Quantitative Precipitation Forecast
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FOUS30 KWBC 141319
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
919 AM EDT SAT OCT 14 2017

...VALID 15Z SAT OCT 14 2017 - 12Z SUN OCT 15 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
DUH 30 WSW ASW 10 SW TAZ 30 NNW UNO 10 NE ADH PVJ 15 NE SWW
15 S MAF 50 NNW MDD 50 SW LBB 20 SE PVW 10 NE CDS 10 NW PNC
10 E AIO 10 NNW RYV CAD 25 NE OSC 30 ENE CYVV 15 WSW CYBN
25 WNW CYKF 20 SSE PHN DUH.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 ESE OZW 10 NW JYM 20 ESE IKK 25 NNW SPI 20 WSW MQB 15 SW MLI
15 E SFY 30 ENE RAC 20 NE MOP 15 SE P58 20 NW CYZR 15 ESE OZW.


...CHANGES TO THE EXCESSIVE RAINFALL OUTLOOK DEFINITION AND
THRESHOLD PROBABILITIES...

NOTE TO USERS...ON FRIDAY OCTOBER 14TH WPC IMPLEMENTED CHANGES TO
THE EXCESSIVE RAINFALL OUTLOOK. THE DEFINITION OF THIS PRODUCT
CHANGES FROM A POINT PROBABILITY TO A NEIGHBORHOOD PROBABILITY. BY
CAPTURING MORE AREA AROUND A POINT...THE PROBABILITIES ASSOCIATED
WITH EACH RESPECTIVE CATEGORY WILL NECESSARILY INCREASE. SEE THE
GRAPHIC LEGEND FOR THE NEW PROBABILITIES. THE PRODUCT WILL LOOK
AND BEHAVE THE SAME AS IT ALWAYS HAS...BUT STUDIES SHOW THAT THE
RISK CONTOURS WPC DRAWS ARE BETTER CALIBRATED TO THE NEW
DEFINITION AND HIGHER ASSOCIATED PROBABILITIES. THESE CHANGES
BETTER ALIGN THE EXCESSIVE RAINFALL OUTLOOK WITH OTHER OUTLOOKS
FROM NATIONAL CENTERS...AND THE PRODUCT IS NOW DEFINED AS THE
PROBABILITY OF RAINFALL EXCEEDING FLASH FLOOD GUIDANCE WITHIN 40
KM OR 25 MILES...OF A POINT.


...LOWER MISSOURI VALLEY TO THE LOWER GREAT LAKES...

AT MID MORNING THUNDERSTORMS WITH HEAVY RAIN WERE STRETCHED ALONG
A FRONTAL ZONE FROM NORTHERN ILLINOIS TO LOWER MICHIGAN. THIS
ACTIVITY WAS OCCURRING IN THE ENTRANCE REGION OF A 130-150 KNOT
JET STREAK. MESOSCALE DISCUSSION 0885 CONTAINS MORE
INFORMATION...VALID THROUGH 1730Z. A SECOND AND GROWING FOCUS FOR
DEEP CONVECTION AND HEAVY RAIN WILL BE HEIGHT FALLS COMING QUICKLY
OUT OF THE ROCKIES INTO THE PLAINS...AND SPURRING CYCLOGENESIS.
THE ASSOCIATED DEEP LAYER FORCING...WORKING WITH RETURN
MOISTURE...HAD ALREADY YIELDED CLUSTERS OF THUNDERSTORMS AND HEAVY
RAINFALL FROM EASTERN KANSAS TO ADJACENT PARTS OF MISSOURI AND
NEBRASKA INTO SOUTHERN IOWA.

VEERED LOW LEVEL FLOW AND PROGRESSION OF THE DEVELOPING SURFACE
LOW WILL PLACE THE EMPHASIS FOR PERSISTENT AND REPEATED LOW LEVEL
MOISTURE TRANSPORT AND HEAVY RAINFALL DOWNSTREAM...FROM FAR
NORTHEAST KANSAS TOWARD THE ONGOING ACTIVITY IN THE LOWER GREAT
LAKES. THE RISK OF EXCESSIVE RAINS WILL END FROM WEST TO EAST AS
THE SURFACE COLD FRONT SWEEPS THROUGH THIS REGION DURING THE
EVENING AND OVERNIGHT. POTENTIAL MAY BE LIMITED ON THE EASTERN
END...HOWEVER...BY A NARROW INSTABILITY AXIS AND RAPID CONSUMPTION
OF IT VIA WIDESPREAD PRECIPITATION. THIS IS THE REASON WE SHAVED A
LITTLE OFF THE SLIGHT RISK AREA IN LOWER MICHIGAN...AS IT APPEARS
INSTABILITY WILL NOT BE SUFFICIENT TO SUSTAIN INTENSE RAIN RATES
WITH EASTWARD EXTENT TOWARD DETROIT. BACK WEST...HOWEVER...WE DID
EXTEND SLIGHT RISK TO CAPTURE THE ACTIVITY BREAKING OUT OVER
NORTHEAST KANSAS. WE ALSO EXPANDED THE RISK AREA SOUTHWARD THROUGH
THE INSTABILITY AXIS EXPECTED IN WESTERN MISSOURI THIS
AFTERNOON...WHICH WAS ALREADY EVIDENT IN THE SPC MESOANALYSIS AS
OF MID MORNING. WHILE THE SURGING COLD FRONT SHOULD ENSURE FORWARD
MOTION...THE PROCESS MAY BE SLOW ENOUGH...AND CELL MOTIONS ALIGNED
CLOSELY ENOUGH WITH THE FRONTAL STRUCTURE...TO ALLOW FOR A 60-90
MINUTE PERIOD OF TRAINING AT ANY GIVEN LOCATION. THE 00Z NSSL WRF
SOLUTION BEST REPRESENTS THIS THINKING. DEEP SOUTHWESTERLY INFLOW
IS FORECAST TO SUPPORT PW VALUES AT OR ABOVE 1.75 INCHES (2-3
STANDARD DEVIATIONS ABOVE NORMAL)...WITH THE NAM INDICATING CAPE
VALUES IN EXCESS OF 1000 J/KT WITH NO INHIBITION FOR MIXE LAYER
PARCELS...SUGGESTING RAIN RATES MAY BE INTENSE ENOUGH TO EXCEED
FLASH FLOOD GUIDANCE DURING A FEW HOUR WINDOW LATER TODAY ACROSS
WESTERN MISSOURI.

BURKE/PEREIRA

$$





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