MESOSCALE PRECIPITATION DISCUSSION 0733
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
715 PM EDT WED OCT 12 2016
AREAS AFFECTED...WESTERN WA....NORTHWEST OR
CONCERNING...HEAVY RAINFALL
VALID 122315Z - 130915Z
SUMMARY...HEAVY RAINFALL IS EXPECTED TO DEVELOP OVER THE COASTAL
RANGES TONIGHT AS ATMOSPHERIC RIVER CONDITIONS COMMENCE.
DISCUSSION...THE LATEST BLENDED-TPW AND CIRA-LPW PRODUCTS CONFIRM
A WELL-DEFINED LONG-TRACKED ATMOSPHERIC RIVER TRAVERSING THE
PACIFIC OCEAN AND TAKING AIM ON THE PACIFIC NORTHWEST. A DEEP
UPPER TROUGH AND ASSOCIATED FRONTAL OCCLUSION IS ALREADY FOCUSED
OVER THE GULF OF AK...WITH A STRONG UPPER LEVEL JET OF UP TO 150
KTS ROUNDING THE BASE OF IT ALONG WITH STRONG EMBEDDED VORT
CENTERS. THIS APPROACHING ENERGY WILL ALLOW A COLD FRONT TO
APPROACH THE PACIFIC NORTHWEST OVERNIGHT WHICH WILL LEAD THE WAY
FOR STRENGTHENING WARM ADVECTION AS A SOUTHWEST LOW LEVEL JET
APPROACHING 50 KTS AIMS INTO THE WA/OR COASTAL RANGES AHEAD OF THE
FRONT.
LIGHT RAIN IS EXPECTED OVER WESTERN WA INITIALLY WITH THE
INCREASED ISENTROPIC ASCENT AND PASSAGE OF A WARM FRONT...BUT THEN
HEAVIER SHOWERS WILL ARRIVE AFTER 06Z AND ESPECIALLY TOWARD 12Z
ACROSS THE COASTAL RANGES OF WESTERN WA/NORTHWEST OR IN
ASSOCIATION WITH INCREASINGLY MOIST SOUTHWEST LOW LEVEL FLOW AND
250 MB LEFT-EXIT REGION JET DYNAMICS AHEAD OF THE COLD FRONT. IN
FACT...BETWEEN 06Z AND 12Z THE 12Z GFS SUGGESTS A RIBBON OF
850/700 MB MOISTURE FLUX ANOMALIES REACHING 3 TO 4 STANDARD
DEVIATIONS ABOVE NORMAL ACROSS THESE AREAS ALONG WITH 1.2 TO 1.4
INCH PWATS WHICH SHOULD FOSTER INCREASING RAINFALL EFFICIENCY.
OFFSHORE RAINFALL RATES AHEAD OF THE COLD FRONT AS NOTED WITH A
1930Z AMSU PASS...ARE ALREADY AS HIGH AS 0.50 TO 0.60 INCH/HR IN
ASSOCIATION WITH SOME CONVECTIVE ELEMENTS AND THIS WILL APPROACH
THE COAST OVERNIGHT.
THE LATEST ARW/NMMB AND NSSL-WRF GUIDANCE ALONG WITH RECENT RUNS
OF THE HRRR ARE IN VERY GOOD AGREEMENT ON 1 TO 2 INCH RAINFALL
TOTALS ACROSS THE COASTAL RANGES OF NORTHWEST OR AND WESTERN WA
OVERNIGHT...WITH ISOLATED HEAVIER AMOUNTS TO NEAR 3 INCHES
POSSIBLE OVER THE OLYMPICS WHERE SLIGHTLY STRONGER MOISTURE
TRANSPORT AND SOUTHWEST UPSLOPE FLOW WILL BE NOTED. ADDITIONAL
HEAVY RAINFALL WILL BE ANTICIPATED BEYOND THIS PERIOD FOR
THURSDAY. WILL CONTINUE TO MONITOR.
ORRISON
ATTN...WFO...PQR...SEW...
ATTN...RFC...NWRFC...
LAT...LON 48722435 48682334 47902257 45512242 44502307
44662419 45662431 46712442 47832481
Last Updated: 715 PM EDT WED OCT 12 2016