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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2014Z Jun 16, 2019)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
414 PM EDT Sun Jun 16 2019

Day 1
Valid 1802Z Sun Jun 16 2019 - 12Z Mon Jun 17 2019

1800 UTC update

Some minor changes to the previous Excessive Rainfall Outlook
based on radar trends with convection across Texas and Louisiana.

Oravec


1600 UTC update

The 1200 utc based hi res guidance has all trended to pushing
heavy rains farther to the south into eastern TX.  While run to
run variability in the hi res guidance is often great...we did
also trend toward the consensus with respect to expanding the
slight risk area farther southward from the previous issuance. 
While the convection may be fairly progressive to south as
southerly low level flow intersects the outflow boundary
stretching from northeast TX into northwest LA...there are still
enough uncertainties with respect to mesoscale details to preclude
heavy rains resulting in localized runoff issues.  Hourly rates in
the 1200 utc hi res cycle are over 2" in spots as the convection
builds southward along the outflow or re-develops farther to the
northwest and drops south and southeastward. 

Across the Ohio Valley...the slight risk area was expanded
westward through the remainder of southern Indiana and into
southeast Illinois.  This was to cover potential for additional
late afternoon into this evening convection that may push across
areas that have received heavy rains over the past 24 hours. 

...Southeast Nevada into Southwest Utah...
A small marginal risk area was added over southeast Nevada into
Southwest Utah. This is in the vicinity of several weak vorts
forecast to push east across central NV into north central UT
Sunday.  There is potential for slow moving cells in the axis of
above average pw values...2 to 3 standard deviations above the
mean...resulting in locally heavy rains and isolated runoff
issues...especially in more sensitive topographic regions. 

Oravec



...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OH VALLEY/LOWER GREAT LAKES REGION AS WELL AS PORTIONS OF THE
SOUTHERN PLAINS...

...Ohio Valley/Central Appalachians...
At the start of the forecast period, a decaying MCS is moving out
of the Upper Ohio Valley toward the Hudson River Valley. 
Convection along the trailing southwest trough is oriented
favorably to mean confluent mid-level flow, will keep best
moisture/instability directed across the Ohio Valley.  A northern
stream decaying MCV is pressing across IL currently as well, with
additional thunderstorms developing in warm advective pattern
across the Central MS valley into the Lower Ohio River Valley.
This warm advection will maintain these thunderstorms through the
early morning, likely to traverse areas affected by heavy rainfall
last evening.  This corridor from HUF to CVG is represented by
1/3hr FFG values of less 1/1.5" respectfully...so with rates of
1-1.5"/hr and training orientation is likely to continue flash
flooding threat this morning, mainly across S IND into SW OH.  As
such the Slight Risk of excessive rainfall was expanded from
central WV back through S IND. 

The northern stream frontal zone does remain well north across N
IL/IND/OH with modest moisture remaining.  Clouds from ongoing
convection across the Ohio River are likely to be reduced enough
to support a narrow ribbon of instability between the frontal zone
and the cloud line.  Preferred Hi-Res CAMS (ARW/ARW2/NMMB) suggest
new development in the late afternoon in response to convergence
along the front.  Yet it is uncertain the magnitude and coverage
will be sufficient for a Slight Risk, so have left a Marginal Risk
area to account for this area, including into the Central MO.  


...Eastern portions of the Southern and Central Plains...
A strong/decaying MCS is currently tracking across Eastern OK with
strong convection developing along the connecting 850mb boundary
across S KS into southern MO, along the southern portion of the
MCS.  Heavy rainfall is likely to be occurring at the interface
but should be weakening quickly after 12z with the exhaustion of
remaining instability.  The resultant MCV will remain fairly
stationary throughout the day and may lead to localized heavy
rainfall but it is unlikely that instability will build back due
to extensive cloud cover (due to the MCS) and reduced return
moisture/warm advection from the western Gulf due to obstruction
from convective complex further south in Eastern TX throughout the
day.  Though if cloud cover does break, a Slight Risk may have to
be added for the resultant stronger updrafts that would be
expected with increased instability. 

Further south, the southern outflow of the MCS is flattening a bit
across northeast TX.  Return western Gulf moisture is currently
intersecting with the boundary with 20-25kt supporting southward
propagation.  However, there is large model uncertainty in the
magnitude/timing of this southward propagation.  This directly
affects the level of excessive rainfall probabilities.  Hi-Res
CAMs such as the ARW/ARW2/NMMB/RAP and ECMWF support solid forward
propagation toward the Gulf, reducing duration of heavy rainfall
across E Texas, through the 16.12-00z. Yet short-burst (less than
1hr) 1-1.5" totals are possible at least initially posing a
possible flash flood risk even though the morning hours.  Midday
increased instability is likely to reinvigorate the squall line
(if it maintains) by afternoon with stronger rates, but higher FFG
values across the Coastal Plain of S TX should reduce overall
coverage of excessive rainfall to isolated and short-duration.

The other possibility, though less likely, is presented by the 00z
GFS/NAM, NAM-CONEST and GEM-Regional; where the outflow boundary
stalls but still results in a cold pool/dome sufficient for strong
overrunning throughout the morning, across northeast TX into
eastern OK.  This could potentially result in extreme totals
(perhaps as much as 6-8" in spots) warranting an upgrade.  Again,
this scenario is less likely but there are sufficient amount of
reliable guidance members that present this evolution to present
sufficient uncertainty. 
By late afternoon, the main shortwave will still be slow moving
across central to eastern OK, supporting veering low level
profiles across West TX where the air mass remains untapped
allowing for daytime heat to rebuild CAPES back into the 3000-4000
J/kg range across northwest TX.  Strong moisture convergence along
remaining boundaries are likely to result in the development of
another convective complex in the vicinity of the Red River that
will propagate eastward.  So even, if the first complex in the
morning does not result in excessive rainfall, this secondary
complex is expected to move across similar areas in northern TX. 
As a result a large/broad Slight Risk Excessive Rainfall area is
posted across much of northeast TX, southeast OK and portions of
southwest AR to best account for these different, yet plausible
unfolding heavy rainfall scenarios today into early Monday.   


...Central High Plains...
Large scale elongated closed low across the southern Canadian
Prairies will direct broad westerly flow out of the Northern
Rockies today leaving upslope regions of WY in right entrance
quadrant to the 50kt speed max across the Dakotas.  In response,
sfc/850mb south/southeasterly flow will increase to 15-20kts
across the NEB panhandle increasing modest low level moisture with
sfc Tds into the low 50s. Insolation should increase instability
back to around 1000-1500 J/kg, and typical upslope convection will
roll off the higher terrain by late afternoon and tap the
available instability/moisture for increase in convective depth
and rainfall efficiency.  There is solid QPF signal from Hi-res
CAMs as well as global guidance with even HREF probabilities
exceeding 3" around 40% in far SE NEB panhandle with modestly high
probabilities of 1"/1hr in SE WY/NW NEB (to the SD line).    With
hourly FFG as low as an inch and portions of this region receiving
>200% of normal precipitation the last 2 weeks, a Marginal Risk is
warranted for this area.  Small adjustments to account for the
very high FFG in the Sand Hills region carve out the odd shape to
the risk area.

Gallina



Day 2
Valid 12Z Mon Jun 17 2019 - 12Z Tue Jun 18 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE EASTERN OHIO VALLEY/MID-ATLANTIC REGION, IN ADDITION TO, THE
SOUTHERN PLAINS/LOWER MISSISSIPPI VALLEY...

...Mid-MS Valley/OH Valley/TN Valley/Mid-Atlantic...

Confidence on where the axis of heaviest QPF sets up and the
intensity of the rainfall for this period is lower than average,
as it is dependent on where the frontal boundary is and any
additional boundaries established from convective complexes on day
1. A surface wave traveling along the frontal boundary will track
toward the Mid-Atlantic region Monday morning. Aloft, mid-level
energy will move atop the aforementioned boundary from the OH
Valley eastward helping to enhance precipitation across portions
of West Virginia into the Mid-Atlantic.  The best forcing,
instability and moisture will become aligned over southern
portions of the Ohio valley and into the central
Appalachians/Mid-Atlantic region Monday afternoon/evening. Models
are not as vigorous regarding the potential MCS and resultant QPF
amounts, though the high-resolution models still depict a complex
moving through MD and off the coast later in the day. 

Meanwhile, farther southwest, across the Mid-MS Valley into the
OH/TN valley, a deepening surface low and associated trough will
lift northwest attempting to undercut the strong ridging along the
the southeast coast.  Instability (with CAPE values above 2000
J/kg) and moisture (as noted by precipitable water >2 inches) will
pool to the east of the trailing cold front and south of the
stationary boundary draped from the Mid-MS Valley northeast into
the Mid-Atlantic. With plenty of surface/boundary layer
convergence, moisture and instability in place, mid-level
vorticity and the right entrance region of the upper level jet
will help to promote large scale lift resulting in convective
developing along and just south of the boundary.

Therefore, there will likely be a two main areas of convection
that will impact this corridor through Day 2.  The first will be
associated with the potential MCS moving across WV into the
Mid-Atlantic and the other associated with the surface low
development and instability pooling across portions of eastern
MO/AR into the OH/TN Valleys. 

