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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0806Z Aug 17, 2019)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
406 AM EDT Sat Aug 17 2019

Day 1
Valid 01Z Sat Aug 17 2019 - 12Z Sat Aug 17 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE CENTRAL
PLAINS INTO THE MID MISSISSIPPI VALLEY...AND ACROSS THE BIG BEND
AREA AND NORTHWEST FLORIDA...

...Central Plains into the Mid MS Valley...
Only made a few changes here with respect to either areal coverage
or placement of the Marginal or Slight Risk areas.  Between
agreement in the global- and meso-scale models about heavy to
excessive rainfall developing later tonight in roughly the same
area...and convection developing over parts of the western High
Plains early this evening, saw little reason to make too many
changes. 

00Z upper air data showed southerly low level winds already
becoming established over parts of Kansas that was leading to an
area of convergence in the 850 mb to 700 mb layer.  This was on a
par with the hi resolution guidance...and the CAPE values shown by
the RAP were largely supported by area soundings.  Thus confidence
is fairly good that a complex will form and propagate eastward
overnight...accompanied by heavy downpours and the potential for
excessive rainfall amounts.  While the models agree well on the
big picture, the details are still difficult to pin down where the
genesis region will be located. Thus, will keep the slight risk
rather broad in nature to account for the spread in potential
solutions.

Also saw little reason to change the marginal risk area which
extends as far west as eastern CO. There have already been
scattered cells that have produced heavy rainfall rates and
rainfall amounts. Given wet antecedent conditions, additional
localized flash flooding is certainly a possibility, even if areal
averaged rainfall will likely not be all that high.


...Florida and the Southeast...
Tropical moisture streaming into the northern Panhandle area will
continue overnight, however the area looks to be in a lull between
daytime convection and anticipated re-development later tonight. 
The atmosphere has not significantly changed during the day with
Precipitable Water values running at or above 2.25 inches and CAPE
values lingering off-shore at or above 3000 J per kg.  As
mentioned in the previous discussions, additional rainfall will
likely result in a growing flood threat based on how much rain has
been observed in a few places over the past 24 hours.  Maintained
the Slight Risk area for this region. There are still signals in
the models for small scale area of very heavy rainfall between
06z-12z as the low level convergence tightens near the coast.

Maintained the Marginal Risk area along the coastline of Georgia
and the Carolinas. One area of rainfall was beginning to move away
from the shoreline in the early evening.  However, there were some
signals that a wave will ride up the coast...which could spread
some of the better concentrated convection very close to or just
inland of the coast in the early morning hours.


...Pennsylvania into New York...
Persistent moisture transport into the frontal/trough axis
supports the potential of additional convective rounds during the
evening.  Some repeat activity or cell mergers remain a
possibility. Thinking isolated instances of flash flooding will
again be possible today across the region.

Bann


Day 2
Valid 12Z Sun Aug 18 2019 - 12Z Mon Aug 19 2019

...THERE IS A MARGINAL RISK ACROSS PORTIONS OF THE DEEP
SOUTH...THE EASTERN KANSAS AND MISSOURI...AS WELL AS THE OHIO
VALLEY INTO THE NORTHEAST...

...Deep South...
Tropical moisture will continue to impact a portion of the Deep
South, most notably adjacent to the Gulf of Mexico and the
Southeast Coast.  This is where surface convergence and a narrow
axis of vorticity remains which will be the focus for convection
and weak waves of low pressure as they ride this boundary.
Northwest Florida has been hit fairly hard as of late with
locations receiving well over 300% of normal over the past week
associated with this aforementioned weak boundary with the surface
ridge keeping it in place for days on end.  However, conditions
should improve across this region as the ridge builds north
helping to shift this axis of heaviest rain north across the
central Gulf Coast and into the Florida Panhandle. This axis will
also be adjacent to the southeast coast.  Heavy rain along these
coastal regions may result in nuisance/isolated flooding impacts
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 resultant waves of low pressure varies quite a bit
which continues to result in notable QPF differences. Multiple
models are showing two distinct regions of potential wave
development; (1) along the east-central Gulf Coast and (2) along
the NC/SC coast- which should be lifting out Sunday morning.
Uncertainty remains with respect to location and intensity of
these features.  With more model consensus and continuity,
anticipate QPF confidence and potentially amounts will increase at
subsequent updates.   

Regardless of model output, when looking at the parameters, a
ribbon of precipitable water values of 2.25+ inches will be
transported by 10-15 knots of southwesterly flow which 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 convection.  Divergence is
limited given the jet stream is well north, however there is
plenty of mid-level impulses to assist forcing for ascent which
would thus assist with more organized convection.

Given model variability, did utilized the ensembles yet again to
highlight the region of heaviest rain. Based on this, the WPC
model blend of choice (00Z GFS/EC/ENSBC) and the overlapping
parameters mentioned, kept the heaviest rain along or just
offshore across the northeast Gulf of Mexico and off the NC coast.
Areal average precipitation  along the northwest FL and LA coast
is around 1-1.5+ inches with locally heavier amounts expected.
Precipitation along the OBX of NC is around 0.5-1+ inches with
locally higher amounts expected here as well.  A lot depends on
the aforementioned waves of low pressure and the position of the
convergence axis of where these lows will likely develop. 

