Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
911 PM EDT Wed Apr 24 2019

Day 1
Valid 01Z Thu Apr 25 2019 - 12Z Thu Apr 25 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
EASTERN TEXAS AND FAR SOUTHEAST OKLAHOMA, THROUGH THE ARKLATX, AND
INTO WESTERN-CENTRAL ARKANSAS...

...0100 UTC update...

...Central to Northeast Texas into Southwest Louisiana...
Continued to trim back the western edges of the Slight and
Marginal risk areas across central TX, based on the latest
observational trends as the mid level drying and associated
negative PW advection ahead of the mid level trough pushes
eastward. The latest HREF neighborhood 1/3/5 hourly QPF exceedance
probabilities show lowering probs of >2-3" within 3 hours after
03Z -- due largely to the forward propagation (low-mid layer
directional shear) along with modest deep layer instability
overnight. Back-building, compact warm conveyor belt (WCB) and
sharpening deformation axis toward the mid level trough will
nevertheless maintain an enhanced threat of training convection
within the Slight risk area -- despite the forward propagation of
individual line segments.

Hurley

...previous discussion follows...

...Central to Northeast Texas into Southwest Louisiana...
Another day of active convection likely across central to
northeast TX...into the Lower Mississippi Valley region...along
and ahead of the cold front expected to push eastward across these
regions.  There are no significant changes expected day 1 to the
recent overall favorable pattern for active convection in the
vicinity of this front.  The upstream closed low moving across
northern Mexico into southwest Texas will maintain a very difluent
upper flow pattern ahead of it across much of Texas into the Lower
Mississippi Valley in an axis of pw values 1.5+ standard
deviations above the mean.  There are some qpf axes differences
among the latest suite of guidance...with WPC favoring the more
southern max solution given the gradually lowering mid to upper
level heights which should support the next round of convection
taking a more southern track across Central to Northeast Texas. 
While some of the shorter term qpf details are lower
confidence...the latest WPC qpf axis for day 1 is closer to the
solution offered by the HREF mean.  While simulated radar
reflectivity from the hi res guidance are often very
poor...typically being underdone versus reality...the expected
movement is often better.  The latest simulated radars from the hi
res guidance suggests the slowest moving convection possible in
the 1800 to 0000 utc time period over central Texas...with
increasing progression to the east after this during the overnight
hours across northeast Texas into Louisiana.  Much of the latest
qpf guidance reflects this with generally lesser amounts across
northeast TX into southwest LA...compared to central TX.    At the
moment...one broad slight risk was denoted given uncertainties
with convective details.

...Lower Arkansas into the Lower Ohio Valleys...
While we favored an overall farther south max in qpf for the day 1
period...moderate to locally heavy rains still likely to extend
northeastward into the Lower Arkansas and Lower Ohio Valley region
as above average pw values stream northeastward this period across
these areas.  With stream flows generally above normal across
these regions from heavy precip over the past few weeks...isolated
runoff issues possible...with a marginal risk area maintained.

Oravec


Day 2
Valid 12Z Thu Apr 25 2019 - 12Z Fri Apr 26 2019

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM SOUTHWESTERN
LOUISIANA INTO SOUTHERN ALABAMA AND THE FLORIDA PANHANDLE...


The models were struggling with the degree of interaction between
a southern stream shortwave will be transitioning from a close
mid-level low into an open wave as it moves out of Texas.  While
this does impact the QPF a bit, there was a agreement in the
models that Gulf moisture coming northward would be tapped by the
mid/upper level dynamics, leading to some 1.5 to 3 inch rainfall
amounts on Day 2...with greatest amounts closest to the coast. 
Precipitable water values feeding northward into the system are in
excess of 1.75 inches are forecast along the Gulf Coast with
values near 1.5 inches farther north toward Tennessee. The region
has been abnormally wet over the past 2 weeks, with pockets of
200+ percent of average rainfall extending from the Lower
Mississippi Valley to the Alabama/Georgia line.  Because of that,
saw little reason to make many changes to the on-going Slight Risk
area.  The Marginal Risk area was extended a bit more to the
northeast along and ahead of trough in the northern stream.

Bann


Day 3
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...

...2030Z Update...
Only several minor changes were needed to the on-going Marginal
Risk area.

...Previous Discussion...
The deep moisture associated with the heavy rain over the Gulf
Coast on Days 1-2 will continue to stream northward as the leading
surface cyclone tracks north and east from  the Tennessee/Ohio
Valley to the Northeast. Organized convection is expected to
develop along and east of the trailing cold front, spreading rain
from the southern Appalachian chain northward into northern New
England. FFG is fairly low for much of these regions with 3-hr
guidance as low as 1 to 2 in the Mountains and 0.75 ti 1.50 inches
in the Northeast. There is a multiple model signal suggesting this
sector could receive more than 1 inch, which would meet or exceed
local FFG. A Marginal Risk for excessive rainfall with an
increased threat for flash flooding was introduced from western
North Carolina to central/Downeast Maine.

Campbell/Bann



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