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FOUS30 KWBC 010027
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
726 PM EST Fri Dec 31 2021
Day 1
Valid 01Z Sat Jan 01 2022 - 12Z Sat Jan 01 2022
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEAST
OKLAHOMA INTO PORTIONS OF THE LOWER OHIO AND TENNESSEE VALLEYS...
01Z Update...
Maintained the Slight Risk area extending from parts of Oklahoma
and Arkansas into the Tennessee and Lower Ohio River Valleys
without change from the earlier outlook. There is general
agreement in the models that there will be increasing coverage of
increasing rainfall rates later this evening and overnight. This
will lead to an increasing risk of excessive rainfall, especially
after 06Z, due to potential of training and repeat convection.=20
Isolated maximum rainfall amounts of 3 to 5 inches are possible
within a broader 1 to 3 inch band. Latest guidance also continues
to support the slight southeastward adjustment made this morning.
Across Arizona into extreme southwest New Mexico..convection with
brief moderate to heavy rainfall rates has developed ahead of a
compact mid and upper level feature. Felt confident enough to
trim a small portion of the Marginal Risk area based on short-term
radar and satellite imagery but left the remainder of the Marginal
Risk stand without change. WPC Mesoscale Precipitation Discussion
1178 is in effect until 01/03Z.
Bann
16Z Update...
The 12Z HREF guidance along with recent HRRR runs continues to
support the idea of a broad axis of locally heavy showers and
thunderstorms developing this evening and overnight from the
ArkLaTex and lower MS Valley initially and then expanding well off
to the northeastward across portions of the lower OH/TN Valleys.
The latest hires model guidance has overall shifted the axis of
heaviest rainfall a little south and east of the 00Z cycle, but
the meteorological set-up remains the same as there are expected
to be bands and/or clusters of convection that evolve that will
locally train/repeat over the same area. Localized streaks of 2 to
4 inches of rain appear to be likely tonight through early
Saturday morning, and this is expected to result in some areas of
flash flooding. The Slight Risk area has been trimmed just a tad
and adjusted overall to the southeast based on the latest hires
model consensus. Additionally, the Marginal Risk over the TN
Valley has been pulled southeast into northern GA and adjacent
areas of the southern Appalachians to account for some pockets of
convection that will be occurring over this area this afternoon
this evening with the initial surge of stronger warm air advection
and moisture transport.
Elsewhere, no changes have been made to the early morning forecast.
Orrison
Previous Discussion...
...Lower Mississippi into Lower Ohio/Tennessee River Valleys...
A vigorous upper level trough tracking over Baja California midday
Friday will result in a surge in positive vorticity advection atop
the South Central U.S. by mid-late afternoon. Farther north, an
intensifying jet streak over the Great Lakes and Central Plains
places its right entrance quadrant directly over the Mid-South,
fostering ample upper level ascent aloft. At the surface, a dome
of high pressure in the northwest Atlantic and a deepening surface
low in the Four Corners region tightens the pressure gradient,
causing a 50 knot southwesterly low level jet (LLJ) to develop
over the Lower Mississippi Valley Friday evening. Precipitable
water levels are forecast to range between 1.0-1.6" across the
threat area, which is on the order of 3 to 4 standard deviations
above normal. The LLJ transports rich Gulf of Mexico moisture and
corresponding theta-e advection to prompt an increasingly unstable
environment from North Texas to the Ohio/Tennessee Valleys. MUCAPE
is generally expected to top out in the 250-500 J/kg range with
the higher values likely to set up in the southern periphery of
the hazards area. Showers and storms are expected to fire in north
Texas, eastern Oklahoma, and western Arkansas first, then as the
LLJ ramps in in intensity, additional showers and storms look to
form from the Middle Mississippi Valley to the Ohio/Tennessee
Valleys.
Mean flow within the 850-300mb layer appears fairly unidirectional
from early evening into the overnight hours, supporting the
potential for training showers and thunderstorms. Model averaged
soundings into the overnight hours indicated warm cloud layers as
deep as 10,000 feet AGL. This along with modest skinny CAPE aloft
can support warm rain processes as far east as eastern Kentucky
and Tennessee. The latest 00Z HREF shows a signal of highest
probabilities for hourly rates exceeding 1 inch per hour from the Oklahoma-Arkansas border to northern Tennessee and southern
Kentucky. Given the increased signal for heavier rainfall rates
over areas of eastern Kentucky and Tennessee that are lower in
FFG, have extended the Slight and Marginal Risks east. Some
locations in northeast Tennessee received 150-200% of normal
rainfall over the last 7 days, making them susceptible to
potential flash flooding. In addition, recent ensemble guidance
depicts the chance for heavier rainfall Friday night into southern
Illinois and Indiana, which lines up well considering the endless
supply of moisture available and the synoptically driven support
overhead. These same areas have also witnessed as much as 150-200%
of normal rainfall totals in the past two weeks. Given these
trends and antecedent conditions, an extension of the Marginal and
Slight Risk areas was made into southern Illinois and Indiana.
...Mid to Lower Slopes of Mogollon Rim of Central Arizona into
Southeast Arizona and Extreme Southwestern New Mexico...
The upper low moving into the northern Baja at the beginning of
this period will open up and become increasingly progressive as it
pushes across northwest Mexico later today. No notable changes
were made as the forecast totals of roughly 0.5-1.5" (locally up
to 3") still appears to be on track. Precipitable water values
start out today roughly 2 to 3 standard deviations above normal
and the approaching upper low supports diffluent flow atop the
atmosphere courtesy of positive vorticity advection ahead of the
trough. Antecedent soil moisture conditions remain overly
saturated with as much as 200-300% of normal rainfall over the
last 7 days to the north of Tuscon. These factors should promote a
chance for localized flooding, especially where soil moisture in
southeast Arizona is greatest.
Mullinax
Day 2
Valid 12Z Sat Jan 01 2022 - 12Z Sun Jan 02 2022
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN ARKANSAS, SOUTHEAST MISSOURI, NORTHWEST TENNESSEE AND
MUCH OF THE OHIO VALLEY...
21Z Update...
There is little change to the previous forecast outlook with
respect to the Slight and Moderate Risk areas stretching generally
from portions of the lower MS Valley northeastward across the
OH/TN Valleys. The guidance remains generally consistent with the
overall expected axis of strong convection this period that will
initially take place over areas of the ArkLaTex and lower MS
Valley and then become increasingly organized as it evolves
northeastward with time ahead of the vigorous shortwave trough
ejecting out of the Southwest and shearing downstream toward the
OH/TN Valley region through the period. Heavy rainfall rates
associated deep moisture transport and forcing will combine with
an excellent set-up for training/repeating convective cells for
locally enhanced rainfall totals. Just like with the 00Z cycle,
the 12Z HREF guidance reflects the potential for seeing
swaths/streaks of 2 to 4 inches of rain with isolated heavier
amounts where cell-training occurs. Areas of flash flooding will
become likely as a result with a notable concern for the more
sensitive urban centers and adjacent suburbia.
The Marginal Risk area was extended somewhat off to the
east-northeast to include portions of the northern Mid-Atlantic
region and the greater NYC metro area (including Long Island) to
account for some increased trend in the 12Z HREF guidance for some
linear bands of convection to develop and align itself generally
parallel to the surface front draping across the region as low
pressure approaches by Saturday evening from the OH Valley and
crosses the region Saturday night. Thus, there will be some
localized concern for some training convective cell activity that
may increase the potential for some runoff problems.
Orrison
Previous Discussion...
An amplifying upper trough emerging out of the Rockies and into
the Great Plains is working in tandem with upper level ridging in
the Southeast to lay the groundwork for a hazardous first day of
2022. A robust jet streak over the Great Lakes and far southeast
Canada places its right-entrance region directly over the
Mid-South, fostering large scale ascent within the atmosphere.
Meanwhile, a strong LLJ in excess of 50 knots will tap into the
remarkably warmer than normal Gulf of Mexico and direct it into
the Lower Mississippi, Ohio, and Tennessee Valleys. Some global
model guidance indicates dew points 12Z Saturday around the
Nashville area could be in the mid 60s, which would be among the
highest observed there for the month of January, and are more
commonly observed in late spring and during the summer months.
This LLJ will intersect a frontal boundary oriented from the
Missouri/Arkansas border northeastward into the Ohio Valley.
Integrated Vapor Transport (IVT) according to the latest 00Z GFS
Saturday morning starts out near 1,000 kg/m/s and increases to
1,400 kg/m/s by late afternoon over Tennessee and southern
Kentucky. The abnormally warm temperatures aloft and deep theta-e
advection into the Tennessee and Ohio Valleys are leading to deep
warm cloud layers which contain precipitable water levels that are
3 to 4 standard deviations above normal. In addition, mean layer
850-300mb flow is unidirectional out of the SW, providing a
favorable setup for training. Add in skinny layer MUCAPE that can
become as high as 500-1,000 J/kg from central Arkansas to the Tennessee/Kentucky border, and the atmosphere is primed to produce
excessive rainfall rates from central Arkansas into the Upper Ohio
Valley.
These initial conditions are largely supported across all forms of
global guidance, and there is growing consensus that areas from
eastern Kentucky into western West Virginia and southern Ohio
could be at risk for flooding. The ECMWF EFI indicates a very
unusual heavy rainfall event may unfold in the Ohio Valley,
positioned at the nose of the aforementioned IVT Saturday morning
and afternoon. Still, an axis of training cells embedded within a
highly anomalous moisture stream aloft and modest instability is
oriented as far southwest as th Oklahoma/Arkansas border. These
areas, of which, will have been already contending with periods of
heavy rain and thunderstorms since Friday night.
This long duration period of rainfall originating on Friday and
continuing into Saturday forces soils to become increasingly
saturated leading up to the heavier rounds of rain and
thunderstorms late morning and into the afternoon hours. This
supports the concern for rapid runoff within the Moderate and
Slight Risk areas, as well as urbanized flooding where surfaces
act in a more hydrophobic manner. By Saturday evening, a cold
front trailing a surface low near southern Indiana will trigger strong-to-severe thunderstorms capable of producing excessive
rainfall rates across the Mid-South. Meanwhile, the IVT becomes
oriented at the central Appalachians, potentially causing
localized flooding concerns into northern West Virginia, far
southwest Pennsylvania, and western Maryland. In total, the swath
of heaviest rainfall is likely to occur in and just around the
Moderate Risk area with 2 to 4 inches of rain likely. Isolated
totals as high as 6 inches are possible where training of bands of
heavy rain and thunderstorms transpire.
Mullinax
Day 3
Valid 12Z Sun Jan 02 2022 - 12Z Mon Jan 03 2022
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN PORTIONS OF
THE SOUTHEAST AND PACIFIC NORTHWEST...
21Z Update...
No major changes to the previous thinking. The models continue to
bring a cold front gradually down into the Southeast coastal plain
that will have some bands of convection focused along it. The 12Z
guidance supports a rather strong shortwave trough approaching the
region Sunday afternoon which will be interacting with the front
and will favor the development of a wave of low pressure along the
boundary which will begin to deepen and lift east-northeast. Some
locally concentrated areas of heavy rainfall will be possible as
this wave of low pressure arrives. For now, the Marginal Risk area
will be maintained.
Elsewhere, a deep upper trough/closed low over the northeast
Pacific involving the Gulf of AK is again forecast to impact the
Pacific Northwest by later Sunday and into Monday. Shortwave
energy rounding the base of the deep trough coupled with
increasingly strong onshore flow into the coastal ranges will
promote heavy rainfall with locally a few inches of rain expected
by the end of this period and more beyond the period as the core
of the system arrives. A Marginal Risk is maintained for the time
being across the coastal ranges of western Washington and
northwest Oregon. However, some consideration may need to be given
for the placement of a Slight Risk, and especially for the
following period given the multi-day heavy rainfall potential
set-up for the region.
Orrison
Previous thinking...
...Coastal Southeast Stretching from Southeast Virginia to Eastern
Georgia...
As the storm system responsible for the deluge of rainfall in the
Ohio Valley and Mid-South advances east, so will the axis of deep
atmospheric moisture aloft. A potent upper level trough quickly
moving across the Deep South will provide plenty of upper level
ascent aloft while tapping into Gulf moisture. Precipitable water
levels will still be 3 to 4 standard deviations above normal and
Integrated Vapor Transport (IVT) averages around 1,000 kg/m/s.
Surface dew points will also be above the 90% moving average in
the Carolinas. There is the potential for training showers and
storms close to where a developing coastal front sets up and
unidirectional 850-300mb flow allows for training convection to
ensue, but operational guidance remains at odds on the depth,
speed, and vertical orientation of the approaching trough. The GFS
is a little deeper with the trough and allows heavy precipitation
to advance as far north as southeast Virginia, while the EC and
CMC are closer together in terms of QPF output.
Antecedent conditions are quite dry for much of the region.
Eastern Georgia is closer to neutral with soil moisture in the
30-70% range according to NASA SPoRT-LIS analysis, but much of the
Carolinas and southeast Virginia are drier than normal. There was
a consideration of adding a Slight Risk due to the anomalous
moisture present and potential training of deep moist convection.
The potential for adding a Slight Risk will likely be revisited in
future forecasts, but due to lingering model spread and parched
soils in the region, have chosen to stick with a Marginal for this
forecast period at this time.
...Pacific Northwest...
A storm system in the northeast Pacific is primed to bring more
unsettled weather into the Pacific Northwest this weekend. A
potent upper trough will direct Pacific moisture at the region
with IVTs ranging between 400-600 kg/m/s. The onshore flow out of
the southwest favors upsloping orographic lift locations, but
falling heights will also result in crashing snow levels
throughout the day. This means the immediate coastal areas, most
notably from western Washington on south along the Oregon coast
are most susceptible for excessive rainfall. Precipitable water
anomalies are not overly impressive for this event. However, the
persistent on shore flow late afternoon and into the overnight
hours supports a steady accumulation of rainfall for the Marginal
Risk area. Antecedent soil conditions are close to normal; not too
dry or too wet. Given that the latest WPC 24-hour rainfall totals
are currently forecast to be 1 to 2 inches spread out over an 24
hour timeframe, have chosen to go with a Marginal Risk as there is
still the potential for heavy rainfall to potentially cause some
localized flooding concerns in poor drainage areas.
Mullinax
Day 1 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!opUJf21_5aQWk4aQUdl9aoUmjxyzBahFzM9-ykHD1VhT= PhaG43oKilXqwZaMeMSviLNEjQhW$=20
Day 2 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!opUJf21_5aQWk4aQUdl9aoUmjxyzBahFzM9-ykHD1VhT= PhaG43oKilXqwZaMeMSviKQCUlqy$=20
Day 3 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!opUJf21_5aQWk4aQUdl9aoUmjxyzBahFzM9-ykHD1VhT= PhaG43oKilXqwZaMeMSviDsFhN8T$=20
$$
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