With such uncertainty in placement for heavy precipitation, kept a
Marginal Risk across this region.  If a stronger wave of low
pressure develops and moves farther inland, especially across
portions of the eastern Gulf Coast (western FL), then a Slight
Risk will be warranted.  In addition, we will have to monitor the
trends across this region given heavy rain will impact antecedent
conditions.     


...Eastern Kansas into Missouri...
A weakening frontal boundary could become the focus for additional
convection in the wake of an MCS late on Day 2 into early Day 3.
As the morning activity weakens and moves east with a mid level
short wave (or perhaps an MCV) across IA into IL, cloudiness in
the wake of the system could delay solar insolation. Because of
this, it is not clear just how quickly 1500/2500 J/KG of MLCAPE
can develop along the boundary (or shear axis) extending from KS
across MO into IL, which peaks near 19/00z.

Storms that develop in the instability axis will tap the 1.75/2.00
inch precipitable water air, which could allow for hourly rainfall
rates over 1.50 inches. Convection that develops should track
southeast (as depicted by the propagation vectors), possibly
moving over areas that received heavy rainfall during Days 1/2.
The 12z NAM is by far the most bullish with the convective
potential, showing an axis of 4.00/8.00 inch rainfall amounts
centered over western MO. The 12z ECMWF/GFS showed 1.50/3.00 inch
rainfall amounts associated with the storms, but in much different
locations, suggesting that the storms form on outflow from earlier
convection.

There remains a fair amount of spread concerning the placement of
any leftover boundaries, which could be convectively modulated by
earlier activity. However, there appears to be enough evidence
that additional convection following the early morning MCS could
pose at least a low end flash flood threat over a portion of the
region.  While QPF totals have increased in amounts and coverage
from the previous forecast, the position of the heaviest rain may
fall just south of the anomalously high precipitation observed
over the past several days.  If this is the case, the soils may be
able to handle the expected heavier rain early Monday morning.
Also the progressive nature of the potential convective complex
may limit flash flooding concerns. Given these factors, the
Marginal Risk area was expanded and maintained.

...OH Valley/Northeast...
Multiple rounds of showers/thunderstorms are possible Sunday
afternoon/evening ahead of a strong cold front moving through the
Great Lakes. Divergence aloft is present as two jet streaks will
exist with lift. In the mid-levels a series of impulses will be
rounding the deepening trough approaching from the northwest.
Precipitable water values around 1.5-1.75 will be transported into
this region from 10-20 knot southwesterly flow.  Given these
factors and  instability around 1000-2000+ J/kg across this
region, expect afternoon convection to develop (though cloud
debris may slow storm growth). Areal average precipitation  is
generally below 1 inch, but multiple high resolution models are
signaling values as high as 2-3 inches.  With low FFG across
portions of northwest PA and NY, felt a Marginal Risk was
warranted. 

Pagano/Hayes

Day 3
Valid 12Z Mon Aug 19 2019 - 12Z Tue Aug 20 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS A PORTION
OF THE DEEP SOUTH...

A weak boundary and vorticity axis remains across northeast Gulf
of Mexico draped northeast just off the coast of the Carolinas. 
This will be the focus for tropical moisture and convection yet
again Monday into early Tuesday.  Given this region has dealt with
multiple rounds of heavy precipitation, the soils across portions
of northwest FL are fairly saturated.  Therefore, this region is
sensitive to additional rainfall. 

The surface ridge remains in place resulting in a stationary
vorticity axis from the northern Gulf of Mexico across north FL
off the Carolina coast.  Mid-level impulses riding atop the
surface ridge and convergence zone, especially in the northeast
Gulf of Mexico, will help to promote another round of convection
migrating inland into northwest FL. Multiple models show low
pressure developing at the surface helping to supply this tropical
moisture inland. The 00Z UKMET is the strongest of the models
suite, which appears to be an outlier.  Regardless, anticipate a
weak wave of low pressure to form near northwest FL where the best
surface convergence and mid-level energy coincide. 

With respect to parameters that are present, precipitable water
values start to drop off through the period from 2.25 to 2 inches
transported by light 10-15 know southwesterly flow. Conversely,
instability is slightly higher than the previous day with values
above 2000 J/kg MUCAPE. When adding the mid-level impulses acting
as another mechanism for ascent, fully expect another wave of low
pressure to migrate northeast along the aforementioned
boundary/axis region.  Uncertainty continues in regard to
intensity and location of heaviest precipitation. 

Given this region has been inundated with rain for several days
and it is expected to continue through Day 2, it is fair to assume
some locations will not be able to handle much more precipitation.
 However, a lot of variability remains in where precipitation will
fall up to this point and then where it will fall on Day 3. 
Therefore, felt a Marginal Risk would suffice at this point and if
confidence increases for the flash flood threat, then an upgrade
will likely be necessary. 

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