Severe Thunderstorm Warning 93, NWS Cheyenne, WY

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
127 PM MDT THU SEP 6 2001

SHOWERS AND THUNDERSTORMS HAVE DEVELOPED AS EXPECTED SINCE LATE MORNING
OVER PARTS OF SOUTHEAST WYOMING...IN DIFFLUENT FLOW AHEAD OF THE
EXTREME NORTHERN UTAH UPPER LOW.  THIS ACTIVITY SHOULD BECOME MORE
WIDESPREAD THROUGH THE EVENING AND BECOME MORE STRATIFORM.  MAIN
THREAT FROM STORMS WILL BE SMALL HAIL EARLY THIS EVENING...WITH UP TO
ONE INCH DIAMETER HAIL IN THE SOUTHERN HALF OF THE NEBRASKA
PANHANDLE.  THE 12Z RUN OF THE MODELS ARE MORE SIMILAR THAN PREVIOUS
RUNS...WITH THE AVN STILL DIFFERING IN ITS FURTHER NORTH PLACEMENT OF
THE UPPER LOW AND LOWER HUMIDITIES.  12Z ETA LOOKED CLOSE AT 18Z WITH
THE CURRENT EXTREME NORTHERN UTAH UPPER LOW AND ITS DEPICTION OF THE
MORE SOUTHWARD MOVEMENT OF THE FEATURE THROUGH 12Z FRIDAY.  AT THE SAME
TIME...A COLD FRONT WILL MOVE THROUGH THE REST OF THE CWA BY 23Z.
FAVORABLE PATTERN FOR A 5 TO 9 HOUR PERIOD OF RAINFALL OVER THE CWA AS
THE COOLER BOUNDARY LAYER AIR MASS IS OVERRUN BY THE DIFFLUENT SOUTHWEST
FLOW ALOFT.  PRECIPITATION SHOULD ALSO CHANGE OVER TO LIGHT SNOW AS THE
NIGHT GOES ON AT ELEVATIONS ABOVE 9500 FEET.  THE PRECIPITATION THEN
ENDS FROM NORTH TO SOUTH OVER THE CWA LATE TONIGHT AND EARLY FRIDAY
MORNING AS THE UPPER LOW MOVES FURTHER SOUTH...ATMOSPHERE DRIES AND
SURFACE UPSLOPE WEAKENS SOME.  PRECIPITATION WILL INCREASE AGAIN OVER
THE CWA LATE FRIDAY AFTERNOON AND CONTINUE UNTIL SHORTLY AFTER 06Z
FRIDAY NIGHT AS ANOTHER SURGE MOVES THROUGH THE CWA.  THE SURGE IS
BROUGHT SOUTHWARD BY A DIGGING 250 MB JET DURING THAT TIME.  THE JET
AND REINFORCED BOUNDARY LAYER UPSLOPE FLOW WILL AGAIN HELP DEVELOP
PRECIPITATION LATE FRIDAY AFTERNOON THROUGH 06Z FRIDAY NIGHT.  THE
PRECIPITATION SHOULD END BY 09Z FRIDAY NIGHT...WITH THE RAIN CHANGING
OVER TO A WET SNOW BY LATE EVENING.  700 MB TEMPERATURES AND THICKNESS
VALUES SUPPORT THE CHANGEOVER.  SINCE IT WILL BE AT THE TAIL END OF
THAT EVENT...WILL NOT MENTION ANY AMOUNTS AT THIS TIME...BUT AN INCH
OR TWO ON GRASSY SURFACES IS POSSIBLE.  THE CLOUDS...WINDS AND
PRECIPITATION FRIDAY NIGHT WILL HELP KEEP THE TEMPERATURES UP SOME
AND GENERALLY ABOVE FREEZING.  AFTER A DRY AND COOL DAY ON SATURDAY
THE TEMPERATURES START TO REBOUND ON SUNDAY.  MAV AND FAN GUIDANCE
NUMBERS ARE COOLER THAN THOSE OF THE FWC AND FIT WITH THE ETA AND
WILL GO CLOSER TO THOSE NUMBERS.

.EXTENDED DISCUSSION...TYPICAL LATE SUMMER/EARLY AUTUMN PATTERN IN
THE EXTENDED WITH SEASONAL TEMPERATURES AND A PROGRESSIVE UPPER FLOW
PATTERN. A FLAT RIDGE WILL ADVECT OVER THE REGION MON FOR DRY
CONDITIONS. A RAPIDLY MOVING SERIES OF VORT DISTURBANCES WILL
FLATTEN THE RIDGE AND BRING A CHANCE OF RAIN TUE AND WED AS THE FLOW
BACKS TO SOUTHWEST. THU LOOKS DRY AS THE REGION WILL BE IN BETWEEN
SHORTWAVES.

.CYS...
WY...NONE.
NE...NONE.

WEILAND/ROBINSON