Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1200 PM EDT Thu Apr 25 2019

Day 1
Valid 16Z Thu Apr 25 2019 - 12Z Fri Apr 26 2019

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE
OHIO VALLEY AND CENTRAL GULF COAST...

...16Z update...

...Central Gulf coast/Deep South...
Some weakening of the main convective line propagating through
LA/MS has been observed since 12Z with warming cloud tops on GOES
16 infrared imagery and a gradual reduction in the coverage of
0.5+ in/hr rates. The line has remained progressive overall but
where bowing segments have aligned with the 850-300 mb mean wind,
training and localized flash flooding have occurred. The heaviest
instantaneous rainfall rates have remained closer to the Gulf
Coast where MLCAPE has been closer to 1000 J/kg (per LCH/LIX RAOB
and the SPC mesoanalysis page), with these values of CAPE dropping
off with northward extent. While MLCAPE is expected to rise during
the day, the greater and more significant values are expected to
remain within a couple hundred miles of the Gulf Coast later today.

The mid-upper level trough axis in place over the ArkLaTex will
continue to advance east through 00Z acquiring a neutral and then
negative tilt across MS and AL/GA which will continue to allow for
a very diffluent flow pattern aloft across the Gulf Coast states,
but also should tend to keep convection propagating toward the
east and this is reflected in Corfidi vector motions of 20-30 kt
toward the east. The concern for flash flooding remains with any
convection out ahead of the main line with brief training or
repeating cells as the subsequent line follows, through the
afternoon with the threat likely lowering into the evening.

...Ohio Valley...
Satellite imagery shows a well developed comma pattern over the
Ozarks/Lower Mississippi Valley with a dry slot punching into AR
and southeastern MO. Some clearing of cloud cover is expected
across the confluence of the Ohio and Mississippi Rivers as the
dry slot moves in. This should allow MLCAPE into the 500-1500 J/kg
range per modification of KILN 12Z sounding and RAP forecasts
across portions of the Ohio Valley. Just to the east of an
advancing surface low located near the MO/AR border at 15Z,
forecast Corfidi vectors are less than 10 kt which may allow for
brief stalling of any small storm clusters that attempt to
organize.

The main concern for flash flooding across the Ohio Valley is the
wet antecedent conditions with AHPS showing over 300 percent of
normal rainfall over the past week for northwestern KY into
southern IN and southwest OH. Generally 1-2+ inches of rainfall is
expected here through about 06Z.

Otto

...previous discussion follows...

...Central Gulf coast/Deep South...
There was agreement that Gulf moisture coming northward to the
southeast of an MCV moving across central AR at 08z, leading to
heavy rains with the greatest amounts across portions of the
central Gulf coast/Deep South.  The 00z GFS shows 700 hPa
temperatures (6C) in decline from here on out, with the effective
capping inversion sinking south to at least I-10 with time. 
There's indication that the cap edge might be close to the Gulf
coast itself in recent radar imagery from southeast TX.  The
guidance shows a significant amount of spread, which is not
particularly uncommon with precipitation within the warm sector
during the warm season.  Precipitable water values (PWs) feeding
northward into the system are in excess of 1.75 inches along the
Gulf Coast.  Portions of the South have been abnormally wet over
the past 2 weeks, with pockets of 200+ percent of average noted. 
Due to the uncertainty, lowered amounts in the QPF somewhat and
attempted to keep some degree of continuity with the risk areas
inherited, other than to shift them somewhat north and east to
account for radar/model trends.  Local amounts of 3-5" are
possible here, with as much as 2" in an hour anticipated given the
moisture and instability available.  The heaviest rain could occur
due to cell training, cell mergers, or mesocyclone formation. 
Leaned modestly towards the 00z HREF mean and 00z/06z high
resolution NAM in this area, as the mesoscale guidance has the
best chance of verifying in this sort of regime.

...Ohio Valley...
There was a pronounced northward shift in the QPF, apparently due
to an MCV moving through central AR at 08z Thursday.  Both the 00z
GFS and 00z ECMWF made the largest shifts to the north.  This is
occurring despite mid-level height falling region wide.  The
guidance does suggest that a northern stream shortwave is expected
to merge/phase with the MCV over time which maintains some
amplification to the mid-level flow pattern.  Instability is
expected to ramp up to 1000-2000 J/kg, which when combined with
PWs rising just above 1.5" could lead to hourly rain totals to
1.75" where either cell training or mesocyclones develop. Recent
RAP runs and the 00z GFS show the best moisture to be present near
or just north of the Ohio river.  At least two rounds of
precipitation are expected -- one with the outflow boundary near
the Ohio river and another with a front moving in from the
northwest.  Local amounts of 2-3" are possible here.   The fairly
broad range in the guidance has not been helpful -- thought the
00z Canadian Regional, 00/06z high resolution NAM, and 12z ECMWF
QPF looked the most reasonable when considering 00z GFS boundary
layer moisture convergence field which also allowed some degree of
continuity to be maintained.  However, did broaden the marginal
risk area northward to account for the 00z GFS/00z ECMWF QPF
solutions as a precaution.  A new slight risk area was added near
and north of the Ohio River where two week precipitation anomalies
are 200%+ of average and a recent batch of rain fell Wednesday and
Wednesday night.  This was coordinated with the Paducah KY,
Indianapolis IN, Louisville KY, and Wilmington OH Weather Forecast
Offices.

Roth


Day 2
Valid 12Z Fri Apr 26 2019 - 12Z Sat Apr 27 2019

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FROM
APPALACHIAN REGION TO THE NORTHEAST...

Showers and thunderstorms will be moving through the
Ohio/Tennessee Valleys and into the Appalachian region, northern
mid-Atlantic and the Northeast as a surface cyclone lifts into the
Great Lakes/Northeast. The deep moisture associated with the
recent heavy rain over the Gulf Coast and Lower Mississippi Valley
will continue to stream northward feeding into the leading edge of
the system, spreading rain from the southern Appalachian chain
northward into northern New England. The convection will mainly be
in ahead of, and within the proximity to the trailing cold front
and the lifting warm front; tracking over regions were 1-hr/3-hr
FFG is 0.50 to 1.00 inch in the mountains and 1.5 to 3 elsewhere.
Guidance continues to show a signal that this sector could receive
1 or more inches of rain, which would meet or exceed local FFG. A
Marginal Risk for excessive rainfall with an increased threat for
flash flooding was maintained but trimmed the southern boundary
back to southern West Virginia.

Campbell

Day 3
Valid 12Z Sat Apr 27 2019 - 12Z Sun Apr 28 2019

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Campbell


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt