Mesoscale Precipitation Discussion 0757
NWS Weather Prediction Center College Park MD
141 AM EDT Tue Jul 30 2024

Areas affected...Eastern VT...Northern NH...Northwest ME...

Concerning...Heavy rainfall...Flash flooding likely

Valid 300540Z - 301100Z

SUMMARY...Stronger thunderstorms directly under upper low will
limit cell motions with some potential for further development in
proximity and additional potential for flash flooding over the
next few hours.

DISCUSSION...Regional RADAR mosaic denotes a band of thunderstorms
has developed within the northern Connecticut River Valley. 
Highly anomalous and fairly symmetric mid-level closed low has
wobbled through Northern New England but appears to be slipping
eastward.  This has brought the core of colder air aloft overhead
steepening the lapse rates; concurrently RAP analysis supported by
regional VWP suite has shown a backing of low-level flow with
15-20kts of 850-700mb southwesterly flow angling through the
Connecticut River Valley both providing some low level warm
advection, further enhancing unstable environment but also
moisture flux as noted by the CIRA LPW sfc-850 and 850-700mb
layers with values of .75 and .4-.5" in each layer respectively. 

Thunderstorms in the northern valley have utilized the enhanced
unstable air and with the flux have generated a few observations
of 2"/hr, which in complex terrain is producing MRMS FLASH signals
of 250-500 cfs/smi, quite indicative of ongoing flash flooding.

As the closed low continues to lift north-northeast, low-level
advection should maintain ample moisture/unstable air given 1000
J/kg of upstream CAPEs, while cell motions will be less than 5kts,
eventually turning more northeastward across central NH into NW
ME, with increasing southwesterly flow increasing with the
north-northeastward exiting upper low.  Winds will increase from
20-25kts in the low levels with some increasingly unidirectional
850-500mb flow to support potential repeating/cross tracks of
thunderstorms with 1.5"/hr rates in short terms across S VT,
central NH into NW ME.

Closer to the upper-low across NE VT/N NH, cells will remain
fairly stationary (~5kts). It is uncertain, if low-level winds are
southerly enough to replenish instability/moisture flux to
maintain more than a few more updraft cycles, but an additional
hour or so is likely to maintain rates over 1.5"/hr and continue
to produce likely flash flooding in the near term.

Gallina

ATTN...WFO...BTV...GYX...

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

LAT...LON   45527018 45396983 45026970 44507004 43887063
            43267120 43267229 43497260 44237262 44897225
            45217154 45497093