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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2015Z Sep 14, 2019)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 PM EDT Sat Sep 14 2019

Day 1
Valid 16Z Sat Sep 14 2019 - 12Z Sun Sep 15 2019

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE UPPER MIDWEST AND SOUTHWEST...

...Upper Midwest...
A warm advection pattern strengthens this period as a low pressure
area moves by to the northwest. Precipitable water values surge to
around 1.25" in the pre convective environment. Temperatures at
700 hPa imply there is no significant capping inversion present.
Although convection could be slow moving at first across southern
MN, for the most part it should be progressive to the southeast
per convective propagation vectors and the 1000-500 hPa thickness
pattern. 
Models were not handling upstream convection in the warm advection
pattern over eastern Nebraska this morning, but radar trends at
15Z suggest this should wane over the next couple of hours,
allowing the risk of later development to the north and east to
remain intact.

Overall, recent runs of the NAM CONUS Nest best reflect our
expectations for afternoon/evening thunderstorms, particularly
placement if the storms should form. The deep layer lift, however,
is not clear cut, and despite the absence of a sharp capping
inversion, storms may struggle within an environment marked by
deep layer height rises and mean shortwave ridging through mid
evening. Presuming at least a few cells can take hold and reach
maturity, we would expect a slow increase in convective scale as
activity marches along over eastern MN into WI, IA after 20Z.
There are some concerns for non-traditional cell training where
activity along the warm front aligns as west-southwest inflow at
850 hPa of 25+ knots exists over a broad area and activity moves
southeast to south-southeast. ML CAPE up to 3500 J/kg is forecast
to exist upstream to the west.  Hourly rain totals to 2" are
possible with locally higher event totals. The big problem across
MN, WI, northern IL and portions of northeast IA are the two week
precipitation anomalies, which are 200-600% of average, implying
saturated soils. The three hourly flash flood guidance values
agree, with 1-1.5" in three hours necessary for exceedance.


...Southwest...
A disturbance aloft moving in from northwest Mexico is forecast to
sharpen as it crosses the AZ border at the end of the period.
Precipitable water values of 1.25-1.5" are expected to move in
near and ahead of this feature. Daytime heating is expected to
bring ML CAPE values up to greater than 1000 J/kg during the late
morning and afternoon, although the CAPE will be distributed
unevenly owing to pockets of thick mid level cloud cover. Inflow
at 850/700 hPa increases with time, to at least 30 knots by the
end of the Day 1 period over southeast AZ and southwest NM. This
same area, however, will experience the greatest cloud cover
inhibiting the growth of instability. Wherever robust
thunderstorms do form, the mean 850-400 hPa wind is not forecast
to exceed 15 kts, which should set up precipitation efficiency.
Where cells train or merge, hourly rain totals to 2" should be
possible, which would be most problematic in urban environment and
washes.

In the 15Z update there was enough of a consistent model signal
between the NCEP hi-res runs and the University of Arizona WRF -
to break the coverage of the Slight Risk around the forecast area
of minimum CAPE over the AZ/NM border. A gradient of Slight to
Marginal Risk does extend westward into the low desert, but expect
decreasing coverage of new updrafts as outflows move toward that
drier and more weakly forced environment this evening. Meanwhile,
there is a possibility of activity continuing into the night
across the southernmost Rockies and the terrain of southwest NM as
the sharpening inverted trough maintains southeasterly inflow and
low level convergence.


...Pacific Northwest...
An early season Atmospheric River is progged to lift onshore
beneath a 110+kt jet streak rotating into British
Columbia/Washington State late Saturday night. GFS progs for the
AR suggest a high probability for IVT 500-750 kg/m/s moving into
Washington, with PWATs exceeding 1.25" on 30 kts of 850mb flow
with 200+ J/kg of CAPE present. This robust moist advection will
be wrung out via ascent within the RRQ of the pacific jet streak,
combined with height falls and surface convergence along/ahead of
a mid-level trough driving a cold front onshore. This will support
rain rates which at times may exceed 0.5"/hr. As flow briefly
becomes parallel to the front, especially into the Olympic Range
of Washington, training and upslope enhancement could produce
isolated instances of flash flooding across the terrain. The
marginal risk area remains despite the expected short temporal
duration of favorable dynamics for excessive rain rates. 

Roth/Burke


Day 2
Valid 12Z Sun Sep 15 2019 - 12Z Mon Sep 16 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
ARIZONA AND NEW MEXICO...

...Southwest...
20z update...Not much in the way of change here, with the greatest
threat for flash flooding expected to be tied to where the best
lift with short wave energy and moisture intersect. Much of the
high resolution guidance showed the highest rainfall amounts over
southeast AZ and adjacent far western NM, with local 2.00 inch
rainfalls possible. This would be sufficient to initiate flash
flooding in more susceptible areas, such as dry washes and burn
scars. The limiting factor could be instability, which would
suppress convective coverage below what high resolution guidance
is currently showing.
The threat should wane as the instability weakens and the best mid
level lift exits into NM and CO, especially after 16/00z.

Previous Discussion...
Potent shortwave/MCV lifting out of Mexico around the western
periphery of a mid-level ridge will provide robust ascent for
convection on Sunday. This feature is progged to move out of
Mexico very late on day 1 or early on day 2, and then lift
steadily northward through Sunday night. PWATs across the
Southwest should rise to 1.25-1.5", more than 2.5 standard
deviations above the climatological mean, supporting the heavy
rainfall threat on Sunday. As the shortwave lifts northward,
guidance is in good agreement that a low-level circulation will
develop in the form of a wave of low pressure, which will act in
tandem with PVA and modest upper diffluence to enhance ascent in
the region. 0-6km mean wind in the vicinity of this low will be
generally 5-10 kts, so storms that develop should move slowly, and
may lead to storm mergers supporting rain rates above 1"/hr, which
could exceed the 1-hr FFG. Although forcing is robust and PWATs
are anomalous, there is some concern that cloud cover may hinder
convective development due to later initiation. This is echoed by
SREF mean MLCape remaining only around 500 J/kg, and late CINH
erosion. However, the anomalous column moisture should still
permit excessive rain rates once thunderstorms develop, and the
model signal is strong enough that a small SLGT risk has been
added after coordination with FGZ/PSR/TWC/ABQ. A MRGL risk
surrounds this slight risk, and extends northward into southern CO
where moisture is less robust and 14-day rainfall departures are
less conducive for flash flooding.


...Pacific Northwest...
20z update...Not much change here either, as the best thrust of
moisture is expected during the first part of the period. The 12z
NAM CONUS Nest indicated the best potential for hourly rainfall
rates of 0.50 inches or greater before 15/18z across the OR
Cascades, and this is reflected in the 12z HREF probabilities. The
higher rainfall rates should drop south and exit the coast after
that time, as the best of the moisture plume slips offshore.

Previous discussion...
A modest Atmospheric River (IVT < 500 kg/m/s) will advect onshore
ahead of a deepening trough and surface cold front early Sunday.
The strongest overlap of forcing through upper diffluence within
the RRQ of an increasingly poleward oriented jet streak and height
falls to produce ascent will occur early on day 2, which will
exist concurrently with the highest PWATs of 1-1.25", more than 2
standard deviations above the climatological mean. Although
instability will be modest, with MUCape peaking near 250 J/kg,
there appears to be enough ascent and moisture transport to
produce rain rates to 0.5"/hr, especially into the terrain of the
Coastal Ranges of Oregon. The temporal duration of favorable
conditions should be limited, so the risk area is capped at MRGL,
but isolated flash flooding will be possible as training develops
in response to 0-6km mean wind becoming progressively more
oriented parallel to the approaching cold front.


...Illinois/Indiana..
The remnants of an MCS crossing portions of northern IL during the
first part of Day 2 may still be robust enough to pose a low end
flash flood threat, mainly through 15/15z or so. The low level jet
should be weakening ahead of the system as it drops southeast
ahead of a cold front dropping south across the Upper MS Valley.
Enough elevated instability may still be available to sustain
rainfall rates between 1.00/1.50 inches (as depicted by the 12z
NAM CONUS Nest) from northern IL into northwest IN in this time
frame.

Three hour flash flood guidance values are as low as 1.00/1.50 due
to earlier heavy rainfall, and the rainfall amounts mentioned
above would be enough to pose a low end flash flood during the
morning hours. After collaborating with WFO LOT, a Marginal Risk
was placed over northern IL and a portion of northern IN to cover
the threat.


Weiss/Hayes

Day 3
Valid 12Z Mon Sep 16 2019 - 12Z Tue Sep 17 2019

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST...AS WELL AS THE TEXAS COAST...

...Southwest...
20z update...Not much change needed here, as the best forcing
exits NM/CO before 17/00z. The best moisture is expected across
eastern NM into southern CO, where 1.00/1.25 inch precipitable
water air is expected to be in place as the best of the lift
exits. This is reflected in much of the 12z guidance. Further west
across AZ, the best mid level lift exits, and the remaining threat
becomes nebulous after that. For now, the Marginal Risk was left
in place, but it may be removed in later forecasts should the
threat continue to dwindle.

Previous discussion...
Return flow around mid-level ridge will continue to drive
monsoonal moisture into the Southwest Monday. A robust shortwave
from Sunday will become absorbed into the westerlies as it lifts
northward into Colorado early on day 3, leaving a much more weakly
forced environment in its wake. PWAT anomalies will remain +2
standard deviations, especially early in the day and across
northern NM and into CO, and this could support briefly heavy
rainfall especially in the terrain where some upslope enhancement
is possible near the San Juans, but otherwise the probabilities
for heavy rain will diminish from SW to NE through day 3. Have
included a MRGL risk for much of AZ/NM behind the departing
shortwave where PWATs are greater and instability recovers to
support briefly heavy rain rates which may exceed the hourly FFG,
but the greatest threat Monday does appear to be focused across
the N/NE portions of the Four Corners region.


...Texas Coast...
Deep moisture and instability in advance of a mid level
circulation and it surface reflection are expected to fuel
convection that produces heavy to locally excessive rainfall
across much of the TX coast during Day 3. Ahead of the mid level
system, the low level southeast flow transports 2.00+ inch
precipitable water air (which is between two and three standard
deviations above the mean) across the coast from KIAH to KBRO,
especially after 16/18z. NAM and GFS soundings showed an axis of
1000 J/KG of MLCAPE in the airmass extending along this stretch of
the coast, peaking near 16/21z.

There is a multi model signal for bands of storms to move west
across the TX coast near the time of peak heating, and 12z NAM
simulated IR images showed very cold cloud tops within the axis of
best instability. Considering the depth of the moisture in the
column, hourly rainfall rates near 2.00 inches are possible with
the storms, and weak propagation vectors suggest that cell mergers
are possible. At this point, it appears as though the threat for
heavy rainfall and flash flooding will be limited to the coastal
plains, where the instability is highest.

As the mid level and surface features approach the south TX coast
toward 17/12z, the bands of storms may be able to penetrate
further inland, as the instability profile may be more favorable
at that time. Much of the 12z model guidance showed less than 1.50
inches of rainfall through 17/12z, which seems low given the
moisture ahead of the system. After collaborating with WFOs
BRO/HGX, a Marginal Risk was placed over much of the coastal plain
of TX. The potential for heavy rainfall could increase beyond the
Day 3 period as the mid level and surface systems come ashore over
the south TX coast.


...Northern Plains/Upper Mississippi Valley...
Moisture and instability increasing in the wake of a warm front
crossing the Upper MS Valley during Day 3 could support scattered
convection producing heavy rainfall. The warm front moves over the
Northern Plains and Upper MS Valley before 17/00z, taking its
synoptic scale cloudiness with it. Model soundings behind the from
IA into MN showed a broad area of 1500/2500 J/KG of MUCAPE in
place, peaking between 16/21z and 17/03z.

Ahead of short wave energy ejected from a long wave trough over
the Northern Rockies and Great Basin, a 25/35 knot low level
southwest flow transports 1.50 inch precipitable water air across
the Upper MS Valley. The combination of moisture and instability
should be sufficient to support at least scattered thunderstorm
activity ahead of a cold front approaching from the west late.
Difluence ahead of the tightly wound short wave could allow
convection that develops to become better organized as it tracks
northeast.

The 12z model suite offered generally modest rainfall amounts in
this scenario, with the 12z GFS the most bullish with rainfall
amounts (near 0.50 inches). It is possible that capping shown in
the model soundings could keep convective activity scattered
enough to preclude any organized flash flood threat. Three hour
flash flood guidance over portions of MN and WI is as low as 1.50
inches due to earlier heavy rainfall, but even the most aggressive
guidance is not close to these values. As such, no excessive area
was placed over the Upper MS Valley for Day 3, but if later
guidance shows a more pronounced QPF signature, a Marginal Risk
(or greater) could be needed here in later forecasts.


Weiss/Hayes


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