Quantitative Precipitation Forecast
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461
FOUS30 KWBC 211351
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
949 AM EDT THU SEP 21 2017

...VALID 15Z THU SEP 21 2017 - 12Z FRI SEP 22 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
55 ESE U24 15 SE SGU 50 NW SGU 60 NE TPH 50 SE EKO 30 NNW ENV
65 S 77M 15 ESE SLC 55 ESE U24.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 NW CWGD PTK 15 SSE GRR 10 NE LDM 20 WSW CVX 15 NE MCD
30 NW CYZE 10 SSW CWBE.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 E BJI BFW ARV 10 W RRL 30 ENE EAU 10 WSW OEO 15 WSW DXX
35 SSW ABR 30 SW FAR 20 E BJI.



1500 UTC UPDATE

NO CHANGES MADE TO THE PREVIOUS EXCESSIVE RAINFALL POTENTIAL
OUTLOOK.


ORAVEC


INITIAL DISCUSSION


...GREAT BASIN...

A LARGE TROUGH WILL SWING FROM POSITIVE TO NEAR NEUTRAL TILT
TODAY...PRODUCING STRONG HEIGHT FALLS NEAR ITS BASE ACROSS EASTERN
NEVADA AND MUCH OF UTAH. THE PATTERN WOULD BE FAVORABLE FOR
THUNDERSTORMS OVER SOUTHERN NEVADA AND MUCH OF UTAH THIS
AFTERNOON...EXCEPT FOR VERY DRY LOW LEVEL AIR WHICH WILL LIMIT
INSTABILITY AND THE EFFECTIVENESS OF ANY STORMS THAT MANAGE TO
FORM. THE HI-RES MODELS INDICATE LITTLE...IF ANY...CAPE. THERE IS
SOME INDICATION OF DEEPER MOISTURE AND HIGHER SURFACE DEW
POINTS...IN THE MIDDLE 40S...ADVECTING UP THROUGH SOUTHEAST NEVADA
INTO SOUTHERN UTAH BY AFTERNOON. STILL...FORECAST SOUNDINGS
INDICATE EVEN THIS WOULD BE INSUFFICIENT TO PRODUCE MUCH
INSTABILITY. GIVEN AN INHERITED MARGINAL RISK AREA...WE CHOSE TO
KEEP IT IN PLACE ON THE OFF CHANCE THAT DEEP SYNOPTIC ASCENT CAN
COMPENSATE FOR POOR LOW LEVEL MOISTURE TO PRODUCE AN ISOLATED
HEAVY RAIN THREAT OVER SOME OF THE SLOT CANYON / FLASH-FLOOD PRONE
TERRAIN IN THIS REGION.


...LOWER MICHIGAN...

AT 06Z THUNDERSTORMS WERE ONGOING OVER PARTS OF IA/IL AT THE TAIL
END OF A SHORTWAVE LIFTING UP THROUGH THE GREAT LAKES. THE SAME
WAVE IS FORECAST TO PRODUCE ENHANCED LOW LEVEL MOISTURE TRANSPORT
INTO LOWER MICHIGAN DURING THE DAY...WHERE SYNOPTIC LIFT AIDED
PERHAPS BY DIFFERENTIAL HEATING / LAKE BREEZES MAY BE ENOUGH TO
SPUR CLUSTERS OF THUNDERSTORMS. MODERATE TO STRONG CAPE VALUES AND
GREATER THAN 1.50 INCHES PRECIPITABLE WATER SUGGESTS A HEAVY RAIN
THREAT TODAY OVER LOWER MICHIGAN.


...UPPER MIDWEST...

JUST AS THE DIURNAL ACTIVITY OVER MICHIGAN WINDS DOWN DURING THE
EVENING...A WARM FRONT WILL SHARPEN BACK WEST ACROSS WI/MN.
ISOLATED INITIATION MAY OCCUR NEAR THE WARM FRONT...WITH MORE
WIDESPREAD INITIATION EXPECTED IN THE STRENGTHENING WARM ADVECTION
REGIME / NOSE OF THE PLAINS LOW LEVEL JET ACROSS CENTRAL MINNESOTA
AND ADJACENT PARTS OF THE DAKOTAS. MODEL QPF SIGNAL IS LACKING IN
COVERAGE HERE...BUT STRONG MOISTURE TRANSPORT AND PW VALUES 1.5 TO
2.5 STANDARD DEVIATIONS FROM CLIMATOLOGY SUGGEST AN EXCESSIVE
RAINFALL THREAT WHEREVER POCKETS OF ORGANIZED CONVECTION OCCUR.
THE RISK CATEGORY IS KEPT AT MARGINAL.

BURKE
$$





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