River Flood Advisory 7, NWS Denver/Boulder, CO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DENVER/BOULDER CO
445 AM MDT FRI MAY 30 2014

.SHORT TERM...(TODAY THROUGH TONIGHT )
ISSUED AT 444 AM MDT FRI MAY 30 2014

OUR FORECAST AREA WILL BE CAUGHT BETWEEN THE UPPER RIDGE AXIS OVER
WESTERN NEBRASKA AND KANSAS AND A SHORTWAVE TROUGH MOVING THROUGH
THE LIGHT SW FLOW ALOFT TO OUR WEST. WHILE THE WINDS ARE LIGHT AT
ALL LEVELS...THERE IS A BROAD AREA OF LOW/MID LEVEL CONVERGENCE
ROUGHLY OVER THE FRONT RANGE. AT THE SURFACE THIS APPEARS TO BE
FOCUSED ALONG A LINE FROM ROUGHLY LAMAR TO CHEYENNE...BUT STEERING
WINDS FROM SHOWER MOVEMENT PLACE THIS MORE OVER THE SUMMIT OF THE
FRONT RANGE. LOW LEVEL WINDS ON THE PLAINS SHOULD BECOME A BIT
STRONGER FROM THE SOUTHEAST AS THE TROUGH MOVES BY...MAINTAINING
THE CONVERGENCE AND INCREASING THE AVAILABLE MOISTURE A LITTLE.
MEANWHILE THE BEST LIFT WITH THE WAVE IS PRODUCING A HEALTHY BATCH
OF SHOWERS NEAR THE WESTERN COLORADO BORDER...AND SHOULD ARRIVE IN
THE NORTHERN MOUNTAINS BY MIDDAY.

MODELS HAVE GENERAL AGREEMENT ON THE EXISTING WESTERN COLORADO
BAND MOVING EAST WHILE ADDITIONAL THUNDERSTORMS DEVELOP OVER THE
NORTHERN MOUNTAINS LATER THIS MORNING. DETAILS VARY AFTER THAT
THOUGH AS CONVECTION ON THE PLAINS VARIES. SOME COMMON THEMES ARE
SOME HEAVY RAIN BY EARLY AFTERNOON IN THE MOUNTAINS NORTH OF
INTERSTATE 70...ESPECIALLY FROM BOULDER COUNTY UP TOWARDS
LARAMIE...AND EITHER THIS MOVING EAST FROM THERE OR NEW CONVECTION
DEVELOPING IN A DENVER CONVERGENCE ZONE ON THE SOUTHEAST SIDE OF
THE CITY OR IN THE SURFACE CONVERGENCE AREA NORTH AND EAST OF
DENVER. SOME QUESTION AS TO WHETHER THE FRONT RANGE CITIES WILL
GET SKIPPED OVER IN THIS...BUT WITH SUCH LIGHT WINDS AND THE
EVENTUAL EASTWARD MOVEMENT OF EVERYTHING THERE SEEMS A DECENT
CHANCE THAT THERE WILL BE STORMS GETTING INTO DENVER FROM ONE SIDE
OR THE OTHER.

FLASH FLOODING PARAMETERS ARE NOT HUGE...BUT ARE ALL POSITIVE AND
SEEM SUFFICIENT. PRECIPITABLE WATER IS APPROACHING 0.90 INCH
AROUND DENVER AND WILL BE A LITTLE OVER AN INCH ON THE PLAINS THIS
EVENING. THERE WILL BE A WARM CLOUD LAYER OF 2-4 THOUSAND FEET
THAT WILL HELP WITH PRECIPITATION EFFICIENCY. MID AND UPPER LEVEL
WINDS SHOULD EVENTUALLY PUSH STORMS TOWARD THE EAST OR
NORTHEAST...BUT THE INITIAL CELLS MAY SIT FOR A WHILE UNTIL THEY
DEVELOP SOME ORGANIZATION. THE SOUTHEAST WINDS ON THE PLAINS MAY
BE STRONG ENOUGH TO ENCOURAGE SOME BACKBUILDING. ALSO EXPECT ONE
OR MORE SURFACE BOUNDARIES TO DEVELOP ON THE PLAINS THAT WILL MOVE
AGAINST THE STEERING WIND...PROVIDING THE OPPORTUNITY FOR TRAINING
OR STATIONARY CELLS. EVENTUALLY EXPECT ONE OR TWO LARGER AREAS OF
RAIN THAT WILL MOVE SLOWLY EAST THIS EVENING...AND WHILE THEY
COULD PRODUCE SIGNIFICANT RAIN THE FLASH FLOOD THREAT SHOULD BE
LOWER AFTER THAT. THERE IS ENOUGH OF A THREAT IN THE FOOTHILLS AT
MIDDAY TO START A FLASH FLOOD WATCH THERE AT 10 AM...WILL WAIT
UNTIL AFTERNOON FOR THE PLAINS. I WILL END THE WATCH AT 8 PM AND
ONLY EXTEND IT OUT TO MORGAN COUNTY BECAUSE THE STORMS SHOULD BE
MOVING BY THEN...EVEN THOUGH WE MAY STILL HAVE QUITE A BIT OF RAIN
ON THE PLAINS AFTER THAT.

