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WPC Met Watch

 
Mesoscale Precipitation Discussion: #0930
(Issued at 1232 PM EST TUE NOV 14 2017 )

MPD Selection
 


Graphic for MPD #0930
MESOSCALE PRECIPITATION DISCUSSION 0930
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1232 PM EST TUE NOV 14 2017

AREAS AFFECTED...WESTERN WASHINGTON...

CONCERNING...HEAVY RAINFALL

VALID 141730Z - 150530Z

SUMMARY...NARROW PERSISTENT SWLY 7H FLOW WITH SOME INSTABILITY
SUPPORT OCCASIONAL RATES UP TO .75"/HR AND TOTALS OF 5-6" IN THE
LOWER OLYMPIC SLOPES THROUGH 06Z AND SHOULD LEAD TO ENHANCED
STREAM FLOW AND POSSIBLE FLOODING CONERNS.

DISCUSSION...GOES-WEST WV DEPICTS A VERY STRONG UPPER LOW NEAR 50N
136.5W WITH MODEST SW-NE ELONGATED ORIENTATION AS A SHORTWAVE AND
ENHANCED JET STREAK ROUND THE BASE FROM 140W TO 135W.  AN ENHANCED
BAROCLINIC LEAF IS NOTED AS WELL WITHIN THE MODERATELY DIFFLUENT
LEFT EXIT REGION.  WHILE MAIN WARM CONVEYOR IS PRESSED WELL EAST
WITH PRIMARY COLD FRONT IN THE ROCKIES...A SECONDARY TROF/FRONTAL
BOUNDARY IS DENOTED LAYING SW-NE BELOW ON THE NORTHERN SIDE OF
THIS BAROCLINIC LEAF.  HEIGHT FALLS FROM THE APPROACH OF THE WAVE 
HAS ALLOWED FOR LOW LEVEL FLOW TO STRENGTHEN AND BECOME GENERALLY
CONFLUENT AS IT ASCENDS TO AROUND 7H NEAR/JUST ALONG THIS
BOUNDARY.  CAA ALOFT ALSO SUPPORTS MODERATELY STEEP LAPSE RATES
(BELOW -7C/KM) WHILE STILL BEING NEARLY SATURATED THOUGH THE
SFC/650MB...SUPPORTING TPWS UP TO .75" AND SHALLOW CAPES OF 250 TO
500 J/KG SUPPORTING TYPICAL SCATTERED COLD AIR CBS. 

THE MOISTURE IS CLEARLY NOT ANOMALOUS... HOWEVER THE DIFFERENCE
CONCERN FOR FLOODING EXISTS WITH THE FOCUS/CONFLUENCE OF THE
Q-AXIS ALIGNED PERFECTLY WITH THE 7H JET AXIS AND ITS GENERAL
STUBBORNNESS TO ADVECT EASTWARD.  THIS WILL CONTRIBUTE TO REPEAT
TRACKS OF THE CONVECTIVE ELEMENTS CAPABLE OF RATES OF .75"/HR
ACROSS NW PACIFIC INTO WESTERN GRAY HARBOR COUNTIES BEFORE A BROAD
OROGRAPHIC ASCENT INTO THE WESTERN OLYMPIC MOUNTAIN RANGES.  7H
FLOW CURRENTLY NEAR 50KTS FROM SW WILL SLOWLY INCREASE TO 65KT
THROUGH THE EVENING WITH VERY LITTLE VARIATION IN DIRECTION
FURTHER SUPPORTING CONSISTENT OROGRAPHIC ASCENT INTO SAME DRAINAGE
CHANNELS.  PERSISTENT .25-.5"/HR RATES WITH OCCASIONAL .75" TO
POSSIBLE 1"/HR...TOTALS ARE LIKELY TO CONTRIBUTE TO TOTALS OF 5-6"
THROUGH 06Z MAINLY ON SW FACING SLOPES BELOW 4000FT (FRZG LEVEL). 
 THIS MAY CONTRIBUTE TO HIGHER THAN NORMAL STREAM FLOWS WITH SOME
FLOODING POSSIBLE.  HI-RES CAMS PARTICULARLY THE 12Z NMMB AND ARW
APPEAR TO HAVE THE BEST HANDLE ON THE SITUATION TO HAVE GOOD
CONFIDENCE IN PLACEMENT/TIMING OF THE BOUNDARY REACHING THE
CENTRAL OLYMPICS/MOUTH OF COLUMBIA RIVER BY 06-07Z.

GALLINA

ATTN...WFO...PQR...SEW...

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

LAT...LON   48352461 48122349 47982280 47442295 47082276
            46462343 46282388 46302407 46922428 47622457
            48122481


Last Updated: 1232 PM EST TUE NOV 14 2017
 

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