Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
815 AM EDT Fri Jul 20 2018
Valid 12Z Fri Jul 20 2018 - 12Z Sat Jul 21 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE MID TO
UPPER OHIO VALLEY INTO THE TENNESSEE VALLEY AND ALONG THE MID TO
UPPER SOUTH CAROLINA COAST AND LOWER NORTH CAROLINA COAST...
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 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 totals in the 3-5"+
range. Changes to the risk areas across these regions from the
previous issuance for this period were to introduce a slight risk
along the immediate coastal locations.
...Mid-Upper Ohio Valley into the Tennessee Valley...
There were only some slight changes made to the previous outlook
for the upcoming day 1 period with a broad 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.
No significant changes made to the previous marginal risk area
extending across portions of the Southwest. With the mid to upper
level ridge remaining in place this period across this area and
continued above average pw values...additional widespread
scattered convection likely Friday afternoon into Friday
evening...especially along the high terrain from west-central New
Mexico westward along the Mogollon Rim of Arizona...into central
to southern Utah and southern Nevada.
Valid 12Z Sat Jul 21 2018 - 12Z Sun Jul 22 2018
...THERE IS A SLIGHT RISK MARGINAL RISK OF EXCESSIVE RAINFALL OVER
PORTIONS OF THE EAST...
...Great Lakes into the Southeast...
A broad Marginal Risk was maintaied from Great Lakes into portions
of 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 b this time, which could locally
increase the flash flood risk. The will not be as much instability
farther north, suggesting cell intensity will be less. Given that
there will be plenty of moisture near a front that should become
nearly stationary, think that the persistent low level convergence
axis may focus enough cells to pose a localized flash flood
risk...even if the overall storm intensity/associated rainfall
rates do not look to be as high as storms farther south.
...Long Island, Northern New Jersey and southern New England...
Introduced 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. The models
got into better agreement in two respects...there was a
convergence of ideas as to how fast the low will be moving
northeast. The exception was the 20/00Z GFS which ended up being
about the fastest model as other models trended slower. There was
also a consistent signal in the models to push precipitation
farther inland than previous runs. Model instability is closest
along the coast...but onshore flow in excess of 30 kts at H85
suggested at least some possibility for heavy to locally heavy
rainfall to occur a bit deeper inland.
The NAM and GFS continue to depict Precipitable Water values of
1.5 inches or greater over parts of the Southwest U.S. on Saturday
into early Sunday. Deep layer flow is forecast to be light which
suggests that storms which form will tend to be slow movers. At
this point, thinking is that the storms will form over the higher
terrain. The two main concerns are for even moderate rainfall
rates falling on the complex terrain and for rainfall on recent
burn scars to result in localized flash flooding.
Valid 12Z Sun Jul 22 2018 - 12Z Mon Jul 23 2018
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A SMALL
PORTION OF THE ROCKIES...
Introduced a small Marginal Risk area over portions of
central/southern Colorado were the operational models were showing
a signal that mid- and upper-level moisture will interact with a
front approaching from the northwest/north during the day on
Sunday. There were several models pointing to rainfall amounts
approaching an inch in or near the complex terrain. That raises a
possibility of flash flooding...especially over burn scars and in
mountainous terrain from the afternoon into evening.
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