Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
403 AM EDT Sun May 31 2020

Day 1
Valid 12Z Sun May 31 2020 - 12Z Mon Jun 01 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN PARTS OF THE
NORTHERN ROCKIES AND SOUTH TEXAS...

...Northern Rockies...
The impetus for convection rooted in the low levels had faded
overnight, but strong storms had earlier produced a quarter to
half inch of rain in a short time from northeast Washington to
northwest Montana. Additional mid to upper level ascent is
forecast to bring another round of convection, this time rooted
more in the mid levels. The updrafts should not be quite so deep,
but most high-resolution model solutions are still indicative of a
quick burst of additional quarter to half inch rain amounts in
this same region through about 15Z. MUCAPE values remain a
respectable 500 J/kg or more this morning in a region that rarely
achieves such values. All these factors lead us to maintain a
Marginal Risk area into the early morning hours out of precaution,
and perhaps as the two rounds of rain will combine to produce
rapid water rises over complex terrain.


...South Texas...
A broad upper level low was evident in water vapor satellite
imagery over northern Mexico and far south Texas. This feature was
best depicted at 500mb to 250mb, with a weaker reflection in the
low levels. Situated near the tail end of a cold front in the
northern Gulf, this upper low is forecast to drift slowly north to
northwesterward, drawing at least some increase of deep layer
moisture content onshore along the south Texas coast and
eventually up toward the Hill Country and Edwards Plateau. Given
that this is not a deep layer baroclinic feature, the low may not
be too efficient at generating instability, and this is reflected
in the NAM instability fields which quickly wash out after a short
burst of convection that is expected this morning. Storms may then
tend to radiate outward and into more unstable areas farther
inland and up the coast by afternoon. This suggests there is
little focus to place any extreme rain totals over any given
location. Very rich moisture, however, will allow for
tropical-style rainfall efficiency, and a chance for spotty
intense totals of at least 2-inch per hour to occur beneath the
stronger storms. Based on QPF trends we expanded Marginal Risk
southward down to Brownsville, but overall the threat remains
fairly localized.

Burke


Day 2
Valid 12Z Mon Jun 01 2020 - 12Z Tue Jun 02 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL AND SOUTHERN ROCKIES...

...Colorado/New Mexico...
Another round of moisture combines with instability in place under
a weakening mid level ridge to support convection that could
produce heavy to locally excessive rainfall across portions of the
Central and Southern Rockies during Day 2. As short wave energy
crosses portion of Southern California for the Southwest states
during the beginning of Day 2, the mean flow transports the
deepening moisture (with precipitable water values between two and
three standard deviations above the mean) across CO and northern
NM. Convection is expected to develop as 500/1000 J/KG of SBCAPE
is tapped, generally after 01/20z from the Front Range in CO down
to the Sangre de Cristo Mountains in northern NM. 
 
Mid level short wave energy (or possible MCVs in the flow) comes
in under the weakening ridge to support more organized storms
across these areas between 01/21 and 02/03z. Weak storm motions
are expected ahead of the short wave energy, and cell mergers and
short term training are possible, particularly as the storms leave
the terrain and cross the plains. Given the amount of moisture in
the column, hourly rainfall rates could approach an inch where
storms merge or train. Both the 00z NAM/GFS continue to show local
1.50 inch rainfall amounts tied mainly to the terrain, and these
amounts are plausible in this airmass. 
 
Three hour flash flood guidance values are as low as 1.50 inches
across northeast CO, where seven day rainfall totals have been
300+ percent above normal. Elsewhere, antecedent conditions are
not quite as wet, so training or cell mergers would be needed here
for flash flooding to initiate. The threat should begin to wane
after 02/04z, as storms across southern areas consume the
available instability, and storms across northeast CO move into
western KS/western NE. Based on the above, a Marginal Risk was
placed over much of CO into northern NM to cover the threat. 


...Texas...
A south southeast low level return flow from the western Gulf of
Mexico continues to supply deepening moisture to much of west and
central TX during Day 2. Within this synoptic setup, high
resolution models (as well as the 00z NAM) are suggesting that the
moisture and instability become focused on a surface trough
pointing toward the TX Mid Coast. As seems to often be the case
with late night/early morning convection along the TX coast, the
distribution of the instability will be the key as to whether
convection comes ashore. The 00z WRF ARW is the most aggressive
bringing heavy rain ashore here, but the signal is fairly muted
among other high resolution solutions. For now, the 00z WRF ARW
solution is eschewed, and for now the highest rainfall amounts
were kept offshore.

