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000
FOUS30 KWBC 191555
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1154 AM EDT Mon Aug 19 2019

Day 1
Valid 12Z Mon Aug 19 2019 - 12Z Tue Aug 20 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A FEW AREAS
IN THE MIDWEST AND THE EAST...

...Southeast...
Here at 16Z we removed Slight Risk and reduced the coverage of
Marginal Risk. Some of the greater potential had existed early in
the morning when coastal convergence enhanced convective coverage.
Panama city recorded over 2 inches of rain in a short time. By mid
morning much of the activity from Louisiana to Florida had become
outflow dominant owing to weak inflow speeds. Going forward, the
models depict a distinct lack of instability within the weakly
convergent trough axis lying over this region. Simulated
reflectivity from the hi-res models suggests a downward trend in
convective coverage this afternoon with little expectation of any
organized events reforming overnight. Based on these trends we
went ahead and reduced the forecast risk. The main area of
interest may be southern Louisiana where there is more broadly
available instability to support a few cell mergers. Convergence
may also shift inland this afternoon to support a swath of locally
heavy rain over southern Alabama, although flash flood guidance
values throughout this region will be somewhat difficult to
achieve without any organized storm modes.

...Northern Mid Atlantic into Southern New England...
A deep layer shortwave trough exiting the Ohio Valley will push
eastward across PA and southern NY through the early afternoon
hours. This feature, along with a passing cold front, will act as
a focus for convection today into this evening. As is often the
case, there are competing factors in the heavy rainfall equation.
Cell motion is predicted to be quick, from the west at 20 knots.
But the environment is exceptionally juicy. The 12Z sounding at
Upton, NY, sampled an impressive combination of steep low level
lapse rates and total precipitable water of 1.90 inches. This
suggests some high-end short term rain rates could occur, with
isolated spots picking up 2 inches or more in an hour or less.
Some brief training or occasional redevelopment is also possible
wherever CAPE can persist owing to the generally cellular nature /
lack of upscale growth / being forecast. There is somewhat a lack
of model QPF support for placing more than a Marginal Risk here.
We did, however, expand the risk area north and south, and could
certainly see the rapid development of heavy rainfall in highly
populated areas this afternoon.

...South Dakota into Iowa...
Models have trended towards organized convective development this
evening into the overnight hours across portions of SD into IA.
This activity should be primarily elevated in nature, forming in a
corridor of 850mb moisture transport, with the approach of a wave
ejecting from the Rockies. The orientation and persistence of the
low level moisture transport axis would appear favorable for
northwest to southeast training. With that said, there is some
signal in the hi-res models that the activity will quickly begin
to forward propagate owing to cold pool potential and modest
inflow speeds. Some training may set up during the peak low level
jet hours, although it is likely that convection would become
increasingly rooted in the mid levels as instability is consumed.
Given this thinking and relatively modest rain rates from the
hi-res models, the risk category is kept at Marginal. The 12Z
models certainly still support the placement of the original
outlook, so no change there.

Chenard/Burke


Day 2
Valid 12Z Tue Aug 20 2019 - 12Z Wed Aug 21 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL PLAINS INTO THE MID MISSISSIPPI VALLEY......

...Central Plains into the Mid Mississippi Valley...
On going convection from an MCS moving out of IA will be marching
south into eastern MO and western IL. Anticipate this activity to
diminish through the morning hours as the low level jet starts to
decrease with the mid-level impulse shifting quickly eastward.
While precipitation intensity should dwindle, still anticipate
high rain rates (>1.5 inches/hour) through the early morning
hours.  Much of the region can withstand 2-3 inches in a 3 hour
period according to FFG. However, with potential training/back
building based on aligned corfidi vectors and mean wind, this may
increase storm total precipitation resulting in isolated to
localized flash flooding in this region.

