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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2030Z Jun 23, 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
430 PM EDT Sat Jun 23 2018

Day 1
Valid 1757Z Sat Jun 23 2018 - 12Z Sun Jun 24 2018

...SLIGHT RISK ACROSS PORTIONS OF EASTERN OK AND WESTERN AR...

...Southern Plains to the Lower MS/TN Valleys and portions of the
Southeast...
Clusters of thunderstorms were scattered along and north of a
meandering frontal zone from Alabama to Oklahoma as of late
morning. The more organized activity had put down generally 0.50
to 1.50 inches of rain across central and southeast Oklahoma this
morning.

Guidance is in pretty good agreement that we will see another
round of convection developing back west this afternoon or evening
across portions of north TX and eastern OK. This activity should
develop near the lingering surface boundary, and within a region
of strengthening 850 mb moisture convergence owing to the
nocturnal increase in the low level jet. The magnitude and
orientation of 850 mb moisture transport into this area does again
support the potential for a period of backbuilding/training over
TX/OK. Thus while some activity may tend to hang up in these
locations overnight, other storms will likely follow the mean flow
and better right entrance upper jet dynamics off to the northeast
into portions of AR/TN and vicinity.

The early morning MCS was not handled very well in the models,
although the NSSL WRF and NCEP WRF-ARW2 did a decent job. Where
WPC has placed the small Slight Risk area, the 00z HREF
neighborhood probability of exceeding 24-hour, 3" rainfall is as
high as 40-70%, and the probability of exceeding 5" rainfall is
20-30%. This threat is expected to peak tonight with the second
round of storms. Elsewhere across the region we will maintain a
broad Marginal Risk to encompass the area of moderate to high
instability and at least modest low level convergence.

...Northern Mid-Atlantic into Southern New England...
Ther is a Marginal Risk of excessive rainfall across portions of
the eastern Mid Atlantic, with the main player being the
opening/weakening of the closed low that has been making its way
across the country over recent days. This shortwave will be
pushing towards New England on Saturday, which helps lift a warm
front across the Mid Atlantic. The amount of instability
along/south of the front should be enough to support convective
development, particularly along/near the coast. Cells should be
progressive, and although significant flooding is not expected, a
localized urban flash flood threat could develop with this
activity given continued above normal PW values and the wet ground
conditions dating back the past several weeks. To the north of the
boundary instability should be minimal, thus we are not currently
expecting a flash flood threat across central or northern New
England.

...WY into the High Plains...
A strong shortwave trough is forecast to move toward WY on
Saturday, providing a continued favorable environment for showers.
While not extreme, there should also be enough instability for
thunderstorms, which given the increasing PW, will be capable of
dropping locally heavy rains. Given how wet portions of WY have
been, this setup warrants the continuation of Marginal Risk.
Overnight this activity may try to expand/organize over portions
of eastern CO, southwest NE and northwest KS...as the nocturnal
increase in the low level jet ramps up the moisture transport into
the region. Some merging of cells is certainly possible with this
setup, resulting in locally heavy rains capable of exceeding Flash
Flood Guidance.

...Northern Plains...
A shortwave trough pushing southeast across SD on Saturday will
move into 1.25" PW (one standard deviation above normal) and some
afternoon instability over portions of the Northern Plains. Cell
motions may be a bit chaotic given weak mean flow and the presence
of a low level convergence axis. This suggests that we could see
some brief cell mergers as storms pulse up and down. Not expecting
a significant flood threat, but the 00z HREF does suggest we could
see localized 3" totals. This could result in isolated flash flood
issues, especially in areas that have been anomalously wet
recently.

...Lower Michigan...
After discussing with WFO Grand Rapids and monitoring trends in
the high resolution models, we have introduced a Marginal Risk of
excessive rainfall to portions of lower Michigan. A well defined
shortwave trough will slide into the lower Great Lakes tonight,
setting up a deformation axis / inverted trough feature up through
the central to northwest portion of the lower Michigan. This
affords an opportunity for slow moving convective cells or
convectively enhanced precipitation, with mean 0-6 km winds around
zero to 10 knots. Precipitable water values around 1.4 to 1.6
inches are somewhat anomalous, about 1.0 to 1.5 standard
deviations above climatology. The main limiting factor is a lack
of instability, as the region is well up to the north of the
frontal zone that stretches through the southern states, skies
were cloudy Saturday morning, and soundings showed deeply
saturated conditions. This will likely limit the short term rain
rates overnight, but slow movement and strong synoptic support
could lead to a gradual accumulation of 1 to 2 inches and isolated
higher amounts over several hours. This rainfall could approach
especially the 3-hour Flash Flood Guidance values.

Chenard/Burke


Day 2
Valid 12Z Sun Jun 24 2018 - 12Z Mon Jun 25 2018

...SLIGHT RISK ACROSS MUCH OF KANSAS AND INTO OKLAHOMA...

...Central into Southern Plains...
A strong shortwave trough shifts east across Wyoming tonight,
closing into a mid-level low Sunday over the SD/NE border. Between
upper level divergence/difluence east of the upper low and
increasing south to southeast flow which taps high precipitable
water values (1.5 to 2" is 2 to 2.5 standard deviations above
normal), the set up continues to support waves of thunderstorms
that will be capable of producing heavy to excessive rainfall.
Three periods of convection are of concern through this time.
First, east-to-west oriented activity develops over southeastern
KS Sunday afternoon. These will be oriented orthogonal to the low
level southerly flow and lift north with the front. Also Sunday
afternoon, convective storms will break out along the cold front
over eastern CO and develop into an MCS. This MCS will cross
Sunday evening/night over areas previously hit in southeastern KS
(per 12Z guidance consensus). Then the remnant southern boundary
late Sunday night should be orthogonal to the strengthening low
level flow and trigger slow moving convective storms.

With the amount of mesoscale processes taking place, the 12Z
guidance suite did not have a consensus location for this. QPF was
based on the 12Z ECMWF/GFS with inclusion of the 12Z Hi-Res NMMB
and ARW2 which were in good agreement with the global models with
the max over south-central KS near Wichita. The 12Z 3km NAM
focuses precip over northern OK which is only a bit south of the
guidance mean. FFG is rather high here with dry antecedent
conditions (will have to see what develops tonight). Therefore, a
large Slight Risk area was preferred with the notion that a
Moderate Risk is very possible once the focus area is honed.

...Eastern End of the Tennessee Valley into the West Virginia...
Moisture pooling (1.75" PW which is a standard deviation above
normal) along the western extent of a quasi-stationary front
extending from the Carolinas back to the Tennessee Valley will be
available to result in locally heavy rainfall, especially at time
of maximum instability Sunday afternoon. Ongoing activity this
afternoon may further enhance the low flash flood guidance over
this area. Marginal Risk maintained with minor change.

Jackson


Day 3
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 shift ahead of the low to MO
where redevelopment will occur in ample instability and moisture.
Limited instability will lift north of the warm front and across
IA into MN/WI and back to SD. Maintained a Slight Risk of
excessive rain with a bit of contraction south given less
instability north (despite lower FFG). Slight Risk was expanded
south where deep southwesterly flow could allow for repeating
activity and farther east to Chicago where FFG is rather low and
where moderate instability should reach late Monday night.

...Coastal Carolinas...
A cold front sags across the Carolinas Monday night. 2 inch PW
ahead of the front 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 raised for this
potential.

Jackson


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