WPC Day 2 Excessive Rainfall Outlook
Risk of 1 to 6 hour rainfall exceeding flash flood guidance at a point
Updated: 0749 UTC Mon Jun 24, 2019
Valid: 12 UTC Jun 25, 2019 - 12 UTC Jun 26, 2019
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
353 AM EDT Mon Jun 24 2019
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.
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt