THE GOLDEN TRIANGLE
OF SOUTHEAST TX -- IS AN AREA WITH THE LEAST AMOUNT OF FORWARD
PROPAGATION EXPECTED.  VEERING LOW-LEVEL WINDS IN AREA SOUNDINGS
SUGGEST THAT FORWARD PROPAGATION TO THE SOUTHEAST IS EXPECTED AT
ROUGHLY 10-15 KTS.  AS 850 HPA INFLOW APPEARS TO BE NARROW AHEAD
OF THE COLD FRONT, THE RISK OF A WARM ADVECTION THUNDERSTORM BAND
APPEARS MINIMAL NEAR THE LA/AR BORDER.  NEVERTHELESS, CELL
TRAINING IS POSSIBLE IF THE CONVECTIVE BAND OUTRACES THE FRONTAL
ZONE AND NEW CONVECTION FORMS CLOSER TO THE COLD FRONT, CAUSING
MULTIPLE CONVECTIVE LINES.  HOURLY RAINFALL RATES UP TO 2.5" ARE
POSSIBLE, WITH A QUICK 2-4" ANTICIPATED WHICH WOULD CHALLENGE THE
HIGH FLASH FLOOD GUIDANCE IN THE AREA.


SOUTHERN FL
~~~~~~~~~~~
HIGH PRECIPITABLE WATER VALUES -- IN THE 2-2.25" RANGE -- COMBINED
WITH LIGHT SOUTH TO SOUTHWEST FLOW IN THE SURFACE-700 HPA LAYER
SHOULD FORCE SEA BREEZE CONVERGENCE-RELATED THUNDERSTORMS THIS
AFTERNOON TO PREFER THE EVERGLADES AND POTENTIALLY THE GOLD COAST
OF FL.  THE 06Z SSEO-BASED PROBABILITY OF EXCEEDING FLASH FLOOD
GUIDANCE SHOWS SPOTTY AREAS ACROSS THE SOUTHERN PENINSULA SOUTH OF
LAKE OKEECHOBEE ABOVE 50% FROM 17-00Z.  FLASH FLOOD GUIDANCE
VALUES ARE FAIRLY LOW IN THE MIAMI/HOMESTEAD/FLORIDA CITY AREA DUE
TO ~30" OF RAIN MIA AND VICINITY HAVE RECEIVED SINCE JUNE 1. 
AFTER CONSULTING WITH THE MFL FORECAST OFFICE, DECIDED IT DID NOT
QUITE REACH THE SLIGHT RISK CATEGORY.  HOURLY RAIN RATES UP TO
2.5" ARE POSSIBLE, WITH LOCAL AMOUNTS OF 2-4" FORECAST WHERE CELLS
MERGE/TRAIN.  TOMORROW, THUNDERSTORMS WITH HEAVY RAINFALL RELATED
TO SEA BREEZE CONVERGENCE SHOULD FOCUS MORE TOWARDS THE WEST
COAST, WHICH HAS HIGHER FLASH FLOOD GUIDANCE, AND A LESSER FLASH
FLOOD THREAT.


ROTH




Last Updated: 148 PM EDT THU JUL 31 2014