Otherwise, the best forcing for this period is expected across
west TX into the Southern Rockies as an upper low wobbles
northward across that area. Outside of the mid level low, synoptic
scale forcing remains weak, and convection that forms in the Gulf
moisture and 500/1000 
J/KG of MUCAPE across much of west and central TX should remain
unorganized. 
 
Though hourly rainfall amounts could approach 1.25 inches in
spots, there does not appear to be one area that is favored over
another with respect to a flash flood threat. Based on this, no
excessive area was assigned for TX on Day 2. If there is better
agreement concerning the threat for the Mid TX Coast, a Marginal
Risk could be needed here in later forecasts.


...Northern Plains/Upper Mississippi Valley...
There is a model signal for the development of an MCS on the nose
of a low level jet across central ND early on Day 2, which peaks
in intensity as it taps 2000/3000 J/KG of MUCAPE aligned across MN
into WI through 01/20z. As the MCS follows the instability
gradient across these areas, it begins to weaken during the
afternoon hours as it crosses northern lower MI. Though there
could be enough moisture (at least initially, as precipitable
water values top out near 1.50 inches) to support hourly rainfall
rates near 1.25 inches as the MCS tracks from MN into WI, the
forward speed of the system should preclude an organized flash
flood threat. Because of this, no excessive area was assigned.

Hayes

Day 3
Valid 12Z Mon Jun 01 2020 - 12Z Tue Jun 02 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL EXTENDING FROM
THE UPPER MISSISSIPPI VALLEY ACROSS THE UPPER GREAT LAKES...

...Upper Mississippi Valley across the Upper Great Lakes...
Moisture and instability focused on a frontal boundary extending
from the Northern Plains across the Upper MS Valley into the Upper
Great Lakes could support convection producing heavy to locally
excessive rainfall during Day 3. There is a model signal (seen in
the 00z
NAM/GFS/UKMET) for an MCS that develops along the front on the
nose of a 20/30 knot low level southwest flow to track across
southern MN into WI/MI during the first part of Day 3. Unlike the
possible MCS on Day 2, this system does not appear to be moving so
quickly as to negate a flash flood threat.

As precipitable water values jump to 1.50 inches ahead of the
convective system (the 00z NAM appears to be suffering some
convective feedback as it has precipitable water values top out
over 2.00 inches), hourly rainfall rates could top out near 1.25
inches, before the convective system begins to weaken over MI.
There is a multi model signal for 1.50+ inches of QPF along this
axis, though there is some latitudinal difference in the placement
of the highest rainfall amounts (which is not unexpected when
trying to place a largely mesoscale driven system on Day 3). Once
the convective system passes, it is unclear whether convection can
redevelop in its wake, as the airmass may be sufficiently
overturned to preclude this possibility.

Three hour flash flood guidance is as low as 1.50 inches across
this area, with seven day rainfall amounts 300+ percent above
normal, resulting in wet antecedent conditions. Training within
the elements of the convective system would likely be required to
initiate flash flooding, but given the antecedent conditions, even
short term training could be sufficient to cause isolated flash
flooding. Based on this, a Marginal Risk was extended from
southern MN across WI into lower MI. It is possible that the
placement of the Marginal Risk could change as the path of the
convection is handled better in subsequent model runs.


...Colorado...
Convection forming under the reestablished mid level ridge across
the Central and Southern Rockies could produce heavy rainfall and
local runoff issues during Day 3. As is typical in this type of
setup, storms are expected to develop across the terrain from the
Front Range in northeast CO into far northern NM between 02/18z
and 02/21z in an axis of 250/500 J/KG of SBCAPE. The storms are
expected to be slow initially across the terrain, as upslope flow
holds them in place before they become outflow dominated.

Storms tapping into the 0.50/0.75 inch precipitable water values
in the low level easterly flow could result in hourly rainfall
rates of 0.50 inches, especially before the storms start moving
across the plains. Other than the 00z GFS, there is not much of a
model signal for rainfall amounts more than an inch, and the
activity could remain scattered enough to preclude an organized
flash flood threat. For now, no excessive area was assigned here.
However, if there is a stronger model signal for higher rainfall
amounts in later model runs, a Marginal Risk could be needed here
in subsequent forecasts.

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