MESOSCALE PRECIPITATION DISCUSSION 0236
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
407 AM EDT THU SEP 12 2013
 
AREAS AFFECTED...CENTRAL AND NORTHERN CO...SE WY 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY 
 
VALID 120806Z - 121206Z
 
SUMMARY...HIGH-END FLASH FLOODING APPEARED TO BE ONGOING AT
BOULDER COLORADO AND OTHER AREAS OF HEAVY RAIN HAD BEEN OBSERVED
IN CENTRAL AND NORTHERN COLORADO. BY 0730Z THE THREAT FOR INTENSE
RAINFALL HAD LESSENED...BUT POCKETS OF MODERATE RAIN WILL
CONTINUE...RESULTING IN ADDITIONAL RAPID RUNOFF. 

DISCUSSION...THIS FLASH FLOOD EVENT WAS FORCED BY LOW LEVEL
UPSLOPE BENEATH UPPER LEVEL DIFFLUENCE...IN AN ENVIRONMENT THAT
FAVORS VERY EFFICIENT RAINFALL AND LITTLE CELL MOVEMENT.
INSPECTING MASS FIELDS FROM THE RAP MODEL THIS SETUP IS NEARLY
UNCHANGING THROUGH THIS MORNING...WITH NO IDENTIFIABLE WAVE IN THE
ATMOSPHERE TO SIGNAL A MEANINGFUL CHANGE. MORE LIKELY THE
CONVECTIVE HEAVY RAIN MAY NOT ENTIRELY END...BUT WILL SIMPLY REACH
A MINIMUM IN COVERAGE AND INTENSITY OWING TO THE DIURNAL CYCLE.

FOR A FEW HOURS THE SPC MESOANALYSIS HAD DEPICTED AN AXIS OF SFC
BASED CAPE AROUND 500 J/KG ORIENTED FROM THE SE CORNER OF CO OVER
TO NEAR COLORADO SPRINGS AND NORTHWARD ALONG THE FRONT RANGE. THE
HRRR FORECASTS THIS AXIS TO PERSIST SEVERAL HOURS...WITH VALUES
FALLING SLOWLY TO 250-500 J/KG BY 12Z.

THIS SUGGESTS THAT CONVECTIVE UPDRAFTS WITH LOCALLY MODERATE OR
HEAVY RAIN WILL REMAIN POSSIBLE THROUGH THE MORNING HOURS. RADAR
TRENDS HAD CERTAINLY SHOWED SOME WEAKENING STARTING AROUND
07Z...BUT DUAL POL RADAR WHICH HAD PERFORMED WELL BASED ON THE
NUMEROUS LOCAL STORM REPORTS...STILL INDICATED AREAS OF 0.50-1.50
INCH PER HOUR RAINS. THIS WILL LIKELY BE THE CASE ALONG THE
SOUTHEAST FLANK OF A RAIN SHIELD IN NORTHERN CO...WITH OTHER
ISOLATED ACTIVITY FARTHER SOUTH ALONG AN INVERTED TROUGH AXIS THAT
MEETS WITH LOW PRESSURE SOUTH OF COLORADO SPRINGS. MODELS FORECAST
THIS TROUGH AXIS TO SHARPEN AND PUSH 10-30 MILES FARTHER
EAST...PERHAPS OWING TO MOUNTAIN DRAINAGE FLOW / DIURNAL
INFLUENCES. BOUNDARY LAYER MIXING MAY DISRUPT INFLOW AND BRING A
RESPITE AFTER 12Z...BUT THIS AREA WILL BE AT RISK FOR ADDITIONAL
HEAVY RAIN LATER TODAY.

BURKE

ATTN...WFO...PUB...BOU...CYS...

ATTN...RFC...ABRFC...CBRFC...MBRFC...

LAT...LON   39850436 40620401 41140422 41480519 41320597 40790620
            40050623 38810584 38430524 38520455 39230441 39850436