WPC Day 2 Excessive Rainfall Outlook
Risk of 1 to 6 hour rainfall exceeding flash flood guidance at a point
Updated: 0815 UTC Sat Jul 20, 2019
Valid: 12 UTC Jul 21, 2019 - 12 UTC Jul 22, 2019
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
417 AM EDT Sat Jul 20 2019
Valid 12Z Sun Jul 21 2019 - 12Z Mon Jul 22 2019
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE CENTRAL
PLAINS EAST INTO THE OHIO VALLEY...
...Central Plains, Mid-MS Valley, OH Valley...
A front will continue to slowly move into the Central Plains,
Mid-MS Valley into the OH Valley Sunday into early Monday. Models
are coming into slightly better agreement on the orientation and
timing of the front, especially across the OH Valley. The GFS and
now CMC are showing signs, however, of more organized convection
as mid-level energy moves atop the aforementioned front. The rest
of the model guidance remains weaker with the impulses and the
position of the jet axis a bit farther removed to the north and
east. Thus the GFS and CMC have higher QPF amounts across eastern
KS into northern MO as an organized complex moves from SE NE into
MO Sunday evening/night. Given the GFS has shown run to run
continuity, the CMC is now showing similar features and GEFS
probabilities support the higher QPF amounts, felt it should be
incorporated into the model blend.
With respect to the ingredients in place, moisture and instability
will continues to pool right along and south of the boundary, with
precipitable water around 2 inches and MUCAPE values over 3500
J/kg. As previously mentioned, there continues to be uncertainty
with respect to the interaction between the mid-level energy and
the surface features, thus resulting in questionable organized
convection in the central CONUS. At this time, forecasting weak
to modest mid-level impulse atop this surface boundary which
should promote convection along the front from the Central
Plains/Mid-MS Valley across the OH/TN Valley.
Areal average QPF generally ranges between 1-2+ inches with
locally higher amounts associated with convection. The two main
areas of concern are across east KS/northern MO with the potential
for organized convection and portions of the OH Valley where FFG
are lower. Rain rates could climb over 1.5 inches/hour. The
propagation of the convection along the front should limit the
flash flood potential as it sinks south. However, if a complex
forms as depicted by the GFS, some training could occur. There is
enough signal to support isolated to potentially more widespread
flash flooding concerns, therefore a Slight Risk was introduced to
encompass these regions.
As a front continues to dive south through the Northern Plains,
settling into the Central Plains, Mid-MS Valley and OH Valley, a
trailing boundary along the Front Range will act as a catalyst for
convection during the afternoon into the overnight hours Sunday
into Monday. Mid-level high pressure will build overhead with
impulses rounding this feature which too will promote vertical
motion. At the surface, as the Canadian high builds south into the
Northern Plains, easterly flow will enhance upslope flow during
the afternoon. Moisture that pooled south of the aforementioned
front will bank up against the high terrain with instability of
over 2000 J/kg available during the afternoon/evening hours.
These factors should promote convective initiation along the
windward slopes of the Rockies in CO and NM. Given fairly weak
flow, convective cells should have fairly slow storm motion along
the slopes with more erratic propagation as they grow upscale.
Regardless, expect heavy rainfall is given decent moisture
transport across this region. Hourly rain rates may approach 1
inch/hour which is just shy of the one hour FFG (1.5 inches)
across this region. Again, given the uncertainty with respect to
the evolution and storm motion, feel there could be some locations
that see 2+ inches when all is said and done.
Models have trended down with QPF for the 00Z forecast suite and
thus have trimmed amounts slightly. Though ensemble
probabilities, albeit somewhat low, still highlight this region as
exceeding 2 inches. Models tend to do poorly within this flow
regime/pattern and thus will maintain the Marginal Risk across
this area with some refinements made based on WPC QPF.
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt