Quantitative Precipitation Forecast
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FOUS30 KWBC 272342
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
742 PM EDT Wed Mar 27 2024

Day 1
Valid 01Z Thu Mar 28 2024 - 12Z Thu Mar 28 2024

...Southeast...
A swath of heavy rainfall is likely today into tonight across
portions of the Carolinas into southeast VA, which will likely
occur in a warm air advection pattern near a front ahead a
developing low pressure. MU CAPE is 500-1000 J/kg in this region.
Effective bulk shear of close to 40 kts could lead to cell training
near the boundary and potentially mesocyclones. Precipitable water
values of 1.3-1.5" could lead to hourly rain totals to 2", which in
a training scenario could support local amounts to 4". Both soil
saturation and streamflows are near to below average over most of
this area, resulting in high FFG values. This situation remains on
the border of a Marginal and a Slight Risk, so left the risk level
be, merely reorienting the areas based on the 18z HREF probabilities
of 0.5"+ an hour and 2"+ and 3"+ in 12 hours. The thinking is that
with time FFG will lower as conditions saturate, which could
result in a more concentrated flash flood risk in or near the
Slight risk area. Localized flooding is possible.

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 due to lesser
instability which would be easier to erode, justifying keeping that
area in the Marginal.

Roth


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...

20Z Excessive Rainfall Discussion...
Latest trends in QPF allowed for a slight reduction in areal
coverage of the Slight Risk and Marginal Risk areas located over
the eastern portion of North Carolina. Isolated maximum rainfall
amounts approaching 3 inches remains possible with downpours
delivering upwards of an inch per hour before the system translates
off-shore. Also maintained the Marginal over portions of New
England as the system propagates northward and spreads rain
northward.

Bann


08Z Excessive Rainfall Discussion...

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...

20Z Excessive Rainfall Discussion...
Based on the 12Z guidance, there should be increasing rainfall
rates late Friday night/early Saturday morning as a strong closed
low makes its way southward along the California coast. The
magnitude of moisture flux still looks strong enough to warrant
the Slight Risk area...especially in/near the Transverse Ranges.

Bann


08Z Excessive Rainfall Discussion...

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: 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


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