Flash Flood Guidance
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
000
AWUS01 KWNH 210033
FFGMPD
CAZ000-ORZ000-210631-

MESOSCALE PRECIPITATION DISCUSSION 0063
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
732 PM EST MON FEB 20 2017

AREAS AFFECTED...NORTHERN CA...CENTRAL CA

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 210031Z - 210631Z

SUMMARY...ATMOSPHERIC RIVER EVENT PEAKING THIS EVENING AS
PRONOUNCED SHORTWAVE APPROACHES. FLOODING AND FLASH FLOODING ARE
LIKELY.

DISCUSSION...PRECIPITATION HAD BECOME MORE GREATLY CONVECTIVE
WITHIN A WEDGE OF STRONG ASCENT DOWNWIND OF A COMPACT MID LEVEL
SHORTWAVE WHERE 250 MB FLOW WAS BROADLY DIFLUENT. AT 2330Z
INFRARED SATELLITE IMAGERY REVEALED A BROAD CONVECTIVE WEDGE WITH
ITS ORIGINS AROUND 35 N / 125 W...ALONG AN ADVANCING COLD FRONT
AND JUST AHEAD OF THE CORE OF THE 150 KNOT JET STREAK PER THE NAM.
COMPLETING THE IMPRESSIVE SATELLITE PRESENTATION OF THIS SYSTEM
WAS A DEVELOPING BAROCLINIC LEAF CLOUD OVER SOUTHWEST OREGON AND
ADJACENT WATERS...WHERE THE DEVELOPING STACKED LOW WILL BE
TRACKING...AND A CORRESPONDING ANTICYCLONICALLY CURVED BATCH OF
CIRRUS ON THE OPPOSITE SIDE OF THE UPPER JET OVER THE SAN JOAQUIN
VALLEY AND SOUTHERN SIERRAS.

KMUX WSR-88D DATA OUT OF MONTEREY CA HAD SAMPLED ENHANCED
REFLECTIVITY AT CLOSE RANGE...CONFIRMING THERE WAS NOT MERELY
BRIGHT BANDING...BUT LEGITIMATE CONVECTIVE ENHANCEMENT TO THE
ECHOES WITHIN THE WEDGE OF STRONG DEEP LAYER ASCENT. MODERATE AND
LOCALLY HEAVY PRECIPITATION HAD FILLED IN NORTHWARD FROM THE
WEDGE...UP TO NORTHWEST CA / SOUTHWEST OR...IN ADVANCE OF THE
IMMEDIATE PATH OF THE MID LEVEL SHORTWAVE. THIS BEST MATCHED THE
NSSL WRF DEPICTION OF PRECIPITATION AS OF 00Z...AND BOTH THAT
MODEL AND THE HRRR SUGGEST WIDESPREAD SYNOPTIC FORCING THROUGH
ABOUT 05Z OVER CENTRAL AND NORTHERN CA. AFTER THAT TIME THE
ATMOSPHERIC RIVER WILL HAVE BROADENED...WITH PW VALUES DROPPING
OFF CLOSER TO 1.00 INCH...AND PRECIPITATION WILL BECOME MORE
GREATLY TIED TO OROGRAPHIC FORCING. STILL...THE FLOOD AND FLASH
FLOOD THREAT WILL REMAIN PRONOUNCED THROUGHOUT THE NIGHT AS WINDS
VEER MORE DIRECTLY ONSHORE IN THE WAKE OF THE APPROACHING
WAVE...AND MOISTURE REMAINS SUFFICIENT TO SUPPORT LOCALLY HEAVY
RAIN.

IN THE NEAR TERM RAIN RATES OF NEARLY 0.75 INCHES PER HOUR HAD
BEEN ESTIMATED BY RADAR AND OBSERVED IN THE COASTAL AREAS SOUTH OF
SAN JOSE. RATES OF 0.25 TO 0.50 INCHES WILL BE MORE COMMON THROUGH
05Z...WITH STEADY RAIN RATES OF AT LEAST 0.25 INCHES CONTINUING IN
THE FAVORED WEST FACING TERRAIN ALONG THE COAST AND IN THE SIERRAS
AFTER 05Z.

BURKE

...PLEASE SEE WWW.WPC.NCEP.NOAA.GOV FOR GRAPHIC PRODUCT...

ATTN...WFO...EKA...HNX...LOX...MFR...MTR...REV...STO...

ATTN...RFC...CNRFC...NWRFC...

LAT...LON   42392377 41822328 40962324 40682287 41152232
            41502192 40942134 39622068 38431983 37851933
            37001862 36471842 36221850 36591887 37021920
            37461981 37852022 37942057 37962101 37712126
            36632072 35492043 35062068 35912179 36632231
            37582271 39712415 41012490 42132453


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.