A digging trough axis pivoting out of Ontario will help to advect
mid-level impulses through the Plains late Tuesday into Wednesday.
 As a result, a strengthening jet axis moving into the Great Lakes
region will position the right entrance region atop the Central
Plains into the Mid-MS Valley region.  These two factors will
promote sufficient ascent within the atmospheric column.  At the
surface a cold front will be progressing southward.  The low level
jet axis will once again increase through the overnight hours
helping to advect precipitable water values of 1.5+ inches from
the southeast. In addition, impressive instability is expected
across this region ahead of the front with MUCAPE and surface
based CAPE values well over 4000 J/kg. There are some limiting
factors that exist.  The approaching mid-level impulse does not
move atop better instability and surface boundary until early
Wednesday morning.  In addition, the aforementioned upper level
jet appears most impressive early Wednesday.  Therefore,
convective development is expected, but may not be as organized
given these features may no coincide in both time and space.  Most
model guidance do show signs of convective development overnight,
but the evolution, position and intensity are still in question
largely due to differences in mass fields and the timing of the
surface front.

Feel convective development will occur either along the surface
front as it continues to drop south and/or residual outflow
boundaries left in the wake of morning convection.  Areal average
precipitation is around 0.5-1+ inches from central NE into far
southwest IA and northwest MO as the expected convection sinks to
the southeast. Hourly rain rates are around 1-1.5 inches/hour,
though the storm mode could result in slow moving convection
exacerbating the flash flood potential.  Given uncertainties
remain, maintained and refined the Marginal Risk area.

...Deep South...
Tropical moisture will move into the interior portions of GA as
the shear axis lifts north.  At the surface, a tropical wave of
low pressure will impact portions of southern GA later today into
tonight, but the evolution of this feature into Day 2 is a bit
uncertain. Models diverge on the track and intensity of the low
and the resultant QPF.  Therefore, kept a Marginal Risk out of
this area knowing their FFG is fairly high in this region.  We
will need to monitor the precipitation observed over the next 36
hours and the model adjustments to determine if this area is at
risk of flash flooding Tuesday into Wednesday.


...New Mexico/Colorado...
Moisture returning from the western Gulf of Mexico as the mid
level ridge repositions itself across the Southern Plains into the
Southwest could be sufficient to support convection with locally
heavy rainfall across New Mexico and Colorado during Day 3.

Integrated water vapor transport vectors (IVTs) from the GFS
showed the return of mid level moisture from the western Gulf of
Mexico across west TX into NM and CO during Day 3. A 15 knot low
level southeast flow transports 1.00/1.25 inch precipitable water
air (which is approaching two standard deviations above the mean)
into eastern NM, peaking near 21/00z. Model soundings showed
MLCAPE values generally between 1000/1500 J/Km, mainly across the
terrain of southern NM, as well as the Sangre de Cristo Mountains
of northern NM into southern CO. The combination of moisture and
instability is expected to support scattered thunderstorm activity
in these location, mainly between 20/20z and 21/04z.

As the mid level ridge becomes repositioned across NM, the mid
level flow drops below 10 knots (as do propagation vectors). This
is expected to result in slow moving storms that initiate over the
higher terrain. The environment could support hourly rainfall
rates between 0.50/1.00 inches, especially before the storms
become outflow dominated moving out of the terrain. Three hour
flash flood guidance values are as low as 1.50 inches over
portions of the terrain in southern NM. and cell mergers or short
term training could produce rainfall amounts that approach these
values.

It is still unclear just how extensive the flash flood threat will
be. It appears that the moisture is not deep enough to support
more than scattered storms, which could mitigate the threat to
some degree. It is also possible that the threat may be a day away
from reaching the point that would trigger an outlook. For now, no
excessive area was assigned, but if later model solutions show an
increased threat, a Marginal Risk could be needed for Day 2.

Pagano/Hayes

Day 3
Valid 12Z Wed Aug 21 2019 - 12Z Thu Aug 22 2019

...THERE IS A MARGINAL RISK ACROSS PORTIONS OF THE CENTRAL PLAINS
INTO THE MID-MISSISSIPPI VALLEY AS WELL AS THE NORTHEAST...
...THERE IS ALSO A MARGINAL RISK ACROSS PORTIONS OF THE
SOUTHWEST...

...Central Plains/Mid-Mississippi Valley...
Residual convective activity will be migrating from NE into MO
early Wednesday morning, diminishing through the morning hours.
Meanwhile, a surface front will continue to track south,
eventually slowing throughout the day. Expect residual boundaries
from this convection and the surface front to be a catalyst for
shower/thunderstorm activity later in the day. Do not expect
precipitation in the morning to result in widespread flash
flooding issues. However, it is the activity expected during the
overnight hours that may result in a higher potential for flash
flooding.

A trough digging into the Upper Mid-West with mid-level ridging
remaining along the spine of the Rockies will allow mid-level
energy to ride between the ridge axis and trough through the
central Plains toward the Mid-MS Valley.  It appears that a fairly
strong mid-level vorticity maxima is set to cross the region
Wednesday night into Thursday.  This will coincide with the right
entrance region of the jet as it pivots through the Great Lakes
region, maximizing around 110 knots Wednesday evening. Thus
sufficient atmospheric ascent is expected.  Precipitable water of
nearly 2 inches combined with marginal instability of 1000-2000
J/kg MUCAPE should result in convection producing moderate to at
times heavy rain. With the surface front slowing and becoming
stalled somewhere across the region and northwest flow aloft,
expect elevated convection to also develop.  While there is model
discrepancies in the placement and magnitude of QPF, a lot is
hinging on the location of the features aloft interacting with the
surface front.

Areal average QPF is around 0.75-1.75 inches with locally higher
amounts anticipated. It should be noted that the 00Z EC/CMC/GFS
all increased their QPF extent and amounts from the previous model
run as the ingredients appears to become better aligned. In
addition, heavy rain may back-build and train in some locations
that would then exacerbate storm total QPF even more.  Given the
potential for several round of convection across this region, FFG
may be adjusted downward in time. However, as it stands, this
region can withstand 1.5-2.5+ inches of rain within a 3 hour
period.  This combined with remaining uncertainties, a Marginal
Risk was introduced.  If QPF totals increases and/or FFG values
are impacted from prior convection, than a Slight Risk area will
likely be needed.

...Northeast...
An approaching cold front will allow moisture and instability to
pool across portions of the Northeast. This combined with upper
level jet support and mid-level energy will result in convective
initiation during the afternoon hours.  Precipitable water of
1.75-2 inches will be transported by 20 knot southwesterly flow
and pockets of >2000 J/kg MUCAPE will develop across the Catskills
and Berkshires.  Therefore, efficient rain rates associated with
convection can be expected.  With mean wind around 30-40 knots
storms should move steadily eastward. However, with the potential
for locations to observe multiple rounds of storms, this could
lead to isolated flash flooding, especially if storms move over
more urbanized locations. FFG in the Northeast ranges from less
than an inch/hour to well over 2 inches within an hour. Therefore,
a Marginal Risk was introduced across this region.

...Southwest...
Monsoonal moisture will increase from the Gulf of Mexico as the
ridge increases across the Southern Plains into the Rockies.
Precipitable water of over 1 inches will be transported northward
across the higher terrain of NM into CO which should support heavy
rainfall associated with afternoon/evening convective activity.
MUCAPE over 1000 J/kg will be present as differential heating is
expected along the slopes of the terrain.  This combined with
surface convergence and orographic influences, expect convection
to form along the terrain and be slow to move with weak steering
flow in place.  Given there has been a dry spell across this
region, do not expect widespread flash flooding to occur.  A
Marginal Risk was introduced to account for isolated flash
flooding with residual burn scars being the most vulnerable
locations. If QPF amounts increase, this area may need to be
upgraded, especially if convection of Day 2 is more widespread
than originally anticipated.

...Pacific Northwest....
An unusually wet period is expected across portions the Pacific
Northwest as an unseasonable deep trough is expected to approach
and cross the west coast.  This will bring a period of moderate
rain into the Olympic Peninsula and Cascades Wednesday
morning/afternoon.  There are even signs of a brief/weak
atmospheric river that is directed toward the WA/OR coast as seen
by the >700 kg/ms IVT off the GFS.  Precipitable water values of
1-1.5 inches aided by 15-25 knots of southerly flow will combine
with minimal instability resulting in periods of 0.25-0.5
inches/hour.  Expect orographic enhancement to aid in even higher
rain rates across this region. Should note that model differences
still remain in the orientation and amount of QPF.  And the swift
progression of the trough and associated front should limit storm
total QPF and its overall impacts.  Therefore, decided to hold off
on a Marginal Risk area for this region. However, if QPF amounts
increase, a Marginal Risk may be needed.

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


$$