Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
453 PM EDT Tue Aug 20 2019
Day 1
Valid 2039Z Tue Aug 20 2019 - 12Z Wed Aug 21 2019
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST, CENTRAL PLAINS INTO OHIO VALLEY, AND THE
SOUTHEAST...
2100 UTC update
Changes to the previous Excessive Rainfall Outlook were based on
the radar trends with the band of showers/storms having departed
IA and most of IL and now moving south on a pace to reach into
northeast AR and TN this evening. The northern edge of the
previous marginal risk area was trimmed south across the rest of
Iowa and central Illinois and extending further south into
Tennessee. The progressive nature of the convection warrants
keeping the risk at marginal, although slower movement at the tail
end could lead to localized flash flood concerns.
A mesoscale precip discussion has been issued for southern MO,
northeast AR, and adjacent western TN.
Oravec
...Central Plains into the Mid MS Valley and Ohio valley...
An MCS will be ongoing at 12z across portions of IA. It will track
southeastward through the morning hours across the Mid MS Valley
and eventually into the OH Valley. Overall expect this activity to
be progressive enough to keep any flash flood threat localized in
nature. The 06z HRRR showed an interesting feature this evening
into the overnight hours over IL/IN...with a backbuilding/training
convective axis. 850mb moisture transport fields from the RAP do
show an area of enhancement across this region...which in theory
could be enough to generate this nocturnal activity. However, it
should be noted that the 06z HRRR is initializing quite terribly
with the ongoing Iowa convection...and this poor initialization
may very well be impacting this potential downstream development.
So while we will need to monitor this, at this time we would
consider the 06z HRRR evolution a less likely outcome.
Will likely have another round of organized convection tonight
over portions of the Central Plains. The combination of an
increasing low level jet into an axis of extreme instability, and
a southward dropping upper jet placing the region in the favored
right entrance region, should be enough to result in upscale
convective growth out of cells that initiate in the High Plains
this afternoon. This activity will likely exhibit some west to
east training as it moves eastward across NE. HREF neighborhood
probabilities of 3" are pretty high, but 5" probabilities are
quite low...likely indicative of the forward propagation of cells
capping rainfall magnitudes (even with some training). Given the
wet antecedent conditions across portions of this region, was on
the fence of upgrading to a Slight risk. However, think there is
likely enough uncertainty on exact evolution and eventful axis of
heaviest rain, that opted to stick with a Marginal. Will let the
day shift reevaluate as new data continues to come in.
...SC/GA...
Added a Marginal risk across portions of SC and eastern GA.
Continue to track a weak low/mid level wave over this region,
which will only slowly track northeastward today. This feature has
been convectively active overnight, but in general nothing has
been too organized or intense. Confidence in how convection around
this feature evolves today is rather low. But given instability
and some signal of a persistence/enhancement of convergence around
the feature...think at least a Marginal flash flood risk exists.
The 06z HRRR was quite aggressive with QPF amounts near this
low...and while we think this is probably overdone by a good
amount...it did help sway us to introduce a Marginal risk, and
will continue to monitor trends.
...NM...
PWs will be around 1 SD above average this afternoon...and we
should see enough instability for scattered convective
development. Activity will be primarily tied to the terrain, but
could see a few cells drift off with time. Localized 1-2" rains in
a short period of time are probable...and while we are not
expecting any organized flash flood threat...if any cells end up
over recent burn scars or other sensitive locations, then a
localized flash flood could result.
Chenard
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
349 PM EDT Tue Aug 20 2019
Day 2
Valid 12Z Wed Aug 21 2019 - 12Z Thu Aug 22 2019
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL PLAINS...
...Central Plains/Mid-Mississippi Valley...
A challenging forecast across the Plains as a cold front and
several shortwaves of varying intensity interact to produce a
series of MCS across the area.
The first of these will likely be decaying across NE/MO early on
day 2 while a cold front drops into the area. While uncertainty
exists into where this front will lay, and where any residual
outflows may reside from previous MCS, there is high confidence
that a morning MCS, and a subsequent overnight MCS late on day 2,
will both bring heavy rainfall to the area.
An amplifying longwave trough across the eastern CONUS centered
over Canada. This trough will begin to suppress the southern
ridge, and between these features shortwaves will zip eastward
within the westerlies. Each of these shortwaves will have the
potential to produce an MCS, as the forcing due to the impulse
will combine with large scale ascent within the RRQ of a jet
streak lifting to the northeast and a nocturnal 850mb LLJ which is
progged to reach 20-30 kts. This ascent will act upon a
thermodynamic environment favorable for heavy rainfall, with PWATs
climbing above 2" and MUCape reaching 3000 J/kg or higher, and
persisting into the evening/overnight hours of D2. The most likely
period for excessive rainfall will be after 00Z Thursday (Wed
night) when the LLJ intensifies and lifts isentropically atop the
cold front. This interaction will produce an elevated front along
which the s/w should produce an MCS. Although Corfidi vectors are
forecast to be rapid during this time, suggesting rapid motion of
the complex, weak 0-6km mean winds and aligned mean cloud layer
wind vectors should produce training, especially along the
southern periphery of the MCS where an outflow is likely to
develop and become aligned with the mean flow. ECENS and GEFS
probabilities remaining elevated for 3" of rainfall in a large
swath from eastern KS into MO, and this will occur on top of a
region which has received 600% of normal rainfall during the past
7 days. If heavy rainfall should train across the most saturated
soils, flash flooding would be likely, and could necessitate a
higher risk. However, due to uncertainty in placement and
potential rapid speed to limit temporal duration of heavy
rainfall, a SLGT risk was maintained, but drawn a bit southward
from inherited to account for latest NAMNest and ARW2 guidance, as
well as the likelihood that previous MCS should push the
instability gradient southward from the current global consensus
forecast.
...Northeast...
Deep layer southwest flow ahead of a cold front will draw moisture
and instability into the Northeast Wednesday. PWATs are forecast
to climb towards 1.75", with aftn MUCape rising above 1000 J/kg.
This thermodynamic environment will be favorable for heavy
rainfall, with ascent being driven by modest RRQ jet-level
diffluence, height falls ahead of the longwave trough, and
low-level convergence ahead of the front. This lift within the
favorable thermodynamics should produce scattered thunderstorms,
especially during the aftn and evening. A MRGL risk was inherited
and maintained, with subtle expansion northward to account for
somewhat lowered 3-hr FFG across parts of northern New England.
Although briefly heavy rainfall is likely, noted by HREF
neighborhood probabilities for 1"/hr rising above 60%, storm
motion is expected to be rapid. 0-6km mean winds of 40+ kts will
drive rapid cell motion to the northeast, so outside of any
training due to boundary parallel flow, the flash flood risk
should be confined to the lowest regions of FFG. GEFS
probabilities for 1" basin-average rainfall are low, so the MRGL
risk was maintained as the threat appears too isolated for an
upgrade to SLGT.
...Southwest...
A brief return of monsoon moisture around the periphery of a
mid-level ridge over TX will bring thunderstorms with heavy
rainfall to the Southwest. PWATs climb to 1-1.25", coincident with
MUCape rising above 1000 J/kg. This instability will support
scattered to widespread thunderstorms during the aftn/eve of
Wednesday. 0-6km mean winds are very light, 0-5 kts, with
low-level convergence likely, especially across New Mexico in the
lee of the highest terrain. The very light flow suggests storm
mergers will be likely, and as rain rates approach 1-hrly FFG of
1-1.5"/hr, flash flooding will be possible. Limiting a more robust
flash flood threat appears to be a disconnect between the
strongest s/w energy and best instability, suggesting convection
won't be ideally forced. Still expect at least a few instances of
flash flooding due to very slow storm motions with heavy rainfall,
and the MRGL risk was maintained.
...Pacific Northwest....
A brief but robust Atmospheric River will bring heavy rainfall to
the Pacific Northwest, most likely in the 18Z Wed - 06Z Thu
timeframe. Recent GFS probabilities indicated a greater than 80%
chance for IVT > 500 kg/m/s advecting into NW Oregon and W
Washington. This AR will lift onshore and move eastward rapidly
ahead of a mid-level trough digging into the region, and it is the
speed at which this occurs which will limit the total flash flood
potential. As the trough pushes eastward, an embedded shortwave
will combine with the height falls and subtle RRQ diffluence to
drive ascent in a column with highly anomalous moisture. Although
instability will be limited, 250 J/kg or less, the PWATs will
approach all time highs into Oregon, potentially exceeding 1.5",
which would be around 4 standard deviations above the
climatological mean. This exceedingly moist environment will favor
excessive rain rates, but 0-6km mean winds of 20-30 kts will drive
rapid motion of these heavy rain producing echoes. HREF
probabilities for 0.5"/hr are 20% or less, and total QPF will
struggle to 1" outside of the upslope enhanced areas on the
western side of the coastal ranges. After coordination with WFO
PQR and SEW, have opted not to create a MRGL risk area, but note
the potential for isolated flash flooding, especially if an urban
area can receive rain rates above 0.5"/hr.
Weiss
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
350 PM EDT Tue Aug 20 2019
Day 3
Valid 12Z Thu Aug 22 2019 - 12Z Fri Aug 23 2019
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL PLAINS...
...Central Plains/Mid-MS Valley/OH Valley/TN Valley...
A cold front is progged to slow across the area Thursday, becoming
parallel to the mid-level westerlies. Along this front, s/w energy
racing eastward will likely move along this front spawning yet
another MCS, with this one at least subtly displaced southward
from the previous days. This s/w will overlap with a favorable
thermodynamic environment for thunderstorms with heavy rainfall,
noted by PWATs of 2" or more, and MUCape above 2000 J/kg. Ascent
will be driven by subtle height falls as well as RRQ diffluence
from an upper jet streak lifting away to the northeast, and
isentropic ascent as the LLJ amplifies atop the baroclinic zone
during the evening and overnight.
This MCS will again likely move rapidly to the east, as Corfidi
vectors reach 10-20 kts. However, 0-6km mean wind parallel to the
Corfidi vectors suggests training of echoes is likely, so despite
the rapid motion of the MCS itself, heavy rain producing cells
could repeatedly move across the same areas, especially south of
the path of the MCS. While there remains uncertainty into the
placement of this MCS, there appears to be enough of an overlap of
heavy rain signal on top of areas which should receive heavy
rainfall on Day 2 that a SLGT risk was introduced where FFG is
expected to be significantly reduced from previous convection.
Further east along the stalling front, strong confluence of the
moisture transport vectors and 0-6km mean wind aligned with the
boundary suggests heavy rain will spread eastward as far as the
Appalachian mountains. FFG further east is higher, and
probabilities for heavy rainfall are lower, so a MRGL risk remains
sufficient for the Ohio Valley and points east.
...Eastern MT/Western ND...
An approaching trough and associated surface cold front will move
through the region interacting with pooling moisture (1.25-1.5
inch PWAT)/instability (surface based CAPE >2000 J/kg) to the east
of the boundary. With strong divergence aloft and mid-level
vorticity, forcing for ascent will be sufficient and appears to
maximize Thursday evening, coincident with a piece of the
southerly 850mb LLJ rising towards 30 kts ahead of the front.
Therefore, expect convection to break out along and just ahead of
the front Thursday afternoon with activity blossoming during the
evening hours across eastern MT into westerly ND in response to
enhanced ascent and robust moisture transport on the nose of the
LLJ. The progression of the front should be such that it limits
the overall flash flood potential. However, with some training,
could see storm total QPF top out over 1 inch as evidenced by
modest ECENS and GEFS probabilities for 1" in eastern MT and
western ND. FFG across this region, especially in eastern MT, are
quite low, 1-1.5"/3 hours, so flash flooding is possible.
Pagano/Weiss
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
