Mesoscale Precipitation Discussion 0509
NWS Weather Prediction Center College Park MD
315 PM EDT Sun Jun 30 2019
Areas affected…Northern to East-Central IL…West-Central to
Concerning…Heavy rainfall…Flash flooding possible
Valid 301915Z – 010115Z
SUMMARY…An expanding axis of heavy showers and thunderstorms
will be settling south through the afternoon hours and will be
capable of producing very heavy rainfall amounts of as much as 2
inches/hr. Flash flooding will be possible.
DISCUSSION…The latest GOES-16 IR satellite imagery has been
showing an expansion of cooling convective tops across northern IL
along the leading edge of a long-lived MCS that has dropped south
out of southern WI over the last couple of hours. Additionally,
the latest visible satellite imagery shows an axis of increasingly
agitated CU across areas of central and eastern IL where this is
in close proximity to a frontal zone.
The airmass near this boundary is very unstable with MLCAPE values
of locally over 4000 j/kg. There is a warm nose up around 700 mb
as seen in a special 18Z RAOB sounding from KILX, but very strong
solar insolation/boundary layer heating is allowing for convective
temps to be exceeded which is allowing for a steady
development/expansion of convection currently across northern IL.
Over the next few hours, the convection should tend to drop
south-southeast toward central and eastern areas of IL, and also
possibly developing into west-central and southwest IN in close
proximity to the aforementioned front and within a rather narrow
corridor of strong and uncapped instability around the east side
of a mid-level ridge centered just to the west over northern MO
and much of IA. The convection will be in a moist environment with
PWATs of 1.6 to 1.7 inches, and a considerable contribution of
this coming from a very moist boundary layer where surface
dewpoints are in the low to mid 70s.
The latest HRRR guidance supports rainfall totals of as much as 2
inches/hr, with storm totals through 00Z of as much as 3 to 5
inches. This will especially be the case where some localized
repeating of cells occur along the front. Given the heavy
short-term rainfall rates, and expected totals, some flash
flooding will be possible.