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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0749Z Jun 23, 2019)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
349 AM EDT Sun Jun 23 2019

Day 1
Valid 01Z Sun Jun 23 2019 - 12Z Sun Jun 23 2019

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE
SOUTHERN PLAINS, CENTRAL PLAINS, MIDWEST, & MID-SOUTH...

...Southeast CO/southwest KS...
Showers and thunderstorms are developing across a region of
upslope flow within relatively low flash flood guidance values
across the Front Range of CO at this time and starting their
eastward progression.  Hourly rain totals via radar imagery have
peaked in the 1-1.25" range thus far.  Effective bulk shear of 40
kts per SPC mesoanalyses and precipitable water values are
0.6-0.8", which are quite high for their elevation.  Convection is
expected to progress generally eastward with time as precipitable
water values naturally rise toward 1.25"+ as elevation lowers and
the atmosphere locally saturates.  Southeast CO itself has seen
pockets of 2-4 times the average precipitation they'd normally
receive in two weeks during mid to late June, so they're more
sensitive to rainfall.  Hourly rain totals to 1.5" with local
amounts to 3" appear possible here, so the slight risk for
southeast CO remains until the storms can clear the area and move
into KS.

...Southern-Central Plains...Lower-Mid Mississippi Valley...Ohio
Valley...Tennessee Valley...Southeast...
A highly-amplified upper flow pattern with closed mid level lows
over the northern High Plains and eastern Canada and an elongated
ridge in between that will move slowly east of the MS Valley
during the period.  This pattern lends itself to cool-season
synoptic-scale support along and near several surface frontal
boundaries. Complicating matters will be a shortwave currently
moving northward across OK at the present time which is creating a
significant weakness in the mid-level capping inversion across the
region.

Recent thunderstorm development has occurred ahead of the
northward moving shortwave across portions of AR and OK, given
such a broad upper level difluence downstream of the western U.S.
trough, and is forward propagating into AR at the present time. 
Additional storms have grown upscale across eastern KS, northwest
MO, and southwest IA within an area of divergence aloft between
the shortwave moving into OK and the broad trough across the West.
 Deep-layer instability is impressive, with ML CAPEs in the
1500-4500 J/kg range currently across the southern Plains into the
Midwest.  The high-res CAMs all depict a heavy rainfall footprint
ahead of, and then south to southeast of, the incoming shortwave,
with the usual discrepancy concerning the location of the heaviest
rain and the breadth of its footprint.  We edged our areas
northward across TX where little thunderstorm development has
occurred and model guidance indicates rising 700 hPa temperatures
and a strengthening mid-level capping inversion.

Roth/Oravec/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

The Day 3 outlook will be updated by 0830Z.


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