Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
857 PM EDT Mon Jul 22 2019
 
Day 1
Valid 1942Z Mon Jul 22 2019 - 12Z Tue Jul 23 2019 

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS 
OF THE TENNESSEE VALLEY AND CENTRAL APPALACHIANS...AS WELL AS A 
PORTION OF THE NORTHERN MID ATLANTIC...

...Lower Mississippi Valley through the Tennessee Valley into 
Central Appalachians, and portions of the Mid Atlantic and 
Northeast...
The first batch of excessive rainfall continues across portions of 
southern New England early in the period, as the storms follow the 
deepest moisture and edge of the instability gradient offshore 
this evening. Based on extrapolation, the flash flood threat 
across southern New England should wane as storms move offshore, 
along with the best instability and moisture. 

Further south across NJ/PA into the southern Appalachians, There 
is a lull in the activity as the airmass across this region has 
been overturned by earlier storms. However, the airmass change 
lags the frontal boundary, and the moisture and instability remain 
in place through 23/05z along and east of the Interstate 95 
corridor. It remains unclear how much the airmass can recover 
after the previous storms, but in deference to the most recent 
HRRR runs, the Slight Risk remains in place through the overnight 
hours, though it is possible that the greatest threat could end 
before 23/06z. Based on input from WFO PHI, the Moderate Risk was 
left in place over a portion of eastern PA into northwest NJ, as 
this area was hit hard by earlier flash flooding. 

Finally, across KY/TN, the airmass remains relatively unaffected 
by the earlier convection passing to the north, as convection 
follows the best instability axis into eastern TN/eastern KY 
before 23/03z. As the storms track eastward, they will tap in to 
the 2.00+ inch precipitable water air just ahead of the front. As 
the mid level flow parallels to the slowly sinking frontal 
boundary, training remains the primary flash flood threat, as 
hourly rainfall rates could top out near 2.00 inches (which is 
supported by the most recent HRRR runs). High resolution guidance 
suggests that local 3.00/4.00 inch rainfall amounts are possible 
before the instability is shunted southward with the front. Based 
on this, the Moderate Risk was left in place for the evening 
hours. 


...Portions of the Southwest into the Southern High Plains...
Monsoonal moisture across portions of the Southwest into the 
southern High Plains will fuel heavy to excessive rainfall, mainly 
during the evening into the early overnight hours

There are three areas of concern for this outlook period. The 
first is southeast NM, where a low level easterly flow continues 
to feed deepening moisture (as precipitable water air values top 
out near 1.50 inches) and 2000/3000 J/KG of MUCAPE across west TX 
into southeast NM. An MCV dropping south from northeast NM will 
provide sufficient lift to support convection along the 
instability axis. Storms or small convective clusters could train 
along the TX/southeast NM border during the evening hours, 
resulting in hourly rainfall rates near 1.50 inches. 

The MCV drops southeast to the far western TX Panhandle before 
23/06z, as the instability either slowly weakens or becomes more 
elevated. Some of the high resolution guidance (including the most 
recent HRRR runs) have been slow to pick up in the development, 
even though the most recent RAP runs have depicted the placement 
and movement of the MCV well. Thus, the flash flood threat could 
continue into the early morning hours.

The second area of concern is northwest NM, where convection 
forming in the differential heating over the terrain in northwest 
NM. The convection is expected to be more scattered in nature than 
southeast NM, as the mid level forcing under the ridge here is not 
nearly as impressive. However, slow cell motions to the south 
could result in cell mergers and short term training, which could 
support hourly rainfall rates over an inch. Three hour flash flood 
guidance values here are as low as 1.50 inches, so a flash flood 
threat will continue until the activity until the instability in 
place becomes exhausted, which could occur as later as 23/06z.   

The third area of concern is a small portion of south central AZ, 
mainly for the evening hours. There is a multi model signal for 
convection developing over nearby Mexico to track over south 
central AZ this evening. The convection follows a monsoonal 
moisture plume (with precipitable water values nearing 1.75 
inches, which is near two standard deviations above the mean) on 
the broad southeast flow under the mid level ridge in place over 
the Southwest. Being just east of the mid level ridge position, 
slow northwest cell motions are expected (generally under 10 
knots) into south central AZ. The slow cell motions could foster 
an environment for cell mergers and short term training, which in 
turn could result in hourly rainfall rates approaching 1.50 inches 
(which is supported by members of the 12z model suite, as well as 
the last few runs of the HRRR). 

Instability over this area is impressive, with the most recent 
HRRR/RAP soundings showing local 2000/3000 J/KG of MUCAPE in place 
through 23/06z. Model soundings also indicate that the cap remains 
in place for much of the evening into the early overnight, which 
could limit the areal extent of the convective coverage. However, 
with the ingredients of flash flooding in place, a Slight Risk was 
left in place here. 

Hayes

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