Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
357 PM EDT Wed Jul 15 2020
 
Day 1
Valid 1952Z Wed Jul 15 2020 - 12Z Thu Jul 16 2020 

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF 
THE MIDWEST ALONG WITH FROM NORTHEAST NEW MEXICO INTO SOUTHEAST 
COLORADO AND THE TEXAS PANHANDLE...

1952Z Update: Slow moving storms have been forming over the higher 
terrain of southeast Arizona during the afternoon.  Given an 
enviroment with precipitable water values between 1.2 and 1.5 
inches in place with several more hours of daytime heating to help 
sustain the instability, opted to hoist a Marginal Risk area for 
this area.  The Marginal Risk area was extended to cover a second 
area of active convection farther to the north and east along the 
New Mexico/Arizona border...although there appears to be a bit 
more CIN in place.  At this point, thinking is that the biggest 
threat for flooding will mainly be in the moutains and not so much 
in the valleys.  The area should weaken later this evening with 
the loss of insolation.

Bann



1830z Update: A few minor updates were made. Trimmed the Slight 
risk across MO, with the greatest threat this afternoon into 
tonight now focused across central and southern IL and far eastern 
MO. Slightly expanded the Slight risk over southeast CO as 
well...to better cover the area of overlap of lower FFG and 
potential merging cells.

16z Update: Main change with this update was to shrink/focus the 
Slight risk over the MS Valley and Midwest. At this time the wave 
appears to be strung out...with the MCV center hanging back near 
Kansas City, but the corridor of strongest 850mb winds and 
moisture convergence located closer to the northeast MO and IL 
border. In the near term the focus of heaviest rainfall should be 
near the latter area...moving east northeast across central/north 
central IL. Cloud cover will limit destabilization to some 
extent...but south to southeasterly low level flow ahead of this 
feature should advect at least some moderate instability into the 
region through the day. Overall should be able to realize a swath 
of 1-4" of rain along/ahead of this feature today. With time we 
should also see convection expand over east central MO ahead of 
the MCV currently over Kansas City. This activity will likely take 
on more of a linear nature as it intensifies mid afternoon into 
the evening hours. This should limit the flash flood risk to some 
extent...however some training/merging in the development phase is 
probable from east central MO into central IL...locally enhancing 
the flash flood threat.

There is a signal for a second QPF max across portions of southern 
WI along a stationary front and on the periphery of the denser 
cloud deck. This seems reasonable, but think the cells will be 
pretty small scale...with flash flooding likely remaining 
localized. Thus opted to shrink the Slight risk and focus it 
across areas that should see more organized/intense convection, 
resulting in a greater threat of a more concentrated flash flood 
risk.

The Slight risk over the Southern Plains was expanded slightly 
based on the 12z HREF and recent HRRR/HRRRx runs. Coverage of 
cells over CO/NM should be numerous enough, and forcing strong 
enough...that there is a good chance we see merging/consolidation 
of activity this evening over portions of the Slight risk area. 
Where this merging occurs, some flash flooding will be possible.

Chenard

...Previous Discussion...

...East-Central Plains into the Mid Mississippi Valley and Upper 
Great Lakes...
The main change to the previously issued Excessive Rainfall 
Outlook (yesterday's Day 2 ERO) was to broaden the areal extent of 
the Slight Risk -- both west (to include northeast KS) as well as 
south toward the MO/IL/KY tri-state area. 

A weakening MCS is will cross eastern KS and northern MO Wednesday 
morning. The MCV ejected from the cluster is expected to track 
across the western OH Valley into the lower MI peninsula during 
the afternoon and evening hours. The lift generated by the MCV is 
augmented by increasing (albeit fairly weak) difluence in the 
right entrance region of a jet streak moving into Ontario during 
the evening hours. Convection forming along the aforementioned 
instability axis crosses IL, western IN, and lower MI= during the 
late afternoon/evening hours. Much of the high resolution guidance 
suggests that the activity remains mostly progressive during the 
evening. However, deep moisture (with precipitable water values 
between two and three standard deviations above the mean) feeding 
into the convection will allow hourly rainfall rates to peak 
between 1.50/2.50 inches, particularly from mid afternoon into the 
evening, especially over northeast MO into central-southern IL per 
the  high probabilities of >1"/hr and >2"/hr rates from the 00Z 
HREF (80-90% and 40-50% respectively).   

The southern end of the line begins to slow as it becomes a bit 
better aligned with the mid level flow, and slowing storms along 
the front could foster an environment for training along the 
southeast MO-southern IL-western KY tri-state area. Much of the 
high resolution guidance shows this occurring between 16/02z and 
16/08z, with isolated 3 to 6 inch rainfall amounts along this 
segment of the front. Based on this, the Slight Risk was extended 
to include this area. The convection is expected to weaken as the 
instability become exhausted or elevated enough not to feed the 
storms as they move east.


...Southern High Plains...

The main change to the previously issued Excessive Rainfall 
Outlook was to extend the Slight Risk from northeast NM into 
western and central portions of the TX Panhandle. There continues 
to be a multi model signal for bringing a short wave across the 
top of the mid level ridge in place over the Southwest states, 
moving over southeast CO and northeast NM, mainly after 16/22z. 
Ahead of the short wave, a low level south southeast flow brings 
1.00/1.25 inch precipitable water air (which approaches two 
standard deviations above the mean) across the abovementioned 
areas. Model soundings showed 1000/2000 J/KG of SBCAPE in place, 
and the combination of moisture and instability should be 
sufficient to support storms developing after 16/20z. 

Lift from the short wave (and its attendant 50 knot jet streak, 
which increases difluence over the region) should be enough to 
allow the storms to form clusters or possibly a small MCS (as 
shown in the simulated IR images from the 12z NAM). The deepening 
moisture could result in hourly rainfall rates between 1.00/1.25 
inches, especially where localized training occurs before the 
activity become more outflow dominated. There is a multi high 
resolution model signal for local 2.00+ inch amounts here, which 
seem plausible given the setup. Three hour flash flood guidance 
values here are as low as 1.50 inches, due primarily to convection 
over the past couple of days. As a result, the Slight Risk area 
that was included in yesterdays Day 2 ERO was expanded to include 
western and central portions of the TX Panhandle. 


...Central-Eastern Gulf Coast...
Deep moisture and instability in the vicinity of a weakening 
closed mid level low supports convection capable of producing 
heavy to locally excessive rainfall across portions of the Central 
Gulf coast. Model soundings showed 1500/2500 J/KG of MUCAPE in 
place across LA/MS (with the highest values closer to the coast), 
which should be enough to support what should be mainly diurnally 
driven storms. Lift associated with the closed mid level low 
should be sufficient to support scattered to broken areas of 
storms, with the greatest concentration near the LA/AL coast.

A weak flow level flow allows 2.00+ inch precipitable water air to 
pool in the vicinity, which could easily result in hourly rainfall 
rates of 2.00+ inches. Weak mid level flow produces storm motions 
of less than 10 knots. Where short term training or cell mergers 
occur due to the slow storm motions, local 3.00/4.00 inch rainfall 
amounts are possible (though this is not explicitly forecast by 
regional/global models).

Three hour flash flood values are generally above 3.00 inches 
(which does not account for activity during Day 1), so short term 
training or cell mergers would likely be needed to initiate flash 
flooding. Given that the ingredients will be in place, a Marginal 
Risk was placed here to account for the threat.


Hurley/Hayes
 
Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt