Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
420 AM EDT Tue Apr 07 2020

Day 1
Valid 12Z Tue Apr 07 2020 - 12Z Wed Apr 08 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE UPPER MIDWEST & SOUTHERN GREAT LAKES...


...Ohio Valley...
Increasing moisture focused on a frontal boundary extending from
the Upper MS Valley across the OH Valley could be sufficient to
feed elevated convection capable of producing heavy to locally
excessive rainfall from northern IL across northern IN and OH,
mainly after 07/00z.

Surface low pressure develops across the northern High Plains in
response to approaching from the Northern Rockies. As the low
tracks north, the front is dragged slowly north across the Upper
MS Valley into the OH Valley, reaching a position extending from
northern IL across northern IN into northern OH before 07/12z. In
the broad warm air advection pattern associated with the front, a
30 to 40 knot low level west southwest flow transports 1.00/1.25
inch precipitable water air (which is about two standard
deviations above the mean) along the front.  The northern edge of
an instability axis reaches the frontal boundary, with model
soundings showing 250-500 J/KG of MUCAPE in the column, mainly
after 07/06z.

The combination of moisture and instability could support elevated
convection that potentially trains along the front (and the
northern edge of the instability gradient). Weak short waves in
the flow could allow convection to form small clusters. Where
training clusters occurs, hourly rainfall rates could touch or
briefly exceed 0.50", especially across northern IN into western
OH after 07/04z. Three hour flash flood guidance values here are
as low as 1.50", and these numbers are attainable where training
occurs. Since there remains some latitudinal spread on the
placement of the highest rainfall totals, the Marginal Risk was
broadened farther northeast into southernmost MI and north-central
OH.

Roth/Hurley/Hayes



Day 2
Valid 12Z Wed Apr 08 2020 - 12Z Thu Apr 09 2020

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
SOUTHWESTERN UNITED STATES...

...Southeast California into far western Arizona...
The strong closed low moving southeast off the Southern California
coast day 1 will begin to push more towards the  east into the
Southwest during day 2.  Timing differences from previous runs
have resolved with the previously more progressive GFS clustering
well now with the previously preferred slightly slower guidance. 
The anomalous pw axis on the eastern and northeastern side of the
strong closed low will continue for much of the upcoming day 2
period with a broad region of values 1.5 to 3 standard deviations
above the mean stretching from southeastern California into
western Arizona and southern Nevada.  Training of cells in a south
to northerly direction possible early day 2 on the east side of
the closed low over Southeast California into western Arizona. The
slight risk area was drawn for these areas and northward into the
upslope of the southern Sierra where southeast low level flow will
enhance totals.  Hourly rainfall totals of .25-.50"+ possible
early day 2 in areas of training.   Elsewhere..the marginal risk
area was maintained over the remainder of Southern California for
additional locally heavy rainfall amounts from slow moving cells
in the comma head deformation band wrapping cyclonically around
this strong upper low.

Oravec


Day 3
Valid 12Z Thu Apr 09 2020 - 12Z Fri Apr 10 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER SOUTHERN
CALIFORNIA...


...Southern California...
The strong closed low moving into the Southwest day 2 may become
more separated from the northern stream flow passing well to its
northeast across southeast Canada and the northeast U.S. during
the day 3 period.  This may allow for slowing of the eastward push
of the strong closed low with timing differences reappearing
during day 3 with the 1200 UTC Monday and 0000 UTC Tue ECMWF and
UKMET on the slower side with the NAM and GFS more progressive but
still showing the slowing trend.  The EC mean from 0000 UTC has
slowed considerably from its 1200 UTC Monday run and is now on the
slow end with the operational EC.  The more western/slower
solutions of the strong closed low show heavier precip wrapping
cyclonically into Southern California day 3.  While the slightly
more progressive GFS and NAM are not as heavy as the ECMWF and
UKMET, they do both show anomalous PW values wrapping cyclonically
back southward into Southern California.  PW anomalies 2+ standard
deviations above the mean in the GFS and 1.5 to 2 standard
deviations above the mean NAM push southward into Southern
California day 3.  For this reason...can not rule out additional
heavy amounts forming in this anomalous pw axis...with a marginal
risk area maintained for isolated runoff issues, especially over
recent burn scar areas.

Oravec




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