MESOSCALE PRECIPITATION DISCUSSION 0075
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
219 PM EDT SAT MAR 18 2017

AREAS AFFECTED...NORTHEAST OR...SOUTHEAST WA...NORTHERN ID

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 181820Z - 182330Z

SUMMARY...INCREASING CHANCE OF SHALLOW CONVECTION IN ANOMALOUSLY
WET ENVIRONMENT OVER SATURATED GROUND CONDITIONS MAY LEAD TO FLASH
FLOODING OR AT LEAST COMPOUND ONGOING FLOODING CONDITIONS IN THE
AREA.

DISCUSSION...GOES-WV MOSAIC DENOTES INCREASINGLY AMPLIFIED FLOW
PARTICULARLY WITH A UPPER LEVEL RIDGING EAST OF THE CONTINENTAL
DIVIDE.  A SHEARED BUT STRONG SHORTWAVE HAS LIFTED INTO S BC ATTM
WITH ASSOCIATED 110-120 KT 250MB JET.  UPSTREAM A DEEP NORTHERN
STREAM UPPER LEVEL LOW IS ADVANCING FROM THE WEST WITH A
STRENGTHENING POLAR JET STREAK SUPPORTING HIGHLY CONFLUENT FLOW
OVER WESTERN WA/OR. IN THE NEAR TERM THIS YIELDS LARGE SCALE
DESCENT AND CLEARING / INCREASED SOLAR INSOLATION THROUGH PEAK
HEATING. THIS IN CONCERT WITH CAA FROM THE UPPER LOW SHOULD
STEEPEN MID-LEVEL LAPSE RATES TO SUPPORT SBCAPES TO REACH 100-250
J/KG IN THE NEXT FEW HOURS.  GLOBAL TPW ANALYSIS AND SATELLITE
ALSO INDICATE MAIN CORRIDOR OF WARM CONVEYOR BELT POINTED THROUGH
N CA/SE OREGON INTO S ID...PROVIDING MOISTENING LOWER PROFILE AS
WELL WITH GPS TPWS REACHING THE .6-.8" RANGE NEARING 3-3.5 STD DEV
ABOVE NORMAL.

THE ADVANCING UPPER LOW/HEIGHT FALLS WILL SHARPEN SURFACE TO 850
FRONTAL ZONE/FGEN ACROSS CENTRAL WA INTO N ID. MODEST BUT NOT
INSIGNIFICANT SOUTHERLY COMPONENT ACROSS CENTRAL OREGON PROVIDES
CONFLUENT SFC TO 85H FLOW WITH SUFFICIENT MST CONVERGENCE AS THE
FRONT SLIDES SLOWLY EASTWARD (ORIENTED NE-SW).  THIS FORCING ALONG
WITH THE INCREASED INSTABILITY SUGGESTS AN INCREASING PROBABILITY
OF THE GENERATION OF SHALLOW CONVECTIVE DEVELOPMENT.  FAIRLY
UNIDIRECTIONAL STEERING FLOW ALLOWS FOR CONVECTIVE CELLS TO
POSSIBLY TRAIN FOR A SHORT DURATION ESPECIALLY IF CORES BECOME
TERRAIN LOCKED WITH INCREASED OROGRAPHIC EFFECTS.  UNCERTAINTY
REMAINS ON THE TIMING OF THE EASTWARD PROGRESSION OF THE
FORCING/FRONT LIKELY LIMITING VERY LONG TERM DURATION OF THESE
MODEST RATES.

HI-RES CAMS HAVE BEEN TRENDING UPWARD IN CONVECTIVE ACTIVITY
NORTHEAST OF THE BLUE MTN RANGE ACROSS SE WA INTO THE SOUTHERN
BITTERROOT RANGE OF N ID AROUND THE 20-21Z TIME FRAME.  THE 12Z
ESRL EXPERIMENTAL HRRR WAS MOST AGGRESSIVE SUGGESTING 1HR RAIN
TOTALS GREATER THAN .5"/HR SUPPORTED BY THE 12Z NMMB SUFFICIENT TO
EXCEED THE LOW FFG VALUES IN THE REGION. GREATEST PROBABILITY
BASED ON THE GUIDANCE SUGGEST PEAK MATURATION AROUND 23-00Z ACROSS
EXT SE WA/NE OR INTO CLEARWATER COUNTY ID. HOWEVER...RECENT HRRR
AND 12Z ARW ARE A BIT MORE RESERVED IN THE STRENGTH/MAGNITUDE OF
THE RAINFALL RATES AND THEREFORE TOTALS.  STILL ONGOING SATURATED
GROUNDS AND SNOW MELT RUN OFF ALREADY HAS LEAD TO FLOODING
CONCERNS. ADDITIONAL RAINFALL MAY ACCENTUATE THE ONGOING FLOODING
CONDITIONS IN THE VICINITY.

GALLINA

ATTN...WFO...MSO...OTX...PDT...

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

LAT...LON   47831593 46761484 45961529 45371673 44761988
            45272021 46161890 47441714