Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
853 PM EDT Tue Mar 25 2025
Day 1
Valid 01Z Wed Mar 26 2025 - 12Z Wed Mar 26 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHEAST TEXAS INTO SOUTHERN OKLAHOMA AND OVER PORTIONS OF
SOUTHEAST FLORIDA...
...Florida...
Maintained the Marginal Risk area across the southeast Florida
peninsula after realigning it based on even trends in satellite and
radar imagery. Even though loss of daytime heating should result in
diminishing risk of excessive rainfall due to loss of daytime
heating by late evening...it was too early to entirely remove the
Marginal risk area as additional showers and thunderstorms capable
of producing downpours in a highly urbanized area...a few places
which had locally heavy rainfall earlier in the day. The
expectation is that the risk of excessive rainfall will diminish by
26/04Z.
...Texas/Oklahoma...
Convection has initiated across portions of southern OK near and
just north of a stationary front. Southerly flow should continue to
advect moisture northward and result in upwards of 1000-2000 j/kg
of CAPE with a base just above the boundary layer. This elevated
convection will track off to the southeast, generally parallel to
the low level convergence axis...which supports some training
potential. The 3km NAM remained the wettest model, but we have
seen elevated convective setups like this verify towards the higher
end of the model spread, and the 3km NAM has performed decently at
these events in the past. So while it depicts a lower probability
outcome based on all available guidance, and a swath of 1-3" is
most likely, can not rule out a localized swath of 3-6" of rain
across portions of northeast TX into southern OK late tonight into
Wednesday morning.
Overall the flash flood risk at any location is low given the dry
antecedent conditions and narrow nature of any heavy rainfall
swath. However intense rainfall rates and some training could allow
for a localized flash flood threat to evolve, particularly if the
heavy rain overlaps any more sensitive urban or low lying area. We
opted to maintain a focused Marginal risk area to account for this
isolated potential.
Chenard
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
853 PM EDT Tue Mar 25 2025
Day 2
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 speed up the timing of the onset of the
heavy rainfall event over southern TX. It now appears likely that
organized convection will begin growing upscale by later Wednesday
afternoon or evening. 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, with only the 3km and 12km NAM really standing out as
outliers, and those are two of the less reliable models for
situations such as this. 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. For now the MDT risk is centered around the highest HREF
probabilities, and generally on the southern gradient of the global
model QPF solutions. 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 was added 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.
Chenard
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
853 PM EDT Tue Mar 25 2025
Day 3
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...
The event described in the day 2 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 2 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.
The CSU machine learning ERO is showing higher end MDT risk
probabilities (albeit probably a bit too far north), likely
indicating the favorable ingredients that will be in place for
excessive rainfall.
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 2 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.
Overall confidence is high in a flash flood event, but confidence
in the details by Thursday 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 was added 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 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Tue Mar 25 2025
Latest guidance is consistent with the general theme of heavy rain
potential along and near the west-central into central Gulf Coast,
as an upper trough interacts with low level Gulf moisture and a
warm front. The Day 4/Friday Excessive Rainfall Outlook has
introduced a Slight Risk area covering parts of southeastern Texas
and southwestern Louisiana, with models/ensembles now clustering
better for this region seeing the best focus. By Day 5/Saturday,
guidance becomes much more diffuse with the rainfall signal. Recent
ECMWF/ECMWF AIFS/UKMET runs show the most agreeable theme of some
activity from Friday-Friday night continuing into Saturday, with
some instability, before trending lighter. Thus the Day 5 ERO
depicts a Marginal Risk area along and north of the central Gulf
Coast region.
