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FOUS30 KWBC 290826
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
426 AM EDT Mon Jul 29 2019

Day 1
Valid 12Z Mon Jul 29 2019 - 12Z Tue Jul 30 2019

...MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MULTIPLE AREAS
TODAY...


...Eastern Portions of TX-OK through the Lower-Mid MS Valley,
western OH Valley, and into southern Lower MI...
Northern stream shortwave trough will pivot across the mid-upper
MS Valley and eventually the OH Valley and Great Lakes Region
Monday and Monday night. Mid-upper level height falls will be more
pronounced across the upper MS Valley, leading to a more
progressive cold frontal passage and more rapid negative PW
advection behind it. Farther south of the more pronounced height
falls, the front will become more west-east orientated with time
and become quasi-stationary by Tuesday morning.

Overall, a marginal or isolated risk for excessive rainfall exists
over a fairly large latitudinal swath ahead of the upper shortwave
trough/surface front. Over the northern areas (mid MS Valley-OH
Valley-southern Lower MI), quasi-linear segments of pre-frontal
convection will feed off a modest thermodynamic profile -- with
PWs ~1.75" and MUCAPEs ~1000-1500 j/kg). Given the swift frontal
progression, any organized segments will likely forward propagate,
especially given the weak mid-level lapse rates ahead of the front
and (thus) the reliance on surface-based heating to foster any
upscale convective growth -- the result of which would promote
fairly productive cold pools/outflow boundaries.

Farther south toward the lower MS Valley, eastern OK-TX and the
Gulf Coast, higher PWs (~2.00" and more robust deep-layer
instability (mixed layer CAPEs 2000-3000+ j/kg) would favor higher
rainfall rates, as noted by the HREF and experimental HREF probs
of 1-2+ inches within an hour. However, weaker low-level inflow
(greater vertical wind shear) along with the weak mid level lapse
rates over this region as well will too limit the persistence of
intense rainfall rates before the ensuing cold pool/outflow decay.


...Portions of the Southwest...
A modest degree of 850-700 mb moisture flux, with PWs not overly
anomalous for late July underneath the upper ridge will pose a
marginal or isolated risk for excessive rainfall over southeast AZ
into southwest and south-central NM, particularly during the peak
heating hours this afternoon and evening. Weak deep-layer flow/low
0-6km bulk shear values and thus pulse-type storm mode will
inhibit any persistence of the more intense rainfall rates. At the
same time however, the HREF continues to depict small pockets of
30-50% probabilities of 1 hour QPF exceeding 1.00", which given
the current FFG values, would certainly support a localized threat
for flash flooding.


...Portions of the Central High Plains...
Convection firing this afternoon across eastern CO/southwestern
NE/western KS, triggered by the peak daytime heating with an
uptick in southeasterly (upslope) low-level flow and moisture
transport, will propagate fairly swiftly to the SE per the Corfidi
vectors. Regardless, strong deep-layer instability (mixed-layer
CAPEs 1500-2000 j/kg) will lead to some intense pcpn rates
(including hail) for a short period of time, which could cause
runoff issues in areas with lower FFG values. Most of the high-res
CAMs show isolated QPF totals between 1.5-2.5".

Hurley





Day 2
Valid 12Z Tue Jul 30 2019 - 12Z Wed Jul 31 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER THE
SOUTHERN ROCKIES INTO THE SOUTHWEST...A SMALL PORTION OF THE
CENTRAL PLAINS...AS WELL AS PARTS OF THE NORTHEAST...

...Southwest...
As the mid level ridge spreads eastward into CO/NM during Day 2,
the return southeast flow focuses the deeper monsoonal moisture on
portions of southern AZ into far southeast CA. Precipitable water
values of 1.75/2.00 inches (which is between two and three
standard deviations above the mean) are transported into these
areas, with the best thrust of moisture expected after 31/00z.
Model soundings from the 00z GFS showed an axis of 1000 J/KG of
MUCAPE extending across much of southwest AZ into far southeast
CA.

Convection is expected to develop across the terrain far southwest
NM and northern Mexico between 30/18z and 31/00z, tracking
northwest in the mid level flow. There is some model support for
weak short wave energy (or an old MCV?) to move from northern
Mexico into southwest AZ during this time, but not all models
depict it. Without more in the way of forcing, the convection
could remain unfocused as it expands over much of AZ and southeast
CA, peaking between 31/00z and 31/06z. There is a model signal for
hourly rainfall rates near an inch, with the best consensus over
south central AZ, especially where short term training occurs, as
the mid level flow becomes better aligned with the propagation
vectors.

The synoptic setup looks increasing favorable for flash flooding,
and a Slight Risk was considered for a portion of south central
and southwest AZ. However, with some details still to be worked
out, and after collaborating with WFOs TWC/PSR, a Marginal Risk
was placed over much of the AZ into far eastern CA for Day 2. As
details come into better focus across this area, a Slight Risk
could be needed here in later forecasts.


...Central Plains...
Moisture and instability on the nose of an increasing low level
flow could feed a developing cluster of storms (or an MCS) over
portions of NE into northeast KS during the second half of Day 2.
Deepening moisture and instability is drawn northward across the
Central and Northern Plains on the eastern edge of the mid level
ridge cresting over the southern Rockies after 31/00z. There is an
increasing model signal for convection to develop on an
instability axis north of a frontal boundary extending from
western ND/western SD across central NE into northeast KS just
before 31/00z.

A 20/30 knot low level south southeast flow transports 1.75 inch
precipitable water air (which is approaching two standard
deviations above the mean) along the instability axis. The
deepening moisture  feeds the developing convective clusters,
which could grow upscale into the MCS. However, there seems to be
less model support for this scenario than this time yesterday.
Instability could end up being the limiting factor for a greater
threat, both spatially and temporally across NE/KS.

Three hour flash flood guidance values here are generally above
2.50 inches, though a portion of southeast NE has seen 300+
percent of normal rainfall over the past seven days. Based on the
above, a Marginal Risk was placed here for Day 2, but if model
trends continue, this Marginal Risk could be eliminated in later
forecasts.


...Northeast...
Moisture and instability streaming northeast along a slowing cold
front could feed convection producing heavy to locally excessive
rainfall over portions of the Northeast, mainly across western PA
into western NY state during Day 2. An axis of 1000/2000 J/KG of
MLCAPE could support scattered to broken areas of convection
extending from WV across western PA into western NY state during
the 30/18z to 31/00z time frame. Propagation vectors drop below 10
knots across western PA into western NY state ahead of the front,
which could promote short term training or cell mergers in the
afternoon hours. As the storms move through a plume of 1.50 inch
precipitable water air, hourly rainfall rates could peak between
1.00/1.50 inches, especially where training occurs.

The lowest flash flood guidance values were scattered over
portions of northern WV into southwest PA (generally between
1.50/2.00 inches), and slow moving storms could produce rainfall
amounts that could approach these values here. Since here has been
an uptick in model QPF amounts with the storms across much of
western and northern NY with the convection, a Marginal Risk has
been extended from eastern
OH/northern WV across western PA into much of western and northern
NY state for Day 2.


...Lower Mississippi Valley...
Convective clusters (or a small MCS, per 00z NAM simulated IR
imagery) are expected to drop south across far southern AR into
northern LA during the first part of Day 2. The storms are
expected to remain south of the slow moving (or quasi stationary)
front here, as low level feeds the weakening convection in 2.00
inch precipitable water air. There is a multi model high
resolution model signal for the storms to drop slowly south as
they weaken, and the 00z NAM CONUS Nest showed the potential for
hourly rainfall rates near 2.00 inches across northern LA before
this occurs.

The 00z NAM CONUS Nest also showed that storms or small MCS
becoming outflow dominated in the 30/12z to 30/18z time frame,
with the storms fanning out into eastern TX and far west central
MS. Outside of cell mergers or short term training occurring along
the outflow, this does not appear to be an organized flash flood
threat, especially since three hour flash flood guidance values
are as high as 3.50 inches. For now, no excessive area was
assigned, as the threat appears nebulous. If there is better
agreement on the evolution of the storms, a Marginal Risk could be
needed here in later forecasts.

Hayes

Day 3
Valid 12Z Wed Jul 31 2019 - 12Z Thu Aug 01 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE FOUR
CORNERS AREA...THE CENTRAL PLAINS AND A PORTION OF THE NORTHEAST...

...Southwest and the Southern Rockies...
Monsoonal flow affects a larger area across the Four Corners area
during Day 3, as the mid level ridging remains more or less in
place. The low to mid level south to southeast flow on the back
side of the ridge sends deep moisture across AZ/southeast CA into
portions of southern NV/UT and CO. The main thrust of moisture
remains across southwest AZ into far southeast CA, where
precipitable water values reach near 2.00 inches (which is between
two and three standard deviations above the mean), and surface dew
points are expected to rise to near 70 degrees.

Convection could be ongoing across portions of southwest AZ at the
start of Day 3, as there is some indication of short wave energy
(or perhaps an MCV?) tracking from Mexico into this area early in
the period. Outside of this, forcing could be lacking across
AZ/southeast CA/far southern NV during the period. Instability is
expected to peak between 1000/2000 J/KG of MLCAPE before 01/00z,
though convection may be unfocused outside of the presence of
forcing. The deep moisture could support hourly rainfall amounts
near an inch, mainly where training occurs. Since it it not clear
from this distance where the main threat will materialize, a
Marginal Risk was placed here for Day 3. Ingredients for flash
flooding will be in place, with a focus being the missing element.
If forcing becomes more evident, a Slight Risk could be needed in
later forecasts.

As the deeper moisture rounds the ridge across UT into CO, the
threat for flash flooding is expected to increase. Like AZ, the
flash flood threat is tied to some kind of focus. There is some
hint that weak short wave energy rides around the ridge with the
deeper moisture, but from this distance, it is not clear just how
the forcing might unfold. Instability and deep moisture support at
least scattered convection across the higher terrain (including
the Wasatch and Uinta Mountains in UT, and the San Juan Mountains
in southwest CO).

Hourly rainfall rates could approach an inch in the rich moisture
plume, and the slow cell motions could support cell mergers or
short term training. A Marginal Risk was placed over these areas
for Day 3.


...Central Plains/Mid Mississippi Valley...
Short wave energy riding over mid level ridging over the central
Plains interacts with deepening moisture and instability to
support the development of a convective cluster or small MCS on
the nose of a low level jet across portions of NE/KS/MS during the
second half of Day 3. A 30/35 knot low level southerly inflow
across this area transports 2.00 inch precipitable water air
(which is about two standard deviations above the mean) over the
Central Plains, which becomes focused on a low level boundary
extending across eastern NE into western MO, mainly after 01/00z.

There was a multi model signal for strong 700/500 mb moisture
transport to become focused here between 01/00z and 01/06z, which
could support the development of a convective cluster or small MCS
(an idea which is supported by 00z GFS simulated IR imagery) over
southeast NE in this time frame. The cluster or MCS then tracks
across northeast KS into far western MO by late in the period.
There is a window of opportunity for training along the south and
west side of the storms before the system become more outflow
driven after the development of a cold pool.

During this time, training in the deep airmass could support
hourly rainfall amounts between 1.50/2.00 inches, especially where
training occurs (an idea supported by the 00z NAM). Three hour
flash flood guidance values are generally above 2.50 inches
(though a portion of southeast NE has seen 300+ percent of normal
rainfall over the past 7/14 days), so training would be needed for
the initiation of flash flooding. At this point, there seems to be
a flash flood threat, but there is still some spread on where this
might occur. Based on the above, a Marginal Risk was placed over
this area for Day 3.


...Northeast...
A long wave trough crossing the Northeast during Day 3 begins to
take on a negative tilt over Quebec, which allows a cold front
crossing the Northeast to slow as it approaches the coast of
Northern New England. Ahead of the front, a low level south
southwest flow continues to feed 1.50/1.75 inch precipitable water
air from the Mid Atlantic into Northern and central New England,
where model sounding showed 1000/2000 J/KG of MLCAPE in about the
same location. The combination of moisture and instability is
expected to support the development of at least scattered storms
along and west of the Interstate 95 corridor in the afternoon and
evening hours.

Synoptic scale ascent associated with the mid level trough could
aid in organizing the storms into clusters, and the shear could be
sufficient to support more than just pulse convection. In the deep
moisture plume, storms could produce hourly rainfall rates near
1.50 inches, as warm cloud heights are fairly high (based on 00z
GFS model soundings), which could result in efficient rainfall
makers. There is some regional/global support for local 2.00/3.00
inch rainfall amounts (especially where storms training in the low
to mid level southwest flow), particularly over Northern New
England.

However, lower flash flood guidance over portions of eastern PA
into northern NJ could result in these areas being vulnerable to a
low end flash flood threat as well. Based on the above, a Marginal
Risk was stretched from northern NJ across much of interior New
England for Day 3.

Hayes


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.t...
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.t...
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.t...


$$




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