MESOSCALE PRECIPITATION DISCUSSION 0915
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1138 PM EDT THU OCT 26 2017

AREAS AFFECTED...WRN ME...NRN ME

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

VALID 270336Z - 270736Z

SUMMARY...A COMPACT MID LEVEL LOW WILL MAKE EFFICIENT USE OF WEAK
INSTABILITY TO YIELD HEAVY RAINFALL ON SATURATED GROUNDS. FLASH
FLOODING IS POSSIBLE.

DISCUSSION...AT 03Z HEAVY RAINS HAD PROMPTED ONE FLASH FLOOD
WARNING AND OTHER FLOOD ADVISORIES AND WARNINGS ACROSS WESTERN
MAINE...AND FOR GOOD REASON. A COMPACT MID LEVEL LOW...DOWN TO 546
DECAMETERS AT 500MB...WAS LIFTING NORTH...WITH LOWER TO MID LEVEL
RESPONSE AND ASSOCIATED STRONG CONVERGENCE ZIPPING UP ONLY SLOWLY
ACROSS WESTERN MAINE. THE SHORT WAVELENGTH AND DEEP CENTER OF THE
DISTURBANCE WILL PRODUCE INTENSE VERTICAL MOTION...AND DESPITE
LITTLE CAPE IN THE MODELS OR MESOANALYSES...THE RADAR PRESENTATION
WAS INDICATIVE OF SOME CONVECTIVE ENHANCEMENT. ALTHOUGH THERE IS
MOUNTAINOUS TERRAIN IN THIS AREA...UPSLOPE ENHANCEMENT SHOULD BE
LIMITED BY THE TROUGH PASSING DIRECTLY OVER THE MOUNTAINS...AND
THE EFFICIENCY OF THE EVENT WAS TIED MAINLY TO THE INTENSE
SYNOPTIC FORCING.

DUAL POL RADAR-ESTIMATED RAIN RATES FROM KGYX CONTINUED TO CLIMB
AT 0320Z...WITH SOME HOURLY RATES EXCEEDING 1.5 INCHES. THIS
APPEARS BELIEVEABLE...GIVEN ORGANIZED AND SLOW MOVING REFLECTIVITY
GREATER THAN 45 DBZ...HEAVY RAIN CLASSIFICATION FROM THE HYDRO
CLASS ALGORITHM...AND THE FACT THAT MUCH OF THIS WAS CLOSE ENOUGH
TO THE RADAR TO BE SAMPLED BENEATH THE MELTING LAYER.

THE RAP FORECASTS 925-850MB CONVERGENT FLOW AND ASSOCIATED 700 MB
OMEGA TO ZIP UP NORTH AND THEN NORTHWESTWARD WITH THE TRACK OF THE
SHORTWAVE ALOFT. THIS IDEA FITS WELL WITH THE MORE RECENTLY
INITIALIZED 00Z NAM CONUS NEST...WHICH PLACES SWATHS OF GENERALLY
1-PLUS INCH...BUT SOME 2-PLUS INCH...ACCUMULATIONS UP THROUGH
WESTERN AND NORTHERN MAINE THROUGH 08Z. PER RECENT WET WEATHER AND
COMPROMISED FFG VALUES...RAIN RATES OF 0.75 INCH TO 1.00 INCH IN
AN HOUR MAY BE SUFFICIENT TO CAUSE FLOODING / FLASH FLOODING.

BURKE

ATTN...WFO...CAR...GYX...

ATTN...RFC...NERFC...

LAT...LON   47676930 47336852 45586863 44386921 43937026
            44297084 45647082 46497045 47027009