Quantitative Precipitation Forecast
Issued by NWS

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FOUS30 KWBC 280101
QPFERD

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

...VALID 01Z SUN AUG 28 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
60 SSE KMZG 50 ESE KMIU 20 ENE KRP 15 NW SGR JAS 10 NNW LFT
20 SSW HDC 10 E BIX 100 SW AAF.

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
100 S DUG 50 E DUG 10 NNE SVC 55 N SVC 60 NE SAD 55 E IWA
55 NW LUF 50 SE IGM 35 ENE IGM 40 SSE SGU 40 SSW BCE 15 SE PGA
40 SSW RQE 40 S GNT 30 SSW ABQ 20 SSW SAF 35 SW RTN 20 E CAO
10 SE EHA 45 N EHA 45 ENE LAA 20 NNW GCK 25 S DDC 30 W AVK
25 ESE WWR 40 NW CSM 10 E PPA 45 NW PVW 40 S CVS 45 ENE ROW
35 N ROW 25 WNW ROW CNM 35 W PEQ 20 WNW MRF 55 E MMCU.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 NNE PIA 10 W RPJ MKE 40 WSW MKG 10 ESE VPZ 15 SSW DNV
15 SE TAZ 10 SSE SPI 10 NNE PIA.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 NW KVAF 10 NNW KBQX LBX 15 NNW BPT 20 E 7R4 20 NW GAO
25 NW KMIS 10 SW KIPN.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
45 NNE SDL 25 W PRC 40 WSW 40G 50 W GCN 40 WSW PGA 45 SSE PGA
30 NE INW 20 ESE SJN 35 S SJN 45 NNE SDL.


...01Z UPDATE...

...SOUTHWEST STATES OUT INTO THE HIGH PLAINS / WESTERN KS/OK/TX...

WE UPDATED TO FURTHER REDUCED THE SIZE OF THE RISK AREAS OVER THE
SOUTHWEST STATES. FOLLOWING THE ACTIVE MORNING HOURS...INSTABILITY
HAD RECOVERED ONLY BRIEFLY AND THEN BEEN QUICKLY EXHAUSTED AROUND
THE BACK SIDE OF AN UPPER LOW...IN THE MULTI-STATE AREA BORDERING
THE COLORADO RIVER BASIN. A FEW STRONGER THUNDERSTORM CELLS REMAIN
POSSIBLE NEAR AND AHEAD OF THE UPPER LOW IN THE VICINITY OF THE
MOGOLLON RIM...AND LIGHT STEERING FLOW WITHIN THE CLOUD LAYER
COULD ALLOW FOR SOME LOCALLY HEAVY DOWNPOURS AND RAPID RUNOFF
CONCERNS. FARTHER DOWNSTREAM ACROSS NEW MEXICO AND SOUTHWEST TEXAS
CELL MOTIONS WILL BE QUICKER...BUT BOUNDARY COLLISIONS COULD BOOST
RAINFALL EFFICIENCY ON A LOCAL BASIS...ESPECIALLY ACROSS SOUTHERN
NM AND FAR WEST TX...WHERE THE RAP CONTINUED TO INDICATE AN UPPER
DIFLUENCE MAXIMUM FORECAST TO PERSIST THROUGH 06Z.

AT THE SAME TIME...CONVECTION OVER THE TX PANHANDLE AND NORTHWEST
OKLAHOMA HAD BECOME SOMEWHAT ORGANIZED. MID EVENING RUNS OF THE
HRRR PREDICT VERY HEAVY RAIN WITH THIS ACTIVITY...WHICH WILL BE
EXPERIENCING AN INCREASE TO THE LOW LEVEL JET AND ASSOCIATED
MOISTURE FLUX. OBSERVED AND PREDICTED CELL MOTIONS
SUGGEST...HOWEVER...THAT THE ACTIVITY MAY MOVE AND PROPAGATE
SOUTHWARD RATHER THAN REMAINING ANCHORED OVER ANY GIVEN
SPOT...ESPECIALLY JUDGING BY THE DRIER AIR IN THE AMARILLO
SOUNDING. STILL...THERE IS A MARGINAL THREAT FOR SOME BRIEF
TRAINING EPISODES AND LOCALLY HEAVY RAINFALL EXCEEDING PERHAPS 2
INCHES. OTHER CELL MERGERS MAY REPRESENT A BRIEF FLASH FLOOD
THREAT ACROSS NORTHEAST NEW MEXICO AND WESTERN KANSAS.


...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 HAD 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. AT 01Z WE EXPANDED THE MARGINAL RISK TO INCLUDE A SMALL
CLUSTER OF SLOW MOVING THUNDERSTORMS EAST OF SPRINGFIELD AS WELL.


...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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