Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Thu May 1 2025
Day 1
Valid 01Z Fri May 02 2025 - 12Z Fri May 02 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHERN
OKLAHOMA AND FAR WESTERN ARKANSAS, SOUTH-CENTRAL TEXAS, AS WELL AS
FOR PORTIONS OF THE CENTRAL APPALACHIANS...
...01Z Update...
A Slight Risk was added to portions of south-central Texas, from
the U.S.-Mex border eastward through the Austin-San Antonio metros
to College Station, where slow-moving, heavy-rainfall producing
storms have increased in coverage this evening. Supported by moist,
southeasterly low level inflow and high instability, some of these
storms have shown rainfall rates of 2-3 in/hr, with MRMS indicating
estimated totals of over 4 inches in the past 3 hours. There is
concern that conditions will remain favorable for continued
development, with the heavy rainfall threat persisting longer and
across a broader area than most of what the CAMs, including the
HRRR, indicate. Given the observed rainfall rates and the rainfall
already received in some areas, a Slight Risk was added. Refer to
WPC MPD #206 for additional details concerning the threat across
parts of this area.
Also added a Marginal Risk across portions of central and southern
California, where an upper level low interacting with PWs of
0.75-1 inch is supporting showers and thunderstorms, mostly along
the high terrain from the southern Sierra Nevada to the coastal
ranges, including the San Gabriel and Sierra Madre. Some of these
storms are producing torrential downpours and isolated flooding
concerns. Storms are expected to diminish later this evening, but
may pose additional concerns, especially across areas recently
impacted by wildfires.
Elsewhere, more minor adjustments were made to the previous outlook
based on current observations and CAM guidance.
Pereira
...16Z Update...
Only very minor tweaks were needed to the Slight Risk areas across
the Southern Plains and central Appalachians. For the southern
Plains, the 12Z HiRes guidance shifted very slightly (10-20 miles)
north with tonight's expected MCS development across Oklahoma.
Given the high sensitivity of that entire area between Dallas and
Oklahoma City for heavy rainfall, the Slight Risk was maintained.
The 12Z HRRR followed previous runs suggesting there may be
training showers that get hung up on the southern end of the MCS
just north of the Red River, but by then the instability should be
largely used up and therefore rates should come down a bit. Given
that, the Slight was maintained with no upgrades...though it
wouldn't take much of an uptick in future guidance to possibly need
a targeted Moderate, depending on if the guidance can come into
better agreement on where the axis of heaviest rain will set up.
Over to the Central Appalachians, the guidance unfortunately
remains in poorer agreement than in the southern Plains. Expect
more widely scattered convection to impact the area starting this
afternoon, developing into one or more lines of storms by the
evening. Each line will be very progressive/fast-moving, so
individual storms will not have much of a flooding impact. However,
with multiple rounds of showers and storms expected in an area
recently hard hit with heavy rain, the multiple rounds of storms
may collectively add up to widely scattered instances of flash
flooding into this evening. The inherited Slight was trimmed in
MD/WV out of areas east of the Allegheny Front, which are
significantly drier and with higher FFGs than areas along and west.
Wegman
...Previous Discussion...
...Southern Plains...
Shortwave ejection out of the Southern Rockies will interact with a
cold front pressing south through the Central and Southern Plains
by the late-afternoon and evening hours Thursday through Friday
morning. A relevant theta_E differential will be aligned within
the confines of the boundary providing a suitable thermodynamic
alignment such that area convection anchors and pushes along the
west to east oriented instability gradient across Southern OK.
Majority of CAMs are in consensus of a heavy rainfall footprint
along and south of I-40, generally configured to the timing of the
cold front, shortwave interaction, and LLJ convergence signal. This
is primarily right around 00z when the convective threat enhances
with the integration of the nocturnal LLJ, a timing historically
prevalent for this time of year and this setup is no exception.
00z HREF neighborhood probs for >1" are relatively high (50-80%)
within the corridor extending from Southwest OK across the Red
River Basin towards the AR border. >2" is highest just north of the
river from I-35 on east, a testament to the timing of the
shortwave arrival and upscale growth of the convective threat as it
maneuvers to the east-southeast. Rainfall rates between 1-2"/hr are
most likely when assessing the hourly rate probs from the both the
NBM and HREF output, a rate assessment that is more than suitable
considering the blanket low FFG intervals due to the antecedent
moist soils from the past 36-48 hrs of rainfall. The areal QPF
footprint of >1" aligned within those lower FFG's pretty much
perfectly, a scenario that lends to a greater credence of a touch
higher risk category that what is normal the case for this setup.
The previous SLGT risk was relatively maintained with minor
adjustments on the edges pertaining to recent hi-res QPF and in
conjunction with the ML First Guess Fields.
...Interior Mid Atlantic...
Shortwave trough currently positioned over the Southern Plains will
lift northeast through the Mississippi Valley with sights on the
Great Lakes region by the second half of the D1. Downstream, an
improved diffluent signature will align over the Ohio Valley and
points northeast allowing for regional ascent to boost convective
initiation in-of places within that diffluent core. The most
notable area for convective development lies within an established
theta_E tongue riding northward through the Appalachians Front
with poleward expansion into the southeastern Great Lakes. This
allowed for moisture rich air exuding PWAT anomalies between +2 and
+3 within the corridor extending through much of WV up through
Eastern OH and Western PA to the Lake Erie border. SBCAPE between
1500-2500 J/kg will be a common occurrence during peak diurnal
destabilization creating a corridor of enhanced thermodynamic
prospects to coincide with the arrival of the better mid-level
forcing downstream of the approaching trough. Regional shear will
be improving as well, especially within the confines of a shallow
warm front that will be lifting northward through the course of the
afternoon across the interior Mid Atlantic. Cell initiation will
begin between 18-00z today with greater multi-cell cluster coverage
likely to occur towards the back end of the above time frame due
to anchoring along the warm front as it approaches. Areas across
Western PA have been hit relatively hard recently with convective
rainfall adding to the areal decoupling of the FFG indices meaning
the prospects for flash flooding have increased with the arrival of
this next disturbance.
00z HREF neighborhood probs for >2" are between 30-50% across a
large section of Western PA extending south into the northern High
Country of WV, rooted in the Appalachians Front, an area notorious
for flashy rivers and smaller towns located within the valleys.
There was enough consensus to warrant an upgrade considering the
antecedent conditions, atmospheric environmental favorability, as
well as the increased probs denoting a locally heavy rain prospect
over the region. A SLGT risk has been added to much of Western PA
down into portions of the Central Appalachians.
Kleebauer
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Thu May 1 2025
Day 2
Valid 12Z Fri May 02 2025 - 12Z Sat May 03 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS FROM THE
SOUTHERN PLAINS INTO THE LOWER MISSISSIPPI VALLEY...
...20Z Update...
A small southward nudge to the inherited Slight was added with this
update, with the Slight now including all of the upper Texas Gulf
Coast. A higher end Slight (not depicted) is in effect for the San
Antonio metro north and east, including the Houston Metro and into
northern and western Louisiana.
Overall very few changes were needed to the inherited risk areas.
The Slight Risk remains for much of Texas, from portions of the Rio
Grande Valley north and east to the Memphis metro. The heaviest
rains will be associated with a southward moving line of storms
with a cold front that will develop across central Texas Friday
afternoon and progress south and east to the Gulf Coast into Friday
night. Plentiful Gulf moisture ahead of the line with PWATs to 1.75
inches around the Houston metro and MUCAPE values well above 2,000
J/kg will favor very heavy rainfall rates with the strongest cores
of storms capable of over 2 inches per hour rain rates.
Fortunately, most of these areas of Texas have been drier than
areas to the north towards Oklahoma, so the lack of overlap with
previous rounds of heavy rain precluded the need for any Moderate
Risk upgrades this afternoon. There is some agreement in the
guidance that the heavy rain axis associated with the potential for
some limited training will remain just north of Houston in the Big
Thicket region. So long as this remains north of the most populated
areas around Houston then the Slight Risk should be sufficient, but
too much of a southward shift in that heavy rain axis would move
the threat into more urbanized areas, in which case a Moderate Risk
may be needed. 12Z HREF guidance shows an over 70% chance of 3
inches or more of rain in the neighborhood probabilities from just
west of the Louisiana border to Austin, remaining just north of
I-10.
