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Excessive Rainfall Discussion
 
(Latest Discussion - Issued 0844Z Aug 14, 2020)
 
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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
444 AM EDT Fri Aug 14 2020

Day 1
Valid 12Z Fri Aug 14 2020 - 12Z Sat Aug 15 2020

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF EASTERN NORTH DAKOTA INTO NORTHERN MINNESOTA...

...Northern Plains/Mid and Upper Mississippi Valley...
We upgraded a portion of the Slight Risk from yesterday's Day 2
ERO to Moderate from portions of eastern SD into northern MN,
based on the heavy rainfall from the initial MCS early this
morning (higher soil saturation and lower FFGs), along with the
heavy rainfall footprint depicted from a multi-model consensus
with the next round associated ahead of the main upper
trough/surface front. Ahead of the surface front and mid level
short wave energy crossing ND during the early, a 30-40 knot low
level south southwest flow transports 1.50-1.75 inch precipitable
water air across IA into MN before 15/00z. After morning
cloudiness dissipates, model soundings showed 2500-3500 J/KG of
MUCAPE in place along and just ahead of the front, which should be
more than sufficient to support deep convection. As a 90 knot
upper level jet streak pivots into western Ontario near 15/00Z,
about the same time the upper level difluence increases ahead of
the negatively-tilted longwave trough, the enhanced right entrance
region forcing of the upper jet will allow for robust upper
divergence, thus allowing organized convection to expand  into a
line that crosses eastern ND into northern MN in the 14/22Z to
15/06Z time frame.

The fast mid level flow should allow the line of storms to remain
progressive, though the deep moisture in the column could allow
for local 3 to 5 inch rainfall amounts across eastern ND into
northern MN during this time (which is supported by all of the 00Z
CAMs, although the models continue to show some latitudinal
differences with the max QPF axis. WPC QPF leaned toward the more
progressive scenario with respect to timing of the heavier QPF,
with the axis of max totals a bit farther south into towards the
more robust instability. The Moderate Risk was placed where there
is overlap of more vulnerable antecedent conditions (recent heavy
rainfall) along with high probabilities of >3" within a 6 hour
period per the 00Z HREF. Exceedance probabilities per the parallel
HREF (probs of 3/6 hourly QPF exceeding 3/6 hour FFG) are highest
within the Moderate Risk area -- peaking over 40% between 18-03Z
(SSEO exceedance probs were even higher). 


...Mid Atlantic/Ohio Valley/Central and Southern Appalachians...
The main change to yesterday's Day 2 ERO was to expand the Slight
Risk across eastern portions of the TN/OH Valleys and farther east
across VA/NC. As the broad mid level trough lifts toward the OH
Valley, it tries to form a closed mid level low over OH after
15/00Z. Ahead of the closing mid level system, the low level
southeast flow feeds 1.75/2.00 inch precipitable water air (which
is between two and three standard deviations above the mean)
across the region. The upslope flow along the frontal boundary
extending from the TN Valley across the Southern and Central
Appalachians into the Mid Atlantic states should focus the
moisture on the boundary. Combined with the 1000/15000 J/KG of
MUCAPE in place, convection is expected to focus near the terrain
in the afternoon and evening hours.

Storm motions should be slow in the vicinity of the closing mid
level system are expected to remain below 10 knots, and could take
on a cyclonic motion over portions of OH into WV. This could
result in short term training and cell mergers, particularly near
the terrain. Slow cell motions and efficient rainfall makers (with
deepening warm cloud heights) could result in cells producing
hourly rainfall rates near 2.00 inches in spots. Much of the high
resolution guidance shows local 3.00+ rainfall amounts, with an
emphasis on southeast OH into western and northern WV.

Given the three hour flash flood guidance is as low as 1.00/1.50
inches over portions of southwest into western VA, these areas
appear to have an increased flash flood threat, so a Slight Risk
was placed here to cover the threat.


...Lower Mississippi Valley/Southeast...
The combination of morning convection dropping into the Lower MS
Valley and convection forming on boundaries across portions of
MS/AL during the afternoon evening could produce heavy to locally
excessive rainfall today.

Morning convection dropping southeast along and ahead of a
developing mid level trough across portions of AR into northern LA
and nearby MS could pose a low end flash flood threat, as it may
track over areas that saw heavy rainfall during Day 1. Any
boundary left by the storm, or Gulf breeze or pre frontal trough
extending across MS into AL into the evening could become the
focus for instability and deep moisture for the development of
additional convection in the afternoon. Storm motions are expected
to be slow initially, but as the trough approaches, a northeast
motion is expected.

There is a fairly solid high resolution model signal for local
3.00+ inch rainfall amounts in these areas, especially where any
short term training or cell mergers occur. Three hour flash flood
guidance values are generally above 3.00 inches, so training or
mergers would be needed to initiate flash flooding. Based on this,
the Marginal Risk from yesterday's day 2 ERO was adjusted to
account for the low-end threat.


Petersen/Hayes


Day 2
Valid 12Z Sat Aug 15 2020 - 12Z Sun Aug 16 2020

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE CENTRAL
AND SOUTHERN APPALACHIANS INTO THE MID ATLANTIC...

A mid level trough currently moving through the Mid-MS Valley will
track east across the OH Valley toward the Mid-Atlantic region
Saturday into early Sunday. This disturbance will interact with a
residual surface boundary, very moist environment (anomalously
high precipitable water >2.25 inches) and marginal instability
(MUCAPE between 500-1000 J/kg).  There is actually quite a bit of
spread among the guidance is terms of the trajectory and speed of
this system. In addition, models differ with respect to a the
surface boundary which will heavily influence the surface low
track and QPF. The NCEP models and CMC, especially the 12Z/00Z
GFS, are illustrating a more amplified mid-level pattern, lifting
the mid-level low through the central Appalachians into the
Mid-Atlantic (MD/DE/northern VA) and just south of Long Island by
Sunday afternoon (Day 3).  However, the non-NCEP camp shows a much
flatter wave pattern and thus keeps much of the moisture/surface
boundary farther south into NC/VA and off the eastern shoes of VA.
 Given these differences and based on the ingredients (best lift
coupled with moisture/marginal instability), decided to hedge
between these two solutions.  This move was also supported by the
latest high resolution guidance.  As a result, there is an
increased potential for very heavy rainfall across portions of the
central/southern Appalachians into the Foothills/Piedmont of VA as
the surface low tracks east across central VA enhancing upslope
flow north of the low. 

Areal average precipitation is around 1-2+ inches with locally
higher amounts anticipated. Though instability is not impressive,
still anticipate widely scattered shower/thunderstorms that will
produce rain rates over 1.5 inches/hour in some locations. Another
factor to consider is the slow storm motion and perhaps
training/cell mergers which will only serve to exacerbate the
hourly/storm totals. 

Given much of this region has received over 300% of normal
precipitation, and more is expected on Day 1, the soils will be
primed ahead of this next storm system.  While a Moderate Risk was
again considered, model spread remains too high at this point. 
However, if models continue to converge on west/central VA, then a
Moderate Risk may be needed.  For right now, a Slight Risk which
takes into account the antecedent conditions and expected
QPF/higher rain rates was retained/refined. 

Pagano

Day 3
Valid 12Z Sun Aug 16 2020 - 12Z Mon Aug 17 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE MIDDLE ATLANTIC...

A mid-level disturbance and surface low will track along a surface
boundary across the Mid-Atlantic shifting off the coast Sunday
morning into the afternoon.  As one may expect, the model spread
on Day 2 rolls right into Day 3 with the NCEP/CMC guidance
illustrating a more amplified pattern, which is lifting the
mid-level shortwave a bit farther north including the
aforementioned surface boundary.  The non-NCEP solutions are
showing a flatter mid/upper level pattern and thus keeps the
surface boundary a bit farther south into NC/VA.  Given the model
blend decision of Day 2 (hedging between the two camps), this
trend thus continues into Sunday/early Monday. 

With so much uncertainty with respect to the location of the
mid-level disturbance, surface low and boundary and the overall
speed of the system, decided to incorporate some of the ensemble
solutions into the Day 3 blend. As a result, the areal average
precipitation is forecast to be around 0.5-1+ inches along eastern
portions of the Mid-Atlantic, including VA.  While precipitable
water values will be around 2-2.25 inches east of the surface low
with weak instability (MUCAPE <500 J/kg), if the system moves
quickly east, then much of this region will be under subsidence
flow early Sunday morning which will drastically reduce the amount
of precipitation observed.  For now, anticipate ongoing activity
Sunday morning, but for the coverage of shower/thunderstorms to
erode through the day into the overnight hours. 

Given this region has received an abundance of rainfall over the
past couple of weeks (over 400% of normal in some locations) and
the anticipation of more convection on Day 1 and Day 2, felt a
Marginal Risk was necessary at this forecast issuance. However, a
lot can change with respect to the low track/timing and thus the
QPF.  Thus, anticipate the risk area may change a bit before all
is said and done. 

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