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FOUS30 KWBC 300054
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
853 PM EDT Mon Jul 29 2019

Day 1
Valid 01Z Tue Jul 30 2019 - 12Z Tue Jul 30 2019

...MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MULTIPLE AREAS
TODAY...

...Great Lakes to the Gulf Coast...
A mid-latitde trough and an associated surface cold front will
continue to be the focus of showers and thunderstorms during the
remainder of the evening and overnight hours.  Convection out
ahead of the front should be progressive enough to prevent
problems, there will continue to be cases of briefly intense
rainfall leading to problems with run-off or instances of flooding
because of cell training or repeat convection.  The storms will be
in an environment where the models have been showing CAPE values
of 1500 to 3000 J/kg in an area where the Precipitable Water
values ranged from 1.5 to 1.8 inches...with greatest values of
each field in the Tennessee Valley southward towards the Gulf
Coast (a 2 inch Precipitable Water value was observed at 00Z by
the KLCH sounding).  The HREF 40 km neighborhood probabilities
showed the highest values confined to the same area through 08Z
before diminishing.  So opted to maintain the Marginal Risk area
for the short-term.  With lower values of instability and
available moisture, removed the Marginal Risk areas from up around
the Great Lakes for the remainder of the evening.

...Western High Plains...
Early evening radar imagery showed cells developing in and near
both Marginal Risk areas...in Colorado/Kansas and between the
Black Hills of South Dakota and the Pine Ridge of South Dakota and
Nebraska...towards peak daytime heating with a low-level upslope
flow component.  The Marginal Risk south of the Black Hills will
still be in play later this evening as an impulse approaches from
these west and takes advantage of CAPE and deeper layered moisture
(Precipitable Water values on the order of 1.25 inches).  Farther
south, there may still be a short period of time in which there
could be runoff issues in areas with lower FFG values.

...Portions of the Southwest...
A modest degree of 850-700 mb moisture flux, with PWs not overly
anomalous for late July underneath the upper ridge will pose a
marginal or isolated risk for excessive rainfall over southeast AZ
and southwest NM, particularly during the peak heating hours
through this evening. Weak deep-layer flow/low 0-6km bulk shear
values and thus pulse-type storm mode will inhibit any persistence
of the more intense rainfall rates. Any burn-scars in the area
will be most prone to flash flooding concerns...although there
could be flash flooding in any normally-dry wash or stream given
enough rainfall intensity.

Bann



Day 2
Valid 12Z Tue Jul 30 2019 - 12Z Wed Jul 31 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER THE
SOUTHERN ROCKIES INTO THE SOUTHWEST...PARTS OF THE NORTHEAST...AND
THE CENTRAL GULF COAST...

...Southwest...
Mid-level ridge centered over New Mexico will gradually shift
westward continuing to drive monsoon moisture into the Southwest.
PWATs as high 2+" will pool near the Mexico/AZ border, more than 3
standard deviations above the climo mean, before advecting
northward in response to persisting low/mid level southerly flow
and a shortwave rotating around the periphery of the ridge. In
addition, a jet streak is progged to strength across the northern
Great Basin, placing the area within the favorable RRQ for ascent
coincident with the best MUCape which may approach 2000 J/kg. The
best moist advection may lag the strongest instability, but still
expect considerable coverage of convection from Arizona northeast
into SW Colorado.

Although coverage of thunderstorms may be scattered to widespread,
and HREF/GEFS probabilities for 1"/hr rain rates are moderate,
storm motions of 15 kts, utilizing the 0-6km mean wind as a proxy,
may inhibit anything more than localized flash flooding despite
low probabilities for 3" of rainfall focused in southern AZ. While
these rain rates and this amount of rain would likely exceed FFG,
after coordination with TWC/PSR have opted to maintain the MRGL
risk due to concern about storm motions and cloud cover
potentially limiting instability. Should a model signal become
more robust for heavy rainfall and flash flooding, a SLGT risk may
be needed with the day 1 update.


...Northeast...
Moisture and instability streaming northeast along and ahead of a
slowing cold front could feed convection producing heavy to
locally excessive rainfall over portions of the Northeast, mainly
across eastern OH and WV into western NY state during Day 2. A
ribbon of MUCape potentially exceeding 1500 J/kg could support
scattered to broken areas of convection, which in an environment
characterized by PWATs of 1-1.5 standard deviations above the mean
could support heavy rainfall. This will be most likely as
propagation vectors become aligned parallel to the boundary, and
ascent is aided by modest upper diffluence. Training of these
heavier cells could produce flash flooding. This will be most
likely in regions of the lowest FFG, which is as low as
1-1.5"/1hr, or 1.5-2"/3hr from WV through parts of upstate NY.
Although flash flooding is not expected to be widespread due to
cloud layer wind as high as 25kts, training of cells atop these
pre-conditioned soils has prompted a continuation of the MRGL risk.


...Central Gulf Coast...
High-res guidance indicates that clusters of thunderstorms, or an
MCS, on the tail end of a slowly sinking cold front will create
heavy rainfall across portions of LA and into the upper