Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1219 PM EDT Fri May 24 2019
Day 1
Valid 16Z Fri May 24 2019 - 12Z Sat May 25 2019
...A HIGH RISK OF EXCESSIVE RAINFALL TODAY AND TONIGHT FROM
NORTHERN OKLAHOMA INTO SOUTHEAST KANSAS...
16z Update:
HIGH Risk upgrade for portions of northern OK and southeast KS.
Significant and life threatening flash flooding is possible today
into tonight across this region. Convection will develop along a
nearly stationary front from the TX Panhandle into southeast KS.
Mean flow is parallel to this front, which is supportive of
training. Given the persistent moisture transport into the front,
plentiful instability, and persistent broad diffluence aloft south
of the upper jet to the north...think we will see repeated rounds
of convection move across OK and into southeast KS today into
tonight. While we may eventually see some propagation off to the
east into the instability pool, training should last long enough
for significant to locally extreme rainfall amounts...with amounts
as high as 4-7" a possibility at least on a local scale.
Precipitation efficiency will be very good given the very high PWs
over the region. The 12z high res guidance is in good agreement
with the axis of heaviest rains...with both the HREF and HRRR in
good alignment. Thus confidence is increasing that a significant
flash flood event will evolve today within/near the HIGH risk area.
Chenard
...Previous Discussion...
...Central Plains/Midwest...
Low amplitude shortwaves moving around the periphery of the
western U.S. trough are expected to slowly lower 700 hPa
temperatures, leading to a weakening/vanishing mid-level capping
inversion with time across the southern Plains and Midwest, with
eastern IA appearing to be the last place it breaks per 00z
NAM/GFS guidance. Inflow at 850 hPa of 30-45 kts is most
persistent into eastern KS, but the guidance is scatter shot in
its indication of a maximum, showing solutions between northern OK
and Kansas City MO. The degree of instability (up to 3000 J/kg of
ML CAPE implied by 21z SREF mean CAPE/spread guidance), inflow,
and effective bulk shear indicates that training bands of
convection, cell mergers, and mesocyclones would be the causes for
heavy rainfall in this environment. With precipitable water
values exceeding 1.75", hourly rain totals to 2.5" would be
possible. The guidance indicates the potential for local amounts
of 5-7" in or near southeast KS, which would be problematic over
saturated soils and areas with high stream flow/river flooding.
Much of Kansas has seen 14-20" of rain during the past 30 days
which is 3-5x normal. The April 23-May 22 period was the wettest
on record at Kansas City, MCI, in 131 years of records, and was
the second wettest such period at Oklahoma City (129 years) for
that same period (source: Southeast Regional Climate Center
Climate Perspectives Tool). Expect a large percentage of surface
runoff from new rainfall, and rapid escalation of impacts from
flash flooding wherever training thunderstorms develop.
Collaboration with ICT/Wichita KS maintained a moderate risk,
given the lingering uncertainty on placement of the highest
totals. LOT/Chicago IL was agreeable to the extension of the
moderate risk northeast along the WI/IL border, an area where both
the warm conveyor belt circulation and warm air advection over an
instability pool could allow for bouts of thunderstorms with heavy
rainfall -- guidance indicates local amounts of 3-4" in that area.
...Northern Rockies/Northern Great Basin/northeast CA...
A closed mid level low drops south from southwest British Columbia
into OR by the end of the period. With the approach of the cold
mid level system, lapse rates increase, resulting in pockets of
500 J/KG of MUCAPE mainly over the higher of CA/NV/ID. The
activity should be mainly diurnally driven, with the greatest
coverage of storms expected between 24/21z and 25/03z.
Ahead of the mid level low, a modest westerly low level flow
transports 0.75-1" precipitable water values over northern and
central CA. Elsewhere across the Great Basin and Northern Rockies,
upslope flow aids in producing local 0.50" precipitable water
values over portions of NV and the higher terrain of ID (an amount
which is problematic at elevation). The combination of moisture
and instability is expected to support scattered convection, tied
initially to the terrain.
