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Excessive Rainfall Discussion
 
(Latest Discussion - Issued 2242Z Jul 15, 2019)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
642 PM EDT Mon Jul 15 2019

Day 1
Valid 2242Z Mon Jul 15 2019 - 12Z Tue Jul 16 2019

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL TONIGHT OVER
SOUTHERN ARKANSAS...

...Flash Flood risk associated with Barry...
The main concern tonight will be a steady precipitation band
expected to form over southern Arkansas. In the meantime, the
flash flood threat appeared to be waning over southeast Texas. At
2230Z isolated thunderstorms were occurring in the vicinity of
Houston where small scale outflows had collided. This should
remain disorganized, and the deep layer ascent was becoming less
favorable for rainfall as Barry continued to lift northward. The
RAP forecasts mean low level convergence to develop eastward
through Louisiana this evening, with winds relaxing over Texas and
southwest Louisiana. We therefore removed the Moderate Risk area,
but will maintain a Slight Risk in the hard hit areas with
saturated ground and inundation occurring north of I-10 in
Louisiana. Scattered outflow dominant thunderstorms will deliver a
quick half inch of additional rainfall locally, but expect this
activity to diminish in intensity and coverage throughout the
evening, leading to eventually a quiet night in Louisiana.

Attention then shifts squarely toward Arkansas, for the next
nocturnal rain band expected to form within the convergent region
south of Barry's circulation, where deep lift coincides with the
instability axis. Models show a very strong signal for a focused
rain band, and it may only affect a handful of counties, but some
higher-end rain rates would be likely given the tropical
environment. With training and a relative absence of outflow,
expect hourly accumulations to push above 2 inches, with very
isolated overnight totals in excess of 7 inches over southwest
Arkansas per the hi-res model guidance. Suspect this event will
begin toward 04-07Z, but sometimes these types of events start
percolating early and then ramp up in intensity.

Farther out from Barry toward the east, convection had been
smaller in scope and more progressive this afternoon. We shrank
the Slight Risk and Marginal Risk areas, but will maintain Slight
Risk overnight arching along the instability gradient from eastern
LA / western MS up toward southeast Missouri.


...Northern Plains into the Upper Mississippi Valley...
A shortwave trough and seasonably unstable air mass will promote
clusters of strong storms, particularly over parts of MN/WI this
evening. Storm modes, steering flow, and T/Td spreads favor
relatively progressive activity. Flash flood risk will be tied
primarily to occasional cell mergers which boost rain efficiency,
especially where rainfall last night had created wet soil
conditions from southeast MN into western WI, promoting a high
percentage of surface runoff from new rainfall. We trimmed back
the Slight Risk over eastern Wisconsin where less intense
convection had begun to remove instability. Most of the hi-res
model QPF signal focuses back to the west this evening.

We also narrowed the Marginal Risk area, but a few flash floods
could occur in areas of strong thunderstorms stretching back
toward the central High Plains, again with seasonably moist and
unstable air, and relatively wet soil conditions.

Burke


Day 2
Valid 12Z Tue Jul 16 2019 - 12Z Wed Jul 17 2019

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS SOUTHERN
AND CENTRAL ARKANSAS WITH A SLIGHT RISK EXTENDING INTO THE OHIO
VALLEY...

...Lower Mississippi Valley into the Southern Great Lakes...
The remaining circulation of T.D. Barry will gradually lift
northward through the MS VLY before becoming absorbed into the
westerlies and opening late on day 2 and shearing off to the
northeast. South and east of this circulation, the moisture plume
will continue to channel from the Gulf Coast evidenced by higher
PWATs of 1.5-2.5 standard deviations above the climo mean lifting
from AR into MI. The channel will be tightened with the
strengthening/impinging Subtropical ridge pressing west across the
Southeast US, drawing additional moisture, but in a sense is drier
relative to the deeper moisture closer to Barry. This channel will
also be favoring the higher instability axis to the south, and an
evolution similar to today across LA/TX is expected in AR/TN
Tuesday. The combination of elevated MUCape and mid-level
confluence to drive ascent will drive convective development with
the potential for excessive rain rates through the afternoon. The
strong convective elements are likely across AR where the greatest
overlap of instability and PWAT combine, and HREF probabilities
are high for 2"/hr through 18Z along with EC probabilities for 5".
Guidance does differ in the exact location of the heaviest
rainfall due to minor perturbations in the confluence band, but
these types of rain rates on top of pre-conditioned soils due to
antecedent heavy rain has prompted a MDT risk across portions of
southern and central AR.

Further north and east, the moisture axis will shift into the
lower Great Lakes, but instability and PWAT anomalies will be
somewhat lower. Still, this tropical airmass will support bands of
convection with heavy rainfall from the TN VLY northward into the
western OH VLY, and there is good model consensus for 1-3" of
rainfall, with rates above 1"/hr likely at times. Flash flooding
will be possible despite relatively drier antecedent conditions,
and the SLGT risk has been extended NE towards Lake Erie due to
pockets of FFG as low as 1.5"/3hr.


...Northern Plains...
A slow moving frontal zone will drift in the vicinity Tuesday and
Tuesday night, along which ripples of mid-level energy will waver
the positioning due to subtle waves of low pressure developing in
response. At the same time, ascent will be driven through at least
weak jet coupling of the RRQ of a jet streak moving across
northern MN, and the LFQ of a secondary jet streak lifting into
the Central Rockies. Beneath these features, surface wave
development in response to pressure falls will allow low-level
winds to back, driving robust moist advection into the Plains on
25-40 kts of 850mb LLJ, especially after 17/00Z. PWATs climb to
1-2 standard deviations above the climatological normal, which in
conjunction with MUCape rising above 2000 J/kg supports a highly
favorable thermodynamic environment for convection and heavy
rainfall.

While the environment seems favorable for heavy rainfall, some
guidance discrepancies still prevent enough confidence for a SLGT
risk. The available HREF members vary considerably the first half
of day 2 with heavy rainfall placement with convectively enhanced
s/w MCSs moving atop the mid-level ridge. Additionally, some
uncertainty persists as to the placement of the front itself,
which will be the focus for the best rainfall/training potential.
Despite high 14-day rainfall departures, and low FFG across SD,
will maintain the inherited MRGL risk until better confidence can
be achieved in where the highest rainfall and/or rainfall rates
will locate.


Gallina/Weiss

Day 3
Valid 12Z Wed Jul 17 2019 - 12Z Thu Jul 18 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM TENNESSEE
THROUGH THE CENTRAL OHIO VALLEY AND INTO UPSTATE NEW YORK...

...Tennessee Valley northeast into Upstate New York...

Moisture streaming ahead of what was once Tropical Storm Barry
will lift into the Ohio Valley and northeast as the remnant center
shears increasingly into the westerlies.

Instability will be continue to be limited with only 500-1000 J/kg
expected by 00z across much of the Ohio Valley, remaining south
across TN/KY where values will be in the 2000-3000 J/kg range,
slightly higher west where the moisture axis will separate from
the instability axis by midday.  Deep moisture will still be
present with PWATs of 2" or more through the afternoon, before
banking up against the terrain of the Appalachians and reducing
below 1.5" early overnight Thursday.  As such, any confluence is
likely to produce isolated to scattered convection further north
across E OH/W WV becoming more numerous in Central/Eastern KY and
TN.  Still, the ideal unidirectional steering flow will break
toward a greater western component by evening supporting faster
eastward propagation/cell motions and reducing extreme rainfall
totals.  Still, rates of 2"/hr are possible producing 1-3" of
rainfall with locally higher amounts possible. FFG in TN is
reasonable high, 2-3"/3hrs, but the potential for excessive rain
rates in the tropical air mass supports the SLGT risk.

Further north into Ohio and New York state, some enhanced ascent
will be likely in the RRQ of a departing upper jet as tropical
moisture streams northward on an 850mb LLJ of 25-35 kts.
Instability will be limited as noted above, so convective
initiation is is likely to be scattered, but mean steering flow
along a residual front will support training of any convective
cells. As such there is an increasing signal for a west to east
QPF swath across the Great Lakes into the Hudson River Valley. The
guidance is converging on this axis of heavier rainfall, and note
an increase in ECENS probabilities for 6-h and 12-h rainfall. For
these reasons and higher than normal recent rainfall across the
area, especially in Ohio, have extended the SLGT risk northeast
into New York state.


...Upper Midwest...
Ascent through a weakly coupled jet structure and low-level
convergence along a front will likely produce thunderstorm
development on day 3. Weak mid-level impulses rotating atop a
strengthening upper ridge to the south will further drive
convective strength, with clusters of thunderstorms or MCSs
possible, especially late in the evening and at night. These MCSs
will be fueled by an intensifying 850mb southerly LLJ to 30+ kts
which will veer cyclonically into MN/WI and drive anomalous PWATs
reaching 2.5 standard deviations above the climo mean into those
states. Intense moisture flux during this time will combine with
ample instability noted by forecast MUCape of over 3000 J/kg to
persist and intensify convection along the stationary front.
Despite what may be rapid motion of this feature, some training
combined with high likelihood for excessive rain rates of 2"/hr
support an enhanced flash flood risk, especially if the heaviest
rainfall can occur across regions with relatively lower FFG of
1.5"/3hr.

While the dynamics and environment would support a SLGT risk,
there remains considerably placement uncertainty into where the
best overlap of forcing, and MCS track, will occur. The trend this
afternoon has been for a subtle northward push of the max axis in
the guidance, but there is still enough spread, including the
12Z/ECMWF which keeps the heaviest rainfall as far south as the
MN/IA border, to prevent a SLGT risk at this time. However, it is
likely that a SLGT risk will be needed as guidance converges onto
a distinct solution closer in time to the forecast period.


Gallina/Weiss




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