Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
255 PM EDT Wed May 02 2018


Prelim Day 1 QPF Discussion
Valid May 03/0000 UTC thru May 04/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1...


...Central Plains into the Mid Mississippi Valley and Upper
Midwest...

Complex convective evolution today lends a below average
confidence on placement of Slight risk areas and QPF forecast.
Still believe the highest flash flood threat today/tonight will be
from central KS into far southeast NE, southwestern IA, and
western MO. Convection will continue to break out along the
stalled out front by early this afternoon. Storm motions should
initially be parallel to the front, allowing for some flash flood
potential to develop along the boundary from central KS into far
southeast NE, northwest MO and southwest IA. Appears likely that
outflow on the southern periphery of this convection will align
west to east, and with increasing 850 mb moisture transport into
this boundary, appears to be a good setup for a period of west to
east training convection tonight. Think the highest flash flood
potential during the day 1 period will indeed be with this
overnight training activity. Quite a bit of model spread noted
amongst the 12z high res guidance. In general prefer the ARW cores
during events such as today...thus will stick with mentioning only
the HRW ARW, ARW2 and the Experimental HRRR. Both the 12z ARW and
ARW2 move convection rather quickly off the front, which ends up
resulting in a further south maximum QPF axis from central KS into
southern MO and northern AR. Meanwhile the 12z Experimental HRRR
keeps convection along the boundary longer, before eventually
propagating it off to the east...resulting in a further north axis
from central KS into central MO. Confidence is not high, but want
to lean more towards the Experimental HRRR output, as it seems to
be typical for the initial convection to train along the boundary
a bit longer before propagating eastward in these stronger
flow/forcing cases. Did lean WPC QPF towards the Experimental
HRRR, although did incorporate some of the 12z HREF as well.

The uncertainty continues downstream into portions of IL/IN/WI.
Both the ARW and ARW2 show some pretty impressive totals across
portions of central IL/IN, as multiple rounds of convection move
across this area. The evolution depicted by these solutions would
likely produce some flash flood threat and would warrant a Slight
risk. However, noted that the 12z Experimental HRRR was much less
aggressive, and depicted a further north axis, closer to the
frontal boundary. This appears to be partially related to the
convection forming over the central Plains, which as mentioned
above we were leaning towards the Experimental HRRR. However, it
is possible that convection forming this afternoon over IA becomes
surface based, and lays out a boundary further south for the later
activity to track across. Thus, certainly possible that the
Experimental HRRR (which is weaker with this earlier convection)
is too far north over the IL/IN region. With that said, still
preferred something a bit further north and closer to the
Experimental HRRR for the new WPC QPF. Confidence is low though,
and will need to closely monitor convective evolution through the
day.

Convection will again expand in coverage Thursday afternoon over
portions of IA/MO and vicinity. Again some disagreement was noted
by this time. Given the longer lead time and synoptic nature of
the system by this time...did incorporate more of the global
guidance (GFS,ECWMF,UKMET) into this portion of the forecast. Did
blend in some of the 12z ARW and Experimental HRRR as well, as
they generally seemed in pretty close agreement to the global
consensus, and added in some more convective detail.


...Northeast CO into southeast WY and western NE...

Opted to add in a Marginal risk from northeast CO into southeast
WY and western NE. The approach of the closed mid/upper level low
will result in increased synoptic ascent over this area into
tonight. Instability will be limited, but continued cooling aloft
should be able to support some embedded heavier convective cores.
Will be a long duration of showers and these heavier embedded
cores, with repeat and slow moving cells a concern. Given the good
high res agreement for isolated amounts approaching 2.5" over this
area, think an isolated flash flood risk could develop.


...Central TX...

Convection will break out this afternoon and evening along the dry
line in south central TX..spreading northeast with time into
central TX. Generally followed a blend of the 12z HREF members and
recent HRRR runs for QPF here. In general convection should be
progressive in nature, keeping the flash flood threat low. However
still could be some briefly intense rates which could cause issues
in any more susceptible areas. Something to watch will be whether
any storms get anchored to the dry line over south central
TX...which is suggested by the HRW ARW and ARW2. If this were to
occur then a greater flash flood risk could develop, so something
to monitor.

Chenard