Elsewhere, rain and high elevation snow will cross the northern
half of the West late this week, followed by an increasing area of
precipitation from the north-central Plains through Northeast and
extending southward through the Mississippi Valley and points east
during the weekend into early next week as one or more frontal
waves evolve and the overall frontal system eventually pushes
eastward. It may be cold enough to support wintry precipitation
types for a time from the northern Plains through the Upper Great
Lakes into northern New England. To the south, some rain could be
locally heavy over the north central Plains as well as from the
Mississippi Valley eastward. In addition, the Storm Prediction
Center is monitoring the potential for severe weather from parts of
the eastern Plains into a large part of the east- central U.S.
during the weekend. Check latest SPC outlooks for the latest info
on severe threats.
After a brief break, another area of precipitation should spread
across the West from Sunday into early next week. There is still a
decent amount of uncertainty for the specifics of supporting
eastern Pacific low pressure, but currently expect highest totals
to be over the central West Coast/northern California.
Very warm temperatures will prevail in the warm sector ahead of
the wavy frontal system evolving over the Plains and then pushing
eastward. Some areas from the central High Plains into the Midwest
may see highs 20-30F above normal on Friday. The warmth will shift
south and east over the weekend with plus 20F or greater anomalies
for morning lows reaching from the Midwest into the Ohio Valley
while plus 10-20F anomalies for highs extend from the southern half
of the Plains into the Mid-Atlantic and Southeast. Continued
frontal progression will confine above normal readings to the East
Coast by Monday. However the Northeast will be on the cool side, to
the north of a front settling over or near the northern Mid-
Atlantic. Behind the main frontal system, modestly below normal
highs will progress from the West late this week into the Great
Lakes/Ohio Valley by next Tuesday.
Rausch
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Tue Mar 25 2025
Latest guidance is consistent with the general theme of heavy rain
potential along and near the west-central into central Gulf Coast,
as an upper trough interacts with low level Gulf moisture and a
warm front. The Day 4/Friday Excessive Rainfall Outlook has
introduced a Slight Risk area covering parts of southeastern Texas
and southwestern Louisiana, with models/ensembles now clustering
better for this region seeing the best focus. By Day 5/Saturday,
guidance becomes much more diffuse with the rainfall signal. Recent
ECMWF/ECMWF AIFS/UKMET runs show the most agreeable theme of some
activity from Friday-Friday night continuing into Saturday, with
some instability, before trending lighter. Thus the Day 5 ERO
depicts a Marginal Risk area along and north of the central Gulf
Coast region.
Elsewhere, rain and high elevation snow will cross the northern
half of the West late this week, followed by an increasing area of
precipitation from the north-central Plains through Northeast and
extending southward through the Mississippi Valley and points east
during the weekend into early next week as one or more frontal
waves evolve and the overall frontal system eventually pushes
eastward. It may be cold enough to support wintry precipitation
types for a time from the northern Plains through the Upper Great
Lakes into northern New England. To the south, some rain could be
locally heavy over the north central Plains as well as from the
Mississippi Valley eastward. In addition, the Storm Prediction
Center is monitoring the potential for severe weather from parts of
the eastern Plains into a large part of the east- central U.S.
during the weekend. Check latest SPC outlooks for the latest info
on severe threats.
After a brief break, another area of precipitation should spread
across the West from Sunday into early next week. There is still a
decent amount of uncertainty for the specifics of supporting
eastern Pacific low pressure, but currently expect highest totals
to be over the central West Coast/northern California.
Very warm temperatures will prevail in the warm sector ahead of
the wavy frontal system evolving over the Plains and then pushing
eastward. Some areas from the central High Plains into the Midwest
may see highs 20-30F above normal on Friday. The warmth will shift
south and east over the weekend with plus 20F or greater anomalies
for morning lows reaching from the Midwest into the Ohio Valley
while plus 10-20F anomalies for highs extend from the southern half
of the Plains into the Mid-Atlantic and Southeast. Continued
frontal progression will confine above normal readings to the East
Coast by Monday. However the Northeast will be on the cool side, to
the north of a front settling over or near the northern Mid-
Atlantic. Behind the main frontal system, modestly below normal
highs will progress from the West late this week into the Great
Lakes/Ohio Valley by next Tuesday.
Rausch
