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Excessive Rainfall Discussion
(Caution: Version displayed is not the latest version. - Issued 2028Z Jul 20, 2018)
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 PM EDT Fri Jul 20 2018

Day 1
Valid 15Z Fri Jul 20 2018 - 12Z Sat Jul 21 2018


...Carolina Coasts...
Mesoscale discussion 0489 is effective until near 1:00pm Eastern
Daylight Time. No changes were made to the risk areas.

Weak surface low pressure is expected to develop along the
stationary frontal boundary currently lying west to east across
the Southeast and along the South Carolina coast. This will
support increasing convection along the mid to upper South
Carolina coast to the lower North Carolina coast this morning into
this afternoon as low level southerly flow strengthens ahead of
this wave. There has been a general trend in all of the guidance
for the heavy qpf axis to shift farther to the west (more of it
falling inland) over the past few runs. Model consensus is for the
max qpf axis along the immediate coastal areas in the axis of pw
values 2 to 3+ standard deviations above the mean from the mid to
upper South Carolina coast to the lower North Carolina coast, with
event totals in the 3-5"+ range. Any coincidental areas of
repeated cells or merging cells in this environment could yield
spotty extreme totals, such as the 8-plus inches reported at a few
CoCoRAHS gauges near Charleston this morning per communication
from the local office.

...Mid-Upper Ohio Valley into the Tennessee Valley...
Mesoscale discussion 0490 is effective until 3:00pm Eastern
Daylight Time. There was little change to the risk areas in the
15Z update.

Slight Risk area maintained from the mid to upper Ohio
Valley---southwest into the Tennessee Valley. Broadly difluent mid
to upper level flow will persist to the east and southeast of the
closed low moving slowly eastward from the Upper Mississippi
Valley and toward the lower Great Lakes. There is the usual amount
of spread with respect to the qpf details across these
regions...but a clear model signal for heavy precipitation
potential across these areas in an axis of fairly high mu-cape
values 1000-3000 j/kg. Of particular concern is the area of
northern KY and adjacent states. This region will experience the
greatest overlap of synoptic deep layer ascent, low level inflow,
and focused convergence along outflows. The hi-res model QPF
signal bears this out with 3 to 5 inch rainfall being common among
the 12Z runs, and the NAM CONUS Nest, although calibrated heavier
to begin with, does indicate potential for isolated extreme
rainfall above 8 inches, especially as multiple rounds of
convection occur through early evening.

At 15Z we expanded the Marginal Risk area to include the southern
Sierras which had been active on Thursday and where hi-res model
signal again suggests slow-moving afternoon convection. Otherwise
the soundings across the Southwest had dried somewhat in the low
levels as the Southern Plains ridge expands its influence
westward. The trend may be toward shorter-lived diurnal convective
events and less convection moving into the low elevations. Hi-res
model QPFs for Day 1 period are on the minimal side, although any
individual thunderstorm will still pose a risk of flash flooding
given the hydrologic nature of the region.


Day 2
Valid 12Z Sat Jul 21 2018 - 12Z Sun Jul 22 2018


...Great Lakes into the Southeast...
A broad Marginal Risk was maintained from the Great Lakes into the
Southeast. This area is along/ahead of the longwave trough axis
and attendant cold front. Moisture and instability will be
plentiful along this corridor. The best overlap of instability and
shear again ends up on the southern extent of this area. Thus the
most organized activity will likely be here, but again some
questions as to storm motions potentially limiting the flash flood
threat. Some chance that 850 mb flow becomes a bit more favorable
for some backbuilding of cells, which could locally increase the
flash flood risk. Farther north, instability is not as great but
significant moisture will be present along a frontal boundary
dropping through the region which may pose a localized flash flood
threat. However, did add a small slight risk area across southern
Ohio and northern Kentucky given very wet antecedent conditions on
day 1 and more rainfall expected in the day 2 period.

...Long Island, Northern New Jersey and southern New England...
Maintained a Slight Risk of Excessive Rainfall from parts of the
Mid-Atlantic Region northward toward Long Island/Southern New York
as low pressure looks to take shape and hug the coast. Models have
trended slightly faster with the low and are now in better
agreement regarding its placement. There also continues to be a
consistent signal in the models to push precipitation farther
inland, given very strong onshore flow in excess of 30 knots.
Thus, did extend the slight risk westward, including the very
urbanized DC to NYC I-95 corridor. 

Models continue to depict Precipitable Water values of 1 to 2
standard deviations above normal over parts of the Southwest U.S.
and the central Rockies. Initial thinking is that storms will form
over the higher terrain and the forecast deep layer flow is light
which would suggest slow moving storm motions. The concern remains
with even moderate rainfall rates falling on the complex terrain
and for rainfall on recent burn scars to result in localized flash
flooding. Thus, a marginal risk was maintained across parts of
central Arizona. A marginal risk was also added across the
Colorado Rockies and into parts of the high Plains due to the
interaction of monsoonal moisture with the terrain.


Day 3

The Day 3 outlook will be updated by 2030Z.

Day 1 threat area:
Day 2 threat area:
Day 3 threat area: