Severe Thunderstorm Warning 18, NWS Las Vegas, NV

MOJAVE DESERT/SOUTHERN GREAT BASIN AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LAS VEGAS NV
235 PM PDT SUN SEP 12 1999

SYNOPSIS...AN UPPER LOW OFF THE NORTHERN CALIFORNIA COAST WILL SLOWLY
WEAKEN THROUGH MONDAY...THEN MOVE INTO NORTHERN NEVADA ON TUESDAY. IN
ADVANCE OF THIS SYSTEM...LIMITED MOISTURE AND INSTABILITY WILL REMAIN
IN PLACE FOR A SLIGHT CHANCE OF AFTERNOON AND EVENING THUNDERSTORMS
EACH DAY.

DISCUSSION...65 KNOT JET AXIS AT 250 MB EXTENDS FROM NEAR PT.
CONCEPTION CA INTO SRN UT/NRN AZ. THIS BAND OF STRONGER WESTERLIES
CONTINUES TO INTERACT WITH HIGHER PRECIPITABLE WATER (PW) PLUME
WHICH EXTENDS FROM GULF OF CA NORTH INTO SRN NV. GOES SOUNDER IMAGERY
STILL SHOWING NOSE OF HIGHER PWS (AROUND AN INCH) BEING SLOWLY DRAWN
NWD AS UPPER LOW SPINS OFF CA COAST. LOCAL ANALYSIS AND PREDICTION
SYSTEM (LAPS) SHOWS BEST MOISTURE/CAPE/INSTABILITY FOCUSED OVER SE
SAN BERNARDINO/SRN CLARK AND MOHAVE COUNTIES THIS AFTERNOON...RIGHT
WHERE BEST STORMS HAVE FORMED. FORCING FROM JET/DIVERGENCE ALOFT NOT
AS EVIDENT THIS AFTERNOON (AS COMPARED TO SATURDAY) WITH OROGRAPHICS
THE MAIN FORCING/TRIGGERING MECHANISM.

MODELS IN GENERAL AGREEMENT ON KEEPING UPPER LOW OFF CA COAST THROUGH
MONDAY THEN SLOWLY OPEN IT UP AND LIFT IT NE INTO NRN NV TUE.
MOISTURE TO CONTINUE TO BE DRAWN UP INTO NV AHEAD OF THIS FEATURE
THROUGH MON WITH JET SLOWLY WEAKENING AND MOVING N. SIMILAR TO
TODAY...LITTLE IF ANY FORCING EXPECTED FROM JET ON MON SO CONTINUED
ISOLATED/SLIGHT CHANCE MENTION LOOKS BEST FOR ALL AREAS. ETA/AVN
SLOWER TO LIFT LOW NE TUE AND ALSO SLOWER TO DRY ATMOSPHERE AND ARE
PREFERRED. STARTED TO TRIM BACK MENTION OF STORMS TUE...ESPECIALLY
INYO CO AS DRIER SW FLOW WORKS IN. STILL UNCERTAIN HOW QUICKLY LOW
WILL LIFT OUT (PROBABLY SLOWER THAN MODELS INDICATE) AND THEREFORE DO
NOT WANT TO REMOVE PRECIPITATION CHANCES TOO QUICKLY...ESPECIALLY
ACROSS SRN NV AND NW AZ.

CONTINUING TREND OF PREVIOUS EXTENDED...WILL LEAVE A CHANCE OF
THUNDERSTORMS IN OVER THE HIGHER TERRAIN FOR WED AS UPPER TROUGH
REMAINS OVER GREAT BASIN AND CONTINUES TO WEAKEN. DRIER SW FLOW ALOFT
BEGINS TO DEVELOP THU AND FRI AS NEXT SHORT WAVE TROUGH TAKES IT/S
PLACE ALONG THE CA COAST.  HEIGHTS FALL A BIT WED AND REMAIN A LITTLE
BELOW NORMAL INTO FRI. HENCE...WILL TREND TEMPERATURES DOWN A BIT
FROM PREVIOUS EXTENDED.

NEMETH

LAS...NONE.