Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Wed Mar 28 2018


Prelim Day 1 QPF Discussion
Valid Mar 29/0000 UTC thru Mar 30/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Central and Eastern U.S...

Convection continues to expand this afternoon across much of
eastern TX. This activity is forming along and just north of a
well defined outflow boundary and just ahead of an advancing cold
front. The cold front should help push this outflow boundary
slowly southward with time into this evening...before eventually
stalling through tonight. This axis should be the focus for
continued convective development through the overnight period.
CAPE should continue increasing to the south of this
boundary...thus serving as a source of instability being advected
into the boundary. Thus seems likely that convection will be able
to maintain intensity on the southern edge of the merged
boundary...with multiple rounds of storms through the night. And
with limited boundary movement and storm motions exhibiting a
large component parallel to the boundary...training is likely. A
flash flood risk exists through the day in the vicinity of this
boundary from southeast TX into central/northern LA and MS.
Anticipate convection will even increase in intensity and
organization some tonight as the better height falls move in from
the west indicative of stronger synoptic support. This will help
eventually push the convection more progressively off to the
east...but not before already saturating areas receive additional
heavy rainfall. Thus the flash flood risk overnight may be even
higher along a similar axis.

Some uncertainty on exactly where the boundary ends up
anchoring...and thus where the axis of most intense rainfall sets
up. The 12z ARW and ARW2 are further south...with the NMMB and NAM
NEST further north. Usually would prefer the ARWs in setups like
this. However based on recent radar/observational trends...it
appears like they are a tad too progressive in bringing the front
towards the coast. Thus for now prefer to just go with a consensus
of these 12z high res runs. Also, recent runs of the HRRR appear
to be handling the evolution fairly well...so QPF from the 16z
HRRR was incorporated into the WPC QPF. All the high res CAMs are
in agreement with widespread 3"+ amounts...with a corridor of 4-7"
(locally even higher). Based on the set up...think these are
probably realistic depictions. Thus would appear the threat of
significant flash flooding is increasing across portions of
southeast TX into central/northern LA and possibly even into MS.
Not confident enough in location/magnitude/coverage to introduce a
HIGH risk...but this event seems to solidly fit into the MODERATE
category...and will shift the moderate risk a bit southwest to
include more of southeast TX to the Gulf Coast. Also trimmed back
the northern extent of the moderate risk over AR and northern MS
where rates are not expected to be as heavy.

All indications are that the convection will become more
progressive by 12z Thursday as the shortwave ejects eastward
giving the cold front a push. Thus the flash flood threat should
be on the decrease through the day Thursday...although locally
heavy rainfall will still be likely along the Gulf Coast from
southern LA into southern MS/AL. Convection should be intense
enough over these areas to produce locally heavy rates...but the
progressive nature of storms should limit any flash flood threat
to only the more susceptible locations. Will probably end up with
a split in the QPF distribution during the day Thursday. One max
towards the Gulf Coast along the front where instability is
greatest...and another across the OH Valley closer to the stronger
mid/upper level forcing. In between should end up with a relative
minimum...with these areas split in between the better forcing to
the north and instability to the south. WPC QPF during this period
relied on a a global and high res model blend to handle mesoscale
differences...with generally good agreement seen on the large
scale.

Chenard