Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
254 PM EDT Sat May 05 2018


Prelim Day 1 QPF Discussion
Valid May 06/0000 UTC thru May 07/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...TN/OH Valley into the Central Appalachians...

Showers and embedded thunderstorms continue to increase in
coverage from eastern KY into the southern half of WV and western
VA. Will be a rather prolonged period of favorable upper dynamics
(both left exit and right entrance regions of upper jet) for
rainfall over this area, resulting in the possibility of multiple
rounds of locally heavy rains. 850 mb moisture transport, while
not extreme, is forecast to gradually increase over this area
through the day out ahead of a surface low currently over TN. This
should aid in the development over showers/storms near and just
north of the front draped across the area. Main limiting factor
will probably end up being the degree of destabilization that is
able to take place. Note a decent cloud shield over much of the
area, which may tend to dampen instability and cap rainfall rates.
Also working against a more organized heavy rainfall/flash
flooding threat is an initial state of generally below normal
stream flows, and while above normal, PWATs are not all that
extreme. With that said, at least a localized flash flood risk
exists into tonight across much of KT into the southern half of WV
and into western VA.

If convection this afternoon/evening is able to tap into some
slightly better instability and become more intense, then it is
possible that a bit more organized heavy rainfall/flash flooding
risk could develop over portions of far eastern KY into southern
WV and western VA. Noting an increase in activity over this area
over the past few hours, although nothing too intense or organized
as of 18z. Some breaks in the clouds ahead of the surface low is
resulting in some uptick in instability. The question becomes how
organized/intense does convection become from eastern KY into
WV/VA through the afternoon. Am growing concerned that portions of
southern WV and far western VA could see multiple rounds of
convection, with some training. The first round ongoing now, with
additional storms firing ahead of the low as 850 mb moisture
transport increases into the evening. This could result in some
3"+ rainfall totals and an increasing flash flood threat with
time. Will need to continue to monitor trends.

In general, should see a weakening of storms and rates by later
tonight into Sunday as rainfall moves out of the instability pool
and into the Mid Atlantic and portions of the northeast. Will
however continue to see showers underneath the main shortwave as
it tracks across WV. By afternoon a signal for some decent storms
to fire ahead of this shortwave across northeast NC into southeast
VA. These storms could be locally intense and produce briefly
heavy rains, but should generally be progressive in nature.

Overall model clustering was pretty good with regards to QPF
across this region. Thus WPC took a blend of the 12z HREF members,
with some incorporation of the ECMWF/UKMET towards the end of day
1. Recent HRRR runs were also incorporated.


...Coastal NC---Outer Banks...

The anomalous PWAT values off the coast will be drawn northward
ahead of the above mentioned upper trough pushing into the eastern
U.S.  These anomalous PWAT values will impact coastal areas from
the upper coast of SC, northeastward into eastern and coastal NC
Saturday night into early Sunday. Heavy rains are possible in a
region of enhanced upper difluence in the right entrance region of
an upper jet over the Mid-Atlantic. There continues to be some
spread with this rainfall axis. It looks like there will be some
impressive rainfall totals focusing along a coastal front, with
12z HREF probabilities of localized 5"+ amounts quite high.
Fortunately at this time it looks like the coastal boundary, and
thus heaviest rains, will remain just offshore. However, will need
to closely monitor trends into tonight. Either way, some moderate
to locally heavy rains should skirt the coast, even if the most
intense activity were to remain offshore. Generally followed a
blend of the 12z HREF members for QPF here.


...Central to South Florida...

Higher PWAT values will remain over FL into Sunday. Also,
anticipate some instability to develop both this afternoon and
Sunday afternoon. The approaching trough and jet to the north will
produce a slightly more favorable environment for convection with
time as well. Thus not surprising the 12z HREF shows storms both
afternoons, with slightly more widespread storms on Sunday with
the passage of the forcing and front.

Chenard