Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
800 PM EST Tue Feb 07 2023
Day 1
Valid 01Z Wed Feb 08 2023 - 12Z Wed Feb 08 2023
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
WESTERN ARKANSAS, SOUTHEASTERN OKLAHOMA, AND NORTHEAST TEXAS...
The Slight risk looks on track across portions of northeast TX
into southeast OK and western AR. Persistent forcing over the area
will support a prolonged period of showers and embedded heavier
thunderstorms through tonight. Strong low level frontogenesis and
moisture convergence, combined with an approaching closed mid
level low and an upper jet to the north, will provide this
persistent upstream forcing. Meanwhile moisture will be plentiful,
with PWs already as high as 1.5" per GPS, which is around the
climatological 99th percentile. The environment is also pretty
favorable for repeat activity...with the lower level convergence
not moving all that much, deep layer winds generally aligned with
this convergence axis, and the mid/upper forcing
persistent/increasing with time tonight. The main limiting factor
for excessive rainfall is probably instability. We do have
500-1000 j/kg east of the low level front...however most of the
convection is expected right along or on the cool side of the
front. This activity will still have some elevated instability to
work with...but probably more on the order of 500 j/kg or less.
This should be enough to get hourly rainfall locally up towards
1"-1.25", but probably not much above that.
Recent HRRR runs and the 18z HREF are pretty well aligned in
favoring far northeast TX into southeast OK and western AR for
heaviest rainfall through 12z. Neighborhood probabilities of
exceeding 3" through 12z are 30-60% across this area, and recent
HRRR runs also support localized 2.5-3.5" totals. We were able to
trim back some on the west and northwest side of the Marginal and
Slight risk with this update as confidence increases on the
corridor most favorable for heavy rainfall. Rainfall rates high
enough to potentially cause flash flooding should stay confined
near and just north of the front where enough instability will
persist.
Soil moisture and streamflows are generally neutral across this
area. Generally near normal values suggest these ground conditions
won't aid or hamper flood potential compared to a typical Feb day.
Soil/stream conditions are a bit more sensitive as you get closer
to the ArkLaTex region...but through 12z the heaviest rainfall is
expected to generally stay northwest of these more sensitive
areas. So overall the hourly rates seem unlikely to exceed FFG
across the area. But the persistence of the rain mentioned above
should begin to result in 3hr and 6 hr FFG being approached and
locally exceeded as we go through the overnight hours. The
greatest threat is across the Slight risk area, but some localized
risk also exists further south over south central TX where
activity may be a bit less organized, but higher instability may
support some locally more intense rainfall rates.
Chenard
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
326 PM EST Tue Feb 07 2023
Day 2
Valid 12Z Wed Feb 08 2023 - 12Z Thu Feb 09 2023
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF
ARKANSAS AND SOUTHERN MISSOURI...
...2030Z Update...
Few changes were made for this afternoon's update. The guidance
remains in excellent agreement on an area of training convection
that will be ongoing at the start of the period across
northwestern AR, and extending northeastward into southern MO.
Expect multiple rounds of convection through much of the day
across this region. While there's great agreement where the heavy
rain will occur, there are definite discrepancies on the
intensity. The 12Z HREF guidance shows a swath of probabilities
over 40% of exceeding 2-year ARIs extending from central into
northeastern AR. Soil moisture percentiles from NASA Sport shows
soils generally 80-90 percent saturated over the southern half of
AR, with quite a bit drier soils in the northwest corner of AR
into southern MO. With those dry soils, the biggest changed from
the inherited forecast was to trim the northern part of the
inherited Slight and especially Marginal Risk areas north of the
AR/MO border, as these areas should be able to handle more
rainfall before any flooding would occur as compared with areas
further south into AR. Soil moisture continues to decline the
further north from the AR/MO border one goes. One final cold front
pushes all the rainfall to the MS River and points east around
06Z, so the flooding threat from excessive rain will end by then.
Wegman
...Previous Discussion...
A Slight Risk area has been maintained for the new Day 2 outlook,
primarily covering northern Arkansas and southern Missouri (with a
small portion in adjoining states). Models continue to show a
relatively slow forward speed of the main upper system as it lifts
northeastward and takes on a negative-tilt, leading to increased
overlap of QPF on Days 1 and 2. The overall setup and forcing
remains similar to Day 1, with deep layer southerly flow ahead of
the trough increasing tropospheric moisture levels across the
MS/OH Valleys, resulting in precipitable water values reaching as
high as 1.5-1.8 inches. This increased moisture, along with strong
lift and sufficient instability (MUCAPE of 500-1000 J/kg), will
result in the potential for heavy rainfall, with areal average
rainfall amounts expected to range from 1-3" across the Slight
Risk area (and locally up to 3-4 inches in the best overlap areas
on Day 1). NASA SPoRT data shows elevated 0-40 cm relative soil
moisture values across portions of the Lower MS Valley (as high as
the 80-90th percentile across southern AR), while soil moisture
values in the Middle MS and OH Valleys are well below normal (as
low as the 10th percentile across portions of southern MO/IL/IN
and western KY).
Churchill/Bann
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
327 PM EST Tue Feb 07 2023
Day 3
Valid 12Z Thu Feb 09 2023 - 12Z Fri Feb 10 2023
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE
WESTERN FLORIDA PANHANDLE, SOUTHERN ALABAMA, AND WESTERN GEORGIA...
...2030Z Update...
The inherited Marginal Risk area was adjusted primarily on the
eastern side to remove much of the eastern FL Panhandle. A squall
line of convection is expected to stall out over the area, with
the guidance still struggling, but generally agreeing on a more
western (slower) stopping point as compared with inherited. Thus,
the Marginal Risk area was also expanded a bit towards the
northeast to include the Atlanta metro area on the northern end as
training convection is likely to develop starting at the Gulf and
riding the stalled front northeast into the Atlanta area. Where
the heaviest rain falls and whether Atlanta itself is directly
impacted remains to be seen as there is some disagreement to that
level. Convection also looks to be significantly weaker compared
with previous days, so at this point no upgrades beyond the
Marginal Risk are expected.
Wegman
...Previous Discussion...
A Marginal Risk of excessive rainfall has been introduced for the
Florida Panhandle and surrounding portions of AL and GA for the
new Day 3 outlook, mainly due to a stalling cold front moving into
the region. High tropospheric moisture levels are expected ahead
of the front, as indicated by GFS progged precipitable water
values of 1.5"+. The presence of sufficient instability (MUCAPE of
500-1000 J/kg) will increase the potential for heavy rainfall,
with rates possibly exceeding 2"/hr at times. The possibility of
repeating rounds of heavy rainfall is also expected, increasing
the threat of excessive runoff potential (with ECENS probabilities
for 3" exceedance as high as 5% locally). However, antecedent soil
conditions in the region are quite dry, with NASA SPoRT 0-40 cm
soil moisture values indicated to be as low as the 10-20th
percentile. This should significantly limit the areal extent of
any flash flooding, with poor drainage/urban areas the most at
risk of realized isolated flooding impacts.
Churchill
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
