Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1142 AM 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 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt