Excessive Rainfall Discussion

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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1028 AM EDT Mon Jun 25 2018

Day 1
Valid 15Z Mon Jun 25 2018 - 12Z Tue Jun 26 2018

1500 UTC update

...Central to eastern Kentucky...
The only change of note made to the initial excessive rainfall
outlook was to add a slight risk area over portions of eastern KY
and expand slightly northward the marginal risk area into the
upper OH river region.  These changes were for convective
potential ahead of the vort initially over the lower OH Valley
region that is forecast to push eastward into the upper OH river
region by this evening.  FFG values are relatively low across
portions of central to eastern KY ahead of this vort... and with
heavy rains potential likely late morning through this afternoon
ahead of this vort...runoff issues are possible. 



A closed low will slowly shift east across NE today. Likely to
have an expanding area of showers and embedded thunderstorms
around the center of this low, with cell mergers and locally heavy
rain rates likely. Instability will likely be a limiting factor,
but should see just enough to allow for some embedded heavier
convective cores within the broader area of more stratiform
rainfall. Portions of northeast NE, southeast SD, southwest MN and
western IA have seen well above normal rainfall of late, making
these areas more susceptible to additional flash flood issues.
Thus, while extreme rainfall totals are not anticipated today, a
prolonged steady rainfall with embedded heavier rates will still
likely pose at least some flash flood threat. The general
consensus amongst the 0z guidance is for localized rainfall totals
around 2" in three hours, with localized totals through the day 1
period of 3-4" within the Slight Risk area.

The Slight Risk was shifted southward with this update, with all
of the 0z global and high res models shifting in this direction.
Will maintain a broader Marginal Risk around the Slight. Sometimes
with closed lows such as this we see some slow moving heavy cells
develop on the periphery of the system where diurnal heating can
be maximized...and thus some chance we see some localized flash
flood issues extend extend further north in SD/MN...with the
Marginal covering this potential.

Will carry a Slight Risk from portions of northeast OK into
southern MO and far northern AR. This is to account for potential
convective development later this afternoon into tonight. The
closed low to the north will put this area in a corridor of very
strong upper level divergence. Also will have a slow moving cold
front draped across this area, with confluent flow increasing
towards evening as the nocturnal increase of the southerly low
level jet kicks in. This should support an expanding area of
convection along/near this front by this evening into the
overnight hours. Given the slow movement of the front and the
persistent 850 mb moisture transport into it, the setup does
appear to support some backbuilding and training. This scenario is
depicted well by most of the 0z guidance as well...with several of
the 0z HREF members depicting some 3"+ amounts across this
corridor. This area has been anomalously dry of late, resulting in
high FFG. However the training potential suggested by the
environmental setup, and the signal within the 0z HREF, suggests
that short term rates may still get high enough to cause some
flash flood concerns...thus a Slight Risk is warranted.

A tricky convective forecast across this area today. As of 08z
note one convective cluster moving into eastern KY, with another
developing over southern IL and western KY. The 0z HREF members
and recent HRRR/HRRRX runs tend to weaken this lead complex as it
runs east of the instability axis, and thus focus additional
development through the day along/ahead of the trailing complex.
However the lead cluster appears to be maintaining itself more
than the models would suggest. Still think it will tend to be on a
downward trend as it heads into the Appalachians, but it very well
may maintain a bit more than suggested by the guidance. This then
brings up the question of whether remnant cloud cover from this
system impacts instability ahead of the next wave. Either way
likely to have convection with locally heavy rain along this
corridor, just some uncertainty with the exact timing of heaviest
amounts, and whether or not an organized flash flood risk ends up

Will carry a Slight Risk across portions of southeast NC and
northeast SC. This is where the best chance of merging cells and
heavier basin averaged rainfall appears to be. As the MCV
approaches from the west, likely will see at least one organized
area of convection move east into this area. Also may see
diurnally driven storms fire ahead of this feature within a
corridor of low level convergence (possibly aided by sea breeze
boundaries as well). Thus it is the concern that this lead
convection ends up merging with a more organized area of
convection moving in from the west, that warrants a Slight Risk
issuance. More uncertainty upstream across western NC into
southwest VA. Considered a Slight Risk here as well, but enough
uncertainty with how things evolve instability wise, given the
multiple MCVs in play, to keep it at a Marginal Risk for now.
Think some flash flood risk certainly exists, but it may end up
more localized in nature across this area compared to further
southeast. Will need to closely monitor trends though, as may need
to upgrade the risk if enough destabilization appears imminent
resulting in more widespread merging of stronger cells.


Day 2
Valid 12Z Tue Jun 26 2018 - 12Z Wed Jun 27 2018


...Wisconsin/Illinois to Missouri and then Eastward into Ohio
The risk of heavy to excessive rainfall spreads eastward as a
closed mid-level low and associated surface low pressure and front
translates eastward. The biggest forecast problem continues to
revolve around where any on-going convection, remnant MCV(s) or
outflow boundaries will be at the beginning of the period...and
how any of those factors impact eventual threat of heavy rainfall.
While these concern are common in any Day 2 forecast, confidence
is simply too low to make many refinements on mesoscale details
given how much the models have been struggling to depict the
systems evolution. Maintained the broad Slight Risk across much of
the Midwest where low flash flood guidance is found.

A lingering front provides a focus for tropical moisture to
converge over the Carolinas and the possibility for coastal
cyclogenesis near the Southeast U.S. coast.  The model signal has
become more consistent over the possibility of cyclogenesis
along/near the coast which results in a period of heavy rainfall
through a combination of on-shore flow underneath increasingly
favorable dynamics aloft before the system heads out to sea.  The
ECMWF looked to be too strong with the system--something that was
true with the 24/12Z runs.  The Canadian also looked to be
over-developed.  But the signal is strong enough to warrant a
marginal risk at this time. 


Day 3
Valid 12Z Wed Jun 27 2018 - 12Z Thu Jun 28 2018


...Eastern Great Lakes Into New England...

A closed mid level system with a surface low and associated cold
front will be making its way slowly eastward out of the Eastern
Great Lakes at the beginning of Day 3.  The dynamical forcing will
still be decent early on with divergent/difluent flow east of the
mid level system and low level flow ahead of the surface low will
be drawing moisture in place from the southwest. However, the
mid/upper level system will be filling Wednesday night into
Thursday morning.  With precipitable water values still running at
or above 1.75 inches ahead of the system, the atmosphere should
still be capable of supporting areas of heavy rain into parts of
the Northeast U.S.. The question by that point will be whether the
upper support and low level forcing will be focused enough to
support widespread risk of excessive rainfall.  With some of the
global models showing rainfall amounts in the 1.5 inch to 2.0 inch
range, opted to place a Slight Risk.  Note the Slight Risk was
drawn a bit of the model QPF maxima where terrain gets to be a
concern despite fairly high flash flood guidance.

...Ohio Valley into the Appalachians...

A Slight Risk area for excessive rainfall was raised from portions
of the Ohio Valley eastward to the west face of the Appalachians
where Flash Flood Guidance was an inch (or less, in spots).  Deep
moisture should be available for storms to tap from Wednesday
afternoon into Wednesday night/early Thursday morning.  How much
mesoscale forcing will be available is in question given the fact
that the mid-level system over the Eastern Great Lakes Region will
be filling and what that implies to the low level wind flow over
this area.  However, consensus values for QPF from the models
approaches the Flash Flood Guidance values apparently in response
to a combination of deep moisture and low level upslope which will
enhance lift in the low levels.


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

Last Updated: 1028 AM EDT MON JUN 25 2018