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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2243Z Jun 15, 2019)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
643 PM EDT Sat Jun 15 2019

Day 1
Valid 2216Z Sat Jun 15 2019 - 12Z Sun Jun 16 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM CENTRAL
OKLAHOMA-EASTERN KANSAS THROUGH THE MID MISSISSIPPI AND OHIO
VALLEYS.

...Mid Mississippi Valley...Ohio Valley...
2216 UTC Update: Expanded the Slight Risk area farther n-nw to
encompass more of northern IL and the eastern 2/3 of IA, based on
the latest observational trends (convection growing upscale)
within a moderately unstable airmass (mixed layer capes 2000-2500
j/kg). It would also appear based on the upper level RAOB data
that this region is benefiting from subtle upper level forcing
(divergence aloft, low level fgen) within the right-entrance
region of the confluent upper flow/jet streak across northern
MN-Upper MI, underneath the St. James Bay low. The ensuing MCV
(smaller than the one currently traversing central IN) will likely
have implications later on tonight downstream into the OH Valley,
where ongoing convection from the first MCV has lowered the 3
hourly FFG values to less than 1.5" from portions of central IL
through central IN. The Slight Risk area also encompasses the
highest 1 and 3 hourly QPF exceedance probabilities from the
current (18Z) HREF. -Hurley

Previous Discussion...

At the start of the forecast period, a strong closed low will be
be pressing a strong cold front and drier air across the northern
and lower Great Lakes, while a southern stream subtle shortwave
and associated mature MCS are progressing across the Central
Plains into the Lower Missouri River Valley.  Northwesterly flow
out of the Pacific stream, is also providing some stream
confluence across the Central MS River Valley into portions of the
eastern Midwest (IL/IND/OH), adding to focus moisture flux
convergence and channeled mean steering flow from WSW to ENE
across these states.  Warm advection early this morning is pumping
deep moisture pocket with total PWats in the vicinity of 1.75". 
The MCS should continue to roll through the northern Ohio River
valley through early afternoon, but with strong warm advection,
post convective environment will be rebuilding instability as
great as 2500-3000 J/kg upstream in MO/IL that will likely
reinvigorate additional round of thunderstorms along the
rear-inflow jet axis/or mid-level confluence axis across IL-IND. 
This should support some training environments, supporting 2-4"
rainfall corridor(s).   While there is agreement toward a
southward shift of the instability axis and confluence boundary in
the Hi-Res CAMs, the precise latitude remains contingent on many
factors (IE MCS placement) and proximity to the northern stream
pressing cold front.  As such, there is sufficient uncertainty to
precise corridor of heaviest rainfall training, and so WPC has
broadened a Slight Risk of Excessive Rainfall to cover a large
area of central IL/IND and Ohio with a Marginal Risk extending
into northeast PA, remaining locations in OH/IND/IL.

...Central/Southern Plains into Missouri...
Upstream, a more robust shortwave is tracking across the Four
Corners and is expected to eject into the Southern High Plains
around peak heating.  A surface low will respond with solid
deepening supporting isallobaric response with strong
northeasterly flow across SE CO/SW KS and a dry-line bulge across
the TX panhandle.  This will support a strong/moisture convergence
signal breaking out convection that will be initially severe, but
will not be able to meander much along the convergence axis across
SE CO into NW OK/SW KS.  GOES-WV and global guidance supports a
faster trend with this shortwave and so moisture return will
likely limit convection coverage along the front range of the CO
Rockies, so have pulled the Marginal Risk to focus further
southeast.

Toward 00z, LLJ will back to more southerly and aiding moisture
convergence at the northeast inflection of the surface wave, where
instability will also build to around 4000+ J/kg and support Total
PWats around 1.5".  Cell motions will be stalled/support
backbuilding with a fairly stationary convective complex to
develop across S central KS/northern OK.  HREF probabilities
suggest 2"/hr rain-totals in this region around 50-60% with
similar probabilities of 3"/3hrs, which would likely exceed the
corridor of regionally lower FFG values (I-35 corridor).  The
shortwave will continue to progress eastward through the early
overnight and LLJ will strengthen and veer leading to strong
convergence along the boundary left by the tonight's MCS and
general confluence boundary that extends along the OK/KS border
into Southern MO generally along I-44 in MO.  There will be ample
moisture/instability in place and fairly unidirectional steering
flow to support some training, but cold pool and slow southward
propagation may limit overall totals relative to the greater QPF
signal further west.  Additionally, there is some potential for
warm sector convective clusters across N TX/SE OK that may disrupt
best moisture flux convergence per Hi-Res CAMs; still in
coordination with local NWS forecast offices, introduced a Slight
Risk area to connect across to the Slight Risk over the Central
MS/Ohio River Valley.

...Upper Mississippi River Valley...
Pacific stream shortwave currently over the Canadian southern
Prairies continues to be blocked by the deeper closed low
downstream, leading to increased shearing of the wave into
confluent flow across the upper Red River of the North into the
Upper Mississippi River Basins, with the surface frontal zone
generally draped through the region.  Relatively clear skies and
modest moisture lingering along the frontal zone, will support
midday growth of instability across S MN into NE IA, generally
expected to be around/above 2000 J/kg by late afternoon. 
Thunderstorms will have the potential to be slow moving into the
shear axis leading to spotty 1-3" rainfall totals.  FFG is about
1.5/2.25 in 1/3hr time frame across this region and so there is a
lower end risk of isolated flash flooding.  As such, have expanded
the Marginal Risk of Excessive Rainfall to best align with
sufficient instability for strong/efficient updrafts and lowered
FFG.  While instability is a bit more limited near the best
forcing/shortwave across the Red River Valley, the FFG is
considerably lower, and so slow moving cells will have equal
potential for exceeding FFG with possible flash flooding.

Gallina


Day 2
Valid 12Z Sun Jun 16 2019 - 12Z Mon Jun 17 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OH VALLEY/LOWER GREAT LAKES REGION AS WELL AS PORTIONS OF THE
CENTRAL PLAINS...

...Ohio Valley/Lower Great Lakes...

Decaying mesoscale convective systems moving across the Ohio
Valley early in the period (Sunday morning) will likely lead to
ongoing precipitation within this region.  At the surface, a cold
front will continue to sink south Sunday.  This will act as a
focus for shower/thunderstorm activity throughout the remainder of
the afternoon/evening.  Mid-level energy moving atop the boundary
will act to enhance precipitation, as noted previously by the
decaying MCS.  Sufficient moisture and surface convergence along
and south of the front will help supply lift.  Instability and
moisture will also pool south of the front with CAPE values
climbing well above 2000 J/kg and with precipitation water values
around 1.75 inches, respectively.  The 850mb southwesterly flow
will be fairly strong, 35-45 knots, especially early Sunday
morning and again later Sunday night as the nocturnal low level
jet strengthens. This will help organize convection Sunday
afternoon/evening south of the front.  Therefore the placement of
the front will largely impact the overall precipitation extent
with model variability still quite high for Day 2. Trimmed QPF a
bit given the instability and moisture convergence seem a bit
weaker across this region with some subsidence noted with the
exiting MCS.  Also, cloud cover may limit destabilization later in
the afternoon/evening.  While there is quite a bit of uncertainty
with respect to the typical ingredients needed, kept the Slight
Risk across portions of southern OH/PA and much of the windward
side of the high terrain in WV.  This is due to the activity
expected on Day 1 (through tonight) and lower flash flood guidance
as of today.  There may be a chance to remove the Slight Risk area
if QPF amounts continue to dwindle.     


...Eastern portions of the Southern and Central Plains...

A shortwave crossing the Plains will nudge a cold front and
associated surface low south and east toward the Lower MS Valley
Sunday. Convection associated with this mid-level energy (likely a
mesoscale convective system) may be ongoing Sunday morning which
will have large implications on convective initiation and
propagation throughout the day as residual boundaries may develop.
This has resulted in varied high-resolution model solutions in
terms of QPF totals. Regardless, heavy rain ingredients are
present as Gulf moisture will advent north into eastern portions
of the southern Plains and Lower MS Valley. Precipitable waters of
1.75+ inches aided by 40kt low level southerly flow is 1.5
standard deviations above the mean for this region.  This combined
with strong moisture flux across east TX and far southeast OK will
result in pooling of moisture east of the dryline and south of the
aforementioned cold front.  With plenty of instability in place
(>3000 J/kg CAPE) and mid-level vorticity slowly crossing atop
surface/moisture convergence, feel convection will develop across
portions of east Texas with potential redevelopment Sunday late
afternoon into the overnight with a strengthening low level jet.
With rain rates above 1.5 inches and potentially multiple rounds
of convection, especially across the eastern half of TX Sunday
through Sunday night, kept the Slight Risk, though adjusted it
south across southeast OK and northeast TX. FFG values are fairly
high for this region with below normal precipitation observed the
past 2 weeks.  While confidence is a bit higher with respect to
the convective activity across eastern TX, feel southeast OK may
encounter subsidence behind the morning MCS which may preclude
additional precipitation activity.  For right now, etched out the
region with the best overlap of heavy precipitation ingredients.
However, fully anticipate the Slight Risk area to adjust with
subsequent updates.

