Mesoscale Precipitation Discussion 0668
NWS Weather Prediction Center College Park MD
110 PM EDT Wed Aug 15 2018

Areas affected...Rooftop of ME...Ext Northern NH...

Concerning...Heavy rainfall...Flash flooding possible

Valid 151710Z - 152300Z

SUMMARY...Multiple rounds of thunderstorms, with stalled/slow
moving initial storms in rich moisture environment pose possible
flash flooding risk.

DISCUSSION...Favorable upper level pattern exists across northern
ME for slow moving or even stationary cells before main convective
line(s) approach later this afternoon.  Strong but opening upper
level trof exits over the western Gulf of Maine lifting northeast,
with more significant/synoptic scale height falls and associated
cold front crossing southern Quebec.   The remains of a deep warm
conveyor/TROWAL has pooled ahead of the approaching front with
CIRA LPW showing maximized 7-5H moisture across the Rooftop of
Maine with values in the .3-.4 range, and 850-7H values in the
coincident at .5-.6".  This along with deeply rich low level
moisture with Tds in the upper 60s and spot 70F values support
Total PWATs over 1.5" which is around the 95 percentile of for
this time of year.  

Early morning isolation with this moisture profile supported
MLCAPEs to 2000 J/kg and GOES-16 Visible imagery already depicts
multiple bands developing ahead of the cold front that lies around
the St. Lawrence Seaway currently.   While low to mid-level flow
is generally westerly ahead of the approaching trof, the relative
flow to the upper low to the south, has slacked the flow
sufficiently for mean cell motions to be very weak or even support
stationary cells.  This is more likely the case for the line from
HUL to GNR.  Right now the low level forcing is weak to only
sprout scattered cells, but rainfall will be efficient up to
1.5"/hr.  This is near or just below 1hr FFG values in the area
suggesting flash flooding is possible.

The line to the north along the ME/Quebec border is a bit
stronger, in better deeper moisture axis but also shows more
propagation than the line south.  Still, these two lines could
track over similar areas potentially compounding flooding
situation.  Additional cells along the cold front may pose a
similar setup as well toward 21z. 

All considered, the flash flood risk is on the lower end, given
scattered nature of cells and repeat/cross rainfall tracks but low
FFG/anomalous moisture and slow cell motions initially pose at
least a possible threat through 23z.

Gallina

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

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

LAT...LON   47546890 47286783 46816764 45426726 45156876
            44797003 44607098 44807151 45317115 45797057
            46737002 47366949