Skip Navigation Links weather.gov 
NOAA logo - Click to go to the NOAA homepage National Weather Service   NWS logo - Click to go to the NWS homepage
The Weather Prediction Center

 
 

 

Follow the Weather Prediction Center on Facebook Follow the Weather Prediction Center on Twitter
NCEP Quarterly Newsletter
WPC Home
Analyses and Forecasts
   National High & Low
   WPC Discussions
   Surface Analysis
   Days ½-2½ CONUS
   Days 3-7 CONUS
   Days 4-8 Alaska
   QPF
   PQPF
   Flood Outlook
   Winter Weather
   Storm Summaries
   Heat Index
   Tropical Products
   Daily Weather Map
   GIS Products
Current Watches/
Warnings

Satellite and Radar Imagery
  GOES-East Satellite
  GOES-West Satellite
  National Radar
Product Archive
WPC Verification
   QPF
   Medium Range
   Model Diagnostics
   Event Reviews
   Winter Weather
International Desks
Development and Training
   Development
WPC Overview
   About the WPC
   Staff
   WPC History
   Other Sites
   FAQs
Meteorological Calculators
Contact Us
   About Our Site
 
USA.gov is the U.S. Government's official web portal to all federal, state, and local government web resources and services.
 
Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1949Z Mar 25, 2024)
 
Version Selection
Versions back from latest:  0   1   2   3   4   5   6   7   8   9   10   11   12   13   14   15   16   17   18   19   20   
 
Abbreviations and acronyms used in this product
 
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
349 PM EDT Mon Mar 25 2024

Day 1
Valid 16Z Mon Mar 25 2024 - 12Z Tue Mar 26 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE LOWER MISSISSIPPI VALLEY AND SOUTHEAST...

...16Z Update...

Current radar and satellite analysis shows our convective pattern
across east TX into the Lower Mississippi Valley is on track from
the previous forecast issuance. Guidance is in fair good agreement
with overall evolution of the heavy rain footprint(s) expected this
morning and eventually later this afternoon and evening. First
plume of heavy rain is progressing through the Ozarks down into
northeastern TX this morning with general 1-2" totals, so far
across those regions. Limited instability has capped the potential
thus far with regards to hourly rain rates, but steady moderate
stratiform with embedded convection will continue to put down an
additional 1-2" on top of what has fallen with a ensemble
probability fields signaling a high potential for at least 2"
(40-60%) within the Ozarks in MO up through the central portion of
the state and some non-zero 3" probabilities now in-place over the
same areas. This was enough of a signal to expand the SLGT risk a
bit further north to match the latest ensemble trends, as well as
the current observational data supporting a steady climb of
regional totals within the area sporting a lower 3hr FFG interval
(2").

The second adjustment was made in a southeastern expansion of the
SLGT risk across northeast LA as a fairly reasonable consensus has
transpired for a line of heavy thunderstorms to develop later this
afternoon across the area north and east of I-49. This is likely
due to the best theta-E wedge and subsequent instability fields
being most favorable within that corridor as the cold front to the
east approaches later today. Regional bufr soundings off multiple
hi-res deterministic paint a tall, skinny CAPE regime within that
zone of higher instability while also settling nicely within a
region of anomalously high PWATs (+2-2.5 deviations) during the
peak of the convective potential. This will likely cause some
localized 1.5-2"/hr rain rates which is backed up by the latest 12z
HREF neighborhood probability of 20-25% chance for rates to reach
at least 2"/hr and 50-70% for at least 1". Local maximum of up to
4" is plausible across northeast LA through portions of MS today
and overnight before the threat wanes slowly as it enters a more
stable environment across the AL line. Still, some heavier rainfall
is expected near the MS/AL line, so the SLGT risk will suffice from
previous forecast, but trimmed back the eastern edge based on the
latest trends.

Kleebauer

...Previous Discussion...


The inherited Slight risk still looks in pretty good shape.
Convection will be ongoing at 12z across portions of northeast TX
up towards the OK/AR border. As we go through the morning hours we
should see an uptick in convective intensity as this line moves
east into AR and southern MO. Increasing forcing from the west is
expected to tighten and strengthen low level convergence helping
intensify convection. Activity is expected to have an eastward
movement, however cell motions will have a fairly large component
parallel to the low level boundary for a period of time this
morning and afternoon, which should result in some brief training
of cells.

Guidance supports 1-3" of rain along an axis from western to
central AR into southern MO, with both the HREF and experimental
REFS ensembles showing a 20-60% chance of localized amounts
exceeding 3". Hourly rainfall will likely exceed 1", but 2"/hr
probabilities are low...with most high res guidance peaking hourly
rates around 1.5". Limited instability and a limited duration of
heavy rainfall are both minimizing factors for flash flooding.
However do think the degree of moisture transport and forcing,
combined with the brief window of opportunity for training as the
wave to the west digs, all support an isolated to scattered flash
flood risk here. There may be a corridor with a relative min in
QPF over the eastern half of AR, before activity ramps back up
again over MS during the overnight hours. Impressive IVT forecast
over MS/AL tonight, with values approaching the climatological max
for late March. The low level jet is also impressive, with 850mb
wind speeds over the 99th percentile. These factors, combined with
the strong mid/upper forcing, should be enough to overcome the
marginal instability in place and result in strong organized
convection capable of heavy rainfall rates.

High res models easily support 1"/hr rainfall with this activity,
with localized swaths exceeding 2"/hr appearing likely. Thus rates
will be there for flash flooding...with the main question being
whether the duration is there as this convection will likely be
moving off to the east at a decent clip. However, given the very
strong low level southerly inflow, would expect to see some brief
backbuilding/training of cells...especially on the south and
southwest flank of the activity. Given the impressive moisture and
forcing in play, suspect that rainfall magnitudes from this event
will verify on the higher end of the model envelope over far
northeast LA into central MS and western AL. Of the two high res
ensembles, the experimental REFS is wetter than the HREF, and thus
am leaning in that direction, which would support a swath of 3" of
rain (REFS neighborhood probabilities of exceeding 3" are 40-60%).
This should be enough to result in isolated to scattered instances
of flash flooding. In the end the progressive nature of the system
should keep this from getting any higher than a Slight risk event.

The Marginal risk still extends northward into northeast MO, IL
and southern WI...where 1-3" of rain is expected. Rainfall rates
will decrease with northeast extent over this corridor, which is
why the risk level is Slight in southern MO, but just Marginal
further north and east.

Chenard


Day 2
Valid 12Z Tue Mar 26 2024 - 12Z Wed Mar 27 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE FLORIDA PANHANDLE INTO SOUTHEAST AL AND SOUTHWEST GEORGIA...

...20Z Update...

No changes to the previous forecast as model consensus is for a
period of convection across the FL Panhandle and adjacent southern
AL/GA later Tuesday afternoon through the early morning hrs
Wednesday. Some of the hi-res deterministic is a bit more bullish
across the Tallahassee area by Wednesday morning with totals
pushing over 2" within the hardest hit zones. Area instability is
modest, at best but the environment is very moist with 2-2.5
deviations above normal forecast in-of the outlined SLGT area.
Frontal progression will be on the slower side as it trudges east
along the Gulf Coast, so general storm motion is slower, thus
creating the potential for some training as storms fire Tuesday
evening. 12z HREF blended mean is consistent with 1.5-2" located
inside the SLGT risk with the PMM significantly higher thanks to a
couple CAM members going a little more aggressive with the
convective development and associated totals. With a slightly
better depiction compared to previous forecast with more CAMs
involved, there was no reason to deviate from the previous SLGT
risk in place.

Kleebauer

...Previous Discussion...

The risk of flash flooding does look to decrease as this event
moves into Tuesday over the Southeast. The better forcing lifts
off to the north and IVT and the low level jet weaken. With that
said, convection will likely be ongoing at 12z Tuesday over AL
with locally heavy rain, and this activity will move into GA and
the southern Appalachians through the day as well. Eventually the
front and moisture plume are expected to slow down and/or stall
out, so while instability and forcing is weaker by this time, can
not rule out some continued locally heavy rainfall.

One area where a higher flash flood risk could exist remains over
portions of southern AL into the FL Panhandle. Instability is
higher here, and so expect intense convection to persist here
Tuesday morning into the afternoon hours. Activity should be
progressive off to the east, but 1-2"/hr totals are still probable.
The front stalls out by later in the day, and while forcing and
moisture transport decrease by this time, instability will remain
to go along with the lingering low level convergence. So while most
guidance is not that aggressive with convective development by
later in the day into the overnight, the potential still appears to
be there for additional development. Overall this Slight is lower
confidence than day 1. Current QPFs from the models support more of
a Marginal risk...however given the setup and ingredients, there
is some potential for convection to over perform near this slowing
front. Thus think maintaining the Slight risk is the best option at
this time.

Chenard


Day 3
Valid 12Z Wed Mar 27 2024 - 12Z Thu Mar 28 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE FLORIDA PANHANDLE INTO SOUTHERN GEORGIA...

...20Z Update...

Minimal changes were made to the previous forecast as the current
SLGT and MRGL risk areas remains on track. Across the Southeast and
Southern Mid Atlantic, ascent will be maximized over the east
thanks to the progression of a potent shortwave trough exiting out
of the southern plains into the Deep South. Large scale forcing
within a diffluent area of the upper trough will initiate a round
of heavy rainfall across much of the Carolinas with totals of 2-3"
with local to 4" increasingly likely within the D3 period.
Fortunately, despite the rainfall totals in question, much of the
precip will be from stratiform with some isolated, embedded low-
topped convection limiting the threat of rates breaching much above
1"/hr at any point during the rainfall. With agreement from the
local Carolina WFO's, decided to refrain from any upgrades in the
MRGL risk with more time to assess as we march forward in time.

The best rates will be closer to the Gulf Coast where a prevailing
theta-E wedge will be located over the Big Bend of FL extending
inland into southern GA. This is where the best prospects for
significant rainfall rates in excess of 2"/hr are plausible as
instability crosses within a stout 2-2.5 deviations above normal
PWAT anomaly focused over the Southeast into the Mid Atlantic.
Latest global deterministic has ramped up the prospects for heavy
rain within that corridor of instability with totals now exceeding
3" among multiple members with an associated increase in the
accompanying ensembles. GEFS probability for 2" of precip is now up
to 60% across the eastern portions of NC and northern SC with
60-70% now located along the Big Bend of FL. This is sufficient at
range to keep continuity with the previous forecast SLGT.

Out west, only a minor adjustment to the northern extent of the
MRGL risk was made to correlate with ensemble mean QPF which are
below the threshold necessary for even a Marginal within the
southwest edge of the Portland CWA. QPF trends still remain on
track further south where the IVT plume is most likely to provide a
period of enhanced rainfall into an area that has seen plenty of
the past several weeks. The Marginal risk is still at the lower end
of threshold thanks to the forecasted rates, but persistence works
in this pattern.

Kleebauer

...Previous Discussion...


...Southeast...

A swath of heavy rainfall appears likely Wednesday into Wednesday
night across portions of the FL Panhandle, into southeast GA and
north into the central/eastern Carolinas. At the moment the models
are in pretty good agreement with the axis of this swath...although
given we are 3 days out the uncertainty is probably a bit greater
than the 00z deterministic guidance would indicate. Either way,
confidence is indeed growing for a swath of 1-3" of rainfall
across this corridor.

Increasing forcing from the west will move over a stalled out
front...likely resulting in cyclogenesis and a swath of heavy
rainfall. PWs are forecast near or above the climatological 90th
percentile, so we will likely have the moisture in place for
excessive rainfall...and the duration also looks to be there given
the stalled nature of the front. The main uncertainty comes down to
the degree of instability and rainfall rates. At the moment it
would appear like instability may be lacking over most of this
corridor, which may act to temper rainfall rates...and thus even
1-3" of rainfall would probably just be worthy of a Marginal risk.
The 00z GFS is outputting 4-6" of rain over portions of SC/NC...but
instability is limited, and only a small portion of this rain is
convective QPF out of the GFS. Thus tend to think this specific
forecast is overdone.

The best chance of more robust and persistent instability is
probably closer to the Gulf Coast. It is here where some better
moisture coming in off the Gulf may sustain enough instability for
more vigorous convection. Even this is uncertain to a degree, but
confidence is a bit higher here for higher rainfall rates and the
potential for 3"+ total rainfall. Thus for now think the best
strategy for the ERO is to start small with the Slight risk, and
keep it confined to where confidence is higher on better rainfall
rate potential. Then keep the rest of the corridor in a Marginal
risk and continue to monitor trends over the coming days and
adjust as necessary.

...Northwest CA into southwest OR...
We will maintain a Marginal risk across these areas, although this
is probably a lower end risk. IVT is not all that strong, and the
system is progressive, both of which will be limiting factors for
excessive rainfall. However upper forcing is pretty good, and it
does look like we may be able to generate 0.5"/hr rainfall rates
with some weak instability working into the backside of the cold
front. Still look on track for 1-2" of rain, with localized 3"
amounts. Considered removing the risk, but soil saturation and
streamflows are running above normal over the area, so seems
plausible that any embedded higher rates could case some localized
flooding concerns.


Chenard


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