Quantitative Precipitation Forecast
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000
FOUS30 KWBC 140830
QPFERD

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

...VALID 12Z 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...BEGINNING WITH THE 15Z DAY 1 EXCESSIVE RAINFALL
OUTLOOK TODAY...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.


...PLAINS INTO THE UPPER GREAT LAKES/MIDWEST...

A TRAILING FRONT ASSOCIATED WITH A DEEP LOW MOVING ACROSS NORTHERN
HUDSON BAY EXTENDS SOUTHEAST FROM EASTERN CANADA ACROSS THE UPPER
GREAT LAKES INTO THE MID MS VALLEY AND CENTRAL PLAINS.  MID LEVEL
ENERGY INTERACTING WITH DEEPENING MOISTURE ALONG THE BOUNDARY IS
EXPECTED TO SUPPORT SCATTERED SHOWERS AND THUNDERSTORMS FROM THE
MID MS VALLEY NORTHEASTWARD INTO LOWER MICHIGAN ON SAT.
MEANWHILE...A WELL-DEFINED SHORTWAVE CURRENTLY DIGGING INTO THE
NORTHERN GREAT BASIN IS FORECAST TO MOVE PROGRESSIVELY ACROSS THE
CENTRAL ROCKIES INTO THE CENTRAL PLAINS BY SAT EVENING.
RIGHT-ENTRANCE REGION UPPER JET DYNAMICS ARE EXPECTED TO SUPPORT
AN INTENSIFYING LOW THAT IS FORECAST TO TRACK NORTHEAST FROM THE
MID MS VALLEY INTO THE UPPER GREAT LAKES SAT NIGHT-SUN MORNING.
DEEP SOUTHWESTERLY INFLOW AHEAD OF THE LOW IS FORECAST TO SUPPORT
PWS AT OR ABOVE 1.75 INCHES (2-3 STANDARD DEVIATIONS ABOVE
NORMAL).  THIS DEEP MOISTURE INTERACTING WITH THE LARGE-SCALE
ASCENT NEAR THE TRACK OF THE SURFACE LOW IS EXPECTED TO SUPPORT
MODERATE TO HEAVY AMOUNTS FROM THE MID MS VALLEY TO LOWER MICHIGAN
SAT EVENING INTO SUN MORNING.  TRAINING CONVECTION MAY CONTRIBUTE
TO THE POTENTIAL FOR HEAVY AMOUNTS LEADING TO FLASH FLOODING
CONCERNS...WITH A NUMBER OF THE 00 UTC GUIDANCE MEMBERS SHOWING
AREAL AVERAGE AMOUNTS OF 1-3 INCHES FROM NORTHERN ILLINOIS TO
SOUTHERN MICHIGAN.

FURTHER TO THE SOUTH...MOISTURE DEEPENING AHEAD THE TRAILING COLD
FRONT MAY FUEL ADDITIONAL THUNDERSTORM DEVELOPMENT...BUT WITH
GENERALLY LIGHTER ACCUMULATIONS AS THE FRONT PUSHES SOUTH ACROSS
THE CENTRAL-SOUTHERN PLAINS SAT NIGHT-SUN MORNING.

PEREIRA






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