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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2209Z Jun 23, 2019)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
609 PM EDT Sun Jun 23 2019

Day 1
Valid 2141Z Sun Jun 23 2019 - 12Z Mon Jun 24 2019

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST
OKLAHOMA, NORTHEAST TEXAS, CENTRAL TO SOUTHERN ARKANSAS & NORTHERN
LOUISIANA...

Southern Plains/Lower Mississippi Valley...
The most significant flash flood threat exists across southeast
OK, central and southern AR, and northern TX.  A pair of
independent thunderstorm areas across AR and southern OK/northwest
TX are attempting to bridge across southeast OK at the present
time within an area of splayed out 1000-500 hPa thickness and
precipitable water values near 2".  Thunderstorm outflow aloft is
being enhanced by a mid- to upper-level low near the NE/CO/KS
border junction.  ML CAPE values/instability upstream is
significant at this time -- 2500-5000 J/kg.  Inflow at 850 hPa is
25-35 knots per VAD wind profiles, and effective bulk shear is of
similar magnitude.  While radar imagery appears to show a training
band, some of the activity remains pulse and it not moving along
the band quite yet.  This is expected to change as a cold pool
broadens under the convection across northwest TX and
south-central OK and strengthens the inflow/effective bulk shear
at the storm scale into southeast OK which should cause the
convection to become better organized and train within the next
couple of hours.  Additional mesocyclone formation is possible
within this environment.  An area with well above normal
precipitation across northeast TX also lies within an area of
somewhat depressed instability within a moisture streamer which
should allow incoming convection from the west and northwest to
tend to accelerate -- hence left the risk level Moderate which was
coordinated with the Dallas/Fort Worth TX and Shreveport LA
forecast offices.  Activity within the streamer itself should grow
upscale and perhaps merge with activity along the slowly sinking
outflow boundary in AR, which would enhance hourly/storm total
rainfall across northern LA and southern AR with time.

The high-res CAMs show local maxima of 3-8", with the mesoscale
guidance initially showing a southern bias much like yesterday and
last night.  However, this bias should not persist much longer as
the convection accelerates into an area with a weakening mid-level
capping inversion caused by the progressive upper low to the
northwest.  Initially, cell training, mesocyclones, and cell
mergers are considered the main reasons for very heavy rainfall. 
As the convective systems move east to southeast as wavy lines of
thunderstorms, embedded mesocyclones are expected to play a more
significant role in enhancing rainfall totals this evening and
tonight.  The overall excessive rainfall/flash flood risk is
expected to fade due to increasing movement of thunderstorm areas
after 1 am CDT/0600 UTC.


Southeast GA/Northeast FL...
Outflow boundary mergers within a moist air mass with adequate
instability have led to hourly rain totals of 2-3" per radar
estimates, with the overall cluster sinking south-southeast. 
Believe issues here will fade early tonight as the complex
attempts to move offshore the Savannah and St. John's river deltas.


Central Plains/Midwest/Great Lakes...
Showers and thunderstorms have been spurred by a nearby upper
level low near the NE/CO/KS border junction a frontal zone farther
to the east.  While moisture availability and instability are
sufficient for heavy rainfall, the overall system is progressive
which has led to the depiction of a marginal risk of excessive
rainfall across much of the area.  The exception is across
IL/IN/western KY where low-level inflow will be harder to dislodge
-- a slight risk of excessive rainfall remains in this area which
has been left sensitive due to recent heavy rainfall across
southern IL, southern IN, and KY.


Roth






Day 2
Valid 12Z Mon Jun 24 2019 - 12Z Tue Jun 25 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE GULF COAST AND PORTIONS OF THE GREAT LAKES INTO THE OH
VALLEY/CENTRAL APPALACHIANS/NORTHERN MID ATLANTIC...

...Texas/Louisiana...
20z update...
No significant changes were made here for Day 2. The main forecast
problem remains just how impactful the remnants of the weakening
MCS is on the mid and upper TX coast, mainly before 25/00z. Much
of the high resolution guidance shows some high hourly rainfall
rates (near 1.50 inches) with the remains of the system as it
becomes outflow dominated and cells ride along the outflow to the
coast between 24/12z and 24/18z. Three hour flash flood guidance
values are fairly high (generally above 3.00 inches), as the area
near the TX coast has been fairly dry over the past seven days.

Based on the above, the Marginal Risk was left in place across TX
into LA. If high resolution guidance shows a better signal for
organized heavy rainfall (especially near the Houston metro area),
a Slight Risk could be needed here in later forecasts.

Hayes

Previous discussion...
A decaying MCS dropping south into portions of east-central TX and
AR will be the boundary/front by which convection will likely
initiate later in the afternoon/evening. Expect precipitation from
the convective complex that moved through portions of northeast
TX, southwest AR into norther LA Sunday night to diminish quickly
during the morning hours on Monday. The atmospheric instability
will try to rebound through the morning/early afternoon hours
allowing convection to likely form along the residual outflow
boundary later Monday.  Residual moisture/instability will pool
along and just south of this boundary which will likely become
draped along or just inland of the TX coast extending across the
Lower MS Valley.  It should be noted there is still quite a bit of
uncertainty where this outflow boundary will reside as it is
heavily influenced on convection and its overall propagation on
Sunday (Day 1). 

Surface base CAPE values will try to climb above 3000 J/kg behind
the decayed MCS by the afternoon despite the expected debris
clouds.  Though moisture transport is not as impressive by Day 2,
precipitable water of 2+ inches aided by 25-35 knot low level
southwesterly flow indicates there is still plenty of moisture to
work with; likely some of which is withstanding from the previous
MCS.  As the mid-level trough pivots through the MS Valley through
the day, there will be pockets of vorticity moving atop the
surface boundary.  While the synoptic scale forcing will diminish
through the overnight, the right entrance region of the jet and
thus divergence aloft may promote additional forcing for ascent
through the afternoon/evening hours across south/east TX into LA. 
This should be enough to assist with convective development
resulting in areal average precipitation 0.5-1.5 inches with
locally higher amounts associated with discrete storms. 

Given fairly high FFG across this region, felt a Marginal Risk of
flash flooding would suffice at this point.  However, a lot is to
be determined with respect to the convection propagation/MCS
activity expected on Day 1 that will heavily influence this
forecast period.  The outflow boundaries will play a role in
frontal placement and where heavy rain will fall, thus influencing
the antecedent conditions/FFG amounts.  Another area of concern is
Houston- if the aforementioned boundary sets up near Houston this
may promote convective development with seemingly slow cell
motion.  However, given uncertainty continues regarding the
outflow boundary, do not feel confidence is high enough to issue a
Slight Risk for this region at this forecast package. 

Pagano


...Great Lakes/OH Valley/Northern Mid Atlantic/Central
Appalachians...
20z update...
There are three areas of interest across the region during Day 2,
but at this point there was not enough confidence to upgrade any
of these areas to a Slight Risk.

The first is across WI. As the surface low pulls into the Upper
Great Lakes and Ontario during Day 2, there should be enough
moisture and instability to support convection ahead of the front
into WI. Some of the more aggressive high resolution model (such
as the 12z NAM CONUS Nest) indicated the potential for hourly
rainfall rates to touch 1.50 inches over central WI before 25/00z.
However, convection is expected to remain progressive (for the
most part), with rainfall amounts topping out near 1.50 inches or
so. Three hour flash flood guidance values are fairly high, as the
area has been dry for the past seven days. Based on the above, WI
was left into the Marginal Risk for Day 2.

