Mesoscale Precipitation Discussion 0711
NWS Weather Prediction Center College Park MD
522 PM EDT Wed Aug 22 2018

Areas affected...Northern UT into western WY

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

Valid 222125Z - 230225Z

Summary...Convection training along a moisture axis extending from
northern UT into southwest WY could pose a flash flood threat into
the late afternoon.

Discussion...Regional radars indicated an increase in convective
coverage this afternoon across northern UT into southwest WY. The
storms are developing in an axis of 1500-2000 J/KG of MLCAPE...as
the last of the low level CIN is wiped out. Upper level divergence
ahead of a closed low over southern ID is allowing the storms to
develop rapidly, and dry air in the mid levels (as seen on recent
HRRR soundings)...as well as HCA products from KMTX...indicated
the presence of hail.

The storms are aligned with a moisture axis extending from central
and northern UT into western WY...where precipitable water values
are topping out between 0.75/1.00  inches. While the KMTX radar
has shown hourly rainfall rates nearing 1.50 inches...it is likely
that these totals are hail contaminated.

The storms are also aligning with the 850-300 mb winds, and this
could result in training...especially over northern UT.  Training
could pose a flash flood threat into the late afternoon as the
mean wind  becomes aligned with the propagation vectors. The most
recent HRRR suggests that rainfall amounts will remain below an
inch with this activity...but the 12z WRF ARW indicated the
potential for local 1.00-2.00 inch rainfall amounts over northern
UT into southwest WY.

Three hour flash flood guidance is generally around 2.00
inches...so training would be required to initiate flash flooding.
While the storms could remain wind/hail dominated, the combination
of a moistening column and training pose a flash flood threat.
Because of the scattered nature of the convection...flash flooding
is only considered possible.

Hayes


ATTN...WFO...CYS...GJT...PIH...RIW...SLC...

ATTN...RFC...CBRFC...MBRFC...NWRFC...

LAT...LON   43010858 42330711 41640710 40030918 39491184
            39651363 41411402 42191351 42811181