244
FOUS30 KWBC 301550
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1150 AM EDT Tue Apr 30 2024

Day 1
Valid 16Z Tue Apr 30 2024 - 12Z Wed May 01 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHEAST KANSAS AND NORTHERN AND NORTHEASTERN OKLAHOMA...

...16Z Update...

Generally only minor tweaks were made to the inherited forecast.
The large scale pattern described in the previous discussion below
remains valid.

...Kansas and Oklahoma...

The primary change made in this area was to expand the inherited
Slight Risk area south to include much more of northern and
northeastern Oklahoma with this update. CAMs guidance is in good
agreement that convection that forms in Kansas and Oklahoma this
afternoon will congeal into an east-west oriented line of storms
largely on the Oklahoma side of the border this evening, persisting
well into the overnight hours. The line itself may be nearly
stationary for a few hours before slowly weakening and dissipating
in the predawn hours Wednesday. There remains considerable
uncertainty, as is typical, with exactly where the line of storms
forms and how quickly it moves to the south, so the inherited
Slight in Kansas was left largely intact, though the flash flooding
threat there has diminished somewhat. Northern and northeastern
Oklahoma was hit hard in recent days with multiple inches of rain
from strong storms, so this area also has elevated river levels and
reduced FFGs, making it more vulnerable to flash flooding.

The storms are likely to focus in a relatively small area that in
a north-south sense should only cover a row or 2 of counties. Both
the expected small size of the highest risk area and inherent
uncertainty with the ultimate behavior of the expected convection
precluded any consideration of a further upgrade. However, should
the storms impact the hardest hit areas, locally significant and
considerable flooding could occur.

...South Florida...

A Marginal risk area was introduced for portions of south Florida.
Afternoon convection is expected to form in southwest Florida this
afternoon as PWATs spike above 1.5 inches in weak steering flow. A
frontal system is approaching the state`s Gulf Coast now, and when
added to the moisture-rich atmosphere in place across the state,
slow-moving storms are likely to develop, then gradually shift east
towards the I-95 corridor. Conditions have been drier than normal
across much of the Florida Peninsula lately, which should help
preclude any widespread flash flooding, but slow moving storms in a
moisture-abundant atmosphere have the potential to produce rates
that may locally exceed FFGs. For this reason, a low-end Marginal
was needed.

...New York and Pennsylvania...

The Marginal Risk area in the Northeast was trimmed out of the
Hudson Valley with this update. Afternoon convection is likely to
form on the west side of the Catskills. As the storms grow upscale,
they will encounter a faster steering flow in the upper levels,
which will help accelerate them eastward. So while rates may exceed
FFGs, the time at which those rates will be realized will decrease
with time. Thus, the Marginal remains for the portions of NY and PA
that are most likely to see the storms initially forming, before
they move too fast to be much of a flash flooding threat.

...Elsewhere...

For South Texas and the upper Midwest, no significant changes were
made, see the previous discussion below for more details in these
areas.

Wegman

...Previous Discussion...

...Kansas and Oklahoma...

A surface cyclone will develop over the northern plains with a cold
front extending along the southern periphery of the surface
reflection. A strong southerly component downstream of the surface
low will allow for a pool of elevated theta-E`s to advect poleward
into eastern KS and Oklahoma during the late afternoon time frame
with an associated increase in regional instability within the axis
of higher theta-E`s. There is a growing consensus for convective
initiation across east-central KS towards the 23-01z period where
the LLJ kicks and the aforementioned cold front converges on the
region with rapid CI likely somewhere north of Wichita. A moisture
rich environment thanks to low-level moisture advection coupled
with primed instability in-of the area will lead to locally heavy
rain within any convective development, especially under more
prominent supercellular modes thanks to increasing shear within the
area of positive buoyancy. 00z HREF blended mean QPF has really
become bullish with totals approaching 2.5-3" now across that area
in and north of Wichita with some guidance expanding further as
cold pool propagation to the south would expand some pretty hefty
totals to areas that were just recently impacted. Rates are likely
to approaching 2-3"/hr based on the latest HREF probability fields
(30-40%) which would easily eclipse the lower FFG indices in place
after the recent deluge. The area near the KS/OK border is
conditional to the cold pool propagation and the cold front
expecting to struggle to gain latitude leading to an elongated
front where convection can situate through the end of the period.
Some models are aggressive with totals exceeding 3" if that were to
occur, so the prospects for higher end impact lead to the addition
of a SLGT risk over the area where the threat is maximized.

...Northeast...

A stalled frontal boundary is analyzed across northern NJ,
extending west through northern PA, southern NY, into Ontario. A
cold front is currently moving through the Great Lakes in response
to surface low moving through the northern Midwest. The stationary
front, cold front, and attendant mid-level energy will all aid in
convective development this afternoon with initiation over western
NY and northern PA, moving eastward through peak diurnal
instability. Consensus for scattered thunderstorms has grown across
all guidance leading into the period with the latest CAMs
persistent on an axis of locally heavy rainfall impacting areas
from the Finger Lakes, eastward into the Hudson Valley. 00z HREF
blended mean QPF footprint depicts a large swath of 1-1.5" precip
totals in-of NY state with the northern fringes of PA out near
Bradford and Susquehanna also within the mean QPF swath formed via
the convective regime expected. Hourly rates will be the primary
driver for localized flooding across the area in question with
HREF probabilities for 1"/hr now topping out at 50-60% from a line
extending between Utica down to northeastern PA with the time frame
between 18-00z as the period of interest for convective initiation
and impact. 2"/hr rates are non-zero, but limited in spatial
coverage with probabilities peaking closer to 10-15% over portions
of southern NY state. The area of interest will lie along the
stalled front where the best low-level convergence axis will
reside, as well as where the favored mid-level vorticity maxima
advects overhead. Where these two align will likely lead to totals
approaching 2" within a course of 1.5-2 hrs before convection
steadily moves eastward. Areas with more urbanization will be
subject to the best flash flood potential due to runoff, as well as
areas that can pick up a quick 1.5-2" when impacted. The MRGL risk
was maintained from the previous forecast with some minor
adjustments on the fringes.

...Midwest to Upper Mississippi Valley...
A quick-moving shortwave will eject out of the southern periphery
of a negatively tilted longwave trough over the northern Plains
and southern Canada leading to convective genesis over the northern
and central plains. The latest HREF and associated CAMs highlight
the potential within the mean QPF forecast with a swath of 1-1.5"
possible over portions of MN/IA/Northwest WI with a signal for
local 2-3" amounts given the potpourri of deterministic. There is
less consensus on exactly where the stronger cells will reside, but
the initiation point across southeastern SD and eastern NE is
fairly consistent on guidance. The threat is low to mid-tier for
the MRGL risk, but probabilistic signals for at least 1"/hr and up
to a quick 2-3" are enough for some local impacts to areas who have
substantially lower FFG indices thanks to the previous series of
storms. The MRGL risk was maintained with little deviation from the
previous forecast.

...Southern Plains...

A dryline will focus over west TX into OK during the afternoon
today with scattered convection likely to fire after 20z within the
Stockton Plateau and portions of north TX towards the Red River.
Precip rates will be capped between 1-1.5"/hr which is enough for
some localized flood concerns in small towns with urbanization
factors that could enhance runoff potential. The threat is on the
lower end of MRGL, but resides within the scope of the threshold of
5%.

Kleebauer


Day 2
Valid 12Z Wed May 01 2024 - 12Z Thu May 02 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL IS FORECAST OVER PORTIONS OF
THE MIDWEST & CENTRAL/SOUTHERN PLAINS...

...Midwest/Central Plains...

A broad, closed upper level cyclone will persist over eastern MT
into the adjacent northern plains and southern Canadian Provinces
by Wednesday with a potent, mid-level shortwave rounding the base
of the mean trough and exiting into the northern high plains by the
second half of the period. A vigorous upper diffluent pattern will
transpire by Wednesday afternoon and evening with a corridor of
convection initiating towards nightfall. A stout LLJ will enhance
areal shear in-of the central and northern plains within the Quad
state area between NE/KS/IA/MO. Any convective development will
congeal and grow upscale into a large cluster of heavy
thunderstorms within the above area leading to widespread heavy
rains capable of flash flooding as rates likely reach towards that
1-2"/hr marker with locally higher possible. The one factor that
may limit the higher end potential will be the progressive nature
of the convection as pattern yields faster storm motions limiting
the prospects for training. This does not mean there will not be
some areas that can overachieve and maximize the potential based on
the prime mid-level energy involved, along with a strong meridional
push of some very moist air approaching 2-2.5 deviations above
normal for PWATs based on the latest GEFS and NAEFS output. The
threat remains solidly within the SLGT with a chance at any upgrade
if future CAMs indicate a better potential for totals to exceed 3"
in a larger footprint than what is currently depicted.


...Southern Plains...

The location on a secondary rainfall maximum across TX is becoming more
clear as a quick moving shortwave embedded within the subtropical jet
ejects out of MX and moves to the northeast by Wednesday evening. Guidance
suggests that somewhere across eastern TX would be the primary target
for convective development with the heaviest rainfall located within
the tongue of higher theta-E`s positioned east of the line from ABI
to DRT. A cluster of thunderstorms will develop over west TX with a
steady propagation to the east during the afternoon and early
evening hours. By nightfall, a prominent LLJ aligned across
central and east TX will provide necessary maintenance to any
convective clusters that will migrate over the state after peak
diurnal instability. Surface dew points running into the upper-60s
and low-70s relay the moisture rich environment that will provide
the foundation for significant rainfall potential as the mid-level
vorticity maxima continues to move over the region providing the
additional large scale forcing that will keep the convection
rolling through the evening. Current deterministic output is
consistent with local totals breaching 4" across portions of
central and north TX with the latest ensemble bias corrected QPF
sufficiently in the range of 2-3" over an area extending from
southwest OK down through the DFW metroplex and adjacent surrounds.

This is quickly gaining steam for a higher impact event,
especially when you compound the idea that the northern portion of
the I-35 corridor in TX becomes a focal point. There is some spread
on the exact positioning of the heaviest precip expected, but ML
outputs have been persistent in the area over north TX for the past
succession of runs. Should deterministic guidance converge further
with similar amounts, or if ensemble support becomes more
aggressive with totals and probabilities for exceeding higher end
QPF markers, a Moderate Risk cannot be ruled out in later
issuance`s. Moderate Risk impacts are certainly possible wherever
the heavy rainfall materializes with the highest prospects within
the major urban centers along I-35.

Kleebauer


Day 3
Valid 12Z Thu May 02 2024 - 12Z Fri May 03 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL IS FORECAST OVER MUCH OF THE
MISSISSIPPI VALLEY EXTENDING FROM THE MIDWEST DOWN INTO EAST TEXAS,
AND LOUISIANA...


...Summary: Two distinct areas of interest will arise for heavy
rainfall on D3 as a strong mid-level shortwave continues to march
eastward out of TX into the Lower Mississippi Valley by the middle
of the period. Further north, a powerful, negatively tilted trough
will maintain foothold across the northern plains with a strong
vorticity maxima pivoting around the southern periphery of the
closed upper reflection in place across southern Saskatchewan. Each
upper forcing will play a role in locally heavy rainfall impacts
spreading eastward from the continuation of the previous period.
The primary threat is currently front loaded with the best flood
prospects leaning towards Thursday morning and afternoon before the
threat wanes heading into the second half of the forecast period.

...Midwest...

Overnight convection from Wednesday will continue to march eastward
as the congealed cold pools from the initial cell mergers will
advance through the central Midwest with sights on eastern IA into
WI and MN on Thursday morning. The complex will eventually peter
out as it moves northward into a less favorable environment which
will put an end to the flooding threat as we move into the
afternoon. Later in afternoon and evening, a cold front will
propagate to the east out of the central plains with a
redevelopment of convection over eastern KS, IA, and northwest MO.
This will allow for the secondary peak of flash flooding potential
as thunderstorms migrate through the mid and upper Mississippi
Valley, impacting some areas that were just affected in the last 24
hrs. With the lower FFGs expected, the threshold for flooding will
be much lower than climatological norms for some areas within the
expected secondary convective regime. Totals are currently within
the 1-3" range on guidance which is significant enough to warrant
at least the current SLGT risk in place with some chance at further
upgrades pending prior period(s) evolution and precip outcomes.


...Southern Plains and Lower Mississippi Valley...

A fairly significant complex of thunderstorms will likely be
ongoing in the beginning of the period with the cold pool
propagation forecast to impact areas of east TX into Louisiana,
similar areas that were just hit recently with significant rainfall
and flash flooding. The prospects for flooding are now well above
climatological norm as areal FFG indices are solidly 50% of what
they normally are during the time of year (2-3" compared to 4-5").
Forecast QPF is hovering between 2-4" with locally higher with the
ensemble bias corrected forecast showing a wide swath of 2+" and
some deterministic already ramping up over 5" across east TX into
north-central LA. Considering the repeated nature of these heavy
rain threats, the prospects for flooding is growing with a SLGT
risk in place over the Arklatex down into parts of east TX and the
Lower Mississippi Valley. Pending the forecast from the previous
period with CAM guidance assistance, an upgrade to a Moderate Risk
may very well be entertained.

Further north across KS and OK, the cold front to the north will
move south with a push from high pressure nosing in through the
plains in wake of the low pressure moving to the north. An area of
convection will likely form over the central plains and move south
with the boundary leading to more locally heavy rainfall in
association with the area of convection. The threat is less
aggressive compared to what is expected further south, but the
chances are still prevalent. The SLGT risk was maintained from the
previous D4 connecting the two primary areas of interest to the
north and south.

Kleebauer

Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt