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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1531Z Jun 23, 2019)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1131 AM EDT Sun Jun 23 2019

Day 1
Valid 16Z Sun Jun 23 2019 - 12Z Mon Jun 24 2019

1600 UTC update

The 1200 UTC hi res guidance is showing very good continuity with
the 0000 UTC run with the organized convection forecast to push
southeast from the Southern Plains into the Lower Mississippi
Valley.  Changes to the previous Excessive Rainfall Outlook were
to trim the northwest portion of the risk areas from eastern
KS...southwest MO into central OK based on latest radar trends
showing precip ending across these areas.  The moderate risk area
was extended slightly southeastward into southern AR and far
northern LA to cover very heavy rainfall potential in the 0000 to
0900 utc time frame across these areas. 

Across the Upper TN Valley/Southern Appalachians...the marginal
risk area was extended southeastward across this area to cover
locally heavy rains late morning into early afternoon moving
across this region.

Oravec



...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS
SOUTHEASTERN PORTIONS OF OKLAHOMA, NORTHEAST TEXAS, WESTERN 
ARKANSAS INTO FAR NORTHERN LOUISIANA...

Changes from yesterday's day 2 ERO: Expanded the Moderate Risk
area on the western/northern/eastern flanks based on the current
convective/mesoanalysis trends early this morning, along with the
growing consensus of current high-res CAM guidance (including 06Z
HRRR).

Early this morning, 3 separate areas of concern were noted across
the central U.S., with merging convective systems likely to pose a
more significant flash flood threat across eastern OK, western AR,
and possibly North TX after 12Z this morning. One MCS was over
eastern KS-northern MO was dropping an outflow boundary southward
toward the convection across eastern OK-western AR that is already
growing upscale. The outflow boundary is helping to fortify a more
sloped low-level frontal zone, thus enhancing the moisture
convergence/transport within the 925-850 mb layer where the
south-southwest flow is between 40-50 kts. Meanwhile, a 3rd, also
compact MCS was affecting western OK and western-central KS --
benefiting from stronger synoptic scale forcing closer to the
upper trough axis (stronger upper level difluence with jet-streak
enhanced upper divergence).

Later this morning, as the upper trough shifts slowly east into
the High Plains, widespread convection will regenerate across
areas already getting hit fairly hard during the pre-dawn hours --
particularly over eastern OK-western AR, southeastern KS, and
southwestern MO. Given the antecedent thermodynamic environment
(MUCAPEs 2000-3000+ j/kg and PWs of 1.75-2.00 inches), rainfall
rates over 2" are anticipated underneath the heaviest cores. The
high-res CAMs show max totals between 3-5+ inches, though outside
of the recent HRRR runs, the 00Z CAM consensus may be a bit too
far south, i.e. not as heavy with the QPF into southeast KS and
southwest MO. The 06Z and 07Z HRRRs in fact noted pockets of 7+
inches within 3-6 hours across a small portion of northeast OK
into northwest AR. Helping to foster the excessive rainfall
potential in this region (northern portion of the day 1 Moderate
Risk area) is the low-level jet aligned nearly parallel and of
similar magnitude of the mean 850-300 mb flow. This will result in
very weak Corfidi vectors, implying an elevated risk of cell
training.

Farther south -- across the Red River into North TX, the Moderate
Risk was maintained from yesterday's day 2 ERO, with the
expectation that as the upper trough continues to trudge eastward
across the Plains, convection will continue to fire along the axis
of upper divergence, while also propagating southward toward the
strongest deep-layer instability over eastern TX-western Gulf
Coast. As the pre-frontal boundary (essentially "effective front"
for convective initiation) slows down and flattens Sun night,
expect an increased risk of w-e cell training. This will enhance
the potential for more numerous flash floods Sunday night over the
southern portion of the Moderate Risk area, as the strong
deep-layer instability and high PW pre-convective environment will
foster areas of 2-3"/hr rainfall rates, i.e. similar to what is
occurring over the MO-KS-AR-OK region Sun morning.

Hurley







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...

...Texas/Louisianan...

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. 


...Great Lakes/OH Valley/Central Appalachians...

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.

Hayes/Pagano


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

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Pagano


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