Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
416 AM EDT Sun Jul 22 2018
Valid 12Z Sun Jul 22 2018 - 12Z Mon Jul 23 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE
Precipitation is expected to be much less organized for the
upcoming day 1 time period as the well defined low that rotated
northwestward across the Mid-Atlantic weakens over the eastern
Great Lakes region early this period. However...with the well
defined mid to upper level low remaining nearly stationary over
the Upper OH Valley---additional widespread showers likely to
develop late Sunday morning into Sunday afternoon across a large
region from the OH Valley---through the Appalachians and into the
Mid-Atlantic. A slight risk area was oriented across the
Mid-Atlantic over regions that have received rainfall totals in
the 4 to 7 inch range over the past 24 hours. With soils
saturated---and the national water model showing high flow across
this region...additional locally heavy rains will have the
potential to produce isolated runoff issues. Confidence is rather
low with any qpf details with the widespread showers expected from
the mid to upper level closed low positions over the OH
Valley---eastward into the Mid-Atlantic given the weaker overall
forcing this period. The previous slight risk area oriented from
northern NJ across southeast NY state and into southern to central
New England was removed given weaker model signal for heavy
rainfall potential across these regions---with a marginal risk
maintained across these area.
The stationary mid to upper level closed low over the OH Valley
will maintain strong onshore west northwest low level flow off the
eastern Gulf and into much of Florida this period. 850-700 mb
moisture flux anomalies from the NAM...GFS and ECMWF all show
anomalous values...3 to 5 standard deviations above the
mean...across the northeast Gulf and into Northwest Florida. This
will support potential for bands of heavy precip to affect
Northwest Florida---with the potential for training of bands in a
general west to east direction. In areas of training---precip
totals in the 3-5"+ range possible along with isolated runoff
issues---especially if these bands can push southward toward the
more urbanized areas near Tampa or inland near Orlando.
...Northern Sierra into the Great Basin...
An axis of above average pw values 2 to 3 standard deviations
above the mean will persist this period from northern California
into the Great Basin. In this above average pw axis...models do
show a stream of upper difluence maxima this afternoon into early
this evening--supporting another day of widespread scattered
convection and isolated runoff issues from the Northern
Sierra---east into the Great Basin.
...Central High Plains...
A surface cold front expected to press slowly southward Sunday
from the Northern Plains into the Central High Plains by Sunday
evening into the early hours of Monday. Increasing pw values in
the vicinity of this boundary---values increasing to 2 to 3
standard deviations above the mean---along with strong frontal
convergence...enhanced upper difluence along the southern edge of
the westerlies/top side of the upper ridge and increasing
instability (mu-cape values 1000-2000= j/kg---will support
increasing convective potential late afternoon from portions of
northeast Colorado---western Nebraska and western Kansas.
Isolated heavy precip amounts possible although confidence low
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