Quantitative Precipitation Forecast
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392
FOUS30 KWBC 272227
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
626 PM EDT SAT AUG 27 2016

...VALID 2226Z SAT AUG 27 2016 - 12Z SUN AUG 28 2016...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
105 S DUG 35 ENE DUG 20 NE SAD 20 WNW LUF 45 WSW EED 60 SW HND
10 SSE DRA 75 NE DRA 20 WSW MLF 30 ESE U24 40 W PUC 30 WSW PUC
40 NW 4HV 60 ENE PGA 40 N RQE 20 N CQC 20 WNW EHA 20 E GCK
40 WSW PTT 20 NW HHF 45 SSE CVN 20 WNW MRF 55 E MMCU.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
60 SE KMIU 25 SSE PKV 25 E CLL 35 S SHV 25 ENE ESF 25 SW MCB
10 WSW PQL 85 SW PAM.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 WNW BEH 20 SW VPZ 10 WNW AAA 20 NNE GBG 15 WSW BUU 35 NW MKG.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
40 SSW KEY 20 N NQX 25 W HST 10 W FXE 25 SSW MYGF.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 SSW HOP BWG 35 S GLW 35 SSW MQY 45 SE MKL 30 E MKL
15 SSW HOP.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 NNW KEMK 15 SE LBX 15 NE HOU 20 NNE BPT ARA 10 NE NEW
20 N KMIS KIKT.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 SSE IGM 35 NNW IFP 50 E LSV 15 NNW SGU 25 NNE CDC 35 NNW BCE
20 E BCE 45 SSE PGA 30 NE INW 20 ESE SJN 35 S SJN 50 NNE SDL
30 SSE IGM.


...22Z SPECIAL UPDATE...

...SOUTHWEST...

THE PATTERN HERE WAS MARKED BY AN UPPER LOW OVER THE COLORADO
RIVER BASIN IN NV/UT/AZ...AND SOUTHWESTERLY FLOW OF MODERATE
STRENGTH DOWNSTREAM INTO NEW MEXICO AND ADJACENT PARTS OF
SOUTHWEST TEXAS AND THE OK/TX PANHANDLES. THE PROBABILITY OF SLOW
MOVING CELLS AND EXCESSIVE RAINFALL IS A LITTLE GREATER NEAR THE
UPPER LOW...WITH COLD AIR ALOFT AND STRONG AUGUST HEATING
CONTRIBUTING TO SURFACE BASED CAPE VALUES OF 1000 J/KG FROM THE
MOGOLLON RIM WESTWARD INTO THE RIVER BASIN. PRECIPITABLE WATER
VALUES THROUGHOUT THE SOUTHWEST WERE CLOSE TO THE CLIMATOLOGICAL
AVERAGE...BUT HAD STILL PROVEN SUFFICIENT FOR OBSERVED HEAVY RAIN
RATES OVER ONE INCH PER HOUR EARLIER IN THE DAY. IN THE AREAS
DOWNSTREAM OF THE UPPER LOW CELL MOTIONS WILL BE QUICKER...AND
ASCENT IS NOT FORECAST TO BECOME PARTICULARLY WELL FOCUSED UNTIL
PERHAPS AFTER 03Z WHEN THE RAP DEPICTS A DIFLUENCE MAXIMUM
EXPANDING OVER SOUTHWEST TX AND EASTERN NM. AT THE SAME
TIME...CONVECTION OVER THE OK/TX PANHANDLES AND SOUTHWEST KANSAS
WILL EXPERIENCE INCREASED INFLOW WITH THE LOW LEVEL JET...AND IF A
CONSOLIDATED COLD POOL CAN DEVELOP THERE MAY BE A FOCUS FOR A
LOCAL THREAT OF FLASH FLOODING. THE MARGINAL RISK WAS EXPANDED TO
ENCOMPASS THIS POSSIBILITY...WHICH IS DEPICTED MOST NOTABLY IN THE
WRF-ARW AND WRF-NMM.


...UPPER TX COAST, SOUTHERN LA, & SOUTHERNMOST MS...

HERE THE MARGINAL RISK WAS EXPANDED NORTHWARD TO INCLUDE NUMEROUS
THUNDERSTORMS ROLLING WESTWARD THIS EVENING. CELL MOTIONS WILL
LIMIT THE DURATION OVER ANY GIVEN POINT AND THUS LIMIT THE TOTAL
RAINFALL...BUT STRONG HOURLY RAIN RATES EXCEEDING 1.5 INCHES PER
HOUR HAD BEEN ESTIMATED BY RADAR. A SLIGHT RISK OF EXCESSIVE
RAINFALL IS TIED MORE TO THE EXPECTED DEVELOPMENT OF CONVECTION
NEAR THE COAST OVERNIGHT...AS DISCUSSED BELOW.

