Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1150 AM EDT Mon Jun 24 2019
Valid 12Z Mon Jun 24 2019 - 12Z Tue Jun 25 2019
1600 UTC update
Changes to the previous Excessive Rainfall Outlook based heavily
on the evolution of the latest radar. The marginal risk area was
removed from the Upper Texas coast...eastward into southern
Louisiana in the wake of the squall line moving offshore.
Over the Ohio Valley...the previous marginal risk area was trimmed
on the southwest side over western to central Tennessee into the
Lower Ohio Valley in the wake of the shortwave and well defined
area of associated precipitation pushing northeastward out of TN
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER A PORTION OF
...Middle-Upper Texas Coast to Southern Louisiana...
For much of this area, any excessive rainfall threat will be early
in the period (mainly through 18Z), owing largely to the
evolution/lingering effects of the ongoing MCS early this morning.
0-6km bulk shear values are comparatively weak toward the Gulf
Coast (aob 20 kts), likely to result in more fragmented convection
as the morning progresses. Nevertheless, given the orientation of
the QLCS (largely w-e), along with the alignment of the 950-850 mb
inflow and mean 850-300 mb flow, the resultant weak Corfidi
vectors would support some upwind propagation with an enhanced
risk of backbuilding and cell training. The moisture profile
remains favorable after 12Z Mon -- with PWs between 1.75-2.00" --
however deep-layer instability will be on the downward trend
outside of south TX. This will significantly curtail the rainfall
rates, especially by 15Z when MUCAPEs fall aob 1000 j/kg behind
Airmass will have a much better chance to destabilize during the
peak daytime heating hours south of the stalled frontal boundary,
as mixed-layer CAPES average between 3000-4000 j/kg while PWs
continue to hover between 1.75-2.00". Meanwhile, shortwave energy
traversing northern Mexico Monday will become
convectively-enhanced upon reaching south-central TX toward 00Z,
sparking renewed convection by late afternoon and into the
evening. The upper perturbation will also allow for a wave along
the effective surface boundary -- slowing the southward
progression of this boundary somewhat as it reaches south-central
Given the thermodynamic setup, as per the some of the high-res
CAMs (especially the WRF-ARW and ARW2), scattered strong cells
will be capable of hourly rainfall rates of 2-3 inches by the time
the activity reaches South TX and the Rio Grande Valley (RGV).
This as the cell motions later this afternoon and evening will
average much slower compared to the convection earlier on Monday.
The 00Z HREF shows highest 3 hourly probabilities of exceeding 3"
over South TX between 2100-0300Z -- with 6hr probs of >3" over
50%, and 12hr probs of >5" between 30-40%. WPC included a Slight
Risk over this region as a result.
Vigorous upper level forcing early in the period ahead of a
pivoting upper shortwave and associated surface cold front will
combine with modest deep-layer instability (MUCAPEs ~1000 j/kg) in
supporting a marginal excessive rainfall threat -- despite the
negative PW advection with time today (near 1.5" at 12Z Mon,
falling to near 1.00" by 00Z Tue). Individual cells are expected
to exhibit more downwind propagation than otherwise; however, some
renewed convection within the broad comma head/along the mid level
deformation axis may cause isolated short-term runoff issues,
especially where the 3 hourly FFG values are 2" or less.
...Ohio-Tennessee Valleys into the Mid Atlantic and portions of
Upper level forcing will be weaker across these areas, as the
aforementioned shortwave lifts northeastward from Lower MI into
eastern Ontario. Nevertheless, subtle divergence aloft just behind
the departing upper ridge axis (aided by right entrance region
forcing via an upper jet streak draped over eastern New England)
will aid in convective development/organization. Deep-layer warm
and moist advection will precede the surface warm front, which is
expected to extend across the eastern Great Lakes to the mid
Atlantic region by 00Z Tue. MUCAPEs averaging between 1000-2000
j/kg will provide modest instability, while PWs climb between
1.75-2.00". 0-6km bulk shear increasing to 30-40 kts will aid in
the organization/coverage of convection, while west-southwesterly
upslope effects aid in the development upon reaching the western
slopes of the mountains. For now, based on the low HREF exceedance
probabilities and the fact that the convection is expected to
forward propagate, the excessive rainfall risk over this area is
expected to be marginal/isolated.
Valid 12Z Tue Jun 25 2019 - 12Z Wed Jun 26 2019
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE PLAINS AND LOWER MISSISSIPPI VALLEY INTO THE
Given a fairly weak flow pattern with an exiting shortwave trough
moving through Ontario and a closed low off the Pacific Northwest,
there is little in the way of any large scale forcing expected
Tuesday into Tuesday night (Day 2). As a result, small
perturbations within the mid-level flow pattern combined with
instability and surface convergence will be the main factors worth
considering for excessive rainfall. With such a pattern in place,
it is quite difficult to discern the mid-level vorticity
placement/trajectory and surface boundary orientation even one day
in advance. This is also evident by the large spread in model
QPF. However, when diving a bit deeper into the typical
ingredients for flash flooding, small signals do arise for
potential excessive heavy rainfall.
Return flow west of the subtropical 850mb high will pump moisture
and instability into the southern and central Plains riding along
a weak boundary draped across northern MO into the OH Valley.
While precipitable water values of 1.25 to 1.5 inches is not much
above normal across this region, the 850mb moisture transport
continues to feed moisture into TX/LA north into OK and toward the
Great Lakes region. Strong instability will also be in place as
noted by 4000+ J/kg MUCAPE, with weak capping in the low levels.
In addition, this corridor may also house residual outflow
boundaries which could act as a catalyst for convective
development as mid-level impulses move overhead. So while there is
not a strong signal for exactly where convective development will
occur and the overall propagation, the ingredients are available
for potential MCS development mainly across portions of the mid-MS
Valley (southern IA into northern MO) and/or across OK. Since
convection may initially start elevated and eventually be surface
driven, feel cold pool propagation of convection, especially
across OK, may move fairly quick. This would limit the threat for
Residual troughing across the TX/LA coast will promote convective
development thanks to the pooling of moisture and instability.
There does appears to be potential mesoscale impulses that ride
north and east that may help provide additional support for
convection just inland of the coast. But organized convection
with such weak flow seems unlikely at this time.
Given the above logic, it appears that the best potential for
flash flooding, albeit low, will be across portions of east TX
into LA then northwest into OK and along a weak boundary into the
Mid-MS Valley. Again, quite a bit of uncertainty at this point,
but with the above ingredients and some sensitive soils from prior
heavy rainfall, feel this area has at least a small chance of
observing excessive rainfall resulting in localized flooding.
Therefore, placed this region in a Marginal Risk area.
Valid 12Z Wed Jun 26 2019 - 12Z Thu Jun 27 2019
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
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