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Excessive Rainfall Discussion
 
(Latest Discussion - Issued 0057Z May 24, 2019)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
857 PM EDT Thu May 23 2019

Day 1
Valid 01Z Fri May 24 2019 - 12Z Fri May 24 2019

...A HIGH RISK OF EXCESSIVE RAINFALL HAS BEEN EXPANDED TO COVER
MORE OF TX/OK/KS/NE...

...Central and Southern Plains...
Daily episodes of heavy rain and flash flooding will continue as
the large scale pattern remains locked in place. Each new rain
event is life-threatening in many parts of the central United
States until further notice. The next event was underway as of
early evening, with supercell thunderstorms rooted in the
near-surface based instability over the Texas panhandle to
southern Kansas. Numerous other strong convective updrafts rooted
at mid levels peppered the regional radar mosaic across western
and central Kansas up into Nebraska. Largely meridional steering
flow will carry individual storms northward. Upscale growth may be
delayed by such favorable wind profiles leading to updraft
rotation and discrete cells. The storms are so numerous, however,
and winds so unidirectional in the mid and upper levels, that
there will be cell mergers and seeder mechanisms acting to enhance
rain rates.

Eventually as a well-defined upper shortwave trough and jet streak
brush across the region from the west, expect more upscale growth
into a broken line or multiple lines of storms. The HRRR has been
depicting this consistently, and the HRRR has performed quite well
during this active weather regime since Monday. The situation is
very supportive of training, with downwind Corfidi vectors not
varying from the 0-6 km mean wind, and backward propagating
Corfidi vectors decreasing to under 10 knots during the height of
the low level jet cycle over the High Risk area.

Expect areal average rainfall 1 to 3 inches, but with local
concentrations of 4 to 5 inch amounts wherever cell mergers are
particularly well focused - most likely in the High Risk area.
These expectations are based primarily on the HRRR, but we do have
concerns that the HRRR may be underestimating the resulting
rainfall amounts given duration of ascent, available instability,
and deeply unidirectional steering flow.

There can be some fatigue when constantly under warning and when
constantly included in Moderate and High Risks. These are,
unfortunately, very unusual circumstances, including exceptionally
wet soils and high stream flows. Much of Kansas has seen 14 to 20
inches of rain in the past 30 days which is 3 to 5 times 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.

...Eastern Ohio Valley...
Moist westerly upslope flow will support a risk of isolated flash
flooding until a few hours after sunset when instability
diminishes and shortwave troughing passes to the east.

...Nevada / Utah...
Cold temperatures aloft / steep lapse rates are the primary factor
supporting scattered convective updrafts over southern Nevada to
southern Utah and adjacent northern Arizona. Mid level flow was
also confluent into this region, with some semblance of a
shortwave trough axis rotating through and loosely supporting
organization in spite of marginal moisture availability and larger
scale height rises. There is a Marginal Risk of flash flooding,
mainly in particularly sensitive areas.

Burke


Day 2
Valid 12Z Fri May 24 2019 - 12Z Sat May 25 2019

...THERE IS MODERATE RISK OF FLASH FLOODING ACROSS PORTIONS OF
NORTHEAST KANSAS INTO SOUTHERN IOWA...

...Southern Plains into the Upper Great Lakes...
Convection forming ahead of a frontal boundary extending from the
Upper Great Lakes into the southern Plains is expected to tap
strong instability and deepening moisture to produce excessive
rainfall, particularly from northeast KS into northern MO and
southern IA during Day 2. There was some differences with the
placement of the QPF maxima during the second half of Day 2, as
would be expected with convectively driven QPF. However, there was
enough agreement to base the WPC QPF and Excessive Rainfall
Outlook on a multi model blend.
 
Scattered to broken convection is expected to develop ahead of the
front from southwest WI into KS during the afternoon hours in an
axis of 2000/3000 J/KG of MLCAPE. The convection could initially
be wind/hail driven, as model soundings showed dry air in the mid
levels. However, after 25/00z, the low level jet strengthens to
45/55 knots across eastern KS into northern MO and southern IA in
the 25/00z to 25/06z time frame. Storms are expected to evolve
into clusters (or perhaps a small MCS), as the 12z GFS simulated
IR images showed cooling tops over this area. 
 
Deepening moisture (as precipitable water values increase to
1.50/1.75 inches, which is between two and three standard
deviations above the mean) should result in a saturating column,
suggesting that the convection could become efficient rainfall
producers. Propagation vectors over portions of northeast KS and
nearby northern MO oppose the mid level flow (per the 12z GFS) in
the abovementioned time frame, increasing the potential for
training and backbuilding in this area. Hourly rainfall rates in
this environment could approach 2.00 inches, especially on the
southern edge of convective clusters, where training is more
likely. 
 
There is a multi model signal for 2.00/4.00 inch rainfall amounts
across northeast KS into northern MO and far southern IA. Three
hour flash flood guidance values here are as low as 1.00/1.50
inches, as 14 day rainfall amounts are 200/300 percent or normal.
Because of the combination of heavy rainfall and wet antecedent
conditions  a Moderate Risk remains in place here. Further
northeast across far northern IL and southern WI, conditions are
also ripe for training convection, but condition here are not as
wet as further southwest. However, if the signal for heavy
rainfall continues here, the Moderate Risk could be extended into
these locations.
  

...California/Great Basin/Northern Rockies...
A closed mid level low drops south from southwest British Columbia
early on Day 2 to a position over OR by the end of the period.
With the approach of the cold mid level system, lapse rates
increase, resulting in pockets of 250-500 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 0.75/1.00 inch precipitable water values over northern
and central CA. Elsewhere across the Great Basin and Northern
Rockies, upslope flow aids in producing local 0.50 inch
precipitable water values over portions of NV and the higher
terrain of ID. 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 12z NAM CONUS Nest
showed the potential for hourly rainfall rates near 0.50 inches,
with 1.00/1.25 inch rainfall amounts where storms train 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 was placed over northeast CA, northern NV
and much of the higher terrain of ID.


Hayes


Day 3
Valid 12Z Sat May 25 2019 - 12Z Sun May 26 2019

...THERE IS MODERATE RISK OF FLASH FLOODING ACROSS FROM CENTRAL
KANSAS INTO SOUTHEAST IOWA...

...Southern Plains into the Upper Great Lakes...
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 (though the 12z NAM may be too far north with the axis of
highest rainfall), 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 was extended from central KS into northern MO/southern IA,
which is about where the previous Moderate Risk was placed.
Further northeast into northern IL and southern WI, antecedent
conditions are not as wet, but deep moisture and strong
instability support a small extension of the previous Slight Risk
here.


...California...
A closed mid level system OR early on Day 3 drops south into
Northern CA during the second half of day. 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 25/15z and 25/21z, 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 result in at least scattered
convection over the higher terrain initially, which drops into the
valleys just after peak heating. With the mid level system nearly
overhead, steering winds drop to under 10 knots, and like Day 2,
this could result in short term training and cell mergers.

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 3.


...NY State into New England
Deepening moisture and marginal instability couple to support
convection across NY state into a portion of northern New England,
mainly during the second half of Day 3. 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 26/03z and 26/09z.
Model soundings across this area showed pockets of 250-500 J/KG of
MUCAPE in the increasing moisture plume, and the combination of
synoptic scale lift associated with the short wave and instability
should aid in the development of low topped convection along a
weakening warm front.

There is a multi model signal for 1.25/1.50 inches of QPF across
much of western and northern NY state into northern VT and
northern NH. The speed of the mid level flow would seem to make
these values difficult to attain, 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
3.


...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, 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
convection to develop (other than embedded elements). If this is
the case, the synoptic scale ascent becomes the main rainfall
producer, lessening the threat of flash flooding.

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 likely, a Marginal Risk
could be needed here in later forecasts.


Hayes



Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt