Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
425 AM EDT Mon Aug 20 2018
Valid 12Z Mon Aug 20 2018 - 12Z Tue Aug 21 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE MID TO
LOWER MISSISSIPPI VALLEY...LOWER OHIO VALLEY AND LOWER TENNESSEE
...Central to northern Iowa..southeast Minnesota...southern
Wisconsin into northern Illinois...
The latest model guidance is very similar with the evolution of
the strong closed low initially over the lower Missouri Valley.
All models show a steady east northeast push into the mid
Mississippi Valley region Monday and toward the southwest Lower
Lakes region Monday night/early Tuesday. With good mass field
agreement among the models...there is also good agreement on an
axis of heavy precipitation associated with what will likely
remain a well defined comma head/deformation precipitation band to
the north of the track of this mid to upper level closed low.
Precipitation areas in this comma head/deformation band will be
slow moving...pivoting this morning over central to northern
Iowa...and this afternoon/evening from southeast
Minnesota/northeast Iowa into southern Wisconsin and far northern
Illinois. In this pivoting regions..there will be widespread
areal average totals in the 1-3" range...with isolated heavier
totals reaching 4-6". No significant changes were made to the
previous outlook for this forecast period.
...Middle to Lower Mississippi Valley...Lower Ohio Valley...Lower
To the south of the expected well defined comma head/deformation
precipitation band with the strong moving into the mid Mississippi
Valley today...additional pre-frontal/frontal convection will
likely reform this afternoon across the Mid to Lower Mississippi
Valley in a region of defined upper difluence and boundary layer
convergence. This activity is expected to then push eastward into
the Lower Ohio and lower Tennessee Valley region by Monday
evening. This activity will likely be fairly progressive---but
with unstable conditions ahead of this front...mu-cape values
1000-2000+ j/kg and pw values above average...locally heavy
rainfall amounts possible. Changes to the previous outlook for
this period were to expand the slight risk area southward into
more of the lower Mississippi Valley.
...Southern Mid-Atlantic...Southern Appalachians into the Central
Gulf Coastal region...
A broad marginal risk area depicted day 1 in the moist unstable
airmass from the Central Gulf Coastal region into the Southern
Appalachians and Southern Mid Atlantic. Over the Central Gulf
coastal region...the previous marginal risk area was expanded
farther to the south into the Central Gulf Coastal Region to cover
areas that have received heavy rains over the past few days from
far eastern Louisiana into southern Mississippi and far southern
Alabama..resulting in the lowering of ffg values and increasingly
anomalous stream flow. Additional localized heavy rains possible
across these regions this period. Farther to the east...the
marginal risk area was extended east from the Southern
Appalachians and across the remainder of North Carolina.
Additional heavy rains possible in the axis of instability to the
south of the west to east front across the Mid-Atlantic.
The mid to upper level trof moving east from the Pacific Northwest
into the Northern Rockies region will enhance upper difluence
across the Northern Rockies this afternoon. This and post frontal
east northeasterly flow will support increasing precipitation this
afternoon from north central Idaho into western to southwestern
Montana and northwest Wyoming. With pw values rising to 1 to 1.5+
standard deviations above the mean and the well defined upper
difluence...locally heavy rainfall amounts and isolated runoff
issues are possible.
The Day 2 outlook will be updated by 0830Z.
The Day 3 outlook will be updated by 0830Z.
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