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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0841Z Mar 27, 2024)
 
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Excessive Rainfall Discussion...UPDATED
NWS Weather Prediction Center College Park MD
440 AM EDT Wed Mar 27 2024

Day 1
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 AND OVER PORTIONS OF
EAST CENTRAL NORTH CAROLINA...

...Southeast...

A swath of heavy rainfall is likely today into tonight across
portions of the FL Panhandle, into southeast GA and northward into
the central/eastern Carolinas.

A Slight risk will be maintained across portions of the the FL
Panhandle into southern GA. Convection should be ongoing across
this area at 12z this morning, with 2"/hr rainfall possible given
the instability and moisture in place. At the moment, it looks
like activity will have enough eastward motion to limit the
duration of the highest rainfall rates and keep any flash flood
risk localized. By this afternoon we see an uptick in mid/upper
forcing and strengthening low level convergence over the area. This
should result in an uptick in convective coverage by this
afternoon into this evening. It is unclear whether this activity
will be organized enough to pose a flash flood risk. Flash flood
guidance exceedance probabilities from both the 00z HREF and 18z
REFS ensembles are very low, and not at the level we would expect
to see for a Slight risk level threat. However we will have a slow
moving boundary overhead, and multiple convective rounds appear
probable into this evening. Given the ingredients, there is some
chance this convection ends up a bit more robust than currently
forecast in the models...and so while this is probably a lower end
Slight risk, not confident enough for a downgrade.

Morning convection over southeast GA into the Carolinas should be
fairly weak. However by this afternoon we are expecting ~1500 j/kg
of CAPE to develop near the stalled out front. Convergence near
the front along with this increased instability should result in
more robust convective development by or just after 18z. Based on
HREF probabilities, expect 1"/hr to 1.5"/hr rainfall with this
activity, but not currently anticipating anything over 2"/hr. Most
indications are that this convective development will aid in
pushing the stalled front a bit eastward with time. Thus while
initial convection is expected more over east central SC/NC, this
activity should push east with eroding instability from west to
east.

By later in the afternoon into the evening hours the focus
for any more intense convection should be gradually shifting east
over the Carolinas. This rainfall over the eastern Carolinas will
likely be reinforced overnight by a developing low pressure along
the front.

Overall, neighborhood probabilities from both the 00z HREF and 18z
REFS indicate a greater than 50% chance of 3"+ rainfall amounts
through 12z Thursday over portions of the central and eastern
Carolinas into southeast GA, however the probabilities of 5"+
amounts drop significantly to only around 5%. Both soil saturation
and streamflows are near to below average over most of this area,
resulting in high FFG values. FFG exceedance probabilities from the
HREF and REFS are low. Overall, a tricky call on whether to stick
with a Marginal or upgrade to a Slight risk. Decided to go ahead
with a targeted Slight risk area over portions of east central NC.
This risk was placed where there is overlap between more coverage
of 3-5" total rainfall, and 1"/hr or greater rainfall rates. The
thinking is that with time FFG will lower as conditions saturate,
which may eventually result in a more concentrated flash flood risk
over the Slight risk area. This Slight risk area has the potential
to see one round of convection this afternoon, and another
overnight as the forcing increases and low intensifies. Localized
flooding is possible with round one, but the Slight risk come comes
into play during the overnight hours when the second round of
possible 1"/hr rainfall could briefly train along the trough axis
extending northeast from the low.

Areas south of the Slight risk over SC and GA have high rate
potential, but shorter durations and less coverage of higher
totals justify keeping the risk at Marginal. Areas north of the
Slight over northeast NC into southeast VA have high total
rainfall, but much lower probabilities of higher rainfall
rates...justifying keeping this area in the Marginal.


...Northwest CA into southwest OR...
We will maintain a Marginal risk across these areas. A cold front
will push south across the region today, with some mid level
shortwave energy and upper jet support moving over the front as
well. This should provide enough forcing to generate rainfall up
to 0.5"/hr, especially given some weak instability working into
the back side of the front. High res guidance is in good agreement
on widespread 1-2" amounts, with pockets of 3"+ rainfall within the
more favored upslope areas. With soil saturation and streamflows
running above normal over the area, it seems plausible that any
embedded higher rates could cause some localized flooding concerns.

Chenard



Day 2
Valid 12Z Thu Mar 28 2024 - 12Z Fri Mar 29 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL OVER PORTIONS OF
EASTERN NORTH CAROLINA...


An area of low pressure moving up the coast will result in a heavy
rain threat to portions of the eastern Mid-Atlantic Thursday into
Thursday night. Highest rainfall totals are expected to be across
eastern NC into southeast VA, where 1-3" of additional rainfall is
probable. By this time instability should be lower across this
area, although strong convergence and weak instability near the low
track should still support rainfall totals upwards of 1"/hr,
especially over eastern NC. Just like described in the day 1
discussion, a Slight risk upgrade was issued over eastern NC where
there is overlap in potential 3"+ total rainfall and 1"+/hr
rainfall rates Thursday morning. Areas north of the Slight risk
should have lower rainfall rates, warranting just a Marginal risk.

A swath of 1-3" of rain is also expected across eastern New
England. Given this region will be on the cool side of the
developing low, not thinking we'll see hourly rainfall any higher
than 0.25". However recent rainfall has resulted in above average
soil saturation and streamflows across this area...so while flash
flooding appears unlikely given the low rates...some areal flood
concerns are a possibility given the saturated conditions. Rain on
snow over eastern ME is also likely, although with temperatures
only in the 30s and 40s with this rain, unlikely to see a rapid
snow melt. Nonetheless, given the wet and or snowy conditions of
late across this region...the additional 1-3" of rain warrants the
continuation of the Marginal risk.

Chenard



Day 3
Valid 12Z Fri Mar 29 2024 - 12Z Sat Mar 30 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHERN CALIFORNIA...

A strong closed low dropping south off the CA coast will bring a
heavy rainfall threat to the state Friday into Friday night. This
is a pretty strong system for the time of year and latitude, with
NAEFS anomalies indicating 850-700mb height forecasts near
climatological record lows. Not the strongest IVT magnitudes with
this system, but still over the 90th percentile, and ~30 kts of
southerly flow at 850mb is aligned well to provide upslope
enhancement into the Transverse Range. Still some timing
uncertainty given we are still 3 days out, but the current model
consensus indicates an uptick in rainfall as low level convergence
increases over the Transverse Range between 06z-12z Saturday.
Likely enough forcing, low level convergence and weak instability
to support hourly rainfall over 0.5" across portions of Santa
Barbara and Ventura counties. The 00z downscaled GFS and ECMWF
indicate upwards of 2-4" of rain across these counties by 12z
Saturday...and while this is higher than the official WPC forecast
(given some lingering timing/strength uncertainties), these amounts
are certainly possible. Thus think the Slight risk over these
counties remains warranted.

This Slight risk is surrounded by a broad Marginal risk over much
of central CA. Less of a focus for heavy rainfall here, and the
cold front will be progressive. Nonetheless enough forcing is
seemingly present to drive a localized heavy rainfall threat,
especially if some instability can work into the system.

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