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000
FOUS30 KWBC 170817
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
416 AM EDT Sat Aug 17 2019

Day 1
Valid 12Z Sat Aug 17 2019 - 12Z Sun Aug 18 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID MS VALLEY AND THE SOUTHEAST COAST...

...Central and Northern Plains into the Mid and Upper MS Valley...
Convection will be ongoing this morning across portions of KS and
MO, with the activity spreading eastward into IL/IN through the
morning hours. In general would expect the flash flood threat to
be on the decline by 12z, however isolated issues may persist.
Additional convective development is likely by later this
afternoon into the overnight hours as stronger forcing moves into
the Plains. As is always the case with summertime convection,
uncertainty exists with exact convective evolution and placement
of heaviest rainfall amounts. Likely to see convection form along
a front/instability gradient this afternoon across portions of the
Dakotas into MN. As we head into the overnight hours strong
synoptic forcing will overspread this front...with an impressive
mid level wave and strong right entrance region upper jet dynamics
in place. This should allow for an expansion of convective
activity along the front tonight...probably more so than some of
the 0z guidance is suggesting. After the better forcing catches
the front, the front will be become pretty quick moving, which
should push convection along. This factor may end up capping the
upper magnitude of rainfall totals, and keep any flash flood risk
isolated in nature. With that said, if convection is able to
develop this afternoon ahead of the better forcing...the approach
of the forcing could develop enough upstream convection to enhance
cell merger potential. Also, if convection organizes into a well
defined MCS over SD/MN/northern IA (which is possible given
forcing/instability), then a slightly greater flash flood risk
could also evolve. Thus for now opted to increase QPF, but keep
the threat at Marginal, but will need to continue to monitor, as
some potential of needing a Slight risk exists.

The other area of focus is from northern MO into IA and western
IL. This region will see the strongest instability and 850mb
moisture transport. And while the strongest synoptic forcing will
stay to the north of this region...indications that a separate
upper jet may help increase upper level divergence in its right
entrance region across this area tonight. The low level focus
mechanism is a bit less clear down here...thus may take some time
for convection to really get organized. Isolated activity is
possible this afternoon near any remnant low level boundaries from
earlier convection...but more likely that we will have to wait
until late this evening into the overnight for an expansion of
more organized storms as the low level jet strengthens. It is the
combination of this strengthening low level jet and moisture
transport, and the aforementioned upper jet streak, that should
support the organized convective threat...and would expect at
least some west to east training to develop. The exact axis of
training remains unclear and will keep the Slight risk rather
broad in nature to encompass the range of most plausible
solutions.

...Florida into the coastal Southeast...
Another wet day expected from FL into the coastal Southeast
along/ahead of a dying frontal boundary. Tropical moisture will
remain present east of this boundary, with PWs over 2.25" common.
The Big Bend area of Fl continues to be a focus, with the best low
level convergence and highest PW axis focused there. Looks like
the best coverage of embedded heavy convective cells will be this
morning across this region...with the environment becoming a bit
less favorable as we head into the afternoon hours. So while
rainfall amounts today may not be as high as what we saw
yesterday, warm rain processes and high PWs will continue to
support very heavy rainfall rates on a localized scale. Flooding
will thus remain possible, especially over areas that have been
hardest hit the past couple days. A Slight risk remains over areas
where we have overlap of heaviest antecedent rainfall and forecast
rainfall.

Another area of focus for heavy rainfall today will be along the
SC and NC coasts. Expect we will see another wave move northeast
along the coast today, which should help focus/organize
convection. While the heaviest totals may remain offshore, recent
trends suggest that at least some heavier convection should make
it along at least the immediate coastal areas. Looks like a pretty
widespread 1-3" will be common along the coast, with localized
totals approaching 6" a possibility. This is pretty similar in
nature to what occurred yesterday, although the coverage of
showers/storms could be a bit better today. Given the rain that
fell yesterday combined with the forecast rain today...think the
flood threat will nudge upward a tad today. Will probably still
not be that widespread given the nature of the soil across most of
this region...but any more susceptible urban locations could see
some issues arise. Also, with rainfall yesterday locally 4-6",
possible that we see some two day totals approach 10" (should
there be overlap of today`s isolated higher totals with
yesterdays), and would think this may begin to cause some
increased flooding concerns as well. Thus think a Slight risk is
warranted for the region today.

...PA/NY into northern New England...
Convection is likely this afternoon and evening from portions of
eastern PA into NY and central/northern New England. Will have a
surface trough across the region, along with persistent
southwesterly low level inflow/moisture transport. Synoptic
forcing is pretty good as well, with a mid level wave and a rather
impressive diffluent upper jet signature moving overhead as well.
Thus assuming we can destabilize enough, should have no problem
getting at least scattered convective development. The
aforementioned low level trough and persistent southwesterly
inflow may support some repeat upstream convective
development...which could result in a few instances of flash
flooding.

Chenard


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: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt


$$