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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2016Z Jun 18, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
416 PM EDT Tue Jun 18 2024

Day 1
Valid 12Z Tue Jun 18 2024 - 12Z Wed Jun 19 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE UPPER AND
MIDDLE TEXAS COASTS...

...Upper and Middle Texas Coasts...

16Z... The morning update from NHC did not make enough change to
require any QPF adjustments, therefore no adjustments were made to
the Moderate, Slight or Marginal Risk areas across Texas and the
Gulf Coast. Another assessment will be made this afternoon.

Campbell

Potential Tropical Cyclone 1 (PTC1) will continue to organize
across the southern and western Gulf today. Associated moisture
originating in the Caribbean will stream northwestward into the
Texas coast around the northern and eastern periphery of the
storm. IVT values are very impressive...building to the upper end
of the scale at 1,500 kg/m/s. This means PWATs with the storm will
consistently increase through the day, approaching 3 inches by the
time the storm's full brunt moves into the coast tonight. PWATs
approaching 3 inches mean the atmosphere will be carrying about as
much moisture as it can possibly carry...and storms that form in
this environment will be capable of extremely heavy rainfall rates
due to highly efficient rainfall processes. Moisture lost by the
rainfall will be quickly replaced as moisture advection (signified
by the extremely high IVT values) will be optimal.

All of this to say, as the plume of moisture associated with PTC1
and supported by advection straight out of the Caribbean moves into
the upper Texas coast tonight, it will contain convection capable
of 3 inch per hour rainfall rates. These rates will easily
overwhelm local streams and creeks as they track northwestward, no
matter how dry/empty they were prior to the start of the rain. This
will likely catch many off guard, so it's important to avoid
crossing flooded roadways.

With the better-forecast organization of PTC1, it's expected that
the associated plume of moisture and heavy rainfall will be more
consolidated as it approaches the Texas coast as compared with
previous forecasts. This means the timing of the heavy rainfall has
been slightly delayed, and is less likely to extend too far north
from the coast. Thus...the inherited ERO risk areas have been
trimmed from the north from inherited. It's likely that it will
take until after 06Z tonight before consistent and steady heavy
rainfall begins moving into the coast, with only scattered
convection expected prior to that. However, given the
aforementioned heavy rainfall rates expected when the directly
associated rainfall with PTC1 moves ashore, numerous instances of
flash flooding are likely late tonight in the Moderate Risk area.
Further west in the Slight Risk area, there will be slightly less
time for the heavy rainfall to cause flash flooding since it will
be starting later.

For Louisiana, the forecast track of PTC1 and aforementioned
consolidation of the precipitation shield should keep most of the
heavy rainfall offshore, so the Moderate Risk area was trimmed out
of the southwestern coast of the state with this update.

...Central Plains into the Midwest...

16Z update... The latest guidance did show an increase in
potential for heavier showers to move through the panhandles of
Oklahoma and Texas with hourly rain rates approaching 2.5/3 inches
per hour. The Slight Risk and Marginal Risk areas were expanded a
couple row of counties to the southwest to account for this trend
and lowered FFG.

Campbell

Ongoing heavy convection across the Dakotas and northern Minnesota
is associated with a low over western Nebraska. The low will track
northward into eastern SD and eventually into the Red River of the
North by tonight. The convection is likely to be ongoing into
northern Minnesota by the start of the period at 7am this morning.
The heavy rain will persist there and far northwestern Wisconsin
for a few hours this morning, resulting in widely scattered
instances of flash flooding. The inherited Slight Risk for this
area is little changed, and will likely be able to be trimmed out
of northern MN and WI with the midday update today.

Further south, the cold front associated with the low is expected
to stall across Kansas, eastern Nebraska, and western Iowa today. 
Late this afternoon, convection is expected to break out along the
front, and increase in coverage and intensity through the evening.
A small area of high pressure over the western Dakotas tonight will
support northerly flow to the north of the front. Meanwhile, the
typical strengthening of the LLJ and southerly flow over the
southern Plains will advect plentiful Gulf moisture into the front
from the south. The clashing of the 2 air masses will keep the
front from moving very much tonight, allowing the clashing to
continue over the same areas for multiple hours tonight. The
resulting convection will also train and backbuild over the same
areas along and immediately south of the front. This will support
widely scattered instances of flash flooding across much of the
rest of the Slight risk area from southwest Kansas northeast to
western Iowa.

The greatest risk of flash flooding will be over Kansas tonight, as
this will be the southern end of the front, where the 2 clashing
air masses will be most opposed to each other. This will keep any
storms that form from moving very much, and the influx of moisture
from the LLJ will support additional convective development. There
has been the typical uncertainty and inconsistency in the guidance
as to where the southern end of the front will be. 24 hours ago it
was expected to be in north-central Kansas and into eastern
Nebraska. Now there's much better agreement across southwestern
Kansas. Thus, this area is at highest risk of flash flooding.
Meanwhile further north and east, amounts should come down a
bit...but unidirectional southwesterly flow parallel to the front
will still support training storms and potential for flash
flooding. Thus, the Slight Risk area from Kansas north and east was
nudged east a bit with guidance trends, but is largely unchanged.

...Central Appalachians...

16Z update... The latest hi-res guidance continues to support
storms capable of producing rain fall rates up to 2 inches/hour
through the afternoon and into the evening hours. There was a trend
for increasing coverage of these storms across portions of central
and southern West Virginia and across eastern New York and
Pennsylvania, especially for the late afternoon and evening hours.
Given these trends the Marginal Risk area was expanded southward
and eastward.

Campbell

"Ridge-running" precipitation is expected to track across eastern
Ohio through central New York today. The storms will be supported
by an air mass with PWATs up to 1.75 inches. The storms will be
capable of local rainfall rates to 1.5 inches per hour, which
depending on where they form may cause isolated instances of flash
flooding. There is somewhat better agreement for a bit better
organization of convection in this area. Thus, a Marginal Risk area
was introduced with this update.

Wegman

Day 2
Valid 12Z Wed Jun 19 2024 - 12Z Thu Jun 20 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN TEXAS AND PORTIONS OF THE MIDDLE AND UPPER TEXAS COASTS...

...South Texas...

21Z update... There was an overall southward trend with the
tropical rainfall across Texas and portions of the Gulf Coast. The
highest rain is expected to focus over south-central and southern
Texas where areal averages of 3 to 8 inches are forecast with
isolated maximums creeping into the double digits. With this
southward shift in the QPF footprint all Excessive Rainfall Outlook
thresholds were adjusted to the South.

Campbell

Potential Tropical Cyclone 1 (PTC1) is expected to turn westward
across the western Gulf on Wednesday and make landfall in the
southern Mexican state of Tamaulipas Wednesday night based on the
4am CDT update from the National Hurricane Center. While the center
will be well south of Texas, a nothing short of impressive plume of
moisture characterized by IVTs pushing the top of the scale at
1,500 kg/m/s will continue advecting northwestward out of the
Caribbean, across the Gulf, and into south Texas on Wednesday. PTC1
will tap into this moisture plume as it tracks westward towards
Mexico. This will greatly expand the associated precipitation
shield north of the center of the storm. Thus, a prolonged period
of heavy rain is likely across all of south Texas Wednesday and
Wednesday night.

Heavy rain will be ongoing across nearly all of the Texas coast at
the start of the Day 2/Wednesday period at 7am Wednesday. With
PWATs approaching 3 inches from Houston/Galveston southwestward
down the entire Texas coast, the convection that will be embedded
within the broader precipitation shield associated with PTC1 will
be capable of extremely heavy rainfall rates as high as 3 inches
per hour with the strongest storms. Rates this high will easily
overwhelm smaller streams and creek watersheds with water falling
close to all at once. Thus, rapid-onset flash flooding is likely
across much of southern Texas with this storm.

For most of the Moderate Risk area, the bulk of the rainfall with
PTC1 will occur during this Day 2/Wednesday period. The rain will
overspread south Texas from east to west. Thus, areas along the
coast will see their heaviest rain during the day Wednesday, while
towards the Rio Grande the heavier rain will be towards evening and
into Wednesday night. Think the greatest rainfall rates will be
with the parent easterly wave associated with PTC1, though heavy
rain will be likely due to moisture availability well before and
after the passage of the wave. As the bands of rain track westward
and inland, they will very gradually weaken with time as they
become separated from their moisture source...the Gulf. However,
the extreme IVT will support them well inland from the coast.
Nonetheless, the highest rainfall totals associated with PTC1 are
likely closer to the coast, with only gradually diminishing
rainfall totals as you move inland, as upsloping into the terrain
plays an increasingly important role in the development and support
of heavy rain. 5 to 10 inches of rain with isolated totals as high
as 15 inches are expected across the Moderate Risk area.

Many portions of the Moderate Risk area have been very dry of late,
so empty rivers and streams will initially preclude much
flooding...though as mentioned the extreme rainfall rates will
locally exceed FFGs, regardless of how dry the area was before the
storm. By the tail end of the event numerous instances of flash
flooding are likely, so the Moderate Risk remains in effect with
few changes from inherited. Note the antecedent dry conditions may
lead many to a false sense of security as regards flooding...and
rapid onset flash flooding will be common.

The surrounding Slight Risk area was dramatically trimmed on the
northern end out of north Texas with this update. As mentioned, the
consolidated nature of the convection should hold much of the
associated rainfall closer to the storm center over south Texas.
Thus, much less rainfall is expected into north Texas, and the
Slight Risk area was cancelled into the DFW Metroplex.

...Central Plains...

21Z update... The latest guidance suggested an increase in
thunderstorm coverage and QPF amounts across portions of Kansas and
northern Oklahoma during this period. As such a small expansion of
the Slight Risk was made on the southern side across south-central
Kansas and into portions of the Oklahoma and Texas Panhandles.
Some the 1hr/3hr FFG guidance is as low as 1.5/2 inches in that
part of the region.

Campbell

A small Slight risk area was introduced with this update across
portions of southwest and south-central Kansas. Ongoing convection
from the overnight period Tuesday night is expected across this
region, with ongoing widely scattered instances of flash flooding
ongoing. The southern/eastern end of the line of convection will
track across the Slight Risk area through the morning Wednesday
before dissipating.

The inherited Slight risk area across portions of north-central
Kansas and eastern Nebraska was downgraded with this update. This
is in regards to the uncertainty with where the southern end of the
front would be from Day 1/Tuesday. Since the general consensus in
the guidance is for the southern end of the front to now be in
southern Kansas, north Kansas into eastern Nebraska will miss out
on the heaviest rain, so the signal for heavy rain in this area has
diminished significantly.

...Midwest...

Elsewhere the large Marginal from the Plains into the Great Lakes
is largely in deference to the various waves of moisture streaming
across the area on the northern end of the large high pressure area
in the upper levels over the Southeast. Convection will likely be
fast moving and largely disorganized, but since various sections of
the Marginal risk area have seen heavy rains in recent days, the
threat for isolated instances of flash flooding from these storms
is there right through central New York.

Wegman

Day 3
Valid 12Z Thu Jun 20 2024 - 12Z Fri Jun 21 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE RIO GRANDE
VALLEY OF TEXAS TO THE LOWER TEXAS COAST AS WELL AS FROM EASTERN
SOUTH DAKOTA TO WESTERN WISCONSIN...

...Texas...

21Z update... The rainfall associated with PTC1 is expected to
continue progressing westward during this period, primarily
focusing over South Texas, Rio Grande, West Texas and eastern New
Mexico. With the decreasing trend across portions of central and
eastern Texas, the Slight and Marginal Risks reduced to the
south/southwest.

Campbell

Residual rainfall from what will likely be dissipated PTC1 will
continue across the Rio Grande Valley Thursday morning. The Slight
was expanded to include the entire Lower Rio Grande Valley and
Lower Texas Coast largely due to expected ongoing flash flooding at
the start of the period Thursday morning. Additional rainfall
during the day Thursday should be mostly light, but abundance of
moisture will still support some widely scattered convection
capable of producing flash flooding. The heaviest rain this period
will be across the Big Bend region, where a higher-risk Slight is
in effect. 2-4 inches of rain with locally higher amounts are
likely in the Big Bend region. This is highly unusual in such a
widespread manner from a tropical cyclone. Thus, this area has a
low, but non-zero potential for further upgrades with future
updates due to the infrequency of this amount of rain in this
usually very dry area. There is a bit more uncertainty with how far
north up the dry line into West Texas/southeast New Mexico the
persistent heavy rain gets as the whole plume quickly weakens, so
future adjustments to the Marginal Risk in this area are likely.

...Northern Plains and Portions of the Upper Midwest...

21Z update... There will be some of the tropical moisture from
PTC1 riding over the ridge and combining with the moisture already
in place over the northern tier states. This will result in a
modest boost in rainfall rates and the overall foot print. Given
the some of the increased trends, the Marginal Risk was expanded
northwest Nebraska and far eastern Wyoming. The Slight Risk had
small expansions south across South Dakota and southern Minnesota.

Campbell

A Slight Risk area was left largely unchanged with this update. A
shortwave and right entrance region of a small 100 kt jet streak
will track over a front over the area with attendant southerly flow
of moisture up the central Plains and into this area. Recent heavy
rain, including in some areas where its ongoing as of this writing
will lead to wet soils that are more susceptible to flooding from
heavy convection. The convection is most likely to impact this
region Thursday night.

...Four Corners Region...

21Z update... A very small eastward bump of the Marginal Risk area
was made across south-central Colorado to account for some of the
guidance showing convection further east from the favored areas of
terrain.

Campbell

The meeting of the remnant upper level energy from PTC 1 will
interact with a slow moving trough over California. Residual
moisture will interact with the terrain in this area. The result
will be scattered showers and thunderstorms capable of heavy rain
that may cause isolated flash flooding since this area is typically
a dry area and not able to handle much heavy rain before flooding
occurs.

Wegman


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt