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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1551Z Jun 18, 2019)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1151 AM EDT Tue Jun 18 2019

Day 1
Valid 16Z Tue Jun 18 2019 - 12Z Wed Jun 19 2019

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE
MID-ATLANTIC & CENTRAL PLAINS...

...Mid-Atlantic/Southern New England...
A strong closed low over SE Canada and subtropical ridge over the
Southeast continues strong confluent flow across the Ohio Valley
into the Mid-Atlantic today.  A lobe of sheared shortwave energy
is spreading across central OH currently into PA this morning and
like yesterday will continue to be a focus for enhanced
convergence along the shear axis across NE PA into CT/RI with very
moist low level environment remaining across E PA into MD with
total PWats at 1.8-2" observed.  Insolation will again drive
instability into the 1000-1500 J/kg range by late morning/early
afternoon especially across SE PA into S NJ.  Global guidance
members continue to enhance strong FGEN, moisture flux for strong
QPF signal across the 850mb boundary in NE PA into southern New
England, however, this is a typical negative bias in summer time
as convection in the warm sector will likely rob best
instability/ascent.  Hi-Res CAMs suggest more individual
thunderstorms across SE PA into MD/N VA by midday that should be
fairly progressive.  This is the second day of heavy rainfall and
there may be increased potential for upstream redevelopment
leading to repeat nature.  Low FFG values across the region
naturally and multiple rounds with capability for quick burst
1-1.5" rates (even faster than 1hr) may produce local flash
flooding concerns.  As a result, a Slight Risk resides in areas
with lower FFG from the Poconos/NYC metro and southeast into
Baltimore/DC region.   Higher guidance and faster cell motions are
likely to limit greater areal coverage threat across SE VA, so in
coordination with local forecast office, have trimmed SE Virginia
out of the Slight but still with the Marginal Risk area.

...Ohio Valley/Cumberland Plateau...
A slow-moving shortwave that has been directing heavy
moisture/instability for daily repeat bouts of heavy rainfall and
flash flooding has sheared significantly and is shifting into the
larger confluent flow farther east.  Sufficient remaining moisture
and some low magnitude instability (near moist adiabatic lapse
rates at this point) may spark isolated thunderstorms capable of
exceeding the highly compromised soil conditions across the Ohio
Valley into this evening.  The storms will likely be shorter/less
intense in duration resulting in reduced coverage.  As a result, a
broad Marginal Risk remains across this area. 

Further south, the shortwave continues to direct a very moist warm
conveyor belt out of the Central Gulf into the central South and
Cumberland Plateau with total PWats to 1.7" to 2" (nearing AL/GA).
 Sufficient upper level divergence from right entrance region
large scale ascent along with slow upglide/convergence along
increasing terrain is likely to spark stronger thunderstorms that
may train for short periods with slow eastward propagation (with
the mean eastward progression of the wave/warm conveyor belt).  As
such Hi-Res CAMs and global guidance support scattered 2-4" totals
across areas of SE KY/E TN and with complex terrain, the
combination yields a Slight Risk of Excessive Rainfall.


...Southern Appalachians and eastern flanks...
A mobile shortwave directs the warm conveyor belt eastward with
time, steadily increasing total PWats over 1.75" even through the
higher terrain which is well above normal.  Shortwave DPVA and
right entrance region ascent to 50kt 3H jet streak crossing the
terrain around 18z should support broad scale ascent and
convective development.  Additionally, southeasterly upslope
components will be strong to support initially slow moving, repeat
convective development across southern Appalachians especially
across NW NC, SW VA.  By 21z HREF Probabilities of 2"/3hr QPF are
around 40-50% from NC along the Blue Ridge in N VA, strongly
supporting a Slight Risk of excessive rainfall.

As cells move off terrain, there is a potential for a
reinforcement of moisture flux convergence around 00z off the
western edge of the tropical shear axis off the SC/FL coast line,
this may intersect with convective outflows supporting a
veering/strengthening LLJ and decreased propagation vectors,
perhaps with backbuilding and training.  Some guidance is
particularly prolific along the eastern flanks of NC/far SW VA
such as the HRRR, GEM Regional and NMMB, and further northeast the
00z NAM.  If this were to result significant flash flooding may
occur and a moderate risk may have to be introduced.  However,
there is still significant uncertainty, model variance to support
a Slight Risk of excessive rainfall for this area as well,
connecting to the other adjacent areas described above.


...Central into Southern Great Plains...
Increasingly strong southerly flow will add to the well of
moisture across the Central Plains.  A draped stationary front
currently caps the northward extension of the area of concern
along I-80 across Nebraska angling northeast across NW IA toward
the strong shortwave/occluded closed low over N MN.  This wave
will continue to exit eastward with very little southward push
throughout the day with exception of cloud cover pressing the
instability gradient that direction as well.  Larger scale
shortwave/MCV across eastern KS will drive the return moisture as
well as forcing/convergence by evening today.  Instability will
build back into the 2000-2500 J/kg range with higher values
further south into N OK/TX panhandle.  This will be sufficient as
profiles will be narrower/skinnier given greater deep moisture
with 1.7-1.8" total PWat available.  Convection will redevelop
along the frontal zone as well as the shortwave trof, likely to
close in/merge along each other across N central KS...shifting
southeast slowly, resulting in large convective clusters/mergers
with very slow cell motions given proximity to the shortwave hub
itself; yielding to large swaths of 3-5" totals with some of the
preferred Hi-Res CAMs such as the HRRR, ARW, ARW2, NMMB.  As a
result WPC QPF is very close to the Bias Corrected multi-model
ensemble.   HREF probabilities also suggest a broad area flash
flooding potential with 2"/hr rates peaking at 40-45% in central
KS at 00z and neighborhood 3"/24hr probabilities over 60-70% over
S Neb into central KS.  As a result a broad Slight Risk remains in
place covering much of KS/Southern NEB with a Marginal Risk
surrounding it.  A shortwave emerging from the central Rockies
will also support another round of upslope forced clusters off the
terrain, eventually merging with other cells in KS.  Convection
across SE CO/NE NM will be faster moving to the east, with strong
quick bursts of heavy rain potentially exceeding 1hr FFG values
even though they will be more progressive.  Have extended the
Slight South into central OK to account for more recent heavy
rainfall where AHPS 2 week precipitation anomalies are 100-300%
above normal, south of which, a broad Marginal extends to about
I-20.


...Central Gulf coast...
A low-level convergence zone has acted as the focus for early
morning convection across portions of southeast LA and southern MS
over the past 2-3 hours.  Precipitable water values are 2-2.2" per
GPS data and low-level inflow is southwesterly at 20-30 kts within
a region of 1000-2000 ML CAPE.  Some eastward shift to the
convection is expected through the afternoon hours as the parent
shortwave in south-central LA forces some forward propagation as
the low-level convergence axis shifts eastward.  Hourly rain
totals to 2.5" and local amounts to 4" are possible into the
evening.


...Florida...
GOES-E WV suite denotes a sheared trough just off shore eastern
Florida with excellent anticyclonic outflow channel to the
northeast and a drying wedge across the eastern Gulf. This is
aiding confluent low to middle flow supporting pooling moisture
and broad convergence across southern Florida today especially
with a weak MCV remnant from recently decaying complex over SW FL.
 While best forcing, moisture convergence will reside along the
coast, there will be continued southerly and southeasterly sfc to
925 hPa flow across South Florida to provide continued moisture
flux with some broad turning to support backbuilding for
thunderstorms that do develop over the urban corridor from VRB to
MIA.  Strong tropical showers with over 2.25" of total PWats and
strong flux will support 2-3"/hr rates, perhaps for one-two hour
stints locally.  In the urban setting, rapid inundation or "flash"
ponding may result.  The Marginal Risk remains along the I-95
corridor and extends northwest to Orlando to account for latest
radar trends.  


Roth/Gallina


Day 2
Valid 12Z Wed Jun 19 2019 - 12Z Thu Jun 20 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MIDDLE MISSISSIPPI, TENNESSEE AND OHIO VALLEYS...

...Middle/Lower Mississippi Valley/Tennessee and Ohio
Valleys/Mid-Atlantic...

Shortwave energy will track across the Plains and into the
Mississippi Valley on Wednesday. The associated surface low is
expected to deepen through the day as it begins to lift north and
east into the western portions of the Tennessee/Ohio Valley. Deep
moisture from the Gulf of Mexico will be drawn northward into the
approaching surface system. The moisture flux significantly
increasing over Arkansas, southern Missouri, Tennessee and
Kentucky by, and after, 00Z Thursday as the low level jet
strengthens to 40 to 55 knots. The precipitable water values of
1.75 to 2 inches will be on the order of 1.5 standard deviations
above the mean when it encounters the strong instability of 2000
to 4000 J/kg over the Mississippi and Tennessee Valleys.
Additionally, large scale forcing for ascent is noted from the
strong 250mb jet and mid-level impulses.

Model spread for this period has improved somewhat with this
cycle, however models still are struggling with the exact timing,
location and quantity of QPF which results in a lower than average
confidence forecast. Antecedent rainfall and well-saturated soils
across this sector of the country will keep the threat for flash
flooding elevated during this period. With areal average QPF of 1
to 2+ inches possible over this region, where parts some locations
have received 300-600% of normal precipitation over the past week
alone, will have an elevated threat for excessive rainfall and
flash flooding. Therefore, this region will be super sensitive to
additional rainfall. The Slight Risk that was already in effect
was refined to reflect adjustments to the QPF. Given uncertainty
remains in terms of convection evolution, the Marginal Risk
accounts for the expected placement uncertainty.

...Northern Plains...

A closed upper level low will continue to drop across the Canadian
Rockies resulting in mid-level energy ejecting from the trough
axis across Montana. With plenty of atmospheric forcing for ascent
aloft, moisture surging into the Dakotas (precipitable water
values approaching 1.25 inches) and weak instability in place,
this will be enough to develop a surface low and potential
convective complex moving east out of the Rockies. Given flash
flood guidance is a bit lower across this region, felt a Marginal
Risk was needed to account for isolated flash flooding concerns. 

Campbell/Pagano




Day 3
Valid 12Z Thu Jun 20 2019 - 12Z Fri Jun 21 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE EASTERN OHIO VALLEY, MID-ATLANTIC AND NORTHEAST...

...Southern Plains/Mid-MS Valley/Tennessee and Ohio
Valleys/Mid-Atlantic...

The strong moisture flux mentioned on day 2 will persist Thursday
the shortwave lifts trough the eastern portions of the
Tennessee/Ohio Valleys and into the Mid-Atlantic. Strong CAPE
values of  2000 to 4000 J/kg will be present over much of the
central Appalachians and surrounding area. Precipitable water
values of 1.75 to 2+ inches will pass through the Appalachians,
Mid-Atlantic and southern New England; which is about 2 standard
deviations above the mean. Antecedent rainfall and well-saturated
soils across this sector of the country will keep the threat for
flash flooding elevated during this period. A few locations have
FFG as low as 0.50 inch after receiving 300-600% of normal
precipitation over the past week. These flood-prone areas will be
very sensitive to additional rainfall and/or high rainfall rates,
therefore introduced a Slight Risk from the Kentucky/Virginia
border northeastward to far southwest Maine. A Marginal Risk
accounts for the expected placement uncertainty and spans from
northern Alabama to central Maine.

...Northern Plains...

The surface low pressure system that spun up over the Northern
High Plains on day 2 will propagate eastward into the Northern and
Central Plains on Thursday. Precipitable water values of 1 to 1.25
inches will continue to be drawn northward into the system.
Surface convergence and instability (1000 to 2000 J/kg), along
with enhanced lift aloft with an upper level jet streak rounding
the base of the trough, will help maintain convection over the
north-central U.S. A Marginal Risk was introduced for Thursday and
shifted east from the highlighted area on day 3 to account for
isolated flash flooding concerns. 


Campbell




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