The second area is western PA. A low level southwest flow
transports high precipitable water air (generally between two and
three standard deviations above the mean) ahead of a frontal
boundary crossing PA during the evening and early overnight hours.
In this airmass, hourly rainfall amounts could approach 1.50
inches over western PA in the evening hours. This is close to the
lower end of the three hour flash flood guidance, where the
National Water Model showed an expanding area of high stream
flows. The activity is expected to remain progressive, as the mid
level flow remains orthogonal to the storms. After collaborating
with WFOs PBZ/CTP, the Marginal Risk was left in place here, but
if there is a better heavy rain signal in the high resolution
guidance, a small Slight Risk could be needed here in later
forecasts.

The third area of interest is eastern PA and nearby NJ. Ahead of
the front between 25/06z and 25/12z, the high precipitable water
airmass supports mostly elevated convection in this time frame.
The high moisture content of the airmass could support hourly
rainfall rates near 1.50 inches across eastern PA in areas that
received heavy rainfall late last week. As is the case across
western PA, the progressive nature of convection should preclude a
larger flash flood threat, but the Marginal Risk was extended over
these areas to cover a low end flash flood threat toward the end
of Day 2.

Hayes

Previous Discussion...
A deepening surface low associated with the aforementioned trough
will track northeast across the Great Lakes on Monday. 
Precipitable waters of nearly 2 inches (which is 2 standard
deviations above the mean) will advect well ahead of the apparent
low aided by southwesterly low level winds. This combined with
instability (surface base CAPE above 2000 J/kg east of the
trailing front) and strong dynamics (divergence and mid-level
energy) will result in heavy rain makers along and ahead of the
front moving through WV, eastern PA into southwest NY.  Heavy rain
(and potential training) associated with the surface low (as noted
by higher instability from steeper lapse rates) is expected across
portions of northern IL into WI and MI.

With pockets of strong instability taping into the high
precipitable water across portions of OH/WV into eastern PA,
expect efficient rain rates to ensue.  This is noted by hourly
rain rates approaching 1 inches as convection quickly shifts east
ahead of the cold front. Given the depth of moisture and
instability in place, it is possible storms rolling down the front
could train, increasing the flash flood threat. Three hour flash
flood guidance values are as low as 1.00/1.50 inches over
southeast PA and southwest NJ, which saw 6.00+ inches of rainfall
just a few days ago. Should models indicate beefier rainfall
amounts across this area, a Slight Risk could be needed here in
later forecasts.

Pagano


Day 3
Valid 12Z Tue Jun 25 2019 - 12Z Wed Jun 26 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL PLAINS INTO THE MID MISSISSIPPI VALLEY...

...Central Plains into the Mid Mississippi Valley...
Increasing moisture and moderate to strong instability in a low
level west southwest flow could support convective clusters (or
perhaps a small MCS) crossing the Central Plains into the Mid MS
Valley during the last part of Day 3. Most of the 12z guidance
showed a weak quasi stationary boundary extending from eastern NE
across northern MO/southern IA into central IL after 26/00z. A low
level west southwest flow pools 1.50 inch precipitable water air
along the boundary, where most models show an axis of 2000-3000
J/KG of SBCAPE (which becomes more elevated with time after
26/00z).

The convective mode is expected to be cluster based, with the 12z
NAM (which is the most aggressive model with the convective
development) showing the potential for a small MCS to form (which
is depicted clearly in its simulated IR images). Other model
solutions hint at the potential for more organized convection
along the boundary, which could train  as the mid level flow
becomes better aligned with the movement of the cells/clusters.
The best chance for training should occur in the 26/00z to 26/06z
time frame, as the convection could become more outflow dominated
after that time.

The National Water Model showed some streamflow response with the
convection (though the 12z GFS is not as bullish with the
convection), and heavy rainfall during Day 1 has dropped three
hour flash flood guidance values below an inch across portions of
northern MO. While there should be some recovery with respect to
antecedent conditions here before Day 3, the area could still be
vulnerable by the time convection affects it. Because of this, a
Marginal Risk was stretched across portions of southeast
NE/northeast KS into northern MO and far southern IA.

Hayes


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