Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
433 PM EDT Sat Sep 21 2019
 
Day 2
Valid 12Z Sun Sep 22 2019 - 12Z Mon Sep 23 2019 

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL EXTENDING FROM THE 
MID MISSISSIPPI ACROSS A PORTION OF THE UPPER GREAT LAKES AND 
WESTERN OHIO VALLEY...

21Z Update - Based on 12Z model trends, shifted the outlook areas 
farther to the northwest across the mid Mississippi valley and 
Great Lakes regions.  Also extended the areas farther to the 
southwest across eastern Oklahoma into the Red River region.

Across the Southwest, shifted the Marginal Risk area farther 
southeast into southeastern Arizona and far southwestern New 
Mexico.  Southwesterly flow drawing deeper moisture into the 
higher terrain of the region may support some locally heavy 
amounts Sunday afternoon/evening, otherwise the 12Z models 
continued to signal little potential for widespread heavy 
amounts/flash flooding across the Southwest through Day 2.

Previous discussion -

...Mid Mississippi Valley into the Upper Great Lakes and western 
Ohio Valley...
Deep moisture and marginal 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 2. There is still some 
model spread concerning how quickly the front drops south, which 
results in some north/south 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 on a multi model blend, 
including the 00z NAM CONUS Nest and the 00z WRF ARW.  
  
Convection is expected to be ongoing along the front extending KS 
into northern MO during the first part of Day 2, as a surface wave 
crosses the region. Behind the surface wave, the front is expected 
to drop 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) 
across the region. Model soundings from the 00z GFS/NAM showed an 
axis of 500/1000 J/KG of MUCAPE, as cloudiness from the Day 1 
convection could tamp down the instability available for storms 
during Day 2. The newly developing storms could become better 
organized 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 central MO into central 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 threat for 
additional training. 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. Given the potential for at least some training along the front 
in a high moisture content airmass, a Slight Risk was extended 
from southeast KS/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 1. If this occurs, a Moderate Risk across portions of eastern 
KS/northern MO could be needed in later forecasts.


...Arizona...
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 2. 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 2, 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 widely 
scattered convection across portions of southern AZ during this 
time period.  
  
Model soundings from the 00z GFS/NAM showed 500 J/KG of MUCAPE 
ahead of the short wave, which should be enough to allow deeper 
convection to form. 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.  
  
A Marginal Risk was placed over much of southern AZ for Day 2. 
However, the model support for this has diminished over the past 
24 hours (with the exception of the 00z UKMET), and if this trend 
continues, the Marginal Risk could be dropped in later forecasts.

  
Pereira/Hayes
 
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt