Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
835 PM EDT Tue Oct 16 2018
Day 1
Valid 01Z Wed Oct 17 2018 - 12Z Wed Oct 17 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
CENTRAL TEXAS...
...Southern U.S...
Will maintain a Slight Risk across portions of central TX, where
there is an overlap of low flash flood guidance and forecast
additional rainfall overnight. Stability will be the limiting
factor here, although some weak elevated instability will still
allow for localized convective cores capable of dropping amounts
up to 0.5" in an hour. Overall appears like an additional 1-2" of
rain through 12z Wednesday is likely over a good portion of the
Slight Risk area. Did not think a Moderate Risk was justified for
the remainder of the night given the generally weaker rates and
additional rainfall amounts, combined with the fact that the
heaviest overnight rainfall should generally be just north of the
hardest hit areas over the last couple days. With that said, this
area has still been anomalously wet of late, and some recent
colder cloud tops are likely indicative of briefly heavier rates
(which should continue overnight)...and thus think some continued
areal flooding and localized flash flooding is still possible
across the Slight Risk area.
Surrounded this Slight Risk with a broader Marginal Risk. The QPF
signal is weaker across this region overnight, however with wet
antecedent conditions here as well, if any briefly heavier rains
were to develop, a localized flash flood risk would exist.
Chenard
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
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
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 PM EDT Tue Oct 16 2018
Day 2
Valid 12Z Wed Oct 17 2018 - 12Z Thu Oct 18 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER COASTAL SOUTH
TEXAS...
...Texas...
The first area of concern for excessive rainfall is along the
south coast of TX. The precipitable water values of 2- 2.25
inches are forecast to move onshore and into far south Texas on
low level easterly flow. The low level trough and convergence in
coastal areas enhances lift across the area.
With pockets of elevated instability anticipate some convection to
develop off the coast and maintain as it moves onshore. Any
convective clusters will be heavy rainmakers, possibly training
across the same locations given the mean flow may become aligned
with the propagation vectors. This will result in rain rates
approaching 1+ inch/hour which may produce some localized flood
concerns across far south TX. While areal averages are low, around
1-1.5 inches, localized higher precipitation amounts are expected.
Another area of concern is for ongoing precipitation across
central TX as waves of moisture ride along the 850mb front. This
combined with divergence aloft will help precipitation continue in
this region. Given the recent rounds of rain and saturated soils
in place, it may not take much precipitation to cause flood
impacts. So while the areal averages are around 1 inch and a
stratiform rain event is expected with modest rain rates, these
values might be enough to produce localized flooding. Based on
this, a marginal risk remains across central TX.
Petersen/Pagano
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
416 PM EDT Tue Oct 16 2018
Day 3
Valid 12Z Thu Oct 18 2018 - 12Z Fri Oct 19 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER COASTAL SOUTH
TEXAS...
...Eastern to Southern Texas...
Southerly mid level flow develops which promotes warm/moist
advection over the low level front in eastern Texas, and provides
coastal convergence in southern Texas. The onshore flow promotes
continuation of the above normal precipitable water values of
2-2.25 inches (2-2.5 standard deviations above the mean). This
combined with strong surface convergence right along the coast and
upper level support for lift from divergence maxima in the coastal
waters and coastline will likely result in some convective
clusters moving onshore and inland with possible training.
Rainfall rates of 1-1.5 inches/hour are possible. Given prior
rainfall from day 2 likely continuing into day 3, a slight risk
was shown, particularly given the models trended a bit higher with
day 3 rainfall totals than day 2. The UKMET/Canadian
global/NAM/GFS have a forecast of several inches of rain just off
the coast, and the UKMET shows heavier rain making it onshore
during this period.
Petersen/Pagano
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
