Mesoscale Precipitation Discussion 0567
NWS Weather Prediction Center College Park MD
1149 AM EDT Tue Jul 31 2018

Areas affected...Central Gulf Coast

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

Valid 311548Z - 312100Z

Summary...Numerous thunderstorms are expected this afternoon along
the central Gulf Coast, from Louisiana into Alabama and the
Western Florida Panhandle. In some cases, these storms may collide
or backbuild, prolonging heavy rainfall in small areas. This could
lead to localized maximum rain rates to around 3 in/hr and
rainfall totals in excess of 5 inches. Rain of this intensity
could lead to flash flooding.

Discussion...GOES-16 Day Convection RGB shows developing
convection along the Gulf sea breeze, rapidly becoming stronger
(through detection of increasingly small ice particles with
stronger updrafts). This was occurring in an arc from southern
Louisiana east to near Mobile Bay and the western Florida
Panhandle. This is expected to continue, with high confidence in
increasing convective coverage given excellent hi-res model
agreement. The convection should gradually lift inland during the
afternoon, as is typical with the sea breeze during the summer
season, and with the aid of southwesterly 10-15 knot mean flow
(850-300mb). This will favor flash flood potential in portions of
southern Alabama, where the current orientation of increasingly
organized convective clusters and lines is roughly parallel to
this mean wind (SW-NE). Hi-res models may be underestimating the
potential for localized heavy rainfall in these areas prior to
21Z. Further west, better hi-res ensemble agreement exists for
heavy rainfall in the next six hours. This may be favored with an
eastward propagating outflow boundary (passing BTR around 1530Z on
GOES-16 visible satellite) interacting with existing storms and
sea breeze boundaries near the Gulf of Mexico and Lake
Pontchartrain. Colliding boundaries would be expected to lead to a
greater concentration of convection, and thus a greater likelihood
of heavy rainfall. The 12Z HREF and 00Z HRRRE may be slightly too
far west with placement, however, given the notable eastward
progress of the outflow. HRRRE and HREF focus the highest
probabilities of 2+ inches of rainfall just north of Lake
Pontchartrain, but it is possible this could end up being
displaced further to the ENE.

Throughout the region, backbuilding Corfidi vector magnitudes
should be around 5 knots or less, suggesting the potential for
cell mergers and upshear storm growth to contribute to localized
very heavy rainfall. Numerous GPS-PW observations in the region,
and the 12Z soundings from LIX, TLH, LCH, and BMX, had PWAT values
around or above 2 inches. Combined with the expected slow storm
motions and moderate instability, this could yield rain rates to
around 3 in/hr. This may be sufficient to produce some flash
flooding by mid-late afternoon along the central Gulf Coast.

Lamers

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

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

LAT...LON   33278597 32168540 30648668 29948840 29559007
            29529158 30359229 31359159 32258874