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Excessive Rainfall Discussion
(Caution: Version displayed is not the latest version. - Issued 2028Z Aug 16, 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 Thu Aug 16 2018

Day 1
Valid 1906Z Thu Aug 16 2018 - 12Z Fri Aug 17 2018


...Central and East...
Made some additional modifications to the Risk areas. The Slight
Risk was cut back in size across the OH Valley and PA/NY. Recent
observational and HREF/HRRR trends suggest that higher rainfall
rates should end up more localized in nature across this corridor.
Thus appears less likely that an organized flash flood threat will
materialize...instead anticipate more of an isolated threat. Thus
a marginal should suffice across this corridor for now.

More uncertainty further southwest from OK/KS into the lower and
mid MS Valley. A well defined MCV is moving across OK this
afternoon. Some of the 12z HREF members expand convection ahead of
this feature from OK into AR/TN. The atmosphere is highly unstable
(especially across AR), but is also decently capped. With the MCV
undergoing a weakening trend, unclear how much the cap will be
broken. Given the locally heavy antecedent rainfall across
portions of OK, southeast KS, northern AR and western TN...there
is a conditional flash flood threat this afternoon into the
overnight. Should convection form in this instability axis then an
increased flash flood threat would result. Confidence is low, but
given the conditional threat, feel the Slight Risk remains
warranted for now. But some chance a downgrade could be needed if
development does not materialize as shown by the 12z HREF. Recent
HRRR runs are much less aggressive across this corridor. Instead
they focus development further north from eastern KS into MO,
closer to the upper low to the north and southward sinking cold
front. This is certainly possible, and results in the lowered
confidence in this forecast. However areas further north have been
drier even if this solution pans out, the flash
flood threat will probably remain localized...and covered by the
broader Marginal Risk in effect.

The Marginal risk across the Upper MS Valley is maintained.
Anticipate we will see good coverage of slow moving pulse
convection across portions of IA/MN/WI/IL by this afternoon.
Forecast PWs, instability and slow cell motions support localized
heavy totals. However the pulse nature and small scale nature of
cells may limit the overall duration of heavy rates. Still would
anticipate pockets of 2"+ amounts, warranting a Marginal flash
flood risk, with isolated warnings expected.

...Southwest U.S....
Upgraded to a Slight Risk across portions of southern CA, southern
NV central/southern AZ. Looks like an active Monsoon day, with
high PWs, plenty of instability, and multiple MCVs helping focus
activity. 12z HREF probabilities are more impressive than on past
days...and even those past days have been somewhat active. Thus
expect this afternoon/evening to be as active or more active than
the past few days. Local WFOs have issued flash flood watches,
another reason to upgrade to a Slight Risk. See MPD 675 for more


Day 2
Valid 12Z Fri Aug 17 2018 - 12Z Sat Aug 18 2018


...Northeast to Ohio Valley and Mid-South...
Models continue to advertise a surface wave tracking across
upstate New York to northern Maine on Friday and Friday night.
This will be accompanied by abundant tropical moisture with
precipitable water values near 2 inches. 850-mb moisture flux
anomalies are also rather impressive, with values as much as 3 to
4 sigma above climatology. The signal for a slight risk remains
strongest near this surface low and given better agreement in the
guidance for a heavy QPF axis, the slight risk area was extended
westward to include much of Vermont and portions of eastern
upstate New York.

Farther south, an extensive marginal risk remains in place across
the northern Mid-Atlantic to the Ohio Valley and mid-south as
anomalous moisture pools along the trailing slow-moving cold
front. Its likely somewhere will see pockets of excessive
rains/flash flooding but its difficult to pin-point where the best
chance for that is right now and thus the best location for a
slight risk. Until a stronger signal comes in from the guidance,
prefer to keep this region in a very broad marginal risk.

...Southeastern Arizona/New Mexico into southern Colorado and the
Oklahoma/Texas panhandles...
Forcing shifts from Arizona to the Southern Rockies as the upper
high retrogrades westward. Expect diurnally forced convection to
first take shape across elevated heating sources before convective
outflow takes the activity toward the surrounding lower
elevations. 700-mb winds remain on the weaker side although the
low-level jet picks up to the east across the High Plains. The GFS
and the NAM (along with the in-house bias corrected model and the
National Blend) continue to show the potential for and axis of 1
to 3+ inches within the region, but the ECMWF and the UKMET remain
more conservative on the amounts. Given the continued signal in
the models, will maintain the inherited slight risk area.


Day 3
Valid 12Z Sat Aug 18 2018 - 12Z Sun Aug 19 2018


...Great Plains region into the Southern Rockies...
A shortwave initially emerging out of central Idaho is expected to
drop southeastward into the Central High Plains by 19/1200Z. The
guidance supports a marked increase in the nocturnal low-level jet
with convection breaking out across eastern Colorado and
translating downstream into the adjacent High Plains. The guidance
shows appreciable elevated instability across the central high
Plains down into the Oklahoma/Texas panhandles within an area of
strong moisture transport and anomalous precipitable water (1-2
standard deviations above normal). Accordingly, most of the
guidance has a fairly impressive swath of 1-2+ inch 24-hour
amounts. As such, the slight risk area in this area remains, while
a marginal risk extends northward into the Central/Northern Plains
and back down into areas of the Southern Rockies.

...Lower Mississippi Valley eastward into the mid-South and
Mid-Atlantic region...
Continued moisture pooling along a west-east boundary will provide
a focus for additional bouts of heavy rainfall from the lower
Mississippi Valley to the mid-Atlantic. Flash flood guidance
values in some locations are fairly low (especially across the
Mid-Atlantic), and excessive rainfall somewhere along the boundary
certainly seems possible, but the latest model runs continue to be
rather scattered with the exact placement and intensity of the
highest QPF. At some point would expect a slight risk somewhere,
but at this time opted to keep a broad marginal risk in place
until a more consolidated solution comes into place.


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