Flash Flood Warning 29, NWS Las Vegas, NV

MOJAVE DESERT/SOUTHERN GREAT BASIN AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LAS VEGAS NV
855 AM PDT TUE AUG 25 1998

SYNOPSIS...A MOIST SOUTHERLY FLOW ALONG WITH DAYTIME HEATING WILL
HELP TO PRODUCE A FEW SHOWERS AND THUNDERSTORMS MAINLY THIS AFTERNOON
AND EVENING ACROSS SOUTHERN NEVADA...SOUTHEAST CALIFORNIA AND
NORTHWEST ARIZONA. AN UPPER LEVEL TROUGH OVER NORTHERN CALIFORNIA
WILL BRING SOUTHWEST WINDS THAT WILL HELP TO DRY THE MOJAVE DESERT
AND SOUTHERN GREAT BASIN BEGINNING WEDNESDAY AND LASTING THROUGH THE
BEGINNING OF THIS WEEKEND.

DISCUSSION...LAST NIGHT AND THIS EARLY MORNING AN URBAN AND SMALL
STREAM ADVISORY WAS ISSUED FOR NOCTURNAL WARM-TOP HEAVY RAIN
CONVECTION THAT OCCURRED OVER NE CLARK...EXTREME SE LINCOLN AND THE
NW PLATEAU OF AZ. THIS ACTIVITY HAS SINCE DIMINISHED AND MOVED NE
INTO UT. URBAN AND SMALL STREAM ADVISORY STILL IN EFFECT FOR THESE
AREAS TIL 10 AM WITH RUNOFF STILL OCCURRING FROM RAIN THAT PRODUCED
BETWEEN .75 TO 1.25 INCHES OVER THE VIRGIN RIVER REGION.

SATELLITE IMAGERY SHOWED TWO STORM-SCALE INDUCED VORTICITY CENTERS
OVER THE FA THIS MORNING. ONE IS FROM YESTERDAYS CONVECTION IN LA PAZ
COUNTY THAT DRIFTED SLOWLY NORTHWARD INTO SOUTHEAST MOHAVE COUNTY AZ.
THE OTHER IS JUST WEST OF BOULDER BASIN IN EXTREME SOUTHERN NEVADA.
SEVERAL QUESTIONS ARISE TODAY...WHEN AND WHERE WILL CONVECTION OCCUR
AND WHAT FACTORS WILL HELP SUSTAIN OR PROHIBIT THE CONVECTION.

PROS...1) LATEST ISODROSOTHERM ANALYSIS SHOWS THERE IS PLENTY OF LOW
LEVEL MOISTURE AROUND...ESPECIALLY SOUTH ALONG THE COLORADO RIVER AND
IN MOHAVE COUNTY. 2) DRA SOUNDING VERY MOIST AND UNSTABLE AT
12Z...AND WITH DAYTIME HEATING THE ATMOSPHERE WILL DESTABILIZE EVEN
FURTHER AS THE DAY PROGRESSES. 3)S-SELY FLOW WILL CONTINUE FROM
SURFACE TO 700 MB. SFC HIGH INDICATED BY LATEST RUC SURFACE ANALYSIS
OVR NORTH CENTRAL AZ WITH THERMAL LOW NORTH OF GULF OF CA.  THIS WILL
KEEP FAVORABLE E-SELY FLOW OVR SRN THIRD OF FA.

CONS...1) AREAS WHERE THERE IS A MID LEVEL CLOUD COVER...SOLAR
HEATING WILL BE LIMITED...PROHIBITING LAPSE RATES TO INCREASE WITH
TIME. 2) AREAS ON THE BACK SIDE OF THE STORM INDUCED VORT
CENTERS...HOW MUCH SUBSIDENCE WILL THEY OBTAIN TODAY. 3)  HOW MUCH IS
THE SWLY WINDS ABOVE 700 MB GOING TO DRY THE MID LEVELS...ESPECIALLY
ACROSS SRN NV AND SAN BERNARDINO COUNTY CA.

WITH SUBSIDENCE AND DEBRIS CLOUDS AROUND BEHIND LAST NIGHTS
CONVECTION...MAY DELAY OR HAMPER CONVECTION ACROSS SOME LOCALES
OVER CWA.  SO ISOLD TO SCT COVERAGE INDICATED BY THE FCST PACKAGE
LOOKS GOOD FOR RIGHT NOW.  LOOKING AT WATER VAPOR THIS MORNING THERE
SEEMS TO BE SOME TYPE OF BROAD SYNOPTIC LIFT OCCURRING...
PARTICULARLY OVER WESTERN CLARK...SRN NYE AND LINCOLN COUNTIES
WHERE THEY SEEM TO BE IN THE RIGHT ENTRANCE REGION OF WEAK 3H JET.

WILL MAKE SOME MINOR COSMETIC CHANGES TO WORDING OF THE FCST AND
BEEF UP THE AREAS WHERE THE BROAD SYNOPTIC LIFT SEEMS TO BE
OCCURRING. LAS 110. CORDERO

A URBAN AND SMALL STREAM FLOOD ADVISORY HAS BEEN ISSUED. PLEASE REFER
TO THE LATEST URBAN AND SMALL STREAM ADVISORY BULLETIN FOR DETAILS.