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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1459Z Aug 16, 2018)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1059 AM EDT Thu Aug 16 2018

Day 1
Valid 15Z Thu Aug 16 2018 - 12Z Fri Aug 17 2018

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL STRETCHING FROM
OK/KS NORTHEASTWARD INTO THE OHIO VALLEY...

15z Update:
Made some minor modifications to the Risk areas. Expanded the
Marginal risk across the Upper MS Valley near the closed low
moving eastward. 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.

Also expanded the Slight Risk across portions of TN/AR/OK/KS. This
is to cover ongoing activity over western TN...and the forecast
additional development this afternoon and evening further west
over OK/KS. This expansion covers areas that have seen locally
heavy rains over the past few days, and are likely to see
additional locally heavy convection in their vicinity again today.

Chenard


Previous Discussion from 09z...

...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.

Oravec


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

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN THE SOUTHERN
HIGH PLAINS...LOWER OHIO VALLEY...AND MAINE...

...New England into the Mid-Atlantic...
Models continue to advertise a surface wave tracking across
upstate New York on Friday evening while eventually crossing into
northern Maine during the overnight hours. This will be
accompanied by abundant tropical moisture with precipitable water
values in the 1.75 to 2.00 inch range. Further, 850-mb moisture
flux anomalies remain impressive, generally in the 3 to 4 sigma
above climatology range. The signal for the Slight Risk remains
strongest across northern Maine although the 00Z ECMWF did drag
heavier amounts back into northern New York. Until additional
guidance joins this solution, feel compelled to leave the region
in a Marginal Risk. Across the Mid-Atlantic region, kept a
Marginal Risk area in play given the saturated soils noted from
all the heavy rainfall this month. Thus, any additional
precipitation could lead to pockets of future flash flooding.

...Ozarks eastward to the Central Appalachians...
The signal for continued heavy rainfall from the Ozarks to the
mid-South remains as a frontal zone stalls across the region.
Forecast soundings show quasi-unidirectional flow setting up with
anomalous moisture pooling along it. A max QPF axis generally sits
along and just south of I-64 across sections of the Lower Ohio
Valley where the Slight Risk remains. Given heavy rainfall
expected during the preceding outlook period, would anticipate
flash flood guidance values to drop leading into the Day 2 period.

...Southeastern Arizona/New Mexico into the Oklahoma/Texas
panhandles...
Forcing shifts eastward 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 take 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
00Z NAM/GFS show an impressive signal of 1 to 3 inches over the
region while the 00Z/12Z ECMWF remain more conservative in nature.
Will maintain the inherited Slight Risk area given the signal
depicted in the NCEP guidance.

Rubin-Oster

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

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN THE CENTRAL
HIGH PLAINS INTO WESTERN OKLAHOMA...

...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 19/0000Z. The
guidance support 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 00Z GFS
shows appreciable elevated instability with 1000 to 1500 J/kg
indicated across western Kansas down into the Oklahoma/Texas
panhandles. Accordingly, the guidance has a fairly impressive
swath of 2 inch plus 24-hour amounts, namely the 00Z GFS/ECMWF. As
such, a Slight Risk area has been introduced 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. The guidance are
rather scattered about with placement and intensity which
precludes anything above a Marginal Risk for flash flooding.
Farther north, decided to extend the risk area up into the
Mid-Atlantic given the 2 to 3 inch 24-hour amounts suggested by
the 00Z ECMWF/UKMET. This is actually two consecutive runs shown
by the ECMWF although its 6 hour QPF projection appears a bit
wonky. While flash flood guidance values remain low over this
region, do not feel comfortable raising the area to a Slight Risk
until a more consolidated solution exists.


Rubin-Oster





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