Steering currents become weak as the mid level system approaches,
with storm motions under 10 knots expected. This could foster
short term training and cell mergers through peak heating, which
becomes less of an issue after 25/03z. The 00z HREF probabilities
of 1"+ an hour rose towards 30%. Hourly totals of 1.00-1.25" are
possible where storms train, backbuild, or merge, especially over
CA. This much rain could pose a low end flash flood threat, which
is predicated on storm coverage. Based on the above, a Marginal
Risk remains over northeast CA, northern NV and much of the higher
terrain of ID.
Roth/Burke/Hayes
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
420 PM EDT Fri May 24 2019
Day 2
Valid 12Z Sat May 25 2019 - 12Z Sun May 26 2019
...THERE IS MODERATE RISK OF FLASH FLOODING ACROSS FROM WEST
CENTRAL OKLAHOMA INTO SOUTHEAST IOWA...
...Southern Plains into the Ohio Valley...
Short wave energy tracking along a frontal boundary interacts with
strong instability and deep moisture to support the development of
convection that produces heavy rainfall over areas with saturated
soils. There was generally good model agreement with the overall
synoptic setup, as well as the placement of the higher rainfall
amounts, so the WPC QPF and Excessive Rainfall Outlook were based
on a multi model blend.
A cold front dropping southeast across the Upper Great Lakes into
the Mid and Upper MS Valley serves as the focus for developing
convection extending from KS into northern MO and central IL
between 25/18z and 26/00z. Initially, the convection could outflow
dominated as it develops in an axis of 3000-4000 J/KG of SBCAPE
extending from western OK into southern IA. However, short term
training or cell mergers along the developing line of storms could
pose a flash flood threat.
The greater threat for flash flooding occurs after 25/00z, as the
convection becomes focused on the strengthening low level jet. A
40/50 knot low level southerly flow transports 1.50/1.75 inch
precipitable water air along the front. Some of the 12z guidance
suggests that the convection could form into clusters, then move
northeast along the front with the mid level flow. This idea is
supported by the 12z GFS simulated IR images showing clusters of
cooling cloud tops, especially after 26/06z.
After 26/06z, storm or cluster motions slow across northeast KS
into southern IA, when the propagation vectors begin to oppose the
low level flow. This should foster training or backbuilding in
these areas, with much of the 12z guidance showing the potential
for 3.00/5.00 inches of rainfall occurring along the front (though
the placement of the maxima varies, as would be expected with a
convectively driven system).
Antecedent conditions here are wet, with three hour flash flood
guidance as low as 1.50 inches. and two week rainfall amounts over
300 percent of normal. The combination nearly saturated soils and
the potential for more than 3.00 inches of rainfall, a Moderate
Risk extended from west central OK into northern MO/southern IA,
which is about where the previous Moderate Risk was placed. Given
the spread of high resolution models, no High Risk was assigned.
Further northeast into northern IL and southern WI and the
northern Ohio Valley, antecedent conditions are not as wet, but
deep moisture and strong instability support training convection.
The greatest threat is expected after 26/06z, as convective
clusters along the front possibly training. The Slight Risk was
left in place here to cover the threat.
...Northeast...
Deepening moisture and instability couple to support convection
across NY state into a portion of northern New England during Day
2. Ahead of embedded short wave energy in a fast mid level flow, a
low level west southwest flow transports 1.25/1.50 inch
precipitable water air (which is between two and three standard
deviations above the mean) over NY state and northern New England,
peaking between 25/21z and 26/06z. Behind a warm front moving
toward New England a slow moving cold front crossing the Upper
Great Lakes, an axis of 1000-1500 J/KG of MUCAPE extends into
portions of western PA and far western NY state, generally before
26/00z.
The convection is expected to produce hourly rainfall rates near
1.25 inches in the deep moisture in place over these areas. The
convection is expected to be progressive, with storms motions from
northwest to southeast at 10 to 15 knots. However, three hour
flash flood guidance values here are as low as 1.50 in posts, and
any short term training could pose a flash flood threat. Because
of this, the Slight Risk in place was left largely unchanged.
After the convection over western PA/western NY weakens during the
evening hours, short wave energy in the fast zonal flow interacts
with marginal instability associated with a weakening warm front
crossing northern New England between 26/03z and 26/09z. Moisture
is deep across the region, but as the mean 850-300 mb flow becomes
better aligned with the propagation vectors, training becomes a
possibility. Three hour flash flood guidance values are as low as
1.50/2.00 inches, since 14 day rainfall amounts of have 150/250
percent of normal. With marginally wet antecedent conditions over
this area, a Marginal Risk was placed here for Day 2.
...California...
A mid level system OR early on Day 2 weakening as it drops south
into Northern CA during the first half of day, followed by a much
stronger mid level system, reaching northwest CA by the end of the
period. Steepening lapse rates are expected to support 250-500
J/KG of MUCAPE across much of the higher terrain of Sierra Nevada
mountains, with the best instability between 25/18z and 26/03z,
after which time the mainly diurnally driven instability becomes
consumed.
A low level westerly flow aids in pooling 0.75/1.00 inch
precipitable water air (which is between two and three standard
deviations above the mean) over this area. The combination of
moisture and instability should support in at least scattered
convection over the higher terrain initially, which drops into the
valleys just after peak heating.
In this airmass, hourly rainfall rates could approach 0.50 inches,
with local 1.00 inch amounts possible, especially over the
northern Sierra Nevada Mountains into the Shasta Trinity
Mountains. Based on this, a Marginal Risk was placed over the
Sierra Nevada range for Day 2.
...Northern Rockies...
Synoptic scale ascent on the east side of the strengthening closed
mid level low over the Pacific Northwest and Northern CA combines
with deepening moisture and marginal instability to pose a heavy
rain threat across portions of northern ID into northwest MT,
especially over the lower terrain. A strengthening low level east
southeast flow transports 0.75 inch precipitable water air into
this region (which is about two standard deviations above the
mean), which is focused on the higher terrain.
Model soundings indicated marginal instability at best (generally
under 250 J/KG of MUCAPE), and it is possible that this area is
too far removed from the better lapse rates associated with the
closed mid level system to allow scattered convection to develop
in the expected area of mainly stratiform rain.
Conditions are wet across the Northern Rockies, with melting snow
and earlier heavy rainfall resulting in some high stream flows. If
instability does not occur, this could be more of a flood versus
flash flood event. Because of this, no excessive area was
assigned. If instability becomes more of a component of the
rainfall producing process, a Marginal Risk could be needed here
in later forecasts.
Hayes
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
424 PM EDT Fri May 24 2019
Day 3
Valid 12Z Sun May 26 2019 - 12Z Mon May 27 2019
...THERE IS SLIGHT RISK OF FLASH FLOODING ACROSS PORTIONS OF THE
SOUTHERN AND CENTRAL PLAINS...AS WELL AS PORTIONS OF THE OHIO
VALLEY...
...Southern and Central Plains...
Short wave energy ejected from a strong closed low over southern
NV taps deep moisture and strong instability along a frontal
boundary extending from the Central Plains into the Mid MS Valley
to support convection producing heavy rainfall over wet soils.
While there was decent model agreement concerning the placement of
the synoptic scale system, there was some spread in the placement
of the highest rainfall amounts (not surprising given the
convective nature of the QPF). With this in mind, the WPC QPF and
Excessive Rainfall Outlook were based on a multi model blend.
Convection is expected to initiate along the dryline/cold front
extending from central NE into west TX, mainly after 26/18z, in an
axis of 2500/3500 J/KG of SBCAPE. Given the amount of instability
in place, the storms are expected to be predominately hail and
wind producers, especially through 27/00z. However, short term
training or cell mergers along the dry line/cold front could pose
a short term flash flood before that time.
The convection is expected to increase in coverage as difluence
increases ahead of the closed mid level system over the southern
Great Basin during the second half of the period. A strengthening
low level jet transports 1.50/1.75 inch precipitable water air
across the Southern and Central Plains, peaking near 27/06z. By
that time, the propagation vectors slow to less than 10 knots
across portion of central KS into southeast NE, which could allow
storms or clusters to train as they move ahead of the front.