...Central High Plains...

Shortwave activity moving through WY into NE will imitate
convection across portions of the Front Range moving into the
central Plains Sunday afternoon evening.  With modest
instability/moisture in place and differential heating at the
surface with some upslope convergence noted, expect
shower/thunderstorm initiation across the lee side of the Rockies,
quickly moving into the Plains Sunday evening into the overnight.
Hourly rain rates above 1 inch are possible as the convection
moves into a more favorable environment across southwest NE.  With
hourly FFG as low as an inch and portions of this region receiving
>200% of normal precipitation the last 2 weeks, felt a Marginal
Risk of flash flood was warranted.   


Pagano

Day 3
Valid 12Z Mon Jun 17 2019 - 12Z Tue Jun 18 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO VALLEY/LOWER GREAT LAKES REGION IN ADDITION TO THE
SOUTHERN/CENTRAL PLAINS...

...Ohio Valley/Lower Great Lakes...

A surface wave traveling along the frontal boundary will cross the
Ohio Valley/Mid-Atlantic region. Aloft, mid-level energy will move
atop the aforementioned boundary helping to enhance precipitation
across portions of WV into the Mid-Atlantic.  The best forcing and
moisture alignment will be over southern portions of the Ohio
valley and into the Appalachians/Mid-Atlantic region where
instability and moisture will pool Monday afternoon/evening. While
there is a multi-model signal for a convective complex to cross WV
and move offshore Monday night, there is still quite a bit of
model spread that would result in precipitation intensity and
placement to meander.  There is also the boundaries that set up
from activity expected on Day 2 that could play a vital role in
terms of convective development on Day 3. 

At this point in the forecast process, it appears QPF amounts of 1
to 2.5+ inches over the higher terrain of WV into portions of
northern VA, MD and DE is possible which may be problematic for
these flood-prone areas. With hourly FFG around 1-2 inches and
hourly QPF expected above this threshold for a portion of this
region, the flooding risk will be elevated. Per collaboration with
the local offices (RLX/LWX/PHI) a Slight Risk was expanded across
most of West Virginia into southern PA, northern VA into most of
MD/DE.


...Eastern Southern Plains/Lower MS Valley...

The shortwave crossing the Plains will begin to lift north and
east into the Midwest/Middle Mississippi Valley on Monday. The
associated surface cold front will slow over the eastern Plains
and become quasi-stationary over portions of the Lower MS Valley
into TX. Meanwhile, strong moisture transport will be present over
east Texas, Oklahoma, Louisiana, Arkansas and Missouri- with a
local max precipitable water value of 2 inches near the
AR-LA-TX-OK border. Strong instability will increase northward
from Texas to Oklahoma (CAPE values of ~3000 to 5000 J/kg) and
will tap into the pooled moisture. The environment will be
conducive for widespread organized convection that will be capable
of producing very efficient rain rates. Multiple models are
signaling the highest QPF to occur over extreme northeast Texas,
eastern Oklahoma and southern Arkansas in the amount of 1 to 3
inches. These amounts may reach or exceed local FFG in a short
period of time. A Slight Risk for excessive rainfall and elevated
flash flooding threat was maintained and refined based on the
latest QPF placement/totals.

...Central High Plains...

Shortwave activity will continue to trek east Monday afternoon
into the overnight across the central high Plains (northeastern CO
into NE).  With upper level troughing across the northern Rockies
and height falls into the High Plains, a surface low will develop
helping to promote moisture advection and instability from the
south.  Precipitable water values will climb to over 1 inch which
is nearly 2 standard deviation above the mean. This combined with
marginal lapse rates/CAPE values >1000 J/kg will supply this
region the mesoscale features needed for heavy rainfall and
efficient rain rates, especially for this region. With weak
mid-level flow and plenty of atmospheric forcing for ascent,
convection will blossom and move just off the Front Range into the
high Plains of NE into west KS.  Rain rates will likely approach
1.5 inches and with Day 2 activity potentially saturating the
soil, feel this region may see isolated flash flood concerns. 
Therefore, a Marginal Risk for excessive rainfall was introduced. 

Campbell/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