Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
323 PM EDT Sun Apr 20 2025

Day 1
Valid 16Z Sun Apr 20 2025 - 12Z Mon Apr 21 2025

..THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE CENTRAL
US TODAY INTO TONIGHT...

16Z Update: Closed circulation over the TX Panhandle is beginning
to lift northeast with a steadier frontal progression to the east
as noted via the theta_E advection panels via recent mesoanalysis.
Large scale ascent is still plentiful in-of the front and just out
ahead leading to scattered convective elements from south-central
TX up through the Mid-Mississippi Valley. The previous forecast
outline remains consistent with what is occurring and general
expectations moving forward. Two main areas of concern this
afternoon and evening will be separated by two distinct forcing
mechanisms. The first will be the northern precip extent over
eastern OK extending up through MO/IA through much of the forecast
period. Much of this convective scheme will be driven by
significant height-falls and primed mid and upper ascent driven by
the shortwave trough/closed ULL pattern propagating to the
northeast along with primed RER jet dynamics from a broad anti-
cyclonically oriented jet streak positioned over the Upper Midwest.
The maturation of the surface cyclone across the Plains and points
northeast will benefit to some training potential within the
developing triple-point likely located over northern MO into
eastern IA by the evening, correlating well with a zone of enhanced
neighborhood probs of >3" (50-80%) located in the aforementioned
region. Heavy rain signature is forecast within those zones in
particular with rates reaching 1-2"/hr at peak intensity within
organized convective schema. Areas of MO are already well saturated
due to previous periods of rainfall the past 24-48 hrs, so
inherited FFG's are fairly low upfront for the setup. This allowed
for a continuation of the SLGT risk across the above area with
little deviation in the risk area outline. For more information on
the initial stages of afternoon flash flood concerns over OK/MO,
please refer to MPD #0149.

Secondary focus will align with the flow oriented out of the
Western Gulf up into east TX providing reasonable theta_E advection
within the confines of a slowing cold front, eventual quasi-
stationary boundary forecast by late-afternoon through the evening.
Radar/Sat composite already indicates convective streaks of cumulus
moving north-northeast just north of the TX Gulf coast. This
outlines a sufficiently buoyant environment capable of convective
development once initiated via either thermodynamic processes,
surface convergence, or a combination of the two. At this juncture,
expectation is for an area of thunderstorms to develop east of the
the terrain focused region of the I-35 corridor with the target
being closer to the Piney Woods area of east TX. Convective
initiation will likely settle along the frontal boundary with some
training potential due to mesovorticies becoming more anchored to
the theta_E gradient positioned in proxy to the front. HREF blended
mean output is very aggressive in forecast totals of 2-3" and
locally higher in those areas that see the slow-moving convective
pattern that initiates. The only good news is the area that will
see the heaviest precip is relatively dry with FFG's running near
and above climo for all relevant indices (1, 3, and 6-hr FFG).
Considering the low probability (10-30%) of even seeing 2"/hr rates
and 3 or 6-hr FFG exceedance probs <20% at any point, the
prospects for significant flash flooding are low. This led to
maintaining the MRGL risk across the area with only some minor
eastward adjustments to reflect the current hi-res trends in
heaviest precip placement.

Kleebauer


Day 2
Valid 12Z Mon Apr 21 2025 - 12Z Tue Apr 22 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE LOWER MISSISSIPPI VALLEY...

20Z Update: Sufficient buoyancy across the Southeast will enable
periods of scattered convection through much of Monday into early
Tuesday morning across the Lower Mississippi Valley with the
greatest coverage over LA/MS/AL. A slow-moving front will bisect
the central portion of the region with prevailing southeast flow
along and ahead of the front thanks to persistence within the
western flank of a broad surface ridge extending from the Western
Atlantic. Modest PWATs entering the 75-90th percentile will pool
over the above area creating a better environment for pulse
convective potential, as well as some organized convective elements
within the confines of the front. Storm motions are forecast to be
pretty weak for the first half of the forecast leading to a risk
of training thunderstorm activity upon initiation. 12z HREF probs
for >3" has increased closer to 45-70% within the corridor from
southern MS up through portions of western AL. Despite drier
antecedent conditions compared to points north and west, the
environment is still favorable for locally heavy rainfall in
portions of southeast LA up through the southern half of MS into
western AL, a region susceptible to flash flooding prospects due to
a plethora of urbanized areas. In coordination with the local
Jackson, MS WFO...a MRGL risk has been added across the
aforementioned area(s).

Across coastal TX, the front mentioned in the previous discussion
above will be co-located over southeast TX, slowly migrating to the
southeast which will eventually press off the TX coast. A deeper
pool of moisture will be situated along the coastal plain within
the confines of the front as convergent flow in proxy to the front
will generate a period of scattered to widespread convective
clusters that will take time to move away from impacted areas. The
good news in this setup is the rates anticipated are generally
well-below FFG indices, especially for the 1 and 3 hr markers
(~4 and 5" respectively) that would limit the flash flood
concerns. However, the slow nature of the precip could still yield
some local totals reaching 3-5" over a span of 2-4 hrs prior to the
full convective pattern moving away from a given area. The best
threat is likely within the immediate Houston metro given the lower
FFG's locally due to urbanization factors. Outside the metro,
sandier soils will limit flash flood threats below the threshold
necessary for a risk, but still non-zero overall. After some
coordination with the local Houston WFO, will opt to maintain a nil
ERO signature over the area, but will mention the threat is non-
zero and could spur a highly localized flash flood threat.

Kleebauer

Day 3
Valid 12Z Tue Apr 22 2025 - 12Z Wed Apr 23 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF TEXAS AND/OR THE ADJACENT SOUTHERN PLAINS...

20Z Update: Relatively unchanged thoughts for the period as
guidance remains spread on the exact evolution of the convective
pattern across the Southern Plains. The greatest threat will likely
be within the overlap of the lower FFG indices due to previous
rainfall the prior 72 hrs and the area where the approaching
shortwave ascent pattern will yield a greater convective coverage.
As of this time, the best chance will lie over north-central TX
towards the Red River Basin down into the northern Hill Country.
Ensemble bias corrected QPF output remains very bullish over those
areas with >1.5" of precip forecast in that zone. That would be
sufficient for flash flood potential given saturated grounds from
this past series of convective impacts. Until we get a clearer
picture across guidance, it will be difficult to pin-point any
targeted upgrades. Until then, maintained general continuity of the
MRGL risk with some trimming on the northern and southern flank of
the risk area given a combination of ensemble probabilities
favoring less of a threat for flash flooding to the north, and very
high FFG's co-located within lower probs for any precip totals >2".

Kleebauer


..Previous Discussion..

Largely maintained a Marginal Risk area over portions of the
Southern Plains from Tuesday into early Wednesday morning due to
the suite of 00Z numerical guidance continuing to show the
potential for repeat/training of cells during the day and with
overnight activity within an emerging moist and unstable return
flow into and over several frontal boundaries. With the models
struggling to latch onto a coherent signal...as shown by some
guidance focusing heaviest QPF near the Gulf coast in proximity to
a weakening cold front and other guidance developing the threat
closer to the Red River in response to the approach of a mid-level
wave...have opted to highlight concern for excessive rainfall in
broad and rather unfocused Marginal Risk area close to a consensus
placement of the precipitable water axis. With PW values generally
being 1.25 inches or less...would not be expected numerous
torrential downpours. The cumulative effect of what has already
fallen from this weekends event and what falls on Tuesday/early
Wednesday needs to be watched.

Bann

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