FXUS65 KVEF 011754
AFDVEF

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LAS VEGAS NV
955 AM PST TUE DEC 1 2009

.SYNOPSIS...HIGH PRESSURE WILL KEEP A DRY WEATHER PATTERN OVER THE 
SOUTHWEST MUCH OF THE WEEK...BUT COOLER TEMPERATURES AND LOCALLY 
GUSTY NORTH WINDS CAN BE EXPECTED ON THURSDAY. THE HIGH PRESSURE 
RIDGE WILL BREAK DOWN GOING INTO THE WEEKEND...AND THERE ARE 
INDICATIONS THAT A STRONG PACIFIC STORM COULD IMPACT THE REGION LATE 
IN THE WEEKEND THROUGH EARLY NEXT WEEK.
&&

.UPDATE...THE 12Z GUIDANCE SHOWS LITTLE CHANGE THROUGH THURSDAY FROM 
PREVIOUS RUNS AND NO UPDATE IS PLANNED. NORTH WIND SPEEDS COULD 
BE A LITTLE HIGHER THURSDAY THAN CURRENTLY INDICATED BY GUIDANCE AND 
THE GOING FORECAST. THE LOCALLY RUN 12Z 4KM ARW WILL PROBABLY SHOW 
HIGHER SPEEDS...ESPECIALLY DOWN THE COLORADO RIVER...WHEN IT IS 
AVAILABLE. ANY CHANGES CAN WAIT UNTIL THE ROUTINE AFTERNOON PACKAGE.
&&

.PREV DISCUSSION.../254 AM PST TUE DEC 1 2009/

.SHORT TERM...SATELLITE SHOWS MOSTLY CLEAR SKIES AREAWIDE...BUT A 
LARGE CIRRUS SHIELD WAS SLOWLY ADVANCING FROM THE WEST. SURFACE OBS 
SHOW LIGHT WINDS AREAWIDE AS THE NORTH SOUTH PRESSURE GRADIENT WHICH 
WAS IN PLACE YESTERDAY HAS COLLAPSED AS EXPECTED. RIDGING OVER THE 
EASTERN PACIFIC AND THE WEST COAST WILL BE THE MAIN FEATURE TODAY 
AND TOMORROW...WITH DRY CONDITIONS /BUT QUITE A BIT OF CIRRUS/ AND 
TEMPERATURES SLIGHTLY ABOVE NORMAL. A WRINKLE GETS THROWN INTO THE 
PICTURE ON THURSDAY...AS THE MODELS HAVE COME INTO AGREEMENT ON A 
DEEPER LOW PRESSURE SYSTEM OVER THE NORTHERN PLAINS ALONG WITH MORE 
WESTWARD EXTENSION OF THE ASSOCIATED TROUGH. THE TAIL END OF THIS 
TROUGH WILL BUTT UP AGAINST THE HIGH AMPLITUDE RIDGE OVER THE WEST 
COAST...PRODUCING A CHANNEL OF STRONG NORTH WINDS ALOFT WHICH WILL 
BRING ANOTHER ROUND OF LOCALLY GUSTY WINDS IN THE FAVORED AREAS. THE 
SOURCE REGION OF THE AIR WILL BE ALMOST PURELY CONTINENTAL...SO 
TEMPERATURES WILL ALSO GET KNOCKED DOWN SEVERAL DEGREES. THE AIRMASS 
WILL ALSO BE VERY DRY...SO RED FLAG CONDITIONS WILL BE POSSIBLE.

.LONG TERM...AND AGAIN STILL A GREAT DEAL OF UNCERTAINTY IN THE LONG 
RANGE FORECAST AS MODELS DIFFER SIGNIFICANTLY AND CHANGE RUN TO RUN 
TO RUN. FAIRLY GOOD AGREEMENT STARTING ON FRIDAY WITH THE AREA UNDER 
NORTHWEST FLOW UNDER A SMALL RIDGE. FOR FRIDAY AND SATURDAY DID 
LOWER DEWPOINTS BY 9-14 DEGREES AREAWIDE AS GUIDANCE SHOWS SOME 
STRONG DRYING MOVING IN ALONG WITH COOLER TEMPERATURES BEHIND AN 
EARLIER TROUGH PASSAGE. IT IS FROM SATURDAY ONWARD THAT THE 
UNCERTAINTY INCREASES AS MODELS ARE DIVERGING AND FLIPPING. A STRONG 
TROUGH COMING SOUTH OUT OF BRITISH COLUMBIA WILL MOVE INTO THE PACNW 
OR A BIT FURTHER EAST DEPENDING ON MODEL. ALL AGREE ON IT SPLITTING 
AND FORMING A LOW IN THE PACNW AREA SATURDAY AND SUNDAY. THE ECMWF 
KEEPS THE SYSTEM UP THERE AND ELONGATES IT WHILE THE GFS AND GEM 
START IT ON A SOUTH TO SOUTHEAST TRAJECTORY WHICH WOULD HAVE A 
SIGNIFICANT IMPACT ON OUR CWA. ALL DO AGREE ON A GOOD FETCH OF 
MOISTURE BEING ADVECTED IN BY MONDAY ON THE SOUTH SIDE OF THE 
LOW/TROUGH. HAVE KEPT IN THE POPS FROM THE PREVIOUS FORECAST WITH 
THE BEST CHANCES IN THE NORTH PART OF THE CWA AND ALL AREA 
MOUNTAINS. I DID INCREASE THE POP NUMBERS IN THE SIERRA AS THIS 
LOOKS LIKE A GOOD OROGRAPHIC EVENT. BEYOND DAY 7 MONDAY IS WHEN IT 
GETS DICEY WITH THE DIFFERENT SOLUTIONS THAT IF SOME EVENTUALLY 
VERIFY COULD LEAD TO A GOOD RAIN AND SNOW EVENT LATER. 
&&

.AVIATION...FOR MCCARRAN....AVIATION...FOR MCCARRAN...LIGHT 
WINDS...PRIMARILY FOLLOWING A TYPICAL DIURNAL PATTERN IS ANTICIPATED 
FOR THE NEXT COUPLE OF DAYS WITH NORTHEAST WINDS IN THE AFTERNOON 
AND SOUTHWEST WINDS IN THE EVENING. HIGH CLOUDS WILL INCREASE DURING 
THE DAYLIGHT HOURS TODAY...BECOMING BROKEN BY EVENING.

FOR THE REST OF SOUTHERN NEVADA...NORTHWEST ARIZONA...AND SOUTHEAST 
CALIFORNIA...WINDS WILL REMAIN RELATIVELY LIGHT OVER THE AREA 
THROUGH WEDNESDAY FOLLOWING LOCAL DIURNAL PATTERNS. AN INCREASE IN 
HIGH CLOUDS WILL OCCUR DURING THE DAY AT 20 TO 25K FEET.
&&
&&

.FIRE WEATHER...LOCALLY GUSTY NORTH WINDS WILL USHER IN A VERY DRY 
AIRMASS ON THURSDAY. RED FLAG CONDITIONS WILL BE POSSIBLE...MAINLY 
IN THE COLORADO RIVER VALLEY WHERE NORTH WINDS ARE FAVORED AND FIRE 
DANGER IS HIGH.
&&

.VEF WATCHES/WARNINGS/ADVISORIES...
NV...NONE.
AZ...NONE.
CA...NONE.
&&

$$

ADAIR/CZYZYK/MORGAN/JACQUES

HTTP://WEATHER.GOV/LASVEGAS