MESOSCALE PRECIPITATION DISCUSSION 0932
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
349 AM EST WED NOV 15 2017

AREAS AFFECTED...NRN CALIFORNIA...FAR SW OREGON

CONCERNING...HEAVY RAINFALL

VALID 150848Z - 152045Z

SUMMARY...A SURGE OF MOISTURE ASSOCIATED WITH A DEVELOPING
ATMOSPHERIC RIVER EVENT WILL ARRIVE IN NORTHERN CALIFORNIA AND FAR
SOUTHWEST OREGON AROUND 18Z (10 AM PST). THE THREAT OF HEAVY RAIN
SHOULD INCREASE AROUND THAT TIME...WITH RAIN RATES AROUND 0.5
IN/HR DEVELOPING IN SOME AREAS BETWEEN 18Z AND 21Z...ESPECIALLY IN
COASTAL RANGES...KLAMATH MOUNTAINS...AND THE NORTHERN SIERRAS.

DISCUSSION...AN ATMOSPHERIC RIVER EVENT IS LIKELY TO DEVELOP OVER
THE NEXT 12 HOURS AS INCREASING SOUTHWESTERLY FLOW AHEAD OF AN
APPROACHING COLD FRONT AND DIGGING UPPER-LEVEL TROUGH DRAWS DEEPER
MOISTURE NORTHEAST INTO THE PACIFIC COAST. EXTRAPOLATION OF TRENDS
IN THE 89 GHZ CHANNEL FROM SUCCESSIVE AMSU PASSES (AT 23Z AND 05Z)
SUGGESTS THE ARRIVAL OF DEEPER MOISTURE CAN BE EXPECTED ALONG THE
SRN OREGON AND NRN CALIFORNIA COAST AROUND 18Z. THIS WAS BASED ON
AN ENHANCEMENT IN THE 89 GHZ CHANNEL...AND IS ALSO EVIDENT IN CIRA
BLENDED TPW PRODUCTS WHICH SHOW PWATS ON THE ORDER OF 1.3 TO 1.5
INCHES IN THE SAME AREA. THE DIGGING TROUGH...AND IN PARTICULAR AN
ELONGATED +PV ANOMALY EVIDENT IN A CHANNEL OF WARMER BRIGHTNESS
TEMPERATURES ON GOES-15 WV IMAGERY AROUND 43N/138W...SHOULD DRIVE
THE FORMATION OF THE ATMOSPHERIC RIVER EVENT. AS THE LOBE OF
HIGHER PV DIGS...WINDS SHOULD RESPOND AND INCREASE WITH A STRONG
UPPER LEVEL JET CORE LIKELY TO DEVELOP OVER OREGON. FAVORABLE JET
DYNAMICS IN THE RIGHT ENTRANCE REGION SHOULD CONTRIBUTE TO
SYNOPTIC SCALE LIFT ACROSS MUCH OF THE OUTLINED AREA.
FURTHERMORE...STRONG LOW-LEVEL FLOW IS ALSO EXPECTED WITH 850MB
WINDS REACHING 50-60 KNOTS NEAR A LOW-LEVEL WAVE DEVELOPING NEAR
AN INFLECTION POINT ON THE COLD FRONT (AND NEAR THE REGION OF
HIGHEST PWATS).

THE STRONGEST LOW-LEVEL FLOW MAY ARRIVE LARGELY AFTER THE VALID
TIME OF THIS DISCUSSION (21Z OR LATER)...BUT INCREASING LOW-LEVEL
SOUTHWESTERLIES SHOULD FAVORABLY INTERCEPT TERRAIN FEATURES (AT A
NEAR PERPENDICULAR ANGLE IN MANY CASES) GENERATING STRONG
OROGRAPHIC LIFT IN ADDITION TO THE BROADER SYNOPTIC LIFTING
MECHANISMS. INTEGRATED VAPOR TRANSPORT MAGNITUDES IN THE CORE OF
THE ATMOSPHERIC RIVER SHOULD BE NEAR THE 99TH PERCENTILE
CLIMATOLOGICALLY FOR NOVEMBER IN NORTHERN AND CENTRAL CALIFORNIA.

RAIN RATES APPEAR LIKELY TO BEGIN IN THE 0.10 TO 0.25 IN/HR RANGE
IN THE MID-LATE MORNING AS THE LEADING PERIPHERY OF DEEPER
MOISTURE BEGINS TO ARRIVE. AS LOW-LEVEL FLOW INCREASES
SUBSTANTIALLY (IN CONCERT WITH A CONTINUED INCREASE IN ATMOSPHERIC
MOISTURE) AFTER 18Z...RAIN RATES SHOULD TICK UP AS WELL. HREF
BLENDED MEAN 1-HR RAIN RATES START TO APPROACH 0.5 IN/HR IN THE
18-21Z TIME FRAME...PARTICULARLY IN FAVORED UPSLOPE REGIONS OF THE
NORTHERN CALIFORNIA COASTAL RANGES AND IN THE NORTHERN SIERRAS.
RAIN RATES WELL OFFSHORE WITH THE PLUME OF HIGHER PWATS WERE
ESTIMATED TO BE AS HIGH AS 0.25 IN/HR IN THE BLENDED RAIN RATES
FROM POLAR ORBITING SATELLITES...AND DOUBLE THESE VALUES AS THE
MOISTURE PLUME REACHES SHORE ALSO SEEMS REASONABLE. THE 00Z HREF
INDICATES THE HIGHEST PROBABILITY OF SUCH RAIN RATES BEING
ACHIEVED PRIOR TO 21Z IN THE COASTAL RANGES FROM NEAR WILLOW CREEK
SOUTH TO NEAR THE REDWOOD VALLEY BURN SCAR...AND IN THE NORTHERN
SIERRAS FROM NEAR LASSEN PEAK SOUTH TO NEAR THE I-80 CORRIDOR.

LAMERS

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

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

LAT...LON   43062431 42842384 42192348 41972320 41512325
            41212310 40952313 40792290 41272216 41012168
            40702193 40512167 40302153 40402089 39952057
            39462014 38801993 38431995 38172011 38432066
            38962122 39282163 39422215 38962251 38682292
            38682367 39092391 39652395 40042443 40452456
            40992433 41522428 41922445 42522463 42972461