There continues to be model spread especially over the eastern
U.S., where models struggle with the shortwave activity and thus
convection at the surface. Therefore, the focus was in relation to
the expected frontal position and overlap of the key ingredients
to denote areas where higher QPF and efficient rain rates should
reside.  QPF of 1 to 3 inches may be scattered, though the
probability the highest across the mid-MS Valley into the
Mid-Atlantic.  With many locations within this swath already
receiving above normal precipitation the past week or two and
higher than average uncertainty with respect to MCS activity,
decided to expand the Slight Risk area to encompass this entire
region.  The Marginal Risk bounded around the Slight Risk area is
also to incorporate some uncertainty with respect to convective
activity, especially across the lower MS Valley.    

...Central High Plains...

Shortwave activity will continue to trek east Monday afternoon
through the overnight hours across the central High Plains
(northeastern Colorado into Nebraska).  With upper level troughing
across the Northern Rockies and height falls into the adjacent
High Plains, a surface low will develop helping to promote
moisture advection and instability from the south.  Precipitable
water anomalies of +2 standard deviations above the mean (over 1
inch) in combination with . marginal lapse rates/CAPE values >1000
J/kg will supply this region the mesoscale features needed for
heavy rainfall and efficient rain rates. With weak mid-level flow
and plenty of atmospheric forcing for ascent, convection will
blossom and move just off the Front Range into the High Plains of
Nebraska and western Kansas.  Rain rates will likely approach 1.5
inches and with Day 1 rainfall potentially saturating the soil,
this region may see isolated flash flood concerns.  The Marginal
Risk for excessive rainfall currently in effect was maintained
over this region.

...Florida...
Subtropical airmass will continue to spread into the Florida
peninsula with anomalously high precipitable water values (>2
inches) and instability (pockets of 1000-2000 J/kg).  With the
continued feed of moisture and slow storm motion as noted by the
10-15 knot BL flow, this will likely result in high QPF totals and
high/efficient rain rates. While this region can withstand a large
amount of precipitation, south Florida especially will receive
quite a bit of rainfall through Day 1 resulting in more sensitive
soils.  Therefore, added a Marginal Risk for this area.  

Pagano/Campbell


Day 3
Valid 12Z Tue Jun 18 2019 - 12Z Wed Jun 19 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID-ATLANTIC AND ACROSS PORTIONS OF THE CENTRAL PLAINS...

...Ohio Valley/Appalachians/Mid-Atlantic...

The quasi-stationary front draped from the Mid-Atlantic westward
into the Mid-MS Valley will slowly sag southward and continue to
act as a focus for convection through Wednesday morning. The best
moisture, lift and instability will remain south of the boundary
over parts of the OH Valley, central Appalachians and
Mid-Atlantic. Again, the position of the front, the shortwave
location/trajectory and QPF on day 2 will all impact what is
observed on Day 3. Multiple models are suggesting a QPF swath of 1
to 3+ inches from KY toward Long Island, though the model spread
in terms of QPF placement is strikingly high. There is little
question regarding convective activity and potential MCS riding
the mid-level flow, however, the questions remain- where will this
complex develop and how will it propagate.  According to the
instability axis noted, precipitation water and mid-level
vorticity trends, it seems the best corridor for heavy rain will
likely be from north KY across WV into the Mid-Atlantic region
during the forecast period.  With rain rates expected to exceed
1.5 inches/hour at times, the greatest area of concern will be
across portions of WV into the Mid-Atlantic where FFG us fairly
flow and expected to come down with the expected rainfall on Day
2.  Therefore, the Slight Risk was refined across this region. 
Would not be surprised to see this Slight Risk area expanded even
farther south and west with FFG likely to come down with expected
rainfall through Day 2. 

...Central Plains...

The shortwave energy mentioned on Day 2 will continue to track
across the central U.S. on Tuesday. An associated surface low will
deepen as it rides along the stationary boundary migrating
southeast from NE into eastern KS.  Strong moisture advection will
overspread the frontal zone and pool east of the trailing trough
over portions of the central/southern Plains. Moderate instability
on the order of 2000 to 3000 J/kg will be present over north Texas
into MO/IA with values of 1000 to 2000 J/kg. The environment will
be conducive for widespread organized convection that will be
capable of producing efficient rain rates. Multiple models are
signaling the highest QPF to occur over eastern KS/NE, southern IA
and northern MO where surface and boundary layer convergence
resides. This new QPF maxima is a big jump spatially from the
previous guidance and forecast due to better forcing being farther
north.  Areal average QPF will be around 1-2 inches with locally
higher amounts possible and largely influenced by convective
development.  These amounts may reach or exceed local FFG in a
short period of time. A Slight Risk for excessive rainfall was
adjusted north to account for the latest QPF forecast. 

Campbell/Pagano




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