Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
412 AM EDT Wed Oct 17 2018
 
Day 1
Valid 12Z Wed Oct 17 2018 - 12Z Thu Oct 18 2018 

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF 
CENTRAL TEXAS...

...Texas Hill Country / Big Country / Edwards Plateau...
WPC has maintained a Slight Risk of excessive rainfall over an 
area of saturated ground and ongoing longer term impacts from 
recent heavy rainfall. This includes locations from San Angelo to 
Brownwood and stretching back southwest to Interstate 10. The 
large scale pattern remains relatively unchanged, with an upper 
low situated to the west and deep layer saturated air within the 
broad, sloping frontal zone in Texas. During the Day 1 period 
there will be some adjustment to the large scale flow as northern 
stream troughing pushes off the coast of New England, causing 
sub-tropical ridging to slide westward along the Gulf Coast - all 
while the low over the Four Corners region opens and begins to 
fill. The net effect is for deep layer ascent / moisture to 
migrate northwestward, such that the heavier new rainfall is 
expected to be a little farther north, with 1 to 2 inches on 
average up toward the Rolling Plains west of Abilene. Short term 
rain rates, themselves, are not expected to be heavy, as this area 
is well removed from the surface cold front and any sense of 
instability rooted in the low levels. Still, quarter inch per hour 
rain rates may persist, especially early in the day, over the 
Slight Risk area. Owing to recent heavy rains, Flash Flood 
Guidance values had tanked to around 0.50" / 3-hr. With ongoing 
inundation style impacts and reported rapid rises on some streams 
/ rivers, we opted to maintain Slight Risk over this area to help 
elevate awareness, although rain rates supportive of traditional 
flash flooding are unlikely to occur. Marginal Risk surrounds the 
Slight Risk and extends farther north and west toward areas where 
0.25"-plus / 1-hr rain rates may become persistent by Wednesday 
evening and overnight as the deep layer gradients tighten in this 
zone located at the periphery of the westward migrating 
sub-tropical ridge.

...South Texas...
The polar cold front had pushed well offshore as of Wednesday 
morning. MUCAPE values of any significance were analyzed to be a 
couple hundred miles off the Texas coast. Although the large scale 
pattern will migrate slowly west and northwestward during Day 1, 
it is likely that any surface-based thunderstorms will form 
offshore and move into the instability, taking them farther 
offshore. This is born out in the hi-res model QPF signal which is 
decidedly offshore for the heavier amounts. Rather than entirely 
eliminate Marginal and Slight Risk areas, however, WPC opted to 
maintain a small area of Marginal Risk over South Texas. With the 
frontal zone being shallow, precipitable water values remain quite 
high in the post-frontal regime, with PW greater than 2.00 inches 
migrating onshore Wednesday night. Organized deep layer ascent may 
be the missing ingredient, given that the mid and upper level 
gradients much better favor organized rainfall in northwest Texas. 
Still, the NAM and GFS do forecast weak upper difluence over south 
Texas, and at least one hi-res model, the NSSL WRF, does indicate 
a flare-up of stronger convective rain rates during the overnight 
hours, with cells drifting from the coast to near the Hill 
Country. NAM forecast soundings suggest little lift would be 
required to yield some instability for parcels based around 900-mb 
to 850-mb, and any convection that forms would find itself with a 
lot of available moisture and a fair amount of inflow. For these 
reasons, we will maintain Marginal Risk.

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