Winter Storm in Arizona, April 5 to 6, 2001

SOUTHEAST ARIZONA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TUCSON AZ
224 AM MST THU APR 4 2001

SYNOPSIS...AN UNUSUALLY STRONG STORM OFF THE SOUTHERN CALIFORNIA
COAST WILL CROSS ARIZONA TONIGHT. WIDESPREAD PRECIPITATION AND GUSTY
WINDS WILL BREAK OUT AS IT APPROACHES...WITH SNOW LEVELS FALLING
SIGNIFICANTLY TONIGHT. THE SYSTEM WILL EXIT THE REGION ON FRIDAY...
BUT A SERIES OF DISTURBANCES PASSING ACROSS THE GREAT BASIN WILL
GENERATE SOME CLOUDS. TEMPERATURES WILL ALSO REMAIN SLIGHTLY BELOW
NORMAL THROUGH EARLY NEXT WEEK.

DISCUSSION...SATELLITE SHOWS A SHARPENING DIFFLUENCE AXIS W OF THE
CO RIVER WITH THE SUBTROPICAL PLUME TO THE E AND DEVELOPING FRONTAL
BAND TO THE W. THESE WILL GRADUALLY COME INTO PHASE THIS AFTERNOON
AS LOW LEVEL MOISTURE (DEWPOINTS IN THE 50S OVER SONORA) ADVECTS
THIS WAY. IN SHORT, I THINK IT'S ONLY A MATTER OF TIME. TYPICAL
MODEL BIASES ARE SHOWING UP THIS MORNING: NGM TOO HIGH ON WIND AND
TOO UNSTABLE, ETA A LITTLE TOO WET AND WARM, AVN A LITTLE TOO DRY
AND COLD. AFTER FACTORING THEM OUT, MODELS ARE IN DECENT AGREEMENT.
ISENTROPIC LIFT IN THE SUBTROPICAL PLUME IS ALREADY GENERATING VIRGA
ACROSS THE CWA, WITH A FEW SPRINKLES ON MT. LEMMON. WINDS ALSO
GUSTING NEAR ADVISORY CRITERIA ON MOUNTAINTOPS WITH 88D VWP SHOWING
50KT WINDS JUST ABOVE THE DECK. THE ONLY THING STOPPING ME FROM A
HIGH WIND WARNING IN ZONE 35 IS CLOUD COVER. CLOUDS ARE ALSO GIVING
ME SOME PAUSE IN ZONES 29/33. HOWEVER, AS SHOWERS BECOME MORE
NUMEROUS, THE DOWNDRAFTS INDUCED BY EVAPORATIONAL COOLING COULD GIVE
US A LOT OF GUSTINESS. IF WE GET ANY KIND OF CLEARING OR ORGANIZED
TSTMS, LOOK OUT! ALL MODELS HAVE HIKED THEIR QPF ABOUT 0.25", WITH
NGM AND NCEP NUMBERS A NICE COMPROMISE BETWEEN THE SOLUTIONS:
0.25-0.75" VALLEYS TO 1.25" MOUNTAINS. THAT MAKES SENSE WITH PRECIP
WATERS APPROACHING 1" AND EXCELLENT Q-VECTOR CONVERGENCE/JET
DYNAMICS. WE COULD SEE SOME PRETTY HEAVY RAIN, BUT SINCE I HAVE
EVERYTHING BUT THE KITCHEN SINK IN THE ZONES ALREADY, I'LL LET DAY
SHIFT AND NOWCASTS DO ANY FINE TUNING. AS FOR MOUNTAIN SNOW, ALL 3
MODELS DON'T COLLAPSE THICKNESSES UNTIL AROUND 06Z, WHICH MEANS OVER
HALF OF THE PRECIP WILL FALL AS RAIN. THE REST, THOUGH, IS ENOUGH
FOR A HEADLINE GIVEN THIS IS A RATHER LATE STORM AND ZONES 19/30
WILL KEEP THEIR WINDS LONGER. I'LL OPT FOR "WINTER WEATHER ADVISORY"
TO ACCOUNT FOR COMBINATION SNOW/WIND/DRIFTING. IF YOU BUY THE AVN,
SNOW LEVELS DROP TO 3KFT BY 12Z. I CAN'T RULE THAT OUT, BUT MODEL
WETBULB ZEROS NEVER GET BELOW 4500FT, AND THE AVN LOOKS LIKE IT
INITIALIZED TOO COLD.

SYSTEM WILL KICK OUT QUICKLY FRI MORNING WITH RESIDUAL STRATOCU IN
THE AFTERNOON. WE COULD HAVE A LATE-SEASON FROST IN THE 3 EASTERN
ZONE FRI NIGHT, DEPENDING ON HOW FAST WE CLEAR. NOW THAT WE'RE IN A
MEAN SPLIT FLOW TROUGH AGAIN, ANY WAVES COMING OFF THE PACIFIC WILL
HAVE TO BE WATCHED. THE ONE ON SAT STILL LOOKS LIKE IT'LL SHEAR TO
THE N, BUT RESIDUAL MOISTURE MAY BE ENOUGH TO TOUGH OFF A LIGHT
SHOWER IN THE WHITES. THE TUE/WED SYSTEM IS GOING TO HAVE MORE ROOM
TO DIG, WHICH MEANS A BETTER CHANCE OF BOTH WIND AND PRECIP. 00Z MRF
HAS LEANED MORE TOWARD THE ECMWF/UKMET, BUT IT'S STILL A LITTLE WEAK
AND VERY DRY. THE CANADIAN TAKES IT ON AN IDENTICAL TRACK TO OUR
CURRENT SYSTEM. ALTHOUGH I DON'T LIKE HAVING WIND IN THE DAY 6/7
EXTENDED, IT'S APPROPRIATE HERE JUST TO INDICATE A CHANGE. I'D LIKE
TO SEE ANOTHER MRF RUN AND A NEW SET OF ENSEMBLE PRECIP
PROBABILITIES BEFORE SLIPPING PRECIP INTO THE MIX. THANKS TO ABQ/EPZ
FOR COORD THIS MORNING. PYTLAK

.TWC...WIND ADVISORY TODAY ZONES 19/29/30/33/34/35
       WINTER WEATHER ADVISORY TONIGHT ZONES 19/30.