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Excessive Rainfall Discussion
 
(Latest Discussion - Issued 1459Z Jun 24, 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
1059 AM EDT Sun Jun 24 2018

Day 1
Valid 12Z Sun Jun 24 2018 - 12Z Mon Jun 25 2018

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE GREAT PLAINS AND UPPER MIDWEST... 
 
...Southern Plains to the Tennessee Valley... 
At 15Z we used observational trends to hone in on a couple of
different Slight Risk areas, downgrading other portions of this
area to Marginal Risk. While a better defined surface front was
located across Kentucky and Missouri, looping up toward low
pressure in the central Plains states, MCS activity has tended to
follow the more southern gradient in 850-mb temperatures and a
weak remnant frontal zone from southwest Kansas toward Arkansas.
Mean flow speeds are actually respectably strong for late June at
this latitude, so individual convective systems are forward
propagating / progressive, and the flash flood threat is more
dependent upon repeat activity rather than training associated
with any single event. This being the case, the leading MCS over
Arkansas / southern Missouri this morning should remain
progressive, and poses only a Marginal threat of flash flooding
given relatively dry antecedent conditions over the Tennessee
Valley and lower Ohio Valley.

The greater risk of flash flooding would appear to be back west
where additional organized thunderstorm systems will overlap a bit
with the recent swath of rain over western Kansas and parts of
Oklahoma. The MCS over northwest Oklahoma as of 15Z showed no
signs of stopping, and may plunge to the south/southeast into a
region where the SPC mesoanalysis shows MCS maintenance
probabilities of 70 percent or greater, given strengthened low
level inflow and a plentiful source of instability over Texas and
southern Oklahoma. This will all likely be followed by yet another
round forming over the high plains, eastern Colorado and western
Kansas this afternoon. The high resolution models have not been
particularly helpful, especially regarding timing and evolution -
their QPF swaths are at least in the right vicinity. We would not
be surprised to see the convection make more southerly headway
into the Red River region, and have thus expanded risk areas
southward.

...Central Plains / Upper Midwest...
The model CAPE fields and QPF from some model sources do indicate
a separation between the now reinforced southerly baroclinic zone
in OK/KS versus other activity expected ahead of the difluent
upper trough and near a surface front / developing occlusion from
eastern Nebraska northward. We chose to draw a separate Slight
Risk area over this region. Deep layer flow is relatively strong,
which could lead to forward movement of individual cells, but over
time as the southeasterly low level flow increases the Corfidi
vectors become very light, suggesting that as cells merge and form
clusters or line segments - they may slow down sufficiently to
pose a flash flood threat from the Omaha and Sioux Falls areas up
into southwest Minnesota.

...Wyoming / South Dakota...
Here we downgraded to Marginal Risk owing to cool surface
temperatures and a lack of instability. Lightning and a few slow
moving cells were noted this morning, and there is antecedent
wetness to support a risk of flash flooding. The coverage,
intensity, and duration of the individual storm cells, however,
argue for neighborhood probabilities in the Marginal category.
Based on trends near 15Z, the greatest threat may exist just to
the east of the Black Hills where the tightening mid level
circulation had begun to tap somewhat greater instability.

...Northeast...
A mid/upper level trough and cold front moving across the
northeast will likely be a focus for convection this afternoon.
Storms will be progressive in nature, although locally intense.
Several recent high res model runs show enough cell development
that we see some brief merging and locally intense rainfall rates.
While not expecting any significant flooding issues, this signal
suggests that convection may become just organized enough to cause
some localized urban flooding issues...and will thus carry a
Marginal Risk for portions of southern new England.

Chenard/Burke


Day 2
Valid 12Z Mon Jun 25 2018 - 12Z Tue Jun 26 2018

...SLIGHT RISK REMAINS OVER THE CENTRAL PLAINS/MIDDLE MS VALLEY...

...Southern MN across IA/MO to KS/OK...
Closed mid level low pressure shifts east from the SD/NE border to
the MN/IA border Monday through Monday night. Remnant
activity/MCVs from Sunday night will be able to tap into an
environment with ample moisture/instability as it shifts ahead of
the surface moving into Missouri. Thinking is that this will lead
to redevelopment of new convection that will produce heavy to
potentially excessive rainfall.  Limited instability will lift
north of the warm front and across IA into MN/WI and back to SD.
Largely maintained the Day 3 Slight Risk of excessive rain as it
becomes the latest Day 2 forecast with only minor modifications. 
In particular, made few changes where the area had previously been
expanded south.  One exception was on the northern periphery of
the boundary where we nudged areas northward a bit over concerns
about convection that fires close to the cut off low. The
numerical guidance has been struggling to handle the convection
even in the short term, so confidence is particular low with
respect to the finer scale details.

...Coastal Carolinas...
A cold front sags across the Carolinas Monday night. Precipitable
water values ahead of the front are forecast to be around 2
inches, which is 2 standard deviations above normal. Ample
instability will allow for thunderstorms and weakening mean flow
will lead to slow movement and potential for localized flash
flooding. A Marginal Risk of excessive rain was maintained.

Bann


Day 3
Valid 12Z Tue Jun 26 2018 - 12Z Wed Jun 27 2018

...SLIGHT RISK OVER PARTS OF THE WESTERN GREAT LAKES TO MID
MISSISSIPPI VALLEY AND EASTWARD INTO THE OHIO VALLEY.

...Wisconsin/Illinois to Missouri and then Eastward into Ohio
Valley...
The risk of heavy to excessive rainfall spreads eastward as a
closed mid-level low and associated surface low pressure and front
translates eastward.  The biggest forecast problem will revolve
around where any on-going convection, remnant MCV(s) or outflow
boundaries will be at the beginning of the period...and how any of
those factors impact eventual threat of heavy rainfall. 
Confidence is simply too low to make many refinements on mesoscale
details.  In general, forecast soundings east of the mid level
system and south of the surface boundary show tall, skinny CAPE
values with K values at or above 35, precipitable water values
generally at or above 1.75 inches all point to a risk of flash
flooding even if there is not much confidence in details to help
pin-down exactly where it will occur.  In addition, Flash Flood
Guidance values have been lowered in the past few days by previous
heavy rain events.  As a result, a broad Slight Risk was hoisted,
with the expectation that the area will be refined in subsequent
outlooks.

...Southeast Coast...
The Marginal Risk of excessive rainfall was indicated along the
Southeast U.S. coast...from parts of the Carolinas into Georgia
and far northeast Florida as a boundary sags south.  There was at
least a modest signal for some 1 to 3 inch localized rainfall
amounts from convection near the front. Widespread flooding
problems appear unlikely considering that Flash Flood Guidance
values remain near 3 inches per hour in the outlooked area. 
However, the forcing by a boundary in a moisture rich atmosphere
suggests that short-term rainfall rates may still result in
localized problems.

Bann



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