Quantitative Precipitation Forecast
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FOUS30 KWBC 022006

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
405 PM EDT Thu Jul 02 2020

Day 1
Valid 16Z Thu Jul 02 2020 - 12Z Fri Jul 03 2020


...Lower Mississippi Valley...

16Z update:
Cycling convection this morning over portions of
central/east-central MS has low confidence given the poor
initialization and handling of the activity with the most recent
hi-res guidance. Mid/upper level weakness and ridging over the
Plains will support a southward drift in the current convection
that is expected to gradually work into a better/more unstable
environment. The main concerns being parallel steering flow to
storm motions that could support backbuilding and regeneration as
well as weak but consistent inflow supporting individual cells or
small clusters. There was a signal seen in the 06Z HREF
neighborhood probabilities for the QPF to exceed the 3-hr FFG
(30-40 percent this afternoon). Some of these areas have seen
above normal rainfall the last couple of days resulting in
saturated/wet soils. However, lower confidence in how this will
evolve precludes introducing a slight risk over portions of MS at
this time, and only some minor adjustments were made to the day 1

---previous discussion---
It is a summer-like convective pattern in which the flash flood
threats are not always obvious, even on Day 1. Today it appears
the most worrisome area of organized heavy rain is likely to occur
over parts of southern Mississippi and perhaps nearby parts of LA
/ AR, where an axis of moderate CAPE will align with a frontal
boundary - recently reinforced by outflow. We suspect that diurnal
convection will make use of the CAPE axis, but the evolution
varies quite a bit between hi-res models. It is not clear how
pronounced the potential for cell mergers and training would be.
The evolution in the WRF-NMM and 00Z HRRR tended to make the most
sense based on observational trends and forcing mechanisms, and
these models depicted convection that is relatively progressive or
loosely organized. Given the uncertainty and weak deep layer
forcing, we maintained the Marginal Risk category rather than
adding a Slight. That said, the combination of moderate to strong
instability and rich deep layer moisture will support local 2-inch
per hour rain rates and a risk of flash flooding anywhere from the
Ozarks to near the central Gulf Coast, from late morning onward.

...Southern Plains...
Here Marginal Risk extends out of the lower Mississippi Valley
into parts of Oklahoma and Kansas, primarily to maintain outlook
continuity and give a nod to very slow predicted steering flow.
The offsetting factors, however, are very warm boundary layer
temperatures that will promote strong cold pool production and
rapid transition to outflow dominance, along with very high flash
flood guidance values in light of recent dry weather. Several
models are predicting a small MCS to evolve near the state line
and advance east to southeast this evening. The main flash flood
threat will be early as the various storms congeal, and then
should other cells form along the frontal boundary to boost local
rain totals in advance of the MCS.

A mid level shear axis should again serve as a focus for diurnal
thunderstorm enhancement over parts of Missouri today. Moisture
and instability that is typical of summer will yield intense local
downpours. We may not see much upscale growth, given a lack of
inflow, but isolated flash flooding is possible, especially owing
to the saturated conditions following a wet several days.

...Other Areas Not Meeting Criteria...
In the northern Plains, at least on Day 1, the model signal for
heavy rain has shrunk, concentrating now mainly on areas east of
the Black Hills. Some of the heavier rainfall is expected in the
Sand Hills, so little flash flood threat there, and in the
tri-state CO/NE/KS area where very warm conditions should limit
efficiency and promote gusty outflows. More generally, the
predicted mean 0-6km winds are 15-20 knots along the High Plains
from Nebraska to North Dakota today. While an isolated flash flood
is possible, the environment does not seem to support Marginal
Risk probabilities.

We also noted the likelihood of some heavier downpours in central
Main and Down East Maine this afternoon.  Steering flow from the
north and weak low level inflow does not represent much of a setup
for training, but hourly rain rates above one inch could lead to


Day 2
Valid 12Z Fri Jul 03 2020 - 12Z Sat Jul 04 2020


...Northern Plains...
...20Z Update...
Quite a bit of uncertainty remains with respect to convection
Friday into early Saturday.  Mid-level impulses rounding a
positively tilted trough will shift northeastward toward the ridge
axis anchored in the Canadian Prairie region. This energy will be
the catalyst for strong synoptic ascent.  With plenty of moisture
and instability available (as discussed below), the question is
not if convection will occur but rather to what extent the
convection will be organized and where will it reside. Anticipate
afternoon/evening convection, with the best potential for
organized convection (possible MCV) during the overnight hours
coincident with the strong shortwave.   Given such unknowns at
this point with the expectation that rain rates could climb
upwards of 1.5 inches/hour over very vulnerable soils, refined the
Slight Risk area to account for this and also where there is a
strong multi-model signal for highest QPF probabilities.

...Previous Discussion...
Short wave energy on the southern edge of the westerlies crossing
the Northern Plains interacts with deep instability and moisture
to support convection producing heavy to excessive rainfall
rainfall across ND into SD during Day 2. The main threat appears
to be across portions of north central ND, which could receive two
rounds of convection over areas that received very heavy rainfall
amounts Wed.

The first short wave crosses ND/SD during the afternoon and
evening. Ahead of the short wave and a surface boundary crossing
the region, model soundings showed more than 3500 J/KG of MUCAPE
in place, peaking near 04/00z. The initial round of storms could
be mainly outflow dominated in such a high instability airmass and
increasingly difluence ahead of the short wave could allow the
storms to become better organized. As the storms initiate, short
term training and cell mergers could pose a flash flood threat
before 04/00z.

A 20 knot low level southerly flow transports 1.50/1.75 inch
precipitable water air (which is about two standard deviations
above the mean) into central SD/ND, and the additional moisture,
coupled with increasing warm cloud depths, may allow the storms to
become more efficient rainfall producers. By this time, hourly
rainfall rates could approach 1.50 inches, especially where cell
mergers and training occur. There is a high resolution model
signal (including the 00z NAM CONUS Nest and the 00z FV3 CAM) for
local 3.00+ inch rainfall amounts with the initial areas of storms
before they become more outflow dominated and progressive.

The next short wave in the mid level crosses ND between 04/06z and
04/12z. Model soundings suggest that there is still sufficient
elevated instability to support additional storms that cross north
central ND during that time frame. Though the airmass may be
overturned locally, the additional storms could produce an
additional inch before the end of the period. Three hour flash
flood guidance in these areas is as low as 1.00/1.50 inches, and
this may not have accounted for the heavy rainfall Wed. Based on
this, and collaboration with WFO FGF, a Slight Risk was introduced
for Day 2 across north central ND.

...Mid and Lower Mississippi Valley into the Gulf Coast...
...20Z Update...
Only minor adjustments needed at this update based on latest
QPF/model trends with the previous forecast discussion on track.
Concern still remains across the Mid-MS Valley region where soils
are very saturated and convection is expected to develop Friday.
Therefore, did extend the Marginal Risk area to account for this.
Though, highest QPF/rain rates should be closer to the Gulf Coast
region where best moisture/instability will overlap.

...Previous Discussion...
A boundary extending from the Lower and Mid MS Valley across the
Gulf Coast states will become the focus for instability and a deep
moisture plume to fuel storms that produce heavy to locally
excessive rainfall amounts along the boundary during Day 2.

Across the eastern portion of the front (generally from southern
MS across the FL Panhandle), a positively tilted long wave trough
closes off across this area, and the weak lift associated with the
closing mid level system could help organize the broad area of
storms expected from afternoon into the evening hours. In the
presence of the mid level system, storm motions are expected to be
slow (generally under 10 knots), and the slow storms could result
in short term training and cell mergers. Where this occurs, hourly
rainfall rates could easily top 2.00 inches. Since there is not a
model consensus for a QPF maximum along the front here, a Marginal
Risk was placed here for Day 2 to cover the flash flood threat.

Across the western edge of the front extending across AR into MO,
the moisture plume is not quite as deep (though precipitable water
values top 1.75 inches), Instability is higher, which could
support scattered storms along the boundary in the same time frame
as further east. Because of the higher instability, the storms
could be more outflow driven, as least initially. However, slow
cell motions here could also result in short term training and
cell mergers, with hourly rainfall amounts topping 1.50 inches.
The main axis of storms is expected to remain west of where heavy
rainfall Tue/Wed, so the Marginal Risk was extended from the Gulf
Coast to the Lower/Mid MS Valley to cover the flash flood threat.

...20Z Update...
Expanded the Marginal Risk farther inland into NY/PA to account
for the latest QPF/model trends.  Some of the latest high
resolution model guidance illustrates the potential for more
widespread convection across this region through the afternoon
that may linger into the evening hours, especially across portions
of upstate NY toward the Poconos, where cells could be slow to
move with training possible. Though the best coverage for
convection and thus highest precipitation/rain rates should be
across southern NY, northern NJ into CT coincident with better
larger scale forcing.  Given much of this region is across the
urban corridor, a Slight Risk was considered. However, at this
time QPF amounts and overall rain rates are just shy of reaching
the threshold for more scattered flash flooding.

...Previous Discussion...
Short wave energy tracking through the broadly cyclonic flow
across the Northeast Day 2 will interact with instability and
moisture to support storms with heavy to locally excessive
rainfall. Ahead of the short wave (and its attendant surface
boundary), a pool of 1000/2000 J/KG in place will aid in
convective initiation near 03/16z. Though the storm motion should
be southeast, they could be slow enough prior to the arrival of
the short wave to allow storms to train briefly.

Moisture in place over the Northeast (with precipitable water
values near 1.50 inches) coincides with the instability for a
time, but eventually gets pushed off the coast before 04/00z.
Where the moisture and instability overlap, the storms could
produce hourly rainfall rates between 1.00/1.50 inches (which is
about what happened in a similar air across the Northeast earlier
this week). The activity should weaken toward 04/00z and the
threat for isolated flash flooding should ease thereafter.

There is a high resolution signal for local 2.00+ inch rainfall
amounts, which could include some urban areas along the Interstate
95 corridor, so a Marginal Risk was placed here for Day 2.


Day 3
Valid 12Z Sat Jul 04 2020 - 12Z Sun Jul 05 2020


...Northern Plains/Upper Mississippi Valley...
Minor adjustments made to the risk areas across this region.
Ongoing convection early Saturday will place boundaries which may
act as a catalyst for redevelopment of thunderstorms during the
afternoon.  Also, anticipate more widespread afternoon convection
to develop associated with mid-level impulses moving overhead.
These storms will track eastward over already saturated soils,
especially across ND.  Multi-model agreement on the overall
convective activity, though where more organized development
occurs has yet to be ironed out.  Thus, maintained the risk area,
refining based on the latest QPF/model trends.

...Previous Discussion...
Short waves on the southern edge of the westerlies crossing the
Northern Plains and the Upper MS Valley interact with strong
instability and moisture to support convection that produce heavy
to excessive rainfall during Day 3. An initial area of storms move
eastward across eastern ND and western MN before 04/15z, and
though it should be weakening, could still produce hourly rainfall
rates near an inch before running out of instability. This round
of storms could leave the airmass here temporarily overturned, and
that could slow the destabilization process ahead of the next
short wave in the flow.

As the next short wave crosses eastern MT, it encounters 1500/2550
J/KG of MUCAPE in place, and lift associated with the short wave
is expected to produce storms that could train as the mid level
flow align with the propagation vectors. Moisture should be deep
enough to support hourly rainfall rates near an inch over areas
with three hour flash flood guidance values as low as 1.00/1.50
inches. Though the storms should be moving with the mid level
flow, isolated flash flooding is possible through the passage of
the short wave here.

The aforementioned short wave is expected to cross the western
Dakotas after 05/00z, as it encounters MLCAPE values near 3500
J/KG, peaking just about that time. Storms that develop in the
airmass should be outflow dominated initially, given the depth of
the instability in place. However, as the storms tap 1.50 inch
precipitable water air ahead of them, hourly rainfall rates could
top an inch. There is a multi regional/global model signal for
local 2.00+ inch rainfall amounts here, and these amount seem
plausible given the depth of the instability and moisture in the
airmass. Based on collaboration with WFO FGF, this area was hit
hard with heavy rainfall Wed, and is vulnerable to additional
rain. With this in mind, a Slight Risk was placed here for Day 3.

...Lower Mississippi Valley and the Gulf Coast...
...20Z Update...
Only minor adjustments made to the Marginal Risk are.  Uncertainty
remains in surface low development along the stalled boundary that
will ultimately impact QPF amounts within this region.  Though, at
this point in the forecast, these details are hard to iron out.
Thus, maintained the general placement of scattered/numerous
convection across this region, especially along the Gulf Coast
region where better instability and moisture reside.  Depending on
the QPF observed on Day 2 (Friday into Saturday), this may play a
role in the potential necessity for an upgrade to Slight Risk.  We
will continue to monitor the latest model trends for this

...Previous Discussion...
A persistent surface boundary extending from the Lower MS Valley
across the Gulf Coast states will once again be a focus for
instability and deep moisture that supplies storms producing heavy
to excessive rainfall on Day 3. A meandering closed mid level low
over the eastern Gulf of Mexico could provide enough synoptic
scale lift to some organization to the convection that forms in
the 1500/2500 J/KG of MUCAPE in the column. As the storms form in
the deep moisture plume (with precipitable water values remaining
above 2.00 inches), hourly rainfall rates could again top 2.00
inches, especially where storms merge or train as mid level winds
remain under 10 knots. There is a model signal (albeit is weak)
for a QPF maximum across the Big Bend area of FL, where
convergence near a surface low could help focus rainfall amounts.
Right now, there does not appear to be enough support for a Slight
Risk here yet, so Marginal Risk was placed over the Gulf Coast to
account for the flash flood threat.

Further west across the Lower MS Valley, the instability is again
greater (with MUCAPE values topping out near 3000 J/KG) than
further east, though the moisture is not quite as deep. Being on
the edge of the influence of the mid level trough to the east,
storm motions should be fairly weak, with a southward drift
expected. This could allow hourly rainfall rates to top out
between 1.50/2.00 inches, despite the seemingly modest model QPF
amounts here. The Marginal Risk placed over the Gulf states was
extended over the Lower MS Valley to account for what could be a
low end flash flood threat.

...Mid Atlantic into the Southern Appalachians...
...20Z Update...
Maintained the Marginal Risk, though latest guidance is not
enthusiastic about convective coverage across this area with the
risk for flash flooding looking less likely.  However, given a
lack of model continuity, retained the Marginal Risk for another
cycle to see if QPF trends continue to lower. If anything the best
forcing may be orographic across the Southern Appalachians which
would result in more convective development; though this region
has observed below average precipitation over the past two weeks.

...Previous Discussion...
A frontal boundary crossing the Mid Atlantic states on Day 3
begins to slow as its support shears south to the northeast, and
eventually the front becomes quasi stationary near the VA/NC
border, extending back toward the Southern Appalachians. Though
the synoptic scale support is modest at best early on, there
should be sufficient instability to support convection extending
from the Mid Atlantic coast back toward the foothills of the
Southern Appalachians, mainly after 04/17z. A low level convergent
flow pools 1.75 inch precipitable water air along the front,
peaking between 04/18z and 05/00z.

As the best mid level support exits, steering winds drop to less
than 10 knots along the front, and there are some indications that
propagation vectors could oppose the weak mid level flow,
promoting an environment with cell mergers and possible
backbuilding. Where this occurs, hourly rainfall rates could top
out between 1.50/2.00 inches, and this scenario is supported by
the 00z NAM. In fact, the 00z NAM showed local 2.50+ QPF amounts
along the front, as well as far western NC.

The main limiting factor for a more organized flash flood threat
could be the coverage of storms. However, with the ingredients for
flash flooding in place, a Marginal Risk was extended along the
front to the Southern Appalachians to cover the threat. Depending
on how the position of the front changes, and whether models can
converge on a threat area, the Marginal Risk could change in later


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt


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