FOUS30 KWBC 260819
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
419 AM EDT Wed Mar 26 2025

Day 1
Valid 12Z Wed Mar 26 2025 - 12Z Thu Mar 27 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTH-CENTRAL AND SOUTH TEXAS...

Model guidance continues to become better clustered in terms of the
timing and areal extent of a heavy rainfall signal across SC-South
TX later this afternoon and overnight. Increasing upper level
difluence and DPVA ahead of a slow moving upper trough will
initiate organized convection across along the Rio Grande in SW TX
later this afternoon, which will begin to grow upscale toward the
evening hours given the uptick in SSE low-level inflow (25-40+ kts
at 850 mb). This area has been very dry and could use the rain, 
unfortunately this setup looks good enough to be a case where it's 
beneficial rain initially, but then quickly overwhelms even the dry
soil conditions. Model soundings by Wed afternoon/evening show a 
deeply saturated profile with good directional shear and moderate 
instability. This should support some convective clusters with some
weak mesocyclones and right mover motions are very slow. Combine 
that with the front nearby and anticipate upscale growth of slow 
moving convective clusters through Wed night within an environment 
capable of 2-3" per hour rainfall. Activity may eventually grow 
upscale enough that it begins propagating east and possibly 
offshore, however even if that eventually transpires, there will 
still be a prolonged period where training convection is a possibility.

Overall there is above average model agreement on this overall
evolution. Moreover, the latest (00Z) HREF probabilities have
bumped up over the past few runs, including probabilities >50% of
greater than 5" of rain in 24hrs (which is a good proxy for a
Moderate Risk area based on verification). There is also a chance 
for an upgrade to High Risk within the Moderate Risk area, albeit 
quite narrow. This is where the HREF has 70+ probs of rainfall 
exceeding 5"/24hrs, along with 40-50% probs of >8"/24hrs.

While confidence on the event happening is above average, there is
still uncertainty on the exact axis of heaviest rainfall, which 
will probably be somewhat narrow in nature. The 00Z ECMWF in fact
came down considerably with the QPF during day 1, and as with its 
previous run, again has more of a diffuse look with multiple axes 
of heavier rain on either side of the Moderate Risk area. So again,
the MDT risk is centered around the highest HREF probabilities, 
and generally on the southern gradient of the global model QPF 
solutions. In fact, changes made to the previous (Day 2) ERO 
include pulling the southern boundaries of the Moderate, Slight, 
and Marginal Risk areas a bit farther south across South TX. Flash 
flood coverage over this area is expected to increase Wednesday 
night into Thursday morning, and locally significant impacts are possible.

...Pacific Northwest...
A Marginal risk continues across portions of western OR and WA. 
Convection is expected to develop over portions of southwest OR by 
21z Wed and then track off to the north through the afternoon and 
evening hours. An anomalous amounts of CAPE is forecast, 
potentially upwards of 2000 j/kg, and PWs are forecast over the 
climatological 90th percentile. This combination of CAPE and PWs 
supports intense rainfall rates. These cells should be quick moving
and the bigger hazard is probably severe weather (as highlighted 
by the SPC Slight risk), however the intense rainfall rates 
producing over 1" in an hour may be enough for localized flash 
flood concerns. Areas of terrain, urban areas, and low lying areas 
will be most prone to localized flash flooding driven by these 
short duration intense rainfall rates.

Hurley/Chenard


Day 2
Valid 12Z Thu Mar 27 2025 - 12Z Fri Mar 28 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTH-CENTRAL AND SOUTH TEXAS AND MUCH OF THE TEXAS GULF COAST...

...South Texas...
Minimal changes were made to the Day 2 ERO areas inherited from
yesterday's Day 3 outlook. Based on the latest guidance trends, we
did nudge the Moderate Risk a bit farther up the Middle TX Coast. 

