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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0758Z Aug 16, 2019)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
358 AM EDT Fri Aug 16 2019

Day 1
Valid 01Z Fri Aug 16 2019 - 12Z Fri Aug 16 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL PLAINS...

...Central Plains into the Mid MS Valley...
A new round of convection has been developing over portions of the
Central Plains.  This new convection will have the potential to
produce locally heavy to excessive rainfall over portions of
Nebraska/Kansas into Missouri overnight.  Models point to at least
one more round of convection forming over parts of the Central
Plains later this evening that is expected to propagate
southeastward towards the Mid Mississippi Valley.  Instability had
a chance to recover a bit from convection earlier in the day,
although CAPE values shown by the RAP were in the 1000 J per kg to
2000 J per kg range and Precipitable Water values were around 1.75
inches.  The threat of heavier rainfall may be shifting with time
westward where CAPE is noticably greater and moisture is deeper. 
In addition, the models were showing a vort center in the low
levels approaching from the southwest.  The concern is that the
mid level energy will have outrun the low level energy.  But given
the approach of the low level vort with corresponding low level
frontogensis, and the threat of backbuilding cells, may result in
a flash flood threat which persists from this evening into the
overnight hours from central Kansas into northern Missouri.

Farther north, an amplifying shortwave moving from the Dakotas
into Minnesota early this evening will be helping to focus
convection capable of locally heavy rainfall across the area. 
Narrowed the areal coverage of the Marginal risk boundary because
flash flood guidance values are pretty high here after a dry 2
week period when 25 percent or less of normal rainfall has
occurred and because there cell motions have been fairly steady. 
Opted to not remove the Marginal Risk area considering the amount
of low level Equivalent Potential Temperature advection occurring
ahead ahead of the shortwave trough.

...New Mexico...
Isolated convection is again expected from portions of eastern AZ
into NM. Not expecting anything too intense with only marginal
PWs/CAPE and isolated coverage...however cell motions will be slow
and portions of this area have been anomalously wet of late.  For
these reasons, isolated instances of flash flooding across more
susceptible locations will remain possible for a few more hours
before instability diminishes with the loss of solar heating.

Bann



Day 2
Valid 12Z Sat Aug 17 2019 - 12Z Sun Aug 18 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
MIDWEST AS WELL AS PORTIONS OF THE SOUTHEAST...

...Central Plains/Mid-MS Valley...

...08Z Update...
There remains quite a spread in model QPF which is a result of the
instability axis associated with a boundary anchored across the
Central Plains into the OH Valley, any residual boundaries from
the weakening MCS in the morning and also the trek of the
mid-level energy and how it phases with the northern stream
trough. As a result, the difference between these models are quite
apparent and expand over a hundred miles.  Therefore, relied
heavily on the ensemble means to help highlight the axis of
heaviest QPF.  The deterministic 00Z GFS appears too far south
compared to its instability/moisture axis, meanwhile the ECMWF is
far too north. Though the mass fields are a bit suspect, the 00Z
NAM fell somewhere between these solutions. Therefore, hedged
between which fell in line with the overall ensemble means. In
addition, the Slight was expanded to incorporate more sensitive
soils across portions of eastern KS into MO as multiple organized
areas of convection will impact this region over the next 24-48
hours. 

...Previous Discussion...
A frontal boundary extending from the central Plains into the
western Ohio Valley will become the focus for deepening moisture
and strong instability that supports the development of convective
clusters or an MCS capable of producing a flash flood threat.
While there was generally good model agreement with the overall
synoptic setup, there was some spread with respect to the
placement of the axis of the highest rainfall. In an attempt to
mitigate some of the differences, the WPC QPF and Excessive
Rainfall Outlook were based on a blend of the 12z ECMWF/GFS.

A weakening MCS is expected to cross MO into southern IL during
the first part of Day 3, as the low level jet feeding it slackens,
and remaining instability becomes elevated. A frontal boundary
extending from western OK across MO into IL becomes the focus for
strong instability (with MLCAPE values greater than 3000 J/KG)
during the afternoon hours (though there could be some delay in
the heating due to debris cloudiness). Scattered storms forming in
the axis are expected to be mainly outflow dominated (as it
typically the case in a high CAPE environment) along the front.

