MESOSCALE PRECIPITATION DISCUSSION 0684
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
352 AM EST MON DEC 21 2015
 
AREAS AFFECTED...SW OREGON AND NRN CALIFORNIA 
 
CONCERNING...HEAVY RAINFALL 
 
VALID 210851Z - 211751Z
 
SUMMARY...THE NEXT ATMOSPHERIC RIVER EVENT WILL BEGIN ACROSS SW
OREGON AND N CALIFORNIA AROUND 12Z...THEN PEAK 15-21Z AS THE COLD
FRONT PASSES.

DISCUSSION...AN AMSU 89 GHZ MICROWAVE PASS AT 08Z CONFIRMED A FAST
MOVING WARM FRONT NEAR 130W WITH A COLD FRONT EXTENDING WESTWARD
ALONG 40N...WITH A BROAD REGION OF 6-HOURLY AVERAGED AMSU RAINFALL
OF 0.25 INCHES FORMING NEAR THE LEADING EDGE OF DEEP LAYER PW
PLUME OF 1.25 INCHES.

WHILE CURRENT MODEL GUIDANCE SHOWS STRONG AGREEMENT WITH THE
TRAJECTORY OF THE LOW AND COLD FRONT...AND THEREFORE THE SYNOPTIC
EVOLUTION...THE MAIN DIFFERENCE IS WITH THE VALUES OF MODEL
QPF...WITH THE HI-RES GUIDANCE GENERALLY 25 TO 50 PERCENT HIGHER.
GIVEN THE DEEP COLD FRONTAL PASSAGE THAT IS ACCOMPANIED BY 850 TO
700 MB FLOW OF 40 TO 65 KNOTS NEARLY PERPENDICULAR TO THE
TERRAIN...THAT PRODUCES STANDARDIZED MOISTURE FLUX ANOMALIES NEAR
4 STANDARD DEVIATIONS ABOVE NORMAL...THE HIGHER VALUES OF THE
HI-RES GUIDANCE MAY BE MORE RELIABLE.

CONCERNING RAIN RATES AND POSSIBLE TOTALS...0.5 TO 0.7 INCHES PER
HOUR ARE ENTIRELY POSSIBLE GIVEN THE HIGH PW VALUES AND STRONG
UPSLOPE WIND COMPONENT...AND WITH A DURATION OF POSSIBLY UP TO 6
HOURS...TOTAL RAINFALL DURING THE PEAK OF THE EVENT MAY REACH 3 TO
5 INCHES...WITH LOCALLY HIGHER AMOUNTS. MODEL TIMING IS SIMILAR
BETWEEN THE SYNOPTIC AND HI-RES MODELS...PEAKING 15-21Z AFTER THE
COLD FRONT PASSES.

JAMES

ATTN...WFO...EKA...MFR...REV...STO...

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

LAT...LON   43272402 42612328 41352282 40902128 39312041 
            38582072 38922137 39742185 40562237 40572296 
            39102316 38912407 40482495 42072497