Quantitative Precipitation Forecast
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FOUS30 KWBC 101332
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
932 AM EDT TUE OCT 10 2017

...VALID 15Z TUE OCT 10 2017 - 12Z WED OCT 11 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
50 SW MMRX 15 W MFE 15 NNE T65 25 ENE PIL 50 SE MMMA.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 ESE AVL 1A5 35 W RHP 30 NE CHA 25 SE CSV OQT 6V3 20 SW LWB
20 SE HSP 20 NNE LYH 25 SW FVX 10 NNW TDF 20 ESE AVL.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 E HUF 20 WNW HNB 20 W EHR MDH 10 SSE CPS 25 NW SET
40 SSW UIN 15 NE CDJ 15 SSE ICL 15 W RDK 15 E ADU 25 WSW ALO
15 S PDC RYV 15 W UES MKE 10 WNW BEH 15 NW GUS 20 WSW GUS
20 E HUF.


...MIDWEST / MO/IL/IN/KY...

WITH DEEP SOUTH TO SOUTHWESTERLY FLOW AHEAD OF A COMPACT
SHORTWAVE...THUNDERSTORMS WERE PRODUCING A FEW SMALL SWATHS OF
MODERATE TO HEAVY RAINFALL THIS MORNING FROM WESTERN TENNESSEE TO
SOUTHERN INDIANA AND ILLINOIS. THROUGH THE AFTERNOON AND EVENING
HOURS THE SHORTWAVE WILL SUPPORT WIDESPREAD RAIN AND EMBEDDED
THUNDER ALONG THE PATH OF STRONG HEIGHT FALLS THROUGH IOWA AND
NORTHERN ILLINOIS...WHILE DEEP LAYER FORCING AT THE BASE OF THE
TROUGH AIDES ADDITIONAL THUNDERSTORM DEVELOPMENT IN THE WARM
SECTOR FARTHER SOUTH. IN THE 15Z UPDATE...WE SHIFTED THE MARGINAL
RISK OF EXCESSIVE RAINFALL SOUTHWARD TO BETTER ALIGN WITH THE
AVAILABILITY OF CAPE BY THIS AFTERNOON...AS THE COMBINATION OF
EARLY MORNING OUTFLOWS AND STRONGLY FORCED SYNOPTIC RAINFALL
SHOULD SHIELD IOWA AND NORTHERN ILLINOIS FROM DEVELOPING MUCH CAPE
OR OPPORTUNITY FOR INTENSE RAIN RATES. WE ALSO EXPANDED THE RISK
SOUTHWARD TO INCLUDE SOME OF THE MORNING RAIN SWATHS WHERE
ADDITIONAL AFTERNOON ACTIVITY IS POSSIBLE. STILL...GIVEN HIGH
FLASH FLOOD GUIDANCE VALUES AND LITTLE RESPONSE TO STREAM FLOW
ANOMALIES IN THE NATIONAL WATER MODEL...THE RISK PROBABILITIES FOR
FLASH FLOODING STILL APPEAR TO BE BEST DESCRIBED BY THE MARGINAL
CATEGORY.


...FAR SOUTH TX...

INGREDIENTS FOR HEAVY RAIN WILL BRIEFLY CONVERGE ON SOUTH TEXAS. A
COLD FRONT WAS DROPPING OUT OF THE HILL COUNTRY TOWARD THE LOWER
GULF COAST...WHILE SATELLITE IMAGERY SAMPLED A SURGE OF DEEPER
MOISTURE MOVING ONTO THE COAST. PRECIPITABLE WATER IN THE 12Z
BROWNSVILLE SOUNDING HAD RISEN ABOVE 2.00 INCHES. THE COLD FRONT
WAS RELATIVELY UNSUPPORTED IN THE MID LEVELS...WITH A WAVE LIFTING
WELL NORTH THROUGH THE MISSOURI VALLEY. AS A RESULT...DEEP
EASTERLY FLOW WILL CARRY DEVELOPING THUNDERSTORMS BACK WESTWARD
ALONG THE SLOPING FRONTAL SURFACE. THIS COULD RESULT IN SLOW NET
MOVEMENT AND SOME AWKWARD TRAINING AS CELLS REFORM FARTHER EAST
ALONG THE ADVANCING COLD FRONT...BUT ARE CARRIED BACK WEST BY THE
MEAN WINDS. THE HI-RES GUIDANCE IS SHOWING ISOLATED HOURLY
RAINFALL RATES OF 1-2" POSSIBLE THIS MORNING INTO MID
AFTERNOON...AND FLASH FLOODING IS POSSIBLE...ESPECIALLY WHERE
SOMEWHAT LOWER FLASH FLOOD GUIDANCE VALUES EXIST NEAR THE U.S.
BORDER AND THE CITY OF BROWNSVILLE.


...SOUTHERN APPALACHIANS...

A FRONTAL ZONE WAS STALLING AND STRETCHING WEST TO EAST ACROSS THE
CENTRAL APPALACHIANS AND SOUTHERN PORTIONS OF THE MID-ATLANTIC
STATES. IN THE WAKE OF THE MOISTURE AND TROPICAL WAVE ASSOCIATED
WITH NATE...PRECIPITABLE WATER REMAINS VERY HIGH...MORE THAN THREE
STANDARD DEVIATIONS ABOVE CLIMATOLOGY...AND RANGING FROM 1.75
INCHES OVER THE MOUNTAINS TO GREATER THAN 2.00 INCHES OVER THE
PIEDMONT AND COASTAL PLAIN. FORCING MAY NOT BE OVERLY
FOCUSED...BUT VERTICAL CROSS SECTIONS AND PLAN VIEW Q-VECTOR
CONVERGENCE DO SUGGEST SUSTAINED LIFT AND A SUBTLE SHORTWAVE TO
WASH THROUGH PARTS OF TN/NC/VA LATE THIS AFTERNOON...PERHAPS
LENDING SOME ORGANIZATION TO THUNDERSTORMS AS THEY FORM OFF THE
HIGH TERRAIN AND WITHIN THE FRONTAL ZONE AT LOWER ELEVATIONS. THE
GREATEST THREAT FOR ANY ISOLATED FLASH FLOODING IS OVER THE
SOUTHERN TO CENTRAL APPALACHIANS WHICH RECENTLY RECEIVED HEAVY
RAINS FROM NATE. THESE RAINS HAVE LOWERED FFG VALUES AND INCREASED
STREAM FLOW ANOMALIES IN SOME OF THE RIVERS ACROSS THIS AREA. THE
WPC RISK CATEGORY IS MAINTAINED AS MARGINAL OWING TO THE SMALL
SCOPE / COVERAGE OF THE ANTICIPATED EVENT...AND WE DID NOT EXTEND
THE RISK INTO THE COASTAL PLAIN WHERE FLASH FLOOD GUIDANCE
SUGGESTS UPWARDS OF 3 INCHES IN 1 HOUR OR 4 IN 3 HOURS WOULD BE
NEEDED TO CAUSE ANY SIGNIFICANT ISSUES.

BURKE/ORAVEC
$$





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