Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
309 PM EDT Thu Apr 3 2025
Day 1
Valid 12Z Thu Apr 03 2025 - 12Z Fri Apr 04 2025
...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
LOWER TO MID-MISSISSIPPI VALLEYS WITH MAJOR FLOODING EXPECTED...
..A PROLONGED LIFE-THREATENING FLASH FLOOD EVENT WILL CONTINUE
TODAY WITH HEAVY RAINFALL IMPACTING A LARGE PORTION OF THE LOWER
MISSISSIPPI AND OHIO VALLEY'S ONCE AGAIN...
...16Z Outlook Update...
The overall forecast philosophy is on track and only minor spatial
adjustments have been made to categorical areas. The High Risk
areas remains in place across portions of Arkansas, Tennessee, and
Kentucky, where a nearly stationary frontal boundary should linger
throughout the day and foster multile rounds of deep convection.
12Z model QPFs suggest another 2-4 inches of rainfall generally
from Little Rock northeastward through south-central Kentucky, with
much of that rain falling on sensitive ground conditions (low to
near-zero FFGs) from prior rainfall and flood impacts last night
and this morning. Significant impacts remain possible, with the
only lingering uncertainty for convective coverage tied to
gradually rising geopotential heights from this afternoon onward.
Little Rock metro was added to the High Risk area after
consultation with WFO LZK, with ongoing high-water impacts
continuing as of this discussion issuance.
The Slight Risk area was expanded eastward into more of West
Virginia, southeastern Kentucky, and Middle Tennessee for this
outlook. Portions of West Virginia have experienced impacts from
heavy rainfall already this morning, and an extensive fetch of
moderate to heavy rainfall is expected to persist much of the day
given renewed convective development across the Mid-South per
latest radar mosaic imagery.
Lastly, Marginal risk areas were maintained across 1) west Texas
for overnight redevelopment of convection in areas that experienced
heavy rainfall this morning and 2) expanded into the
Philadelphia/New York City and adjacent areas for banded/training
convective potential in the 08-12Z timeframe. A quick 0.5-1.25 inch
of rainfall is possible across urban areas of the I-95 corridor,
which could pose a few runoff issues.
See the previous discussion below for more information.
Cook
...Previous Discussion...
Significant rainfall over the past 12-24 hrs will only add to the
favor of widespread flash flooding later this afternoon and evening
as the next round of heavy rainfall occurs as the next surface wave
rides up the quasi-stationary boundary in place across the Lower to
Mid-Mississippi and Ohio Valleys. Widespread 2-4" of rain with
maxima between 5-6" has fallen this evening across a zone spanning
from southern Arkansas up through western Tennessee and Kentucky
leading to several flash flood warnings as the first surface wave
lifts north. Some overturning of the atmosphere is anticipated by
morning with areal theta_e's decreasing across areas north of the
Ohio River and a tightening instability gradient positioned along
and south of the quasi-stationary front nestled from southwest to
northeast over the Mississippi Valley. Despite some limiting
factors early on, small mid-level perturbations embedded within the
mean flow will still cause some scattered light to moderate
convective development through the morning and afternoon leading to
a continuation of priming from the prior evening.
The nocturnal period will once again become the main period of
interest as the nocturnal LLJ kicks back in across the Deep South
and banks into the stationary front bisecting the Mississippi and
southern Ohio Valley. The same areas that saw the significant rain
and flash flooding this evening will be hit once again with another
wave of convection that will initiate across the ArklaTex and
southern Arkansas and eventually move northeast as the flow aligns
parallel to the stationary front. Additional precip totals of 2-4"
with local to 6" will once again be in the cards for much of
Arkansas, western Tennessee, and southwestern Kentucky which will
ultimately put 48-hr totals between 4-8" with locally up to 10"
during the time frame. Local FFG's are already very low after
today's precip with much of the corridor spanning eastern AR up
through north-central KY exhibiting 1 and 3-hr FFG intervals
running below 1/1.5" respectively. 00z HREF is very much indicating
that rainfall rates during the peak of diurnal convection will
reside within that 1-2"/hr marker with FFG exceedance probabilities
above 50% for the 1/3/6-hr intervals in place. Neighborhood
probabilities for >3" total are between 40-60% across eastern AR
with as high as 60-80% over northwest TN and southwestern KY. >5"
neighborhood probs are running between 25-40% within the same
corridor, backed up by EAS probs >2" sufficiently above 50% along
the Mississippi north of Memphis to points northeast.
Despite the overall output being slightly lower than what occurred
this evening, the prescience has been set for significant flood
potential given the already saturated soils, expected rates, and
regional rivers running high from the previous period of rainfall.
The previous High Risk inherited was generally maintained with
small adjustments accounting for recent rainfall trends, updated
FFG's, and encompassing elevated probabilities from both the hi-res
and national ensemble blends. A Moderate Risk still in effect for
a large area surrounding the High Risk in place across the Lower to
Mid-Mississippi Valley with a span that covers areas of southwest
Arkansas up to as far northeast as Cincinnati.
This is quickly becoming a high-end flash flood scenario with
another day or two of rainfall expected on top of what has occurred
and what will occur. If you lie within a flood plane or any area
that is prone to flash flooding across Arkansas, western Tennessee,
and western Kentucky, you will want to pay close attention and
have a plan to seek higher ground, if possible.
Kleebauer
Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
|
