Mesoscale Precipitation Discussion 0714
NWS Weather Prediction Center College Park MD
635 PM EDT Thu Aug 23 2018

Areas affected...ND into northeast SD

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

Valid 232235Z - 240335Z

Summary...Developing convection ahead of a short wave could
organize into training clusters, which could result in flash
flooding across portions of ND and SD this evening.

Discussion...Regional radars over ND showed bands of convection
trying to form in an axis of moisture oriented from northwest to
south. The convection is responding to MLCAPE values between
1000-2000 J/KG, but enough CIN remains in place, due to earlier
cloud cover, to keep the convection mainly elevated thus far.

With the approach of a short wave located over eastern MT, the
remaining CIN is expected to be removed as the convection becomes
more surfaced based (per recent RAP soundings extending from west
central ND into northeast SD). Increasing difluence ahead of the
short wave, in conjunction with the instability in place, should
allow the convection to expand across the region this evening.

Precipitable water values increase to near 1.50 inches on a
strengthening low level southwest flow, and this moisture is
wrapped back into the instability ahead of the short wave. The
resulting convective clusters are expected to track northeast with
the 850-300 mb mean flow, though there could be an eastward
component to the clusters (based on the RAP propagation vectors).

The most recent HRRR suggests that the convection forms after
24/00z, when the storms become more surface based. However, based
on how quickly the CIN is being removed, it is likely that the
HRRR is too slow with the development of the convective clusters.
Most of the high resolution guidance (including the most recent
HRRR and 12z WRF ARW) suggest local 2.00 to 4.00 inches of
rainfall, and this is supported by 18z HREF probabilities showing
a 90 percent chance of more than 2.00 inches of rainfall, centered
over south central ND.

Flash flood guidance values are as low as 1.50 inches over eastern
ND, and based on the expected environment, this values could be
easily exceeded. Therefore, flash flooding is likely in areas
where convective cluster training occurs late this evening.

Hayes


ATTN...WFO...ABR...BIS...FGF...

ATTN...RFC...MBRFC...NCRFC...

LAT...LON   47960185 47619952 46069624 45059702 44869871
            45340011 45900127 46310185 47400289