I TRIED TO COBBLE TOGETHER SOME OF THE CONSENSUS INGREDIENTS FROM
THE MODELS...NUDGING THE FORECAST TOWARD THE ABOVE TIMING AND
FOCUSING POPS AND RAINFALL AMOUNTS A BIT MORE FROM DENVER
NORTHWARD.

.LONG TERM...(SATURDAY THROUGH THURSDAY)
ISSUED AT 444 AM MDT FRI MAY 30 2014

ON SATURDAY...A DRIER WEST TO SOUTHWESTERLY FLOW ALOFT WILL
DEVELOP FOLLOWING THE PASSAGE OF THE UPPER TROUGH FRIDAY
NIGHT...BUT STILL PLENTY OF LOW LEVEL MOISTURE AVAILABLE FOR AFTN
AND EVENING THUNDERSTORMS. FORECAST SOUNDINGS IN THE AFTN RANGE
FROM 600-800 J/KG ALONG THE URBAN CORRIDOR TO 1500 J/KG NEAR
AKRON. OVERALL SHEAR PROFILE WILL BE BETTER AS WELL. THE PW VALUES
ARE PROGGED TO DROP TO 0.70 INCHES IN DENVER SATURDAY AFTN WITH
HIGHER VALUES NEAR ONE INCH OVER THE NORTHEAST PLAINS. AS A
RESULT...SHOULD SEE A DRY LINE DEVELOP AS DRIER AIR MIXES DOWN TO
SURFACE FM THE WEST...WITH SOME SEVERE POTENTIAL NORTH AND EAST OF
THE LINE IN THE AFTN AND EVENING. MAIN CONCERN WOULD BE LARGE HAIL
AND DAMAGING WINDS...AND POSSIBLY A TORNADO OR TWO. ALONG THE
URBAN CORRIDOR...THE THREAT OF HEAVY RAIN WILL LESSEN WITH LOWER
PW VALUES AND STORM MOTIONS FM THE SOUTHWEST AROUND 15 KTS. RUNOFF
WILL CONTINUE AT FAIRLY STEADY CLIP IN THE HIGH COUNTRY...THE
WARMEST DAYS WILL BE SUNDAY AND MONDAY...WITH SOME SLIGHT COOLING
TUESDAY THROUGH THURSDAY AS A WEAK TROUGH PASSES TO THE NORTH OF
COLORADO. THERE WILL BE A COUPLE OF WEAK COLD FRONTS THAT WILL
SLIP INTO THE NORTHEAST PLAINS. ONE IS PROGGED TO MOVE THROUGH THE
REGION LATE SUNDAY NIGHT INTO MONDAY MORNING...WITH ANOTHER ON
THURSDAY. OVERALL EXPECT A SLIGHT CHANCE OF THUNDERSTORMS EACH
DAY...BUT IT COULD BE HIGHER THAN THAT NORTH AND EAST OF
DENVER...IN THE TUESDAY TO THURSDAY TIME FRAME.

&&

.AVIATION...(FOR THE TAFS THROUGH 12Z SATURDAY MORNING)
ISSUED AT 444 AM MDT FRI MAY 30 2014

THUNDERSTORMS WILL DEVELOP OVER THE MOUNTAINS THIS MORNING AND
MOVE INTO THE DENVER AREA THIS AFTERNOON...THEN ACROSS THE PLAINS
THIS EVENING. COVERAGE WILL BE HIGH ENOUGH TO AFFECT
APPROACHES/DEPARTURES. INSTRUMENT APPROACHES TO KDEN WILL LIKELY
BE NEEDED AT TIMES...AND LOCALIZED IFR CONDITIONS CAN BE EXPECTED
NEAR SOME OF THE THUNDERSTORMS THOUGH VFR CONDITIONS ARE STILL
EXPECTED TO PREVAIL. THUNDERSTORMS SHOULD BE EAST OF DENVER BY
03Z.

&&

.HYDROLOGY...
ISSUED AT 444 AM MDT FRI MAY 30 2014

A FLASH FLOOD WATCH WILL BE ISSUED FOR THE FRONT RANGE FOOTHILLS
STARTING AT 10 AM...AND THE AREA FROM DENVER AND FORT COLLINS EAST
TO FORT MOGAN STARTING AT NOON...BOTH AREAS RUNNING UNTIL 8 PM.
STREAMS ARE RUNNING HIGH FROM SNOWMELT AND RECENT RAINS AND THE
GROUND IS MOIST. CONDITIONS ARE FAVORABLE FOR THUNDERSTORMS
PRODUCING 1 TO 2 INCHES OF RAIN IN 1 TO 2 HOURS IN SOME PLACES.
LOCALIZED HIGHER AMOUNTS ARE POSSIBLE. THERE MAY ALSO BE ENOUGH
WIDESPREAD RAIN TO IMPACT THE LARGER RIVERS WHICH ARE ALREADY
RUNNING HIGH. THE THREAT OF BOTH HEAVY RAINFALL AND FLOODING IS
HIGHEST IN THE FOOTHILLS OF BOULDER AND LARIMER COUNTIES AND INTO
WELD COUNTY...WHICH ARE THE MOST VULNERABLE AREAS FROM BOTH RECENT
RAINFALL AND SNOWMELT AND THE DAMAGE FROM LAST YEARS FLOODING.
OUTSIDE OF THE FLASH FLOOD WATCH AREA THERE IS STILL A THREAT OF
HAVING ENOUGH RAIN TO RAISE STREAMFLOWS...BUT THE THREAT OF
INTENSE FLASH FLOODING IS LOWER.

THE FLASH FLOOD THREAT WILL LESSEN ON SATURDAY AS A DRIER WEST TO
SOUTHWESTERLY FLOW ALOFT DEVELOPS. BETTER STORM MOTIONS COUPLED
WITH A DRIER AIRMASS WILL SHIFT THE FOCUS OF HEAVY RAIN TO AREAS
EAST OF INTERSTATE 25. IT WILL REMAIN WARM THROUGH MONDAY
RESULTING A STEADY RUNOFF. THUNDERSTORMS ON SUNDAY AND MONDAY
WILL BE ISOLATED AND HIGHER BASED...SO THIS WILL NOT COMPOUND THE
RUNOFF ISSUE. TUESDAY THROUGH THURSDAY WILL BE A BIT COOLER AS A
COUPLE OF WEAK FRONTAL BOUNDARIES PUSH THROUGH FM THE NORTH AND
EAST BUT OVERALL JUST A SLIGHT CHANCE OF STORMS IN THE AFTN AND
EVENING. MINOR COOLING HOWEVER SHOULD NOT BE ENOUGH TO ALTER THE
SNOWMELT NEXT WEEK.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH 10 AM TO 8 PM FOR THE FRONT RANGE
FOOTHILLS...ZONES 35 AND 36.

FLASH FLOOD WATCH NOON TO 8 PM FOR THE ADJACENT PLAINS FROM DENVER
AND FORT COLLINS EAST TO MORGAN COUNTY...ZONES 38>40..42>45.

&&

SHORT TERM...GIMMESTAD
LONG TERM...COOPER
AVIATION...GIMMESTAD
HYDROLOGY...COOPER/GIMMESTAD

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