Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Wed May 1 2024
Day 1
Valid 12Z Wed May 01 2024 - 12Z Thu May 02 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL AND EASTERN TEXAS...

...Southern Plains...

Latest hi-res ensemble and associated CAMs have come into better
agreement on the development of a complex across TX with origins
out in west TX near the dryline, growing upscale through the
evening with aid from a well defined LLJ and shortwave progressive
within the sub-tropical jet over the region. Given consensus now
favoring at least 2" of precip over a large expanse of central and
eastern TX with much higher embedded amounts given the convective
nature of the precip and anomalous tropospheric moisture (2-2.5
standard deviations above normal PWATs), have allowed for an
upgrade to a Moderate Risk over the aforementioned areas. This is
now the primary period of interest with a shift away from the
previous MDT that was forecast for the now D2 time frame.

The reasoning for the shift was due to the progressive nature of
the convective pattern within the latest CAMs suite, as well as
some instance within the ensemble bias corrected mean QPF
footprint. Previous forecast iterations were more robust for the
now D2 time frame, but the addition of CAMs guidance and the
ensemble have allowed for a shift in the timing of when the
heaviest precipitation would occur, as well as the general
locations. 00z HREF EAS probabilities for at least 2" of rainfall
are now upwards of 40% across a large swath of central and east TX,
a good portion now overlapping with the area that saw extreme
rainfall totals over 6" from this past weekend. The conjunction of
heavy rain over those areas impacted and forecasted rates exceeding
2"/hr will likely force easier flash flooding potential due to the
moist antecedent conditions in place. Also of note is the area
over central TX where despite missing the previous rainfall, the
flash instances are climo favored due to the soils and high runoff
potential. With coordination with the local WFO's involved within
both TX and LA, the MDT risk was shifted to account for the abrupt
change in convective timing and impacts.


...Midwest/Central Plains...

A closed upper low is currently analyzed over Big Sky into the
southwestern Canadian Provinces. A robust mid-level vorticity
maxima will pivot around the base of the mean trough carved out by
the upper low with sights on the central plains and adjacent
Midwest allowing for rapid convective development downstream with a
corridor of heavy rain forming later this afternoon through the
remainder of the period. Guidance has trended to a favored heavy
rain footprint across northern KS through eastern NE with the
heaviest rain with a swath of 2-4" of rainfall over the course of
this evening into early Thursday AM. 00z HREF EAS probabilities for
at least 1" have become most aggressive in-of southeast NE which
matches well with the current ML output from the past several runs.
This makes sense synoptically given the best mid-level forcing
will be downstream of the approaching vort max in an axis of
diffluence perfectly aligned overhead. Rates between 1-2"/hr will
be common within the areal extent of convection leading to some
local totals between that 2-4" marker forecast. This should be
sufficient for some higher flash flood potential within larger
urbanized settings and areas that see the higher rates in question.
The progressive nature of the precip will limit the higher end
impacts, so the previous SLGT risk forecast will suffice despite
the alignment synoptically.

Kleebauer