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FOUS30 KWBC 172006
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
405 PM EDT Wed Jul 17 2019

Day 1
Valid 16Z Wed Jul 17 2019 - 12Z Thu Jul 18 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE UPPER MISSISSIPPI VALLEY, THE NORTHERN MID ATLANTIC INTO
SOUTHERN NEW ENGLAND, AND THE MID SOUTH...

...Upper Ohio Valley into the northern Mid Atlantic and southern
New England...
No significant changes were made to the previous ERO across this
region; the primary change was to expand the Slight Risk area
slightly further north based on some shifts in the hi-res model
QPFs. In particular, the 12Z runs of the 3km NAM, WRF-ARW2 and
HRRR seemed to most closely match regional radar trends, and they
all highlight a corridor of enhanced rainfall from S NY into MA.
This also fits with a conceptual model of training bands in the
confluent low-level flow downstream of the remnants of Barry's
circulation. This will be an area of relatively high PW (GPS
observations are around 2 inches as of 15Z), and should also be on
the northern periphery of an instability maximum in the I-95
corridor from SE PA into CT and RI. There may also be some
contribution to the overall vertical motion in the region from a
jet streak in Northern New England, placing the right entrance
region over the Hudson Valley and SC NY to W MA this afternoon.
Given these ingredients near the northern periphery of the ongoing
Slight Risk, the risk outline was expanded northward. Additional
pockets of heavy rainfall will be possible further south and
southwest, from OH and WV into PA and NJ as those areas will still
be within Barry's moisture plume. However, the bands of heavy
rainfall may be less focused as compared to further north. One
exception may be if Barry's circulation center survives
sufficiently in tact during the afternoon to lead to a more
concentrated area of convection in PA. Hourly rain rates may
exceed 2 inches in the strongest convective rain bands and
thunderstorms, and 6-hour rainfall amounts could exceed 4 inches
on an isolated basis.

...Mid South and Lower Ohio Valley...
A Slight Risk focused over northern Alabama (with ongoing
convection) was expanded to the northwest through N MS, W TN, NE
AR and SE MO. The slow-moving convective cluster in N AL has
generally shown a reduction in coverage of 40+ dBZ reflectivities
on KHTX radar, although GOES-16 IR satellite imagery shows a
maintenance of cold cloud tops in a small area. This convective
cluster could persist for another several hours and lead to a
continued flash flood threat. However, additional scattered
thunderstorms are expected to the northwest this afternoon, with
good agreement from the 12Z hi-res model suite. Any convection
would be likely to be slow-moving with relatively weak mid-upper
level winds and an upper-level anticyclone centered near MO. The
slow movement of thunderstorms embedded within an arc of deeper
moisture (PW around 2 inches) could lead to localized flash
flooding particularly given recent rainfall and greater soil
moisture across the region. Neighborhood probabilities of 2 inch
per hour rain rates are as high as 40 percent this afternoon on
the 12Z HREF. The northwestern extent of the Slight Risk was
limited by some drier air evident in the GOES-16 Simple Water
Vapor RGB over Missouri. Surrounding the Slight Risk, the Marginal
Risk was broadened out considerably given convective uncertainties
with an upstream MCS in Iowa that models are not handling well.
More on that in the next section.

...Upper Mississippi Valley...
A strong MCS with a bow echo on radar reflectivity continues to
make progress to the east and southeast over Iowa this morning. As
is sometimes the case with robust convective systems in the summer
months, it is not being handled well by models; many hi-res models
fail to sustain any convection in the area the MCS is currently
located, and obviously struggle to model potential downstream
impacts as a result. Two previous hi-res model runs actually had
the MCS in approximately the correct place -- the 00Z WRF-ARW and
00Z WRF-ARW2. Both models suggest that the MCS could make progress
hundreds of miles to the southeast into MO and IL. The ongoing MCS
is propagating relatively quickly forward, and thus poses
primarily a threat of localized flash flooding. However, it is
important in how it may generate downstream convection (with a
weakness in the upper level flow over MO and IL), and how the
cirrus canopy and outflow boundaries impact convective initiation
later today and tonight across NE, IA and MN. Given these
uncertainties, the Marginal Risk downstream of the MCS was
broadened considerably. Over IA and MN where another round of
convection is possible later, the Slight Risk was adjusted south
to better fit with the forecasts of the 00Z WRF-ARW and 00Z
WRF-ARW2 which show late afternoon or evening development along
the outflow boundary of the MCS and generally had a QPF maximum
further south than other models.

...Southeast AZ...
No changes across this region. The previous discussion is as
follows: will maintain the Marginal risk over southeastern AZ. The
threat appears pretty localized, but should have at least
scattered convective coverage this afternoon, with instability
probably getting just high enough to support rainfall rates strong
enough to produce a localized flash flood risk across any more
sensitive areas.

Lamers/Chenard


Day 2
Valid 12Z Thu Jul 18 2019 - 12Z Fri Jul 19 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE UPPER
GREAT LAKES REGION...

...Northern Wisconsin/Northern Michigan...
20z update: There was not much change with respect to the shape
and orientation of the Slight Risk area over far eastern WI into
portions of the UP and northern portion of the LP of MI. There is
still a strong model signal for MCS development, though there is
still some spread concerning how the MCS evolves (since there is
some latitudinal differences with respect to the axis of best
instability.

The regional/global models continue to be further north that the
high resolution guidance with the MCS (which is not surprising,
considering the high resolution solutions often have the
convection forming/moving along the instability axis, which is
typical further south. In any event, the previous thinking appears
to be solid, and the Slight Risk area was adjusted for trends in
the guidance.

Previous discussion...
At the start of the forecast period, amplifying shortwave at the
nose of unseasonably strong 250mb jet that is flat and stretches
well back into the Northeast Pacific will be lifting out of the
area into Ontario, with a strong surface low pressing the
attendant frontal zone across N MN/Lake Superior.  However, given
the orientation of the deeper flow out of the southwest veering
slight west, the frontal zone flattens out across the UP/N WI into
south central MN.  The 850mb LLJ will be strong across the region
at over 40kts and with solid moisture convergence at the
inflection where the front flattens will support total PWats AOA
2".   The LLJ will continue to be strong throughout day with
slight backing supporting moisture flux anomalies over 4 Std.Dev
from normal.

However, there will be a ongoing convective complex across the
Upper Mississippi River Valley and there is growing uncertainty in
its evolution on D1.  Hi-Res CAMs suggest with high instability
and strong LLJ, propagation vectors will have a stronger southerly
component potentially across NW IA/N IL into NW IND.  Global
guidance is quicker to kill off the MCS and favor stronger
ascent/forcing near the front and therefore supports stronger
convection across N WI into the UP and Northern Portions of the LP
of MI.  The former would disrupt warm advection of necessary
moisture/instability across Lake Michigan for the build up for a
complex Thursday evening into Friday morning.

Given the uncertainty in the placement, but the very strong
synoptic environment for heavy rainfall clearly capable of
producing heavy rainfall rates and short-term totals capable of
scattered flash flooding conditions warranting a Slight Risk.  At
this time, given the stronger larger scale signals will favor the
global guidance slightly in the placement of the Slight Risk,
however, will broaden the Marginal Risk of excessive Rainfall
further south and west to account for the backbuilding system.


...New Jersey/NYC...
20z update: The main focus for flash flood potential still appears
to be short wave energy crossing the Mid Atlantic interacting with
deep moisture associated with the remnants of Barry. Timing has
slowed slightly with the exiting mid level trough, which extends
the threat into the afternoon and possibly evening hours. Given
the broad moisture plume ahead of the short wave, the Marginal
Risk was expanded to include more of southern New England, which
should reside in the moisture plume the longest.

Previous discussion...
By the start of the forecast period, the deep moisture conveyor
belt associated with the remnants of Barry has pressed eastward
offshore into the Northwest Atlantic.  However, its remaining
sheared shortwave that supported a new surface circulation out of
the Mid-Atlantic lee surface trof on Wednesday (not PTC Barry)
will be in the vicinity of the Mid-Delaware River Valley,
maintaining some near surface moisture.  This will result in a
tightening deep moisture gradient with 1.75-2" remaining through
the total depth by midday.  Given a slight trend for eastward
press of the moisture, remaining high clouds are likely to
eastward as well allowing for increased insolation and instability
to build by early afternoon across New Jersey.  Hi-Res CAMs and
even global models suggest SBCAPEs to exceed 1000 J/kg perhaps
reaching as high as 2000 J/kg.  As such there is a solid
clustering of Hi-Res CAMs suggesting 1-3" totals are possible with
slower moving/merging convective cells in the afternoon.  As such
have maintained a Marginal Risk of Excessive Rainfall for NJ and
the low FFG areas near the NYC metro, but have pulled the Risk
area out of southern New England where lingering
clouds/precipitation is more likely to limit sufficient
instability to build back (additionally higher FFG values).

Gallina/Hayes


Day 3
Valid 12Z Fri Jul 19 2019 - 12Z Sat Jul 20 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE
NORTHERN PLAINS INTO THE UPPER GREAT LAKES...

A frontal boundary extending the Northern Plains into the Upper
Great Lakes during Day 3 becomes the focus for moisture and
instability to fuel convection capable of heavy to excessive
rainfall. While there was generally good overall agreement with
the synoptic scale features, there were some differences in the
placement of the front, and associated highest rainfall amounts.
In an attempt to mitigate some of the differences, the WPC QPF and
Excessive Rainfall Outlook were based on a blend of the 12z
ECMWF/GFS/NAM.

A frontal boundary becomes quasi stationary from SD across
southern MN into central WI and parts of the northern MI. The
front lies on the southern edge of a very strong (for the season)
mid level flow, and the flow becomes more or less parallel to the
front. Moisture pools along the front, with pre convective
precipitable water values near 2.00 inches (which is between two
and three standard deviations above the mean) stretched along it.
Model soundings indicated the potential for 1500/2500 J/KG of
MLCAPE just south of the front, peaking near 20/00z.

Convection is expected to develop in the mid to late afternoon
hours, when the bulk of the instability is surface based. Short
wave energy in the fast mid level flow could provide pockets of
lift, resulting in convective clusters tracking south of the front
after 20/00z. In fact, the simulated IR images from the 12z GFS
suggest that a small MCS could track along the boundary in the
20/00z to 20/06z time frame. In any event, as the propagation
vectors become better aligned with the mid level flow, the
potential for training increases with time.

Given the depth of the moisture along the boundary, hourly
rainfall rates could approach 1.50 inches, with local 3.00 inch
rainfall amounts possible (as shown by the 12z NAM). These amounts
seem plausible where training occurs. As mentioned earlier, there
is some latitudinal spread concerning the placement of the front
(with the 12z CMC/UKMET seemingly too far north, given the
distribution of instability depicted by the 15z SREF). So, despite
the relatively modest WPC QPF (due in part to the spread in the
placement of the highest QPF amounts), there appears to be
sufficient moisture and instability to support a Marginal Risk
along the front. Note that there could be some positional changes
with the Marginal Risk as models come into better agreement with
the placement of the front.

Hayes


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


$$




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