Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
314 PM EDT Sat May 19 2018


Prelim Day 1 QPF Discussion
Valid May 20/0000 UTC thru May 21/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1...


...Eastern Carolinas...

Will need to continue to monitor portions of eastern SC and NC
through tonight for the potential of an axis of heavier rainfall.
An axis of near 2" PWATs is expected to focus across this area for
an extended period of time, with at least weakly convergent flow
in the lower levels. Also appears like one or more weak shortwaves
will push north in the southerly flow, locally enhancing lift and
low level moisture transport. Not expecting convection to be too
organized, and instability is not all that great. However, the
persistent convergence axis, higher PWATs and mean flow does
support the potential for a period of south to north moving repeat
cells capable of producing briefly heavy rates. The 12z HREF shows
moderate to high probabilities of exceeding 3" across portions of
the area, especially into the evening, and so think the setup
suggests at least a localized flash flood threat. Will maintain
continuity and have a Slight risk to account for this risk.


...Florida...

Another wet forecast across Florida as well. PWATs and 850 mb
moisture transport should both be higher tonight into Sun than
previous days, potentially resulting in a wetter overall period.
Activity should initially focus along the west coast sea breeze
boundary, before potentially shifting to the east coast overnight.
The potential is certainly there for these heavier totals given
the increased southeasterly flow, strong convergence signature and
near climatological record PWATs. Appears like the high res models
are hinting at a subtle wave riding north, interacting with these
favorable parameters and producing a period of south to north
training. Tough to say for sure if this will materialize, but
certainly something to watch, as given wet antecedent conditions,
a greater flood threat could exist. With the above environmental
parameters in place, 0z HREF probabilities of locally exceeding 5"
in the moderate to high category, and wet antecedent conditions,
will go with a Slight risk across the southeast Florida coast.
Lower confidence with this risk area, but the conditional risk
appears high enough to warrant it at this time and will monitor
trends through the evening/overnight.


...Southern-Central Plains into the Mid MS Valley...

A cold front extends from the Southern Plains into the Mid and
Upper MS Valley, with a dryline across western TX. The mid/upper
levels will remain broadly divergent and generally uncapped -- as
the bulk of the vort energy early traverses the central Plains and
eventually the mid MS Valley. Vigorous deep-layer instability
(surface-based CAPES 2000-3000+ j/kg), buoyed by steep mid-level
lapse rates (7.5-8.0 c/km between 700-500 mb) along/ahead of the
front and dry line across north and west TX will spawn new
convection by late afternoon, with the activity expected to grow
upscale later in the evening along an axis of increased 850 mb
moisture transport as the southerly low-level inflow increases to
40-45 kts. Given the degree of directional shear progged, per the
high-res CAM simulated reflectivity expect the segments to be more
progressive/forward propagating than otherwise. Still, there
continues to be a multi-model signal for hefty areal-average
totals across portions of north TX across the Red River into
southwest OK -- maintaining the higher QPF confidence over this
region. The WPCQPF comprised a blend of the previous forecast
(continuity) with the HREF probability-matched mean, NBM, and
WRF-ARW -- which noted a slight timing difference (a little more
progressive) from the previous WPC forecast.

Farther north -- another concentrated area of heavier rainfall is
still anticipated across the lower MO into the mid MS Valley,
based on the additional destabilization later this afternoon
underneath a thinning canopy of last night's mid/upper level
convective debris, while also ahead of the main vort lobe that
across KS-NE around midday. Areal-average totals up to around one
inch are anticipated given the multi-model consensus; however, per
the high-res CAMs, localized amounts of 3-5+ inches will be
possible within the axis of anomalous PW (deep TROWAL zone). This
especially given the relative weak downwind propagation given the
low-mid layer wind fields. 

Hurley/Chenard