Short wave energy in the fast mid level flow tracks from the
Northern Rockies across the Central Plains and the Mid MS valley
after 18/00z. Ahead of the short wave, the low level jet
increasing to 20/30 knots, transporting 2.00 inch precipitable
water air (which is about two standard deviations above the mean)
along the front. Lift associated with the short wave is expected
to organize the convection, growing upscale into an MCS on the
nose of the low level jet. The MCS follows the instability axis
(which become increasingly elevated with time, per the 12z NAM/GFS
soundings) across northern MO into southern IA, before it begins
to weaken over northern or central IL toward 18/12z.

As mentioned earlier, there is some spread concerning concerning
the path of the MCS (and the axis of highest QPF). The differences
appear to be tied to the placement of the ribbon of best
instability (with 12z model showing some latitudinal differences).
The WPC QPF and Excessive Rainfall Outlook were based more closely
on the southern solutions, where the best axis of instability is
expected to lie as the MCS approaches. The airmass can support
hourly rainfall rates near 2.00 inches, and local 2.00/3.00 inch
rainfall amounts are expected where training occurs on the
southern edge of the MCS track.

Three hour flash flood guidance values are generally between
2.00/3.00 inches (though these amounts could be modulated by
rainfall during Days 1 and 2). Since the higher QPF amounts are
close to these values, and given the potential for training in a
high moisture environment, the Slight Risk was retained for this
forecast, modified to fit the above thinking, as well as guidance
trends. 

...Florida into the Southeast...

...08Z Update...
Only minor adjustments made to ERO with convection expected to
remain focused along the stationary front draped over the northern
Gulf of Mexico across northern FL and off the southeast coast. Due
to the available moisture and instability along the Gulf Stream,
did increase QPF totals which will encroach along the SC coast.
Thus, the Slight Risk was extended to include Charleston with
potential heavy rain impacting a more urbanized location. 
Expanded the Slight Risk a bit across north FL, especially north
of Tampa Bay as this region has received and will continue to
receive an abundance of precipitation over the next 48 hours. 
Therefore, soils will become saturated leading to some locations
being more vulnerable to additional rainfall. 

...Previous Discussion...
A persistent flow of tropical moisture along and ahead of a
weakening frontal boundary extending from the eastern Gulf of
Mexico to the Mid Atlantic feeds convection capable of producing
excessive rainfall, particularly over portions of northern FL and
Southeast GA during Day 3. There was generally good model
agreement with the overall setup, so the WPC QPF and Excessive
Rainfall Outlook were based on a multi model blend.

A plume of deep tropical moisture (with precipitable water values
near 2.25, which is approaching three standard deviations above
the mean) continues to ride northeast along and ahead of a
weakening frontal boundary that extends across northern
FL/southern GA into the interior Mid Atlantic states. Instability
appears to be more continuous along the front (unlike Day 2, when
the instability seems to remain just off the coast of the Gulf of
Mexico and
FL/GA Atlantic coastal waters), with an axis of 1000/2000 J/KG
extending along its length.  Storms are expected to develop along
the front, with the greatest concentration across southern GA into
northern FL.

There is once again a multi model signal for a weak surface wave
to form on the front (as short wave energy drops into the broad
mid level trough extending south from the Ohio Valley to the
central and eastern Gulf Coast). A developing surface wave could
help focus the convection on the low level convergence ahead of
it, resulting in a narrowing band of storms over southern GA and
northern FL.  Given the depth of the moisture in the column (as
well as increasing warm cloud depths, hourly rainfall rates in
excess of 2.00 inches are expected, as the storms become efficient
rainfall producers.

Not surprisingly, there is good model agreement showing an axis of
2.00/4.00 inches of rainfall over these areas (with the 12z NAM
being the most bullish with respect to rainfall amounts),
especially where training occurs. Three hour flash flood guidance
values are generally above 3.50 inches, as portions of southern GA
and northern FL have been dry over the past month or so (per
collaboration with WFO TAE). The Slight Risk from the previous
forecast was left mostly intact, with some truncation across the
abovementioned areas due to longer term dry conditions.

...Northern Plains/Upper Mississippi Valley...

...08Z Update...
Adjustments to QPF/ERO were made based on the progression of the
shower/thunderstorm activity ahead and along the cold front.
Previous thinking remains.

...Previous Discussion...
Instability and deepening moisture ahead of a long wave trough
crossing the Northern Plains and Upper MS Valley (and its
attendant cold front) feeds convection capable of heavy to locally
excessive rainfall during Day 3. As the long wave trough pushes
the cold front across North Dakota and western MN between 17/18z
and 18/03z, it interacts with 1500/2000 J/KG of MLCAPE and 1.50
inch precipitable water air to support scattered to broken
convection across eastern ND/eastern SD and much of northwest MN.

The mid level trough and cold front are expected to remain fairly
progressive, which would limit the overall flash flood threat.
However, three hour flash flood guidance values are as low as 1.50
inches over much of eastern ND into western MN. This could allow
for a low end flash flood threat through the frontal passage in
the early evening hours. The Marginal Risk from the previous
forecast was maintained with little in the way of modification.


Hayes/Pagano





Day 3
Valid 12Z Sun Aug 18 2019 - 12Z Mon Aug 19 2019

...THERE IS A MARGINAL RISK ACROSS PORTIONS OF THE DEEP SOUTH...

Tropical moisture will continue to impact portions of the Deep
South as a surface ridge helps to promote moist southwesterly flow
from off the Gulf of Mexico.  Portions of west-central Florida
will observe anomalously high precipitation through Day 2 as a
weak boundary focuses convection leading to more widespread flash
flood concerns. By Day 3, however, this region will likely see a
reprieve as the surface ridge builds north helping to lift the
washed-out boundary and associated mid-level vorticity axis into
the central Gulf Coast region and across southern GA/SC Sunday
into early Monday.

While the overall synoptic pattern is in fairly good agreement
among the model guidance, the details with respect to mid-level
vorticity and ridging aloft/surface varies which results in
notable QPF differences. Multiple models are showing signs of
wave(s) of low pressure developing along the vorticity/surface
convergence axis which is also wreaking havoc on QPF totals.
Regardless, there continues to be a multi-model signal, ensemble
clustering and continuity that would suggest heavy rainfall will
likely impact the southern Gulf Coast States with weaker impulses
riding along the coast of the Carolinas.

The ribbon of precipitable water values of 2.25+ inches being
transported by 10-15 knots of southwesterly flow will start to
narrow throughout Sunday as the surface ridge lifts north. 
Meanwhile, CAPE values across this region will remain around
1000-2000+ J/kg helping to promote on going convection.  With
limited divergence as the jet stream is well north, mid-level
vorticity will be one of the only features aloft to help instigate
additional forcing for ascent and would thus assist with more
organized convection.

With the aforementioned model variability, relied yet again on
ensemble means to help highlight the area of heaviest
precipitation.  With the overlap of instability, moisture and
mid-level lift, it appears there will be a plume of tropical
moisture that will feed moderate to, at times, heavy rain from
southeastern LA through the panhandle of FL which could also
result in training across this region. Areal average precipitation
is around 1-2+ inches with locally heavier amounts possible.

Since the surface boundary has been draped farther south, the
majority of this region has been and will be spared from the
heaviest QPF through the next 48 hours. Therefore, these soils are
not nearly as sensitive as those locations across west-central FL.
 In addition, flash flood guidance is fairly high across this
region; 2-3 inches/hourly and 3-5 inches/3 hourly. Thus a Marginal
Risk seemed sufficient to cover the overall limited threat. If the
axis of heaviest rain shifts and impacts regions that have been
inundated by recent rainfall, than a Slight Risk may be necessary
for a portion of northwest FL. The Marginal Risk extends farther
north and east to account for potential waves of low pressure to
develop helping to promote heavy rainfall/training resulting in
increased flash flood potential, especially in more urbanized
locations. 

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