WPC Day 1 Excessive Rainfall Outlook
Risk of 1 to 6 hour rainfall exceeding flash flood guidance at a point
Updated: 0716 UTC Sat Jun 10, 2023
Valid: 12 UTC Jun 10, 2023 - 12 UTC Jun 11, 2023
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
317 AM EDT Sat Jun 10 2023
Valid 12Z Sat Jun 10 2023 - 12Z Sun Jun 11 2023
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE PLAINS, ARKLATEX, AND THE MIDWEST...
Northern High Plains...
Anomalous moisture continues early on before dropping off and
southward as an upper level shortwave potentially splits, with
some energy moving north into Canada and some undercutting the
ridge and dropping southward. Instability lingers early on, so
the existing Slight Risk still makes sense, but it was refined
using the latest guidance.
A shortwave drops south around a forming closed low across
southeast Ontario, with the overall system on a strengthening
trend. A low- to mid-level low drops southeast and lures PW
values up to 1.5-1.75" into portions of KS, MO, NE, and IA.
Inflow at 850 hPa and effective bulk shear should be increasing,
particularly late in the period when the low-level jet tends to
ramp up Saturday night into Sunday morning which portends
organized convection. CAPE values upstream are forecast as high
as 4000 J/kg on the 21z SREF guidance during daytime heating,
falling towards 1000 J/kg overnight. The guidance signal is more
scattered than on previous nights in and near the complex border
junction of NE/MO/KS/IA, near where mid-level heights/thickness
pattern are diffluent (eastern NE, western IA, and western MO),
with the guidance (particularly the 00z NAM and 00z Canadian
Regional) still showing a signal for local 3-5" amounts, with the
00z HREF guidance showing a non-zero chance for 5"+ amounts.
Flash flood guidance values, particularly outside MO, are modest,
but the Kansas City urban area would naturally be susceptible to
heavy rainfall. The ingredients available could support hourly
rain totals to 2.5", which would be a problem in urban areas. The
Slight Risk area was re-expanded due to the above, but care was
taken not to go too far into IA per their lesser sensitivity
mentioned in prior coordination. A broad Marginal Risk area
continues to surround the Slight due to some lingering spread as
the guidance is no longer coherent if a convective complex is
going to form, even if all the ingredients appear to be there for
the formation of such a complex. The re-expansion was coordinated
with the DMX/Des Moines IA and OAX/Valley NE forecast offices.
Portions of CA/NV...
An anomalously strong closed low drops into CA Saturday into
Sunday. Precipitable water values remain elevated, with moisture
increasing in the Central Valley as the cold low develops and
moves in. Instability shouldn't be a problem as temperatures
aloft decrease. Believe a Marginal Risk of excessive
rainfall/flash flood should continue Saturday into Sunday. Hourly
rain totals up to 1.5" should be possible where cells backbuild,
train, and/or merge.
ArkLaTex to the Central Gulf Coast...
As a southern stream shortwave phases with a deepening positively
tilted upper trough, PWs rise towards 2" along a section of the
polar front making some progress eastward. Low-level
inflow/effective bulk shear appear to be sufficient for convection
with some level of organization, which would support hourly rain
totals to 2.5" somewhere between eastern TX, LA, and MS. The
NAM/GFS still advertises local 4-6" amounts in eastern TX
especially, so the Slight Risk area remains in place in that
Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt