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FOUS30 KWBC 300954
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
553 AM EDT Tue Jul 30 2019

Day 1
Valid 12Z Tue Jul 30 2019 - 12Z Wed Jul 31 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS
SOUTHERN-CENTRAL ARIZONA AND FAR SOUTHWESTERN NEW MEXICO, ALONG
WITH EASTERN PORTIONS OF TEXAS, MUCH OF LOUISIANA, AND FAR
SOUTHWEST MISSISSIPPI...


...Southwest...
Mid-level ridge centered east of the 4 Corners Region will remain
nearly stationary through the day 1 period (12Z Wed), thereby
continuing to drive monsoon moisture into the Southwest.
Meanwhile, a mesoscale/convectively aided vort max over northwest
Mexico south early this morning continue to revolve around the
closed mid/upper high, tracking up along the eastern Gulf of
California and toward southwest AZ by 12Z Wed. PWATs as high 2+"
will pool near the Mexico/AZ border, which is near 3 standard
deviations above the climo mean, before advecting northward in
response to persisting low/mid level southerly flow along with the
aforementioned shortwave rotating around the periphery of the
ridge.

Across southern and central AZ, and upgrade to a Slight Risk was
made in large part to the uptick in both areal-average QPF along
with HREF/experimental HREF probabilities of >1"/hr rainfall
rates. While not over the same area, all the high-res CAMs
indicate scattered areas with max 1 hour rainfall rates of
1.5-2.5+ inches, which given the soils (peculation vs. runoff
rates) and low FFG, would lead to the possibility of more
scattered vs. isolated flash flooding.

The other area of concern would be across far southeastern CA and
southern NV. Areal-average QPF is quite low over this region,
however given the pooling of greater deep-layer instability on the
western periphery of the deeper cloud canopy, expect an isolated
flash flood risk to branch out a bit farther west-northwest than
the latest guidance would suggest.

...Western-Central Gulf Coast northeast through the TN and OH
Valleys, Appalachians, Lower Great Lakes, and interior portions of
the upper Mid Atlantic and Northeast...
A broad Marginal Risk was noted ahead of the slow-moving upper
trough/surface cold front. Thermodynamics are rather modest --
with PWs of 1.5-1.75" north and near 2.00" south not really
anomalous for this time of year. MUCAPEs between 2000-3000 j/kg
from the Gulf Coast through TN Valley would support more intense
rainfall rates, as evidenced by the higher 1-2+ inch hourly
rainfall probabilities per the HREF and experimental HREF.
However, relatively weak low-level inflow/moisture flux into the
frontal boundary, with 0-6km bulk shear values largely under 25
kts, would imply productive cold pools/outflow boundaries and thus
limit the duration of the more intense rainfall rates.

As such would expect a more localized excessive rainfall/flash
flood risk across most areas, though the most likely area for a
more organized heavy rainfall threat would be across eastern TX
into much of LA and far southwest MS per a multi-CAM blend. Even
across this area, there's still quite a bit of model spread with
the heavier totals; however, there is a sufficient heavy rainfall
signal to justify an upgrade to a Slight Risk. The latest (06Z)
HREF neighborhood probabilities of >2"/hr rainfall rates are now
30-60% within the Slight Risk area, primarily through 18Z over
northern portions and between 18-00Z closer to the Gulf Coast.

Farther north (OH Valley-upper Mid Atlantic and areas north),
deep-layer instability will not be as robust (MUCAPEs averaging
1000-1500 j/kg); however, stronger forcing ahead of the upper
trough (enhanced upper divergence and low-level frontogenesis via
the right entrance region of the broad upper jet streak) will
nevertheless foster widespread, organized pre-frontal convection,
especially with the orographic enhancement on the western slopes
of the Appalachians. Nevertheless, modest at best low-level
moisture transport along with weak mid-level lapse rates will
limit the risk of sustained, prolonged heavy rainfall.

Hurley






Day 2
Valid 12Z Wed Jul 31 2019 - 12Z Thu Aug 01 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
ARIZONA AND UTAH...

...Four Corners area...
Monsoonal flow affects a large area across the Four Corners area
during Day 2, as the mid level ridging moves slowly east. The low
to mid level southerly flow on the back side of the ridge sends
deep moisture across AZ/southeast CA into portions of southern
NV/UT and CO. The main thrust of moisture remains across southwest
AZ into far southeast CA, where precipitable water values reach
near 2.00 inches (which is between two and three standard
deviations above the mean).

Convection could be ongoing across portions of southwest AZ at the
start of Day 2, as there is some indication of short wave energy
(or perhaps an MCV) tracking from Mexico into this area early in
the period. Any remaining convective clusters should be moving
through western AZ, following the ribbon of best moisture before
31/18z. Outside of this, forcing could be lacking across Southern
AZ into southeast CA during the period. Instability is expected to
peak between 1000/2000 J/KG of MLCAPE before 01/00z, though
convection may be unfocused outside of the presence of forcing.
The deep moisture could support hourly rainfall amounts near an
inch, mainly where training occurs.

The short wave energy or old MCV is expected to track north across
western AZ into southern and central UT, mainly between 31/18z and
01/00z. The mid level lift is expected to interact with the deep
moisture plume (where precipitable water values are between two
and three standard deviations above the mean), resulting in at
least scattered storms. There is a multi high resolution model
signal (led by the 00z NAM CONUS Nest) for hourly rainfall rates
near 1.25 inches over much of northern AZ into southern and
central UT, mainly before 01/00z.

The main forecast problem could be instability. Cloudiness in the
deep moisture (and ahead of the mid level support) could at least
delay the recovery of instability, especially before 31/18z. Most
models show 1000 J/KG of MLCAPE developing before peak heating in
these areas, and that should be more than sufficient for
convective initiation, particularly over the higher terrain. Even
in the presence of mid level lift, the areal extent of convection
could be muted by instability, which would lessen the flash flood
threat to some degree.

However, the depth of the moisture and the potential for enhanced
lift could compensate for the lack of more widespread instability.
With this in mind, a Slight Risk was placed over northwest AZ into
southern and central UT, and this was collaborated with WFOs
(FGZ/VEF/SLC).


...Central Plains into the Mid Mississippi Valley...
Short wave energy riding over mid level ridging over the central
Plains interacts with deepening moisture and instability to
support the development of a convective cluster or small MCS on
the nose of a low level jet across portions of NE/KS/MO during the
second half of Day 2. A 30/35 knot low level southerly inflow
across this area transports 2.00 inch precipitable water air
(which is about two standard deviations above the mean) over the
Central Plains, which becomes focused on a low level boundary
extending across eastern NE into western MO, mainly after 01/00z.

Strong 700/500 mb moisture transport across the Central Plans
becomes focused on the front, supporting an increasing model
signal for the development of a small MCS (an idea which is shown
by 00z GFS simulated IR imagery) over southeast NE during this
time window. The MCS then tracks across northeast KS into far
western MO, along the axis of best instability on the nose of the
low level jet late in the period. There is a window of opportunity
for training along the south and west side of the storms before
the system become more outflow driven after the development of a
cold pool, which is expected to occur toward 01/12z.

During this time, training in the deep moisture plume could
support hourly rainfall amounts between 1.50/2.00 inches,
especially where training occurs (an idea depicted by the 00z NAM
CONUS Nest). Three hour flash flood guidance values are generally
above 2.50 inches (though a portion of southeast NE has seen 300+
percent of normal rainfall over the past 7/14 days), so training
would be needed for the initiation of flash flooding. Much of the
00z guidance (including much of the high resolution guidance)
showed a fairly narrow axis of highest rainfall, with the 00z NAM
showing amounts over 4.00 inches, so there could be some
longitudinal spread here. However, the ingredients are expected to
be in place, as well as good model agreement developing an MCS, so
a Slight Risk was placed over northeast KS and western MO for Day
2 (which was collaborated with WFOs OAX/TOP/EAX/SGF).


...Northeast...
A long wave trough crossing the Northeast during Day 2 begins to
take on a negative tilt over Quebec, which allows a cold front
crossing the Northeast to slow as it approaches the coast of
Northern New England. Ahead of the front, a low level south
southwest flow continues to feed 1.50/1.75 inch precipitable water
air from the Mid Atlantic into Northern and Central New England,
where model sounding showed 1000/2000 J/KG of MLCAPE in about the
same location. The combination of moisture and instability is
expected to support the development of at least scattered storms
along and west of the Interstate 95 corridor in the afternoon and
evening hours.

Synoptic scale ascent associated with the mid level trough could
aid in organizing the storms into clusters, and the shear could be
sufficient to support convection maintenance. In the deep moisture
plume, storms could produce hourly rainfall rates near 1.50
inches, as warm cloud heights are fairly high (based on 00z GFS
model soundings), which could result in efficient rainfall makers.
There is some regional/global support for local 2.00/3.00 inch
rainfall amounts (especially where storms training in the low to
mid level southwest flow).

The most recent guidance has shifted the axis of highest rainfall
further south across portions of eastern PA into northern NJ and
the NYC Metro area, where the lowest flash flood guidance is
located (as low as 1.50 inches in three hours). Based on the
above, a Marginal Risk was stretched from northern NJ across much
of interior New England for Day 2.


Hayes

Day 3
Valid 12Z Thu Aug 01 2019 - 12Z Fri Aug 02 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE ROCKIES
ACROSS THE CENTRAL PLAINS INTO THE MID MISSISSIPPI VALLEY...AS
WELL AS A PORTION OF THE MID ATLANTIC...

...Rockies into the Central Plains and the Mid Mississippi
Valley...
Convection following an axis of instability and deepening moisture
on a backing low level flow could feed clusters or a small MCS
tracking from NE into western MO, mainly after 02/00z. A ribbon of
1000/1500 J/KG of MLCAPE (which become more elevated with time) on
the edge of the mid level capping extends from SD across NE into
western MO, which peaks between 02/00z and 02/06z. The instability
should be sufficient for the development of storms on the axis,
which track southeast with the mid level flow.

There is some model support for short wave energy to track over
the ridge position from the western Dakotas into the Mid MS Valley
during the same time frame, which would allow the convective
elements to become more organized with time. The low level flow
backs TO the south and southeast ahead of the short wave (and the
front boundary or surface trough across central KS), feeding 2.00
inch precipitable water air into the developing convection.
Several regional/global models indicate the potential for an MCS
to form on this axis, tracking across eastern KS/western MO before
02/12z.

The airmass could support hourly rainfall amounts near 1.50
inches, especially on the southwest side of the MCS, where
training is possible. Several regional/global models indicated the
potential for 2.00/3.00 inch rainfall amounts (with the 00z NAM
the most bullish with the rainfall), though there is some spread
on where this occurs (which could be tied to the positioning of
the mid level capping, which in turn influences the path of the
convection). Three hour flash flood guidance is generally above
2.50 inches, but an MCS during late Day 2 could pre conditions the
soil here. At this point, with some differences concerning the
placement of the highest rainfall amounts, a Marginal Risk was
placed over
SD/NE into KS/IA/MO. If there is better agreement with the track
of the storms, a Slight Risk could be needed here in later
forecasts.


...Mid Atlantic...
Deep moisture and instability pool along a frontal boundary across
the Mid Atlantic states during Day 3. As the best mid level
support for the frontal shears out to the north across New
England, the front is expected to slow, and could become quasi
stationary across portions of VA/NC after 01/18z. The moisture and
instability should be sufficient to support at least scattered
storms along and south of the front, where MLCAPE values are
expected to range between 1500/2500 J/KG.

Increasing warm cloud depths could allow storms to be fairly
efficient rainfall makers, and given the
2.00+ inch precipitable water axis along the front should support
storms capable of producing hourly rainfall rates near 2.00
inches, especially over far southern Va into much of central NC.
Storm motions could be less than 10 knots, especially across
western VA/western NC, which may foster an environment for short
term training and cell mergers. A weak long wave trough extending
from the Ohio Valley to the central Gulf Coast would result in
steepening lapse rates, but overall, there may not be sufficient
shear for cell maintenance.

There is some regional/global model support for local 3.00 inch
rainfall amounts, particularly where training occurs. Three hour
flash flood guidance values are generally between 2.50/3.50
inches, and training in the higher moisture airmass could allow
some locations to approach these values. Based on this, a Marginal
Risk was placed over much of southern VA and NC for Day 2.


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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