The cold front driving these storms will advect much drier air into
much of Oklahoma and north Texas, which will greatly diminish any
storm threat in that area. For southeast Oklahoma, the threat is
largely Friday morning with lingering convection from the MCS that
will be progressing southeastward into northeast Texas and southern
Arkansas/northern Louisiana.
Storms will be less organized and much faster moving progressing
northeast into the central Appalachians/western Pennsylvania. Thus,
despite low FFGs from recent heavy rains, the Marginal should
suffice for that area. Of course, should the storms slow down or
develop into multiple rounds, then a Slight will be needed.
Nonetheless, potential for heavy rains over saturated soils did
necessitate a small expansion of the Marginal to the southern Tier
of western New York.
Wegman
...Previous Discussion...
Steady progression of a cold front through the Southern Plains will
lead to textbook convergent pattern with an addition of a strong
upper diffluence signature within the bounds of the southern jet
proxy. High areal theta_E averages will denote the significant
instability pattern positioned ahead of the approaching cold front
leading to ample thermodynamic assistance for strong convective
cores and expected heavy rainfall. Recent CAMs have finally come
into view for the primary window of cell initiation with a
widespread convective pattern likely to extend from the Stockton
Plateau to points east within the I-10 corridor up through all of
Hill Country, especially over the I-35 section of south-central TX.
HREF blended mean QPF is over 2" now within a large area
encompassing the terrain focused across the Central portion of the
state. This had led to elevated neighborhood probs of >2" for the
12-00z time frame in the forecast running >70% over pretty much all
of the above region extending into portions of East TX. The East TX
corridor has become a newer development in terms of anticipated QPF
magnitude as the the CAMs and in some instances the global
deterministic are now catching on to the deep moisture advection
regime poking north out of the Western Gulf ahead of the front
arrival. Forecast soundings across the area between Houston up to
the latitude of Austin are now indicating MUCAPE or the order of
3500-5000 J/kg prior to any front arrival leading to significant
heavy cores to transpire due to cap breach from diurnal
destabilization initially, followed by more organized multi-cell
clusters as we move into the evening as the LLJ re-intensifies and
the cold front motions into the region.
Soundings also note a period of mean flow and upshear components
that would lead to potential backbuilding within the confines of
the front. Very strong ascent between 850-300mb is also noted
within multiple CAMs outputs, a testament to the favorability for
the setup to become more organized and providing a vast areal
expanse of heavy convection once the setup matures. PWATs between
1.6-1.9" with locally >2" are being forecast along and ahead of the
front as it moves into Central TX, a forecast that traditionally is
favored for greater flash flood prospects, especially over the
favored I-35 corridor between Austin to San Antonio where high
runoff capabilities are found due to soil type and limestone
foundations. Local totals between 3-6" are likely over the entire
stretch of Hill Country through East TX with the eastern most
extent even shifting into Western LA as the convective pattern
evolves and grows upscale entering into the adjacent Lower
Mississippi Valley. Houston metro is now in play for convective
impacts, especially as thunderstorms to the north provide outflow
generation that could send cold pools south towards the Gulf coast
with additional convective development along the edge of any
approaching cold pool.
The previous SLGT risk was relatively unchanged across Central TX.
Some expansion to the west was made into the Stockton Plateau where
initial cell formations will likely occur as the front arrives and
interacts with a formidable instability axis aligned over the
plateau itself. The SLGT was expanded southeast to cover the
Houston metro and points north where there is growing consensus on
convective impacts over and near the urban center. This is a high-
end SLGT over much of Central TX, especially Hill Country to the
I-35 corridor and points east. A risk upgrade is not out of the
question in subsequent updates given the environmental
favorability and flashy history of places expected to see the
greatest thunderstorm coverage/rates.
Kleebauer
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Thu May 1 2025
Day 3
Valid 12Z Sat May 03 2025 - 12Z Sun May 04 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE CENTRAL
GULF COAST TO THE CENTRAL APPALACHIANS AS WELL AS PORTIONS OF THE
GREAT BASIN...
...20Z Update...
The inherited Marginal Risk was expanded south to the central Gulf
Coast with this update, with no changes towards the Great Basin.
Ongoing convection at the start of the period will impact the
central Gulf Coast on Saturday morning. While this area can take a
lot of rain before there are significant flooding issues, the
urbanization along the coast should support isolated instances of
flash flooding from New Orleans through Pensacola. Additional
afternoon convection will develop all throughout the Marginal Risk
area Saturday afternoon from the Gulf Coast through New York.
Topographic concerns and urban areas will be at higher risk for
potential flash flooding, but the otherwise progressive nature of
the storms should preclude more than isolated flash flooding in the
Marginal Risk area. PWATs over 1.5 inches from the Gulf Coast into
central Alabama and western Georgia would favor the most storms
capable of heavy rainfall. Depending on how much rainfall falls
along the Gulf Coast, especially through Mobile Friday night, the
additional rainfall Saturday could necessitate a Slight Risk
issuance with future updates from southern Alabama roughly to the
Atlanta area. Meanwhile further north once again the storms look to
be progressive enough that only isolated instances of flash
flooding are expected, including into western Pennsylvania.
Wegman
...Previous Discussion...
...Appalachians...
An anomalous upper low center evolving over the Ohio Valley will
generate a strong diffluent signature downstream across the
Appalachians and points east with a significant moisture fetch
poleward from the Gulf, and eventual Atlantic Basin by the end of
the period. Organized convective activity will brew within the RER
of a strengthening upper jet core located over Ontario/Quebec as
the upper pattern becomes closed off to the west after 00z/Sun.
NAEFS PWAT anomalies between +1 and +2 deviations will offer a
suitable moisture rich environment conducive for stronger heavy
convective cores capable of locally enhanced rainfall prospects
within the confines of the great diffluent axis. As of this time,
this places the Appalachians Front from Northern GA up through PA
as the primary region of interest, however some elevated
instability notions are present within some of the recent
deterministic output. As of this time, a general 1-2" is forecast
within the above corridor with some heavier cells even possible as
far south as the Central Gulf Coast, but those are more aligned
within the decaying surface front that is progged to breakdown
before exiting the coastal plain. The area of focus will reside
within that broad ascent pattern enhanced by the evolving amplified
pattern to the east of the Mississippi with the complex terrain
providing a secondary element that would favor isolated flash flood
prospects. As a result, a broad MRGL risk extends from the
Southern Appalachians up through Northeast PA where the current
ensemble mean QPF is highest while also residing within the lower
FFG indices available in the Eastern CONUS.
...Great Basin...
Omega block pattern initiating over the CONUS will lead to yet
another amplified upper low generation across the Western CONUS
through D3. The threat for elevated convection in-of interior NV up
into Southeast OR have been forecast for several days with the
deterministic guidance basically "locked in" to the expected
evolution. Despite rainfall rates rather low grade compared to what
you normally find for flash flooding, this area has a notorious low
FFG bias that tends to accentuate the flash flood prospects in this
area of the CONUS. Recent ensemble means have been hovering between
0.75-1.25" of total QPF within the corridor extending from Reno to
points northeast with local maxima hovering up near 2" due to the
added effect of greater upper forcing thanks to the maturing closed
ULL reflection. This setup has been steady within the First Guess
Fields for the threat and really hasn't wavered much in the MRGL
risk output from the ML. Considering the factors above and a
pattern more favorable for these convective schemes across the
Great Basin, the previous MRGL risk inherited was maintained with a
small expansion northward into Southeast OR to highlight QPF trends
favoring better totals within those zones.
Kleebauer
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 PM EDT Thu May 1 2025
Early next week, two occluded frontal systems are expected to
bring moderate to locally heavy rainfall to portions of the West
and East. In the West, anomalous moisture accompanying the upper
low will support heightened precipitation from the Great Basin to
the Central and Southern Rockies and Plains. Precipitation will be
initially triggered by the surface frontal system, which will push
across the Intermountain West on Sunday then become stationary over
the Plains next week. Once the system stalls, moisture will
continuously pool over the Plains, which will likely result in a
multi-day heavy rainfall event across the Central and Southern
Plains. The heaviest precipitation is forecast to fall over the
Southern Plains Monday into Tuesday, then spread into the Lower
Mississippi Valley Wednesday and Thursday. Portions of the Plains
states will be especially vulnerable to heavy rain/flooding due to
recent heavy rains and saturated soils. This activity is
highlighted in the Day 4-5 Excessive Rainfall Outlooks (EROs) with
a Marginal Risk area on Sunday and a Slight Risk area on Monday
focused on eastern New Mexico and the Texas and Oklahoma
panhandles.
Under the eastern upper low, the frontal system will very slowly
push east across the East Coast, resulting in a stream of onshore
moisture over the Mid-Atlantic and Northeast. This pattern will
result in persistent precipitation across these regions through
Tuesday. Initially, rainfall could be beneficial and bring some
relief to drought conditions, but repeated rounds of rainfall
could produce locally heavy totals and may result in localized
flooding in urban and poor drainage areas. The Day 4-5 EROs show a
Marginal Risk area across these regions on both Sunday and Monday.
By Wednesday, the frontal system should accelerate to the east as
the upper low interacts with the northern stream, shifting
precipitation farther into the Northeast.
The northern High Plains in particular will see well above normal
temperatures on Sunday, with some plus 15-25 degree anomalies for
highs. Temperatures will steadily moderate as this area of warmth
migrates eastward early next week. In contrast, the upper low
tracking into the Southwest will bring much cooler temperatures to
the region. Highs should be 10-20 degrees below normal on Sunday-
Monday, with some record cold highs possible near the upper low's
path. The Southwest will slowly trend closer to normal after
midweek. Persistent clouds and rainfall should keep the southern
High Plains below normal for highs as well. The Northwest will see
a warmer trend next week, with some highs reaching 10-15 degrees or
so above normal. Over the East, locations under and southwest of
the Ohio Valley upper low should see moderately below normal highs
Sunday-Monday and then some moderation. The Northeast will tend to
see above normal lows.
Dolan/Rausch
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 PM EDT Thu May 1 2025
Early next week, two occluded frontal systems are expected to
bring moderate to locally heavy rainfall to portions of the West
and East. In the West, anomalous moisture accompanying the upper
low will support heightened precipitation from the Great Basin to
the Central and Southern Rockies and Plains. Precipitation will be
initially triggered by the surface frontal system, which will push
across the Intermountain West on Sunday then become stationary over
the Plains next week. Once the system stalls, moisture will
continuously pool over the Plains, which will likely result in a
multi-day heavy rainfall event across the Central and Southern
Plains. The heaviest precipitation is forecast to fall over the
Southern Plains Monday into Tuesday, then spread into the Lower
Mississippi Valley Wednesday and Thursday. Portions of the Plains
states will be especially vulnerable to heavy rain/flooding due to
recent heavy rains and saturated soils. This activity is
highlighted in the Day 4-5 Excessive Rainfall Outlooks (EROs) with
a Marginal Risk area on Sunday and a Slight Risk area on Monday
focused on eastern New Mexico and the Texas and Oklahoma
panhandles.
Under the eastern upper low, the frontal system will very slowly
push east across the East Coast, resulting in a stream of onshore
moisture over the Mid-Atlantic and Northeast. This pattern will
result in persistent precipitation across these regions through
Tuesday. Initially, rainfall could be beneficial and bring some
relief to drought conditions, but repeated rounds of rainfall
could produce locally heavy totals and may result in localized
flooding in urban and poor drainage areas. The Day 4-5 EROs show a
Marginal Risk area across these regions on both Sunday and Monday.
By Wednesday, the frontal system should accelerate to the east as
the upper low interacts with the northern stream, shifting
precipitation farther into the Northeast.
The northern High Plains in particular will see well above normal
temperatures on Sunday, with some plus 15-25 degree anomalies for
highs. Temperatures will steadily moderate as this area of warmth
migrates eastward early next week. In contrast, the upper low
tracking into the Southwest will bring much cooler temperatures to
the region. Highs should be 10-20 degrees below normal on Sunday-
Monday, with some record cold highs possible near the upper low's
path. The Southwest will slowly trend closer to normal after
midweek. Persistent clouds and rainfall should keep the southern
High Plains below normal for highs as well. The Northwest will see
a warmer trend next week, with some highs reaching 10-15 degrees or
so above normal. Over the East, locations under and southwest of
the Ohio Valley upper low should see moderately below normal highs
Sunday-Monday and then some moderation. The Northeast will tend to
see above normal lows.
Dolan/Rausch