As mentioned earlier, there is some spread in the placement of the
highest rainfall amounts, which is typical in an environment
dominated (at least initially) by strong instability. However,
there was enough commonality to place the highest rainfall amounts
from west central OK across central KS into western IA, as the
storms follow the best low level moisture plume. There was some
consideration for a Moderate Risk for portions of central KS,
where soils are fairly wet due to convection earlier this week.
However, given the spread in the placement of the highest rainfall
amounts, and after collaborating with WFOS ICT/DDC, it was decided
to hold off the Moderate Risk for now. However, if there is better
agreement on the placement of the highest rainfall amounts, a
Moderate Risk could be needed in later forecasts.
...Ohio Valley...
Short wave energy tracking along a frontal boundary extending from
the Central Plains into the Ohio Valley interacts with deepening
moisture and instability to support convection capable of
producing excessive rainfall, especially over southern
OH/northern WV into far southwest PA. While there was generally
good model agreement with respect to the overall synoptic setup
(including the frontal position), there are some differences in
the amount of QPF in these areas. In an attempt to mitigate some
of the differences, the WPC QPF and Excessive Rainfall Outlook was
based on a multi model blend.
Convection weakening over the Mid MS Valley early in the period
rides along a frontal boundary from IL into southern IN and
southern OH. Short wave energy (or MCV) riding along the frontal
boundary provides synoptic scale ascent over an area with
2000-3000
J/KG of MLCAPE extending along and south of the front, which
should result in renewed convective activity, mainly after 26/18z.
As the storms, they will tap the 1.50 inch precipitable water
moisture plume in place.
As the low level moisture flux peaks between 26/21z and 27/03z,
storms (or convective clusters, as the 12z GFS simulated IR images
showed cooling tops aligned in clusters) could train as the mid
level flow parallels the front. Hourly rainfall rates could
approach an inch, especially where convection trains, with local
1.50 inch amounts possible through 27/06z. Three hour flash flood
guidance values are as low as 1.00/1.50 inches over portions of
southern OH/much of WV into southwest PA, and these amounts could
easily be exceeded by training clusters. Based on the above, the
Slight Risk was left in place, with just some adjustments due to
model trends.
...Montana...
On the eastern side of a strong closed mid level low dropping
south from Vancouver Island into Northern CA, a low level
transports moisture across portions of MT and far northern WY
during Day 3. Steepening lapse rates in conjunction with the mid
level low could support 250/500 J/KG MUCAPE values over much of
central MT, where the low level flow transports 0.50/0.75 inch
precipitable water air (which approaches two standard deviations
above the mean). The combination of moisture and instability could
support convection capable of producing hourly rainfall rates near
0.50 inches in this area.
There is a strong model signal for 1.50 inch rainfall amounts,
especially over south central MT, along and just east of the
divide in the upslope flow. Three hour flash flood guidance values
are as low as 1.00/1.50 inches, which are attainable where storms
train. The Marginal Risk was left largely intact for Day 3, and if
there is a better convective profile near the divide in later
forecasts, a Slight Risk could be needed.
...California/Nevada...
A strong closed mid level system tracks from northwest CA early on
Day 3 to a position over southern NV by the end of the day.
Steepening lapse rates is expected to result in pockets of 250/500
J/KG of SBCAPE over the northern and central Sierra Nevada
Mountains into nearby sections of western NV. Scattered convection
forming in the afternoon will have access to 0.75 inch
precipitable water air to produce locally heavy rainfall through
the early evening hours.
As the mid level system passes overhead, steering currents will be
weak (generally under 10 knots), which could support local
training and cell mergers. Hourly rainfall rates in this
environment could approach 0.50 inches (as indicated by the 12z
GFS), with local 1.25 inch precipitation amounts possible where
training or mergers occur.
Three hour flash flood guidance values are as low as 1.50 inches
over portions of the Sierra Nevada Range, and these values could
be approached. It should be noted that snow levels drop to around
7000 feet with the mid level system, so the potential for runoff
issues should be limited to elevations below this. The environment
could be borderline for a low end flash flood threat, especially
as snow level drop. For now, no excessive area was assigned.
Hayes
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
