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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1956Z Sep 20, 2019)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
356 PM EDT Fri Sep 20 2019

Day 1
Valid 16Z Fri Sep 20 2019 - 12Z Sat Sep 21 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN AND NORTHERN PLAINS...

...Southern Plains and Lower MS Valley...
1600 UTC Update -- Very few, largely cosmetic changes made to the
day 1 ERO, based on the latest observational and high-res guidance
trends.

The remnants of Imelda continues to shift northward across eastern
TX/OK this morning. While we do have a corridor of enhanced
moisture transport associated with this feature...we have been
lacking instability, and so rainfall overnight has not been all
that heavy. Some signs this could change this morning...with a
signal for potential heavy rainfall near the Red River into far
southeast OK. Recent mesoanalysis and HRRR forecast fields show
instability advecting northward across eastern TX this morning.
Thus far this instability has stayed west of the aforementioned
enhanced 850mb moisture transport axis. However there appears to
be a growing signal that as this instability advects towards the
Red River we will get a favorable overlap of upstream instability
and enhanced moisture transport/convergence. This could allow for
an intensification of convection across this area...and with the
persistence of the moisture transport axis and Corfidi vector
orientation opposite the mean deep layer flow...the pattern would
appear favorable for backbuilding/repeat convection. Rainfall
would be very efficient given the tropical airmass in place...and
thus any training would likely result in flash flood concerns. Do
have pretty good confidence that convection will increase in
coverage/intensity over this area through the morning hours...with
a scattered flash flood threat evolving. However, the magnitude of
the threat remains uncertain...as will really depend on how much
instability we can get to overlap with the low level convergence
axis. Certainly some chance that we see a localized significant
flash flood event evolve, but this is far from a certainty and
will closely monitor trends through the morning/early afternoon
hours.

Another area of convective focus is likely further west across the
TX panhandle into southwest OK. Likely to have scattered to
numerous coverage of cells across this region by afternoon. Some
potential for cell mergers is depicted within the 0z high res
guidance...along with some potential for upscale growth as the low
level jet increases this evening. PWs will remain well above
average as well, with at least a weak connection in the mid/upper
levels to the eastern pacific tropical systems. Seeing enough of a
signal in the guidance to suggest that scattered flash flooding
could evolve with time today into tonight across this region as
well...with areas of 3-5" a possibility.

...Northern Plains...
Strong synoptic forcing will overspread the northern Plains today
into tonight. Plentiful instability and moisture will advect north
into the region as well...with PWs near climatological record
values. Thus all the ingredients are in place for widespread
organized convective development. Cell motions will be quick off
to the north...however the impressive magnitude of the southerly
low level moisture transport will favor a southward
backbuilding/propagation of activity. The combination of
progressive mid/upper forcing and an eventual cold pool
propagation...should eventually allow for a rather quick eastward
progression of activity. This should end up capping the upper
magnitude of rainfall totals. However, before this happens there
should be an opportunity for some training of cells...as they
backbuild into the low level flow counter to the deep layer mean
flow. Given the PW airmass in place...cells will be efficient...so
this short term training should be enough to result in at least
scattered flash flood concerns. We should end up with an axis of
2-5" with this setup. Exactly where is uncertain...although
current trends favor ND into northwest MN. Activity is expected to
be a bit less widespread and likely more progressive further south
over SD...which should keep areal averaged rainfall lower there.
However, it has been very wet of late over portions of SD...and at
least some chance that convection ends up organizing further south
than the current consensus suggests. If this were to occur then
the rainfall would end up higher over SD than currently forecast.
Thus even though our QPF is lower here compared to ND...think the
best option is to maintain the Slight risk into SD for now.

Hurley/Chenard


Day 2
Valid 12Z Sat Sep 21 2019 - 12Z Sun Sep 22 2019

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF EASTERN KANSAS INTO NORTHERN MISSOURI...

...East Kansas into Northern Missouri...
...20Z Update...
At this update shifted the heaviest rain slightly south based on
the 12Z model guidance but also associated with expectation that
best forcing will residual across eastern KS, northern MO and even
into portions of SE IA and northeast IL. The aforementioned shift
is influenced largely by the approaching trough/surface front and
the mid-level impulses moving through the strong southwesterly
flow. The interaction of the trough (strong divergence from the
right entrance region), mid-level vorticity, increased LLJ,
plentiful atmospheric moisture, surface front and marginal
instability will result in very heavy rainfall across eastern KS
and portions of northern MO, especially Saturday night. The ERO
adjustments were made to account for these changes.  However, the
previous forecast and logic remains the same, the only difference
being the position. 

...Previous Discussion...
Moisture and instability focused on a surface front with waves on
it feed training convection, resulting in an enhanced threat of
flash flooding across portions of northern KS into northern MO
during Day 2. While there is still some model spread with the
placement of the highest rainfall amounts, there is enough model
agreement to base the WPC QPF and WPC Excessive Rainfall on a
multi model blend (including members of the 00z high resolution
guidance suite).

A frontal boundary extending from the Upper MS Valley into the
Central Plains and the Central Rockies slows ahead of a developing
long wave trough extending from the Northern Plains into CO/NM. As
the front slows, surface waves forming on the front ahead of the
trough should tend to keep the surface front quasi stationary
extending from northern KS across northern MO into northern IL,
mainly after 22/00z. A 20/30 knot low level southwest flow
transports 1.75/2.00 inch precipitable water air (which is between
two and three standard deviations above the mean) along the front,
which is focused by strong low level convergence along the front,
peaking between 22/00z and 22/06z.

Instability ahead of the front should be sufficient to support
convection along the front, which could become better organized in
the difluence in the left exit region of a 90 knot jet streak
moving across CO. As the mid level flow becomes parallel to the
front, storms are expected to train from northern KS into northern
MO, again peaking between 22/00z and 22/06z. Given the strong low
level moisture convergence in place, hourly rainfall rates could
approach 2.00 inches, especially where training occurs. There is
multi model support (including the 00z WRF ARW and 00z NAM CONUS
Nest) for an axis of 3.00/4.00 inches of rainfall extending across
northeast KS into northern MO.

Three hour flash flood guidance values here are generally around
2.50 inches across this area, but the potential for training ahead
of the surface waves tracking along the front could support local
6.00+ inch rainfall amounts. Based on the above, and after
collaborating with WFOs TOP/EAX/DVN, a Moderate Risk was stretched
from northern KS across northern MO. This area was surrounded by a
Slight Risk extending from central KS into a portion of northern
IL, where training may not result in an enhanced flash flood
threat.


...North Dakota...
...20Z Update...
Minor adjustments made to the Marginal Risk area to account for
the updated QPF forecast, which was shifted west due to the trough
position and associated mid-level vorticity maximum interaction
with the moisture advection ahead of the surface front. Increased
QPF amounts and expected rain rates as there is strong forcing and
enough moisture to produce shower/thunderstorms that should be
ongoing Saturday morning (thanks to the LLJ influence) before
lifting into Manitoba. 

...Previous Discussion...
A short wave tracking from eastern MT into North Dakota during the
first part of Day 2 is expected to push a cold front across the
region. Ahead of the front, model soundings showed 500/1000 J/KG
of MUCAPE across central ND, which could support a small line of
storms the crosses central and eastern ND before 22/00z. The
convection is expected to remain fairly progressive in advance of
the short wave, which would limit the residence time of the
storms. That being said, hourly rainfall rates could approach an
inch in locations where the three hour flash flood guidance is as
low as 1.50 inches (without taking into account rainfall during
Day 1).

Based on the above, a Marginal Risk was placed over portions of
western and central ND for Day 2. If the short wave and front slow
during Day 2 (and there has been some hint this could occur in the
00z model suite), a small Slight Risk could be needed in later
forecasts, especially if rainfall during Day 1 results in even
wetter antecedent conditions.

Hayes/Pagano

Day 3
Valid 12Z Sun Sep 22 2019 - 12Z Mon Sep 23 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL EXTENDING FROM THE
MID MISSISSIPPI VALLEY INTO THE UPPER GREAT LAKES...

...Mid Mississippi Valley into Upper Great Lakes and western Ohio
Valley...
...20Z Update...
While the axis of greatest risk for flash flooding was hardly
modified (as noted by the ERO), the QPF amounts were adjusted to
account for the ongoing convection expected early Sunday morning
into early afternoon across eastern KS/mid-MO as well as the
activity expected to shift north and east ahead of the frontal
zone.  Model spread associated with the magnitude and position of
the heaviest rain continues. Relied on the ECENS/GEFS means to
help with the position, but feel the LLJ influence across eastern
KS into MO will still play a role with the heaviest amounts
observed across this region early in the period. As the stacked
upper/mid-level trough pivots energy and the 125+ knot jet streak
across the Great Lakes, feel showers/thunderstorms will develop
along/just ahead of the surface cold front and be slow to
transition eastward Sunday afternoon into the overnight.  Could
see the need to expand the heavier rainfall even farther to the
northeast.  But for now, have trended the QPF in that direction.

...Previous Discussion...
Moisture and instability focused on a frontal boundary slowly
crossing the Upper Great Lakes, western Ohio Valley and the Mid MS
Valley is expected to support convection capable of excessive
rainfall during Day 3. There is some model spread concerning how
fast the front drop south, which results in some latitudinal
differences in the placement of the highest rainfall amounts. In
an attempt to mitigate some of the differences in the location of
the axis of highest rainfall, the WPC QPF and Excessive Rainfall
were based primarily on a blend of the 00z ECMWF/GFS.

Convection is expected to be ongoing along the quasi stationary
front extending KS into northern MO during the first part of Day
3, as a surface wave crosses the region. Behind the surface wave,
the front is expected to drop slowly southeast ahead of a long
wave trough crossing the Northern and Central Plains. A 20/30 knot
low level southwest flow continues to supply 1.75/2.00 inch
precipitable water air (which is between two and three standard
deviations above the mean, possibly enhanced by the remnants of
Imelda) across the region. Model soundings from the 00z GFS/NAM
showed an axis of 1000/200 J/KG of MUCAPE, which should be more
than sufficient to support convection along the front. The
convection could become better organized into clusters in the
difluence associated with the right entrance region of a 110 knot
jet streak crossing the Upper Great Lakes.

The depth of moisture in the column could support hourly rainfall
rates between 1.50/2.00 inches, especially early on across
portions of eastern KS across northern MO into northern IL, where
training occurs along the front. An approaching long wave trough
should provide enough mid level flow to keep the convection moving
(after an initial training before 22/18z), which could limit the
residence time of storms in any one location. As mentioned
earlier, there is some spread concerning how quickly the front
starts moving southeast, which in turn determines where the axis
of heaviest rainfall occurs. At this point, the highest rainfall
amounts were stretched from central MO across central IL into far
southwest MI, based on a slowly progressive frontal boundary.

Three hour flash flood guidance across this area is generally
between 1.50/2.50 inches, with the lowest values across northern
IL. Give the potential for at least some training along the front
in a high moisture content airmass, a Slight Risk was extended
from northern and central MO across central IL into far southwest
MI. Since there is some model spread with respect to the placement
of the front, a slower frontal movement could result in training
occurring in places that receive heavy rainfall during Day 2. If
this occurs, portions of northern MO, far southeast IA and north
central IL could need a Moderate Risk in later forecasts.


...Arizona...
...20Z Update.
Minor adjustment to the QPF to account for moisture moving in
quicker across the southern tier of AZ associated with Lorena. 
Otherwise the ERO remained untouched. 

...Previous Discussion...
Short wave energy crossing the northern Baja Peninsula could
interact with moisture from Lorena to produce convection capable
of producing heavy to locally excessive rainfall over portions of
southern AZ during Day 3. However, the amount of instability
available to the lift and moisture is uncertain, which could have
an impact on the coverage of any storms and the extent of the
flash flood threat. Because of this, the WPC QPF and Excessive
Rainfall Outlook were based on a multi model blend.

Subtle short wave energy crosses the northern Baja Peninsula in
the mid level flow ahead of a developing long wave trough is
expected to provide synoptic scale ascent across much of central
and southern AZ during Day 3, mostly after 23/00z. Ahead of the
short wave energy, a weak low to mid level southwest flow
transports 1.75/2.00 inch precipitable water air (which approaches
three standard deviations above the mean) from Lorena across
southwest AZ, peaking between 23/06z and 23/12z. The combination
of moisture and lift should support the development of at least
scattered convection across much of southern AZ during this time
period.

Model soundings from the 00z GFS/NAM showed 500/1000 J/KG of
MUCAPE ahead of the short wave, which should be enough to allow
deeper convection to form, possibly supporting hourly rainfall
rates nearing an inch over southwest AZ. However, synoptic
cloudiness associated with the short wave could limit the amount
of instability prior to the arrival of the best moisture and lift,
limiting the instability available for deeper convection. This is
reflected in the QPF fields from much of the 00z guidance,
suggesting that instability could indeed the limiting factor for a
flash flood threat.

After collaborating with WFO PSR, a Marginal Risk was placed over
much of southern AZ for Day 3. There are model indications that
the current Day 4 (Monday) could be more convectively active, with
several 00z model solutions indicating 1.00/2.00 inches of
rainfall. It could be that the greater threat occurs just beyond
the Day 3 Outlook time frame.


Hayes/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