AN AXIS OF ABOVE NORMAL PW VALUES EXTENDING FROM THE FL PENINSULA
WESTWARD INTO THE NORTHWEST GULF COAST ALONG THE SOUTHERN
PERIPHERY OF THE SUBTROPICAL RIDGE WILL CONTINUE TO LEAD TO
WIDESPREAD CONVECTION FROM PARTS OF FL WESTWARD INTO THE WESTERN
GULF COAST. A WEAK UPPER LOW WHICH HAD DEVELOPED OVER COASTAL
SOUTHEAST TEXAS IS FORECAST TO SHOW LITTLE MOVEMENT OVERNIGHT.
MOST HIGH RESOLUTION GUIDANCE PRODUCES TWO LOCAL MAXIMA OF
PRECIPITATION DURING THE OVERNIGHT / EARLY MORNING PERIOD...ONE
BENEATH THE UPPER LOW NEAR THE 850-700 MB INVERTED TROUGH AXIS
AFFECTING FAR SE TX AND SW LA...AND ANOTHER OVER FAR SE LA BENEATH
A DIVERGENCE MAXIMUM DOWNSTREAM OF THE UPPER LOW. THE MAIN
CHALLENGE HERE IS IN FORECASTING HOW FAR NORTH THE HEAVIER
CONVECTIVE BANDS WILL DEVELOP. PROXIMITY OF THE UPR LOW ALONG WITH
OFFSHORE SFC FLOW SHOULD KEEP THE HEAVIER CONVERGENCE BANDS NEAR
AND SOUTH OF THE COASTAL WATERS...WITH THE HRRR SUGGESTING FIRST
ECHOES AROUND 05-07Z. MOST OF THE GUIDANCE MAXIMIZES ACTIVITY
OFFSHORE...BUT DOES BRING MULTI-INCH ACCUMULATIONS JUST ONSHORE
THROUGH 12Z.


...CHICAGOLAND...

IN THE WAKE OF EARLY DAY CONVECTION...INSTABILITY HAS RECOVERED IN
THE WARM SECTOR SOUTH OF A STATIONARY FRONT BEING ANALYZED NEAR
THE IL/WI BORDER. WHILE HEIGHT FALLS WILL BE PRIMARILY CONFINED TO
AREAS FARTHER NORTH...AHEAD OF A PROGRESSIVE TROUGH...THERE IS
SOME UPPER JET FORCING EXPECTED TO GRAZE ACROSS NORTHERN
ILLINOIS...WITH MOST GUIDANCE SUGGESTING A SOMEWHAT ORGANIZED
CONVECTIVE EVENT THIS EVENING. FLASH FLOOD GUIDANCE VALUES HAVE
BEEN LOWERED BY RECENT WET WEATHER...SO A MARGINAL RISK OF
EXCESSIVE RAINFALL WAS INTRODUCED HERE. THE ANTICIPATED COVERAGE
OF ROBUST CONVECTION AND HEAVY RAIN RATES IS TOO SMALL TO WARRANT
A SLIGHT RISK AREA...BUT GIVEN THE URBAN SETTING AROUND CHICAGO
AND THE LOW FFG...ONE OR TWO INSTANCES OF FLASH FLOODING APPEAR
POSSIBLE.


...TENNESSEE AND KENTUCKY...

THUNDERSTORMS HAD BRIEFLY BEEN EFFECTIVE AT RAINFALL PRODUCTION
NEAR NASHVILLE...OWING TO A CELL MERGER. SATELLITE IMAGERY AND THE
RAP REVEAL AN UPPER LEVEL CIRCULATION HERE...WHICH MAY SUPPORT
SOME RENEWED DEVELOPMENT ALONG OUTFLOWS AS SUGGESTED BY THE 20Z
HRRR. ALTHOUGH THE LOWER/MID LEVEL WIND FIELDS ARE NOT TERRIBLY
ORGANIZED...AND CELL MERGERS WILL TEND TO BE RANDOM...WE DID
INCLUDE A SMALL MARGINAL RISK AREA GIVEN THE PRESENCE OF THE UPPER
LOW.

...SOUTH FLORIDA...

MOST DAYTIME CONVECTION HAD ALREADY ROLLED THROUGH SOUTH FLORIDA.
CONVECTION ASSOCIATED WITH INVEST 99L...THE DISTURBANCE
APPROACHING THE FLORIDA STRAIGHTS...MAY AFFECT FAR SOUTH FLORIDA
AND THE KEYS BY MORNING...ALTHOUGH THIS WILL BECOME MORE LIKELY
BEYOND 12Z.

BURKE
$$





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