Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 AM EDT Thu Jun 12 2025

Day 1
Valid 12Z Thu Jun 12 2025 - 12Z Fri Jun 13 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHEAST TEXAS...

...Texas through Arkansas...

In coordination with HGX/Houston, TX forecast office, a Moderate
Risk upgrade was introduced with this update. Convection blossoming
over central Texas right now is forming in response to increased
upper level divergence ahead of a sharp upper level trough over
west Texas. This trough is also supporting a potent LLJ advecting
deep tropical moisture out of the Gulf and into east Texas. This
steady supply of plentiful moisture along with a very slow-moving
upper level trough are both primary ingredients for slow-moving
thunderstorms capable of producing very heavy rainfall. Storm
coverage will continue to increase so that by the start of the
period later this morning, a well-defined MCS will have developed
north and west of Houston. The storms will build southeastward
towards the Gulf as they advect towards the moisture and
instability source. Additional storms forming along the coast will
also advect north into the MCS. Since the southerly flow associated
with the LLJ will continue through the day, this will promote
backbuilding storms along the cold pools of the existing storms,
resulting in new activity. The result will be several hours of
heavy rainfall in some areas, which even in areas with higher FFGs,
are likely to be exceeded due to the prolonged duration of heavy
rain in any one area.

On the south side of the MCS, the backbuilding storms may form an
east-west oriented line which could train over the areas that part
of the line moves over. This will locally further enhance any flash
flooding threat, which would be enhanced further should it occur
over Houston or another urban area. Antecedent soil conditions have
already been saturated in many areas of southeast Texas, so this
additional rainfall could cause flash flooding quickly.

The MCS will gradually develop into a southeastward moving line
through the morning hours, which should gradually reduce the flash
flooding threat as the storms approach the Louisiana state line.
Urban concerns in and around Houston will also increase the flash
flooding threat. The good news is by midday or so, the line will be
diminishing and will be moving largely orthogonal to the
orientation of the line, which will greatly decrease the time any
one area sees heavy rain. Further, instability and competing
sources of forcing should disrupt the MCS in typical diurnal
fashion. This too should work to reduce the flash flooding threat.
Thus, the Moderate Risk is really just for the morning and early
afternoon, and will likely be able to be downgraded this afternoon.

Further north into Arkansas, the northern end of the MCS will cause
additional storms to form across the state this morning. Most
areas will see a break through the day, but additional convection
forming this afternoon through tonight behind a developing mesolow
will add to the precipitation totals. Further topographic
influences with the Ozarks may add upsloping to the forcing
mechanisms. Thus, expect a secondary "bullseye" of heavy rainfall
across much of central to southwestern Arkansas. The Slight Risk
area was adjusted eastward with this update as the guidance better
resolves the ongoing shower and thunderstorm activity. The risk
areas were generally reduced from the DFW Metroplex west, and
increased in central Louisiana and central Arkansas. A higher-end
Slight is in effect for the area from the Moderate Risk north and
east into central Arkansas.

...Northern Plains...

At the northernmost head of the LLJ of deep tropical moisture out
of the Gulf across Minnesota, the plume of moisture will run into a
warm front that will add lift to the atmosphere as the moisture
runs up the frontal interface. Divergence aloft in the right
entrance region of a 120 kt jet will also support shower and
thunderstorm development in the fast upper level westerly flow
associated with the jet. Shower and thunderstorm activity have
already broken out along the Minnesota/Iowa border this morning. As
the warm front lifts north through the day, the coverage of
rainfall will diminish a bit with daytime heating disrupting the
synoptic scale lift in favor of much more chaotic local forcings.
By tonight however, as the typical diurnal LLJ reestablishes
itself, showers and thunderstorms will once again redevelop, but
this time further north into central Minnesota, and likely near the
Twin Cities. The strengthening supply of moisture associated with
the LLJ will support backbuilding and training storms, as the cold
pools work to lock where the storms form in place, despite the
continued northeastward movement of the synoptic-scale warm front.
This will support a rather narrow (50-80 mile wide) corridor where
heavy rain on the order of 1-3 inches with locally higher amounts
are expected tonight. This should cause widely scattered instances
of flash flooding. The Slight Risk area was nudged northward with
this update in keeping with the latest suite of guidance, which has
been struggling with where to place the warm-front related
convection on the map for the last few days.

Wegman

Day 2
Valid 12Z Fri Jun 13 2025 - 12Z Sat Jun 14 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF
ARKANSAS...

The very slow moving upper level trough will continue to drift
across Missouri and Arkansas through the period. Continued
northward advection of deep tropical moisture will run into the
Ozarks as a dryline also gradually pushes east out of the Plains.
The combination of each of these forcings over an area with
topography and likely very wet antecedent soil conditions will all
support continued potential for scattered instances of flash
flooding across much of Arkansas. The heaviest rain will likely
hold off until after midnight Friday night, with the diurnal
strengthening of the low level jet, a faster push of the dryline
and associated sharp gradient in moisture, as well as the peak
divergence as the upper level shortwave trough amplifies a bit.
All of these support the continuation of the Slight Risk over
Arkansas, which was shrunk a bit due to better confidence in the
placement of the heaviest rain, allowing areas further north
towards Paducah to be dropped from the Slight Risk. With the
wettest soils in Arkansas from about Little Rock west, this area
was maintained in a higher-end Slight with this update.

Elsewhere, lingering rain from Minnesota east through the U.P. will
support isolated instances of flash flooding in that region,
largely on Friday morning. A new Marginal Risk was introduced from
southeast Virginia through the Carolinas and into Georgia. While
convection in this area will likely remain disorganized and
dominated by cold pool forcings, the abundance of moisture on the
order of over 2 inches of PWAT will still favor any storms that
form having the capability of multiple inch per hour rainfall
rates. The Marginal from Montana to northeast Colorado was
maintained with minimal changes along the western interface of the
deeper moisture over much of the Plains.

Wegman

Day 3
Valid 12Z Sat Jun 14 2025 - 12Z Sun Jun 15 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM CENTRAL
ARKANSAS THROUGH NORTHERN MISSISSIPPI...

...Arkansas through Alabama...

After multiple days of shower and thunderstorm activity in this
area, there should be some decrease in the organization of the
overall shower and thunderstorm activity expected Saturday from
Arkansas east through Alabama. Despite that decrease, very wet
soils through Mississippi and a lingering frontal boundary
providing some forcing should still lead to enough organization to
support widely scattered instances of flash flooding. For this
region, a Slight Risk area was introduced for Arkansas and
Mississippi with this update. The flash flooding risk has likely
decreased a bit due to a lack of upper level support by Saturday,
so the Slight Risk remains a lower end Slight. Plentiful moisture
over 1.5 inches of PWAT will still support any storms that do form
to be capable of heavy rainfall. Further, a lack of forcing and
organization will increase the time any storms may impact any one
area, to include much of the period, day and night. In other words,
there isn't one specific period of time where the threat for heavy
rain and subsequent flash flooding is significantly higher than any
other point.

...Mid-Atlantic...

A second signal for heavy rain has shown in the guidance across
northern Virginia and into portions of the Tidewater. A slow,
southward moving front over PA will interact with the divergence
ahead of the slow-moving shortwave trough that caused the problems
with flooding further west the past couple days. Since this signal
is relatively new and highly subject to change, a Slight risk
upgrade has not yet been introduced, but is being considered.
Should the heavy rain signal drift north to the DMV area, then a
Slight Risk will likely be needed.

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