The event described in the day 1 discussion over south TX will 
continue into Thursday. There is a bit more uncertainty on the 
details by this time as convective evolution will likely be 
impacted by mesoscale influences of the day 1 convection. On the 
synoptic scale, ingredients are likely even more favorable for 
heavy rainfall on day 3 then they were on day 2. Large scale 
forcing should reload and be even a bit stronger as the mid level 
trough axis and associated shortwave energy moves across the area
(enhancing the upper divergence/DPVA). 

However, the extent of the excessive rainfall risk will likely be 
at least partially dependent on mesoscale factors...such as the 
outflow location form the day 1 convection and its impact on 
instability. The expectation is that the outflow boundary will be 
near the coast by this time, and so that is where the greatest 
focus for training convection is expected on Thursday, and thus 
where the MDT risk is in place. There is some chance this is 
offshore by 12z Thursday, but the current consensus suggests it may
struggle to clear the coast, leaving coastal areas susceptible to 
additional heavy rainfall and flash flooding. Even if instability 
is eroded by Thursday, the increase in forcing and southerly flow 
should be enough to destabilize portions of the area again with 
time, and so do think at least some flash flood risk exists even in
a scenario where convection initially propagates offshore,
especially Thu night with the uptick in S-SSE low-level flow. 

Overall confidence is high in a flash flood event, but confidence
in the details by Thursday-Thu night remains low. In a worst case 
scenario outflow stalls near the coast allowing for repeat 
convection and a significant flash flood event could evolve. In a 
best case scenario convection propagates offshore, but 
redevelopment later in the day still results in some flash flood 
threat, albeit probably less significant. When combining day 2 and 
3 it seems probable that some areas receive 5-10" of rain over 
south TX, and localized amounts of 10-15" are certainly possible. 
Confidence is increasing in the coverage of flash flooding and the 
possibility of at least localized significant impacts, warranting 
the Moderate risks on both days 2 and 3. 

...MS and OH Valley...
A Marginal risk continues from yesterday's Day 3 ERO across portions
of northern MO, southern IA, IL and central/southern IN. A 
stationary front draped across this region will gradually lift 
north as a warm front through the period. Southerly flow will 
result in an uptick in instability and moisture, and at least 
scattered convection is expected Thursday into Thursday night. 
There is at least some chance for organized convective development 
near the front, which could briefly train from west to east. Global
model QPF is locally as high as 1-3" along this corridor, and 
given the expected convective nature of activity, this would 
suggest isolated amounts over 3" are probable. A localized, and 
generally minor, flash flood risk could result.

...Pacific Northwest...
Another modest Atmospheric River (narrow ribbon of 500+ kg/m/s
IVTs) will result in pockets of 3-5+ inch 24 hour rainfall totals
along the West Coast from Northern CA northward through OR-WA.
Among the 00Z 3/25 models, the Regional GEM is by far the wettest,
with isolated totals of 6-8+ inches. Despite the lack of
instability, the wet antecedent soils along with the potential of
0.3-0.5 in/hr rainfall rates could lead to localized short-term
runoff issues, particularly along small streams and over burn scars.

Chenard/Hurley


Day 3
Valid 12Z Fri Mar 28 2025 - 12Z Sat Mar 29 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE UPPER TEXAS COAST THROUGH SOUTHWEST AND SOUTH-CENTRAL LOUISIANA...

Slow-moving mid-upper trough will push the area of most favorable
deep-layer forcing (upper divergence/diffluence and DPVA) farther
east along the Gulf Coast. Coincident with the favorable synoptic-scale
forcing, TPWs between 1.75-2.00", along with mixed layer CAPEs
around 1000-1500 J/Kg, will result in a more enhanced excessive 
rainfall threat (i.e. Slight) across the Upper TX Coast to 
Southwest and South-Central LA. Attributing to a lower risk
(Slight) compared to the previous days (Moderate) will be the
weakening low-level flow, which given the more limited deep-layer
shear, would more likely result in more effective convective outflows
and thus shorter duration of the more intense updrafts and heavier
rainfall cores.

Hurley

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