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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1955Z Jun 22, 2019)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
355 PM EDT Sat Jun 22 2019

Day 1
Valid 1810Z Sat Jun 22 2019 - 12Z Sun Jun 23 2019

1800 UTC update

The update to the previous Excessive Rainfall Outlook continues to
be driven by latest radar trends.  Slight risk area over southeast
MO was trimmed back...but extended into western KY and western TN.
 Rainfall rates over the next few hours may reach 1.0 to 2" per
hour as the area moves into western KY and western TN.  Please see
WPCs mesoscale precipitation discussion # 0479 valid until 2336
utc for additional information across this area.

Over the Southern Appalachians..the western portion of the
previous slight risk area over eastern TN was trimmed given
current radar trends showing convection moving into western NC,

Oravec


1600 UTC update

Changes to the previous Excessive Rainfall Outlook based heavily
on current radar trends.  The slight risk area from the Lower Ohio
Valley into the Middle Tennessee Valley was trimmed back given the
progression of the convective complex moving out of the Ohio
Valley into the Tennessee Valley. 

Over the Mid Mississippi Valley...the slight risk area was
extended farther to the southeast to cover ongoing training
convection over portions of east central  Missouri.

No changes made to the risk areas over the Southern and Central
Plains.  The new hi res guidance (12Z) is showing a lot of
continuity issues from the 00Z suite.  Low confidence in some of
the smaller scale details...but still potential for organized
convection taking shape across these areas beginning Saturday
evening and continuing into Saturday night/early Sunday.

Oravec


...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EASTERN
PORTIONS OF THE SOUTHERN-CENTRAL PLAINS INTO THE MID MISSISSIPPI
VALLEY, AND FROM THE UPPER TENNESSEE VALLEY/SOUTHERN APPALACHIANS
AND LOWER MID ATLANTIC REGION...


...Southern-Central Plains...Lower-Mid Mississippi Valley...Ohio
Valley...Tennessee Valley...Southeast...
Somewhat of a chaotic pattern for the first full day of summer
across much of CONUS, owing to 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. 500 mb height
anomalies remain around -2.5 standard deviations over the 4
Corners Region and off the Northeastern U.S. coast -- which will
again lend more cool-season synoptic-scale support along and near
several surface frontal boundaries. Complicating matters will be
the convective contribution on the smaller scale, thereby altering
the coverage/timing of rainfall along with the surface frontal
structure.

Early this morning, a few, relatively compact MCSs were riding
between the 582-588dm 500 mb heights -- a relatively narrow "ring
of fire" region shunted south of the northern tier CONUS given the
aforementioned upper pattern. The models show additional
development during the peak heating hours this afternoon across
the central U.S., given such a broad upper level difluence
downstream of the western U.S. trough. Deep-layer instability will
once again be quite impressive, with Mixed-layer CAPES climbing
above 4500 j/kg across the southern Plains into the mid MS Valley.
Placement/timing of QPF was challenging, given the aforementioned
convective element, particularly with the MCV lifting northeast of
the Rio Grande and across eastern portions of the Southern Plains
and eventually into the mid MS Valley by 12Z Sun. The high-res
CAMs all depict a heavy rainfall footprint ahead of this feature,
though also show quite a bit of discrepancy in terms of where the
heaviest rain will fall. As a result, WPC continued with a rather
broad Slight Risk area -- while maintaining the southern boundary
south of North TX given the likelihood of productive cold pools
and enhanced south propagation overnight given the southerly 850
mb flow peaking between 40-50 kts.

Farther north into the lower MO Valley and mid MS Valley,
additional storms will fire by late afternoon into the evening
along/near the surface triple point along the NE/KS/IA/MO 4-state
corridor, while MCV across northern MO early this morning and
ignites additional rounds of storms over the lower OH Valley and
into the TN Valley. The high-res CAMs show localized totals
between 3-5+ inches over parts of western KY into TN, which
considering the antecedent soil moisture conditions, could
certainly lead to more scattered (vs. isolated) short-term runoff
issues over the Slight Risk area noted from southern IL-IN into
western-southern KY and much of TN.

WPC utilized the recent HREF exceedance probabilities in
delineating the two separate Slight Risk areas west and east.
Currently, the multi-model consensus depicts a relative lull in
the QPF and heavy rainfall probabilities from portions of central
MO into central IL (including STL)after 12Z this morning, thus the
current split in the Slight Risk area. Later shifts will refine as
needed, based on the latest observational and model trends.

Hurley


Day 2
Valid 12Z Sun Jun 23 2019 - 12Z Mon Jun 24 2019

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF OKLAHOMA, NORTH TEXAS AND FAR WEST ARKANSAS...

20z update:
The main changes to the Excessive Rainfall Outlook was modifying
the Moderate Risk to account for model trends, and extend the
Slight Risk into a portion of southwest MI, to account for wet
antecedent conditions.

Across the Southern Plains into the Lower MS Valley, the initial
MCS is expected to be in the decaying phase early on Day 2 across
OK/TX, as the low level jet weakening and instability reaches a
diurnal minimum. There is some model spread concerning who
convection redevelops during the afternoon hours, as the airmass
over eastern OK into north TX could have been overturned by
earlier processes. However, there is a better model signal for
storms to once again form in the 23/21z to 24/00z time frame over
central and southeast OK into southwest AR, as a 30/40 knot low
level jet increases, in an axis of highly unstable air.

Both the 12z NAM/GFS simulated satellite images showed an MCS
forming over south central OK after 24/00z, which tracks toward
southeast OK and northeast TX. The 12z NAM CONUS Nest showed the
potential for hourly rainfall rates near 2.00 inches with the
activity on the southeast side of the developing MCS (where
training is more likely), which could track into southwest AR as
well. The best flash flood threat should occur during this time,
with local 3.00/4.00 inch rainfall amounts possible.

After 24/06z, there is a strong model signal for the MCS to become
outflow driven, as the cold pool should be sufficiently deep to
allow the storms to push out away from the MCS into much of
northeast TX, southwest AR and northwest LA. Hourly rainfall rates
could still top out near 1.50 inches, but the convection is
expected to be progressive enough to allow the flash flood threat
to slowly ebb through 24/12z.

The other change of note was across southwest MI, where seven day
rainfall amounts have been 400+ percent above normal (much of
which occurred in the three to five day range). Convection forming
ahead of the warm front could locally train over this area,
especially during the 24/00z to 24/06z time frame. After
collaborating with WFO GRR, the Slight Risk was extended over
southwest MI to cover the flash flood threat.

Hayes

Previous discussion...
A positively tilted trough will continue to edge eastward across
the Southern/Central Plains through the day Sunday (Day 2).  As
the jet aloft continues to strengthen, mid-level shortwave speed
max will help erode the ridge in place across the Oh/TN Valley. 
At the surface, a front draped from southern MN through the
panhandle of TX will slowly shift east. Ahead of this boundary,
anticipate strong instability and ample moisture to pool across
portions of northeast TX, east OK into western AR into MO.  This
will be the area by which we will see the most precipitation from
convective activity Sunday into Sunday night.  However, it should
be noted that previous and/or ongoing convection and their
resultant outflow boundaries will play a large role in
afternoon/evening convective initiation.  Therefore, a level of
uncertainty exists with respect to the initiation and propagation
of convection and thus where the heaviest rain will reside. 

The synoptic setup is nearly ideal for an enhanced flash flood
threat across the Southern Plains, as deep moisture is transported
north from the western Gulf of Mexico along and just ahead of the
aforementioned frontal boundary. As the low level jet increases in
the 24/00z to 24/06z time frame, convective clusters forming along
the front are expected to grow upscale into an MCS over central OK
during this window. Like on Day 1, the greatest threat for flash
flooding is expected to be on the upwind side of the growing
complex. While many 00z model solutions indicate this would occur
across the eastern half of OK, just ahead of the front on the nose
of the low level jet, there are some models that suggest the
initiation will be farther south and east sinking quickly in
response to the outflow. If this comes to fruition, the axis of
heaviest rain may occur farther south than is current depicted.

Hourly rainfall rates associated with the expected MCS could
approach 2.00 inches over portions of eastern OK into far
northeast TX, before the system weakens and becomes more outflow
dominated before 24/12z. Much like Day 1, and as previously
mentioned, there is some spread concerning the placement of the
highest rainfall amounts. However, there is a better signal for
rainfall amounts between 2.00/4.00 inches here, with higher
amounts where training occurs. As the system weakens, the outflow
could support local 2.00/3.00 inch rainfall amounts over portions
of western AR and north/east-central TX between 24/06z and 24/12z.

The flash flood threat here on Day 2 is enhanced (at least in
part) to the heavy rain threat during Day 1, as the soil becomes
preconditioned ahead of the convection after 24/00z. There is a
growing model signal for local two day rainfall amounts of
5.00/6.00 inches extending from north TX into eastern OK. While
much of eastern OK into northeast TX and western AR has been drier
than normal over the past week or two, soils are still sensitive
as indicated by WFO TSA. Also with two rounds of convection
expected, rainfall totals of 5+ inches are possible, resulting in
likely lower FFG.    After careful coordination with the impacted
offices, adjusted the Moderate Risk to be a bit more broad and
draped farther south to potentially capture are farther south
initiation of the MCS as it moves south and east. 

The Slight Risk was refine farther north and east across the
Mid-MS Valley into the Great Lakes region where convection could
result in excessive rainfall.  FFG at or above 1 inch within an
hour may result in localized flash flooding, especially across
northern MO into IL. 


Pagano

Day 3
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 ACROSS PORTIONS OF THE GREAT LAKES AND
INTERIOR NORTHEAST...

...TX/AR/LA/MS...
20z update:
Not much change made with the update to the Excessive Rainfall
Outlook across TX into the Lower MS Valley. Convection drifting
southeast on the outflow boundary could provide foci for
additional storms during the afternoon and evening hours,
especially across portions of southeast TX into northern and
central LA. A low level southwest flow continues to feed deep
moisture and strong instability along the boundary, which should
be sufficient to support clusters of storms riding the instability
axis.

At this point, there is not support for favoring one location over
another to confidently place a Slight Risk over southeast TX into
northern LA at this point. However, if the convection appears as
though it may become better organized over the Houston metro area,
a Slight Risk could be needed here in later forecasts.

Previous discussion:
Residual moisture and instability pooling along or just south of a
front will result in convection initiating Monday
afternoon/evening.  Earlier on Monday,

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. Residual
moisture/instability will pool along and just south of this
boundary which extends north and east into the TN Valley. 

Surface base CAPE values will climb above 3000 J/kg by the
afternoon despite the expected debris clouds from the decaying MCS
that morning.  Though moisture transport is not as impressive by
Day 3, 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
lose its luster 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 northeast into northern MS.  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 and Day 2.  The outflow boundaries will
play a role in frontal placement and where heavy rain will fall,
thus influencing the antecedent conditions/FFG amounts. 


...Great Lakes/Interior Northeast...
20z update:
Not much change with this update of the Excessive Rainfall
Outlook. It is possible that regional/global models are
underplaying the flash flood threat along the boundary extending
from the Great Lakes into the northern Mid Atlantic states in the
25/06z to 25/12z time frame. Along and ahead of the front, model
soundings indicated 500-1000 J/KG extending from western NY state
into southeast PA and nearby NJ, as precipitable water values peak
as the convection makes the transition from elevated to more
surface based.

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

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 two day total precipitation of 1-3+ inches across portions of
the Great Lakes region and lower FFG values already in place,
anticipate they may be a bit more sensitive to additional
rainfall.  Also, with convection potentially moving through
portions of WV, eastern PA and NY where hourly FFG values are as
low as 0.5 inches, felt this was another area of concern for flash
flooding.  Therefore, a Marginal Risk was issued to encompass
these two regions. 

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