Quantitative Precipitation Forecast
Issued by NWS

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000
FOUS30 KWBC 261153
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
752 AM EDT FRI SEP 26 2014

...VALID 12Z FRI SEP 26 2014 - 12Z SAT SEP 27 2014...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 NNW VGT 60 NNE DRA 75 NNE DRA 65 E TPH 15 NNE P68 10 N EKO
15 WSW AWH 35 E MUO 30 S LLJ 35 E LLJ 30 NNE S14 15 SW WEY
20 E S14 35 WNW IDA 20 E 77M 30 NNE DPG 35 N MLF 25 ENE SGU
30 SSE SGU 60 NNE IGM 35 NE IGM 40 E IGM 30 W PRC 45 SW PRC
65 WNW LUF 30 ENE BLH 30 N BLH 15 WSW EED 30 WNW IFP 15 SE LSV
25 NNW VGT.



SOUTHWEST/GREAT BASIN
~~~~~~~~~~~~~~~~~~~~~
A MID-LEVEL TROUGH AMPLIFIES THROUGH TONIGHT/SATURDAY MORNING AS
STRONG SHORTWAVE ENERGY OFFSHORE NORTHERN CA DIGS INTO THE BASE OF
THE TROUGH AND CLOSES OFF A MID-LEVEL LOW OVER CENTRAL CA/WESTERN
NV BY SATURDAY MORNING.  THIS DYNAMIC SYSTEM COULD BECOME A MAJOR
RAINFALL PRODUCER BEGINNING LATER FRIDAY AFTERNOON AND FRIDAY
NIGHT AND CONTINUING INTO SATURDAY DUE TO A COMBINATION OF HEIGHT
FALLS, INCREASINGLY FAVORABLE UPPER DIVERGENCE, AND STRENGTHENING
850/700 HPA SOUTHERLY FLOW OFF THE GULF OF CA INTO PARTS OF
WESTERN AZ, SOUTHEASTERN CA, SOUTHEASTERN NV, AND SOUTHWESTERN UT.
 FLOW THAT BECOMES INCREASINGLY UNIDIRECTIONAL WITH HEIGHT OUT OF
THE SOUTH WILL BRING PRECIPITABLE WATER VALUES (PWS) OF ABOVE
1.50" AS FAR NORTH AS THE AZ/CA/NV BORDER AND LOCALLY ABOVE 1"
ACROSS SPOTS OF THE GREAT BASIN -- WHICH IS CLOSE TO FOUR SIGMAS
ABOVE THE AVERAGE FOR LATE SEPTEMBER REGIONALLY -- HELPING TO
SUPPORT THE NOTION OF LOCALLY HEAVY RAINS FROM PORTIONS OF WESTERN
AZ INTO EXTREME SOUTHEASTERN CA AND NORTHWARD ALONG THE NV/UT
BORDER INTO SOUTH-CENTRAL ID.

RECENT RAP RUNS INDICATE CAPE VALUES RISE >1000 J/KG ACROSS THIS
REGION AND 850-700 HPA INFLOW ON THE ORDER OF 25 KTS, WHICH SHOULD
BE ENOUGH TO ORGANIZE CONVECTION ACROSS THE REGION.  CLOSER TO THE
DESERT SOUTHWEST, THE MESOSCALE QPF GUIDANCE ADVERTISES LOCAL
AMOUNTS OF 2-4" (CLOSE TO THE ANNUAL AVERAGE RAINFALL FOR THE
REGION), WHILE LOCATIONS FARTHER NORTHWARD ACROSS THE GREAT BASIN
HAVE ADVERTISED MAXIMA IN THE 2" RANGE.  THE FARTHER NORTH YOU GO
WITHIN THIS REGION, THE MORE LIKELY THIS COULD EVOLVE MORE INTO A
LONGER DURATION FLOOD RATHER THAN A SHORTER DURATION FLASH FLOOD
EVENT.  CELL TRAINING IS EXPECTED TO BE THE PRIMARY CAUSE OF ANY
FLASH FLOODING IN THIS REGION -- SOME DRY AIR IN THE LOW-LEVELS OF
FORECAST SOUNDINGS WITHIN THIS REGION SHOULD FOSTER FORWARD
PROPAGATION BEFORE THE ATMOSPHERE SATURATES.  COORDINATION WITH
THE SLC/SALT LAKE CITY UT AND PIH/POCATELLO ID FORECAST OFFICES
CONTRIBUTED TO THE AREA DEPICTED WITHIN THE SLIGHT RISK OUTLINE.


SOUTHERN TX
~~~~~~~~~~~
PWS OF 1.75-2" ALONG WITH INCREASING LOW-LEVEL INFLOW OFF THE GULF
OF MEXICO ON THE ORDER OF 20-25 KTS THIS AFTERNOON AND EVENING
ALONG WITH BUILDING INSTABILITY NEAR A COASTAL FRONT/"PADRE
TROUGH" COULD ORGANIZE AFTERNOON THUNDERSTORM ACTIVITY ACROSS THE
REGION, PARTICULARLY NEAR THE COAST.  THE MESOSCALE QPF GUIDANCE
ADVERTISES LOCAL AMOUNTS OF 3-6" IN THIS REGION, WITH THE BEST
SIGNAL CLOSER TO PORT ISABEL/BROWNSVILLE TX WITHIN THE 00Z SSEO
NEIGHBORHOOD PROBABILITIES OF EXCEEDING THE THREE HOURLY FLASH
FLOOD GUIDANCE VALUES (FFGS), WHERE FFGS ARE CURRENTLY LOWER.
SINCE THE EXACT LOCATION OF THE HEAVIEST RAINS SHOWS SOME
VARIABILITY WITHIN THE GUIDANCE, CHOSE TO MAKE THIS AREA A "SEE
TEXT" HIGHLIGHT RATHER THAN ISSUE A SLIGHT RISK OF EXCESSIVE
RAINFALL FOR THE AREA.

ROTH/TERRY
$$




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