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Excessive Rainfall Discussion
(Caution: Version displayed is not the latest version. - Issued 0827Z 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
427 AM EDT Thu Aug 16 2018

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


...Mid to Upper Ohio Valley into the eastern Great Lakes...
A surface frontal boundary expected to move only slowly east
southeastward day 1 across the lower Great Lakes and to the north
of the Ohio Valley.  A lead area of height falls pushing east
northeastward along this boundary will maintain a generally
favorable upper difluent flow pattern in an axis of pw values 1.5
to 2+ standard deviations above the mean.  This will support
potential for areas of convection pushing east northeast from the
middle to upper Ohio Valley into the eastern Great Lakes region
along and to the south of the slow moving frontal boundary.  There
is model consensus for widespread moderate to heavy totals across
these areas.  Changes to the previous outlook for this period were
to push the slight risk area over the lower Great Lakes farther to
the southeast into the eastern Great Lakes and expand the area
southwestward along and to the south of the slow moving front into
the Upper to Middle Ohio Valley region. 

...Mid to Upper Mississippi Valley...
The well defined closed over central South Dakota early this
morning is expected to push steadily east southeastward across
portions of the northern to central Plains and into the middle
Mississippi Valley day 1.  Scattered convection likely to continue
ahead of this system in a region of enhanced upper difluence. 
Confidence is not high with the qpf details given a fairly large
spread in the model.  However---there is a signal for moderate to
heavy precip totals across portions of the Middle to Upper
Mississippi Valley in a region of above average pw values.  The
marginal risk across this area reflects an eastward expansion of
the previous outlook for the upcoming day 1 period.

...Southeast Kansas...northeast Oklahoma into southwest Missouri
and northwestern Arkansas...
Upper difluence is also expected to become well defined day 1
along the southern edge of the closed low moving into the Mid to
Upper Mississippi Valley and ahead of additional height falls
pushing east southeast across the Southern Plains.  This will
support potential for organized convection Thursday evening/night
into Friday morning along and north of a stationary frontal
boundary forecast to lie west to east across portions of the
Central Plains into the Lower Missouri Valley.  The previous
slight risk area across this region was decreased in size and
centered more over the lower ffg values across these regions.

...Southwest U.S....
The previous marginal risk area over the Southwest U.S. was
expanded into southwest Arizona...far eastern California and much
of NV.  Not a lot of changes expected for the upcoming day 1
period with respect to the current monsoonal precip pattern across
the Southwest.  PW values expected to remain much above average
from southern AZ...southeast CA into much of NV.  Another day of
widespread scattered convective activity likely in this above
average pw axis---supporting isolated heavy precip totals and
isolated runoff issues...especially in more flash flood prone burn
scars, dry washes, and slot canyon regions.


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


...Eastern Midwest to the Ohio Valley...
Timing of a weak remnant low across the lower Michigan peninsula
is a bit faster in 12Z consensus of the GFS/ECMWF/UKMET.
Precipitable water values around 2 inches ahead of the low are 2
to 2.5 sigma above climatology. This tropical surge will spread
abundant rainfall along and ahead of the advancing cold front with
the heaviest amounts focusing over the Lower Great Lakes where
stronger height falls should overspread the region. The Slight
Risk was spread south along the frontal zone over IN/OH to
Cincinnati and KY.

...Central to Southern Great Plains to the Mid-South...
An upper low opens over the northern plains Thursday with
associated locally heavy rainfall spreading from SD to IA where a
Marginal Risk remains warranted. Farther south, featured best in
the 12Z GFS shifts east across OK Thursday. As the impulse
encounters an increasingly moist and buoyant atmosphere
downstream, some convective complex should emerge by the evening
hours. Given wet antecedent conditions in eastern OK, a Slight
Risk was raised there into AR. Should the north and south
complexes merge, it would be near northern AR which also has wet
conditions currently. Therefore, the Slight Risk spans northern AR
into western TN.

...Arizona into western New Mexico...
High precipitable water air continues to lift north out of the
Gulf of California/Sea of Cortez in a monsoonal pattern. Diurnal
heating over the region should force convection over the higher
terrain of Arizona/New Mexico initially although the weak 700-mb
winds suggest the activity may struggle to push off the mountains
into the lower elevations. Little adjustment was again necessary
to the Marginal Risk area.


Day 3

The Day 3 outlook will be updated by 0830Z.

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