Mesoscale Precipitation Discussion 0689
NWS Weather Prediction Center College Park MD
213 PM EDT Sat Aug 18 2018

Areas affected...Portions of Eastern LA...Central/Southern
MS...Central /Southwest AL

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

Valid 181810Z - 190010Z

Summary...Very heavy showers and thunderstorms with rainfall rates
to near 3 inches/hr will result in some flash flooding this
afternoon.

Discussion...Radar imagery is showing a notable expansion of
shower and thunderstorm activity across areas of eastern LA,
central/southern MS, and over into west-central AL as strong
diurnal heating is promoting a moderate to strongly unstable
boundary layer. MLCAPE values are on the order of 2000 to 2500
j/kg, with the instability axis now essentially focused in between
mesoscale boundaries with a seabreeze boundary trying to lift
north from the Gulf Coast and a convectively driven outflow
boundary farther north.

GOES-16 visible satellite imagery shows a highly agitated CU/TCU
field in between the boundaries across southern MS and southwest
AL. Heavy showers and thunderstorms will be developing across
these areas this afternoon, and convection is expected to become
rather organized as it is likely that the aforementioned mesoscale
boundaries will gradually merge later this afternoon.

PWATS remain very high with values near or even above 2.25 inches,
with 12Z RAOB data confirming a highly concentrated degree of
moisture in the 700/300 mb layer. This will promote highly
efficient rainfall processes for enhanced rainfall rates. Given
the degree of boundary layer instability, and expectation of deep
cold convective tops, rainfall rates are expected to be quite
impressive and upwards of 3 inches/hr.

The 12Z hires model consensus and repeated runs of the HRRR
strongly support heavy rainfall totals going through the afternoon
hours. Expect as much as 3 to 6 inches of rain, with the heaviest
amounts occurring in vicinity of cell-mergers. Some flash flooding
will be likely given these totals.

Orrison

ATTN...WFO...BMX...JAN...LIX...MOB...TAE...

ATTN...RFC...LMRFC...SERFC...

LAT...LON   32758649 32578587 32028575 31658598 31318668
            31078730 30828817 30678890 30618926 30478957
            30359013 30479053 30799089 31359090 31889071
            32269004 32368946 32348846 32468786 32708736