Mesoscale Precipitation Discussion 0805
NWS Weather Prediction Center College Park MD
247 PM EDT Sat Sep 08 2018

Areas affected...Ohio River Valley

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

Valid 081846Z - 090045Z

Summary...Widespread heavy rainfall is likely in the Ohio River
Valley, and this should lead to numerous areas of flooding and
flash flooding across the region. Even after the rain ends,
flooding may persist for hours, as it likely still the case back
into southern Illinois and far eastern Missouri. The most likely
area for heavy rain and new/expanding flooding this afternoon
would be in southern Indiana. Some areas could receive in excess
of 3-4 inches of rain in the next six hours.

Discussion...A surface low associated with the remnants of Gordon
was approaching southwest Indiana as of 18Z. This also appeared to
be closely related to a circulation extending into the mid-levels,
evident on some of the area radars such as KVWX. The broad
circulation had inbound and outbound velocities in excess of 30
knots aloft, indicating a fairly vigorous circulation. This
appeared to be helping organize convective rain bands immediately
downstream, and was likely helping focus a band of heavy rain
around 100 miles downstream through an enhancement to the low-mid
level inflow and increased convergence on the nose of the LLJ.
Some of the convective bands near the center of the circulation
have been highly efficient rain producers. One tracked right over
Mount Vernon, IL (KMVN) and the observing site reported 2.41
inches of rain in an hour. Expect fairly widespread 1-2 in/hr rain
rates in this broader region of enhanced rain immediately
downstream of the MCV, with embedded corridors with rain rates up
to 3 in/hr. This should lead to a swath of heavy rainfall within
about 40mi of the track of the low and MCV into southern Indiana,
and it appears likely that some areas should receive over 3-4
inches of rain in the next 6 hours. The localized swaths of higher
rain rates would be very likely to cause flash flooding.
Therefore, the expectation is for widespread flooding issues
developing in southern Indiana this afternoon, with dangerous
flash flooding possible in areas of higher rain rates.

As the rain clears out, widespread areal flooding is likely to
continue for at least several hours. Numerous reports were
recently received of continuing flooding issues in southern
Illinois, where the rain has stopped falling, but over 3 inches
(and locally much more than that) fell over the previous 12-18
hours. Therefore, any flooding that does develop this afternoon
from southeast Illinois into southern Indiana should linger for
some time.

Further to the east, additional rain bands may precede the
approach of the low into northern Kentucky and southern Ohio, and
increase toward 00Z as low-level inflow begins to increase.
Although rain rates may be lower during the valid time of this
discussion, persistent rain could lead to some flooding issues,
and an increase in rainfall into the evening hours should allow
the flooding threat to continue spreading east.

Overall, the risk of significant and dangerous flash flooding
continues, and should be ramping up most notably in southern
Indiana this afternoon.

Lamers

ATTN...WFO...ILN...ILX...IND...JKL...LMK...LSX...PAH...RLX...

ATTN...RFC...LMRFC...NCRFC...OHRFC...

LAT...LON   40228341 39518144 38408260 37978499 37248754
            37308868 38418888 39408811 40068595