Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
509 AM EDT Sun Jun 16 2019

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

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

...Ohio Valley/Lower Great Lakes...

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 Mid-Atlantic region. Aloft, mid-level energy will move atop
the aforementioned boundary helping to enhance precipitation
across portions of West Virginia into the Mid-Atlantic.  The best
forcing and moisture alignment will be over southern portions of
the Ohio valley and into the Appalachians/Mid-Atlantic region
where instability and moisture will pool Monday afternoon/evening.

There continues to be model spread especially over the eastern
U.S., where some of the guidance has QPF reaching southern New
York while a majority keep it over southern Pennsylvania/Maryland.
Guidance suggests QPF amounts of 1 to 4 inches over the higher
terrain of West Virginia into portions of northern Virginia,
Maryland and Delaware is possible which may be problematic for
these flood-prone areas. With hourly FFG around 1-2 inches and
hourly QPF expected above this threshold for a portion of this
region, the flooding risk will be elevated. The Slight Risk area
was maintained and refined across most of West Virginia, southeast
Ohio, into southern Pennsylvania, northern Virginia and most of
Maryland and Delaware.

...Eastern Southern Plains/Lower MS Valley...

A MCS is anticipated to form over the Southern plains and move
into the Lower Mississippi Valley as a shortwave moves through the
central U.S. Strong moisture transport and surface convergence
will be present over eastern Texas/Oklahoma near a stalled frontal
boundary. Through the day on Monday, instability will increase
northward within this moisture rich environment and will continue
to fuel the convective development. Precipitable water values of
+2 inches will spread from northeast Texas to southern Illinois
within the low level jet of 20 to 30 knots. Multiple models are
signaling the highest QPF to occur over extreme northeast Texas,
eastern Oklahoma and southern Arkansas in the amount of 1 to 3+
inches. These amounts may reach or exceed local FFG in a short
period of time. A Slight Risk for excessive rainfall and elevated
flash flooding threat was maintained and refined based on the
latest QPF placement/totals.

...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.

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 THE EASTERN PORTION OF THE
SOUTHERN/CENTRAL PLAINS...

...Ohio Valley, Appalachians and Mid-Atlantic...

The quasi-stationary front draped from the Mid-Atlantic westward
into the Midwest will sag southward and will continue to act as a
focus for convection. The best moisture, lift and instability will
remain south of the boundary over parts of the central
Appalachians and areas east. Again, the position of the front and
QPF on day 2 will affect where it occurs on day 3. Multiple models
are suggesting QPF maximums of 1 to 5 inches over parts of
Del-Mar-Va region. These amounts would meet or exceed local FFG of
1-2 inches. A Slight Risk was introduced for eastern Virginia and
far southern Maryland. A broad Marginal Risk extends from Long
Island to northern North California westward to the Middle/Lower
Mississippi Valley and into the Central Plains.

...Central and Southern Plains...

The shortwave energy mentioned on day 2 will continue to track
across the central U.S. on Tuesday. A surface low and associated
surface front will move south and east through the Plains,
becoming nearly stationary from Missouri to western Oklahoma.
Strong moisture advection will overspread the frontal zone and
pool over much of the Southern and Central Plains. Moderate
instability on the order of 2000 to 3000 J/kg will be present over
Texas and Oklahoma, with values of 1000 to 2000 J/kg creeping
northward into Arkansas and Missouri. The environment will be
conducive for widespread organized convection that will be capable
of producing very efficient rain rates. Multiple models are
signaling the highest QPF to occur over extreme northeast Texas,
eastern Oklahoma and southern Arkansas in the amount of 1 to 2
inches. These amounts may reach or exceed local FFG in a short
period of time. A Slight Risk for excessive rainfall and elevated
flash flooding threat was introduced for southeast Kansas,
northeast Oklahoma, northwest Arkansas and southwest Missouri.

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