Quantitative Precipitation Forecast
Issued by NWS

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FOUS30 KWBC 172333
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
733 PM EDT WED MAY 17 2017

...VALID 01Z THU MAY 18 2017 - 12Z THU MAY 18 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 NE BIL 30 N SHR 20 E PIN BYG PIN 10 WSW SHR 30 WNW SHR
45 NNE GEY 35 SSE BIL 30 S BIL 45 SSW BIL 55 NNW COD 20 E LVM
25 NE LVM 25 SW 3HT 20 W 3HT 15 NW 3HT 20 E 3HT 20 NE BIL.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 SE CYLD CIU 15 NNE MTW 20 S MRJ 15 WSW IOW 15 NW OXV
20 ENE ADU 25 W DNS 10 NW SUX 15 NNE YKN 25 NNE MHE 10 SSW ETH
10 E GPZ 20 NNE CWEC.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 WNW HCD 15 ESE HZX 40 S GNA 30 SW CMX 20 SE SAW 20 NE MNM
15 SW ISW 15 WNW MCW 20 ESE LRJ 15 ESE PQN 10 WNW HCD.


...UPPER MIDWEST AND WESTERN GREAT LAKES...

REPEATING THUNDERSTORM CLUSTERS OR NARROW QUASI-LINEAR CONVECTIVE
SYSTEMS WILL AFFECT AREAS FROM IOWA THROUGH EASTERN MINNESOTA AND
CENTRAL WISCONSIN TOWARD THE U.P. OF MICHIGAN. STRONG FLOW WILL
MOVE PRECIPITATION ALONG QUICKLY...BUT THE REPEATED ROUNDS WILL
ADD UP. GIVEN SEASONABLY LOW FFG NUMBERS...ALONG WITH RECENT
RAINFALL HAVING SATURATED THE SOILS IN CERTAIN AREAS...THERE WILL
BE SOME POTENTIAL FOR FLASH FLOODING OVERNIGHT...WITH SOME SPOTS
RECEIVING MORE THAN 2 INCHES OF ADDITIONAL RAINFALL. EMBEDDED
CONVECTIVE RAIN RATES MAY EXTEND NORTH INTO THE COLD SECTOR TO
AFFECT MORE OF MINNESOTA AND NORTHERN WISCONSIN...AS DEEP SYNOPTIC
FORCING BECOMES STRONGLY FOCUSED IN THE MID LEVEL FRONTAL ZONE
WEDGED BETWEEN A DOWNSTREAM JET ENTRANCE REGION AND THE UPSTREAM
MID LEVEL CYCLONE. REFER TO MESOSCALE PRECIPITATION DISCUSSION FOR
TIMING AND PLACEMENT DETAILS.


...UPSLOPE AREAS OF SOUTHERN MONTANA AND NORTHERN WYOMING...

BY EARLY EVENING OBSERVATIONAL TRENDS CAPTURED THE DEVELOPMENT OF
AN OCCLUDED FRONT/TROWAL SETUP ACROSS THE FOOTHILLS OF THE
GALLATIN MOUNTAINS. ANOMALOUS EASTERLY 850-700 MB WINDS OF UP TO
40 KNOTS WILL PERSIST...FORCED BY STRONG RIDGING TO THE
NORTH...WITH A 1024 MB SURFACE HIGH SPRAWLED ACROSS CANADA...AND
WITH EVACUATION ALOFT OCCURRING TO THE IMMEDIATE NORTH SIDE OF THE
2.5 STANDARD DEVIATION / BOWLING BALL STYLE LOW DIGGING THROUGH
THE INTERMOUNTAIN WEST. THE ONLY THING PREVENTING A MORE
PRONOUNCED FLOOD / FLASH FLOOD THREAT...IS THE RELATIVELY COLD
TEMPERATURES THROUGHOUT THE COLUMN...LEADING TO MAINLY SNOW AT
HIGHER ELEVATIONS...AND COUPLED WITH A VERY NARROW PLUME OF MORE
FAVORABLE PRECIPITABLE WATER VALUES ON THE ORDER OF 0.50 TO 0.65
INCHES WITHIN THE UPSLOPE AREAS FROM NEAR BIG TIMBER MONTANA TO
SHERIDAN / BUFFALO WYOMING. THOUGH RADAR DETECTED AN IMPRESSIVE
AND PERSISTENT PRECIPITATION AREA ACROSS SOUTHERN MONTANA AT
23Z...OBSERVED RATES WERE ONLY AROUND 0.20 INCHES PER HOUR...AND
MUCH OF THE HEAVIER REFLECTIVITY WAS DUE TO BRIGHT BANDING WEST OF
BILLINGS. GENERALLY...THE COMBINATION OF LONG DURATION LIGHT TO
MODERATE RAIN MIGHT LEAD TO SOME FLOODING...AS RECENT HRRR RUNS DO
COME CLOSE TO EQUALING THE 1.5 INCH PER 6 HOUR...FLASH FLOOD
GUIDANCE VALUES THAT EXIST IN SOME PARTS OF THIS AREA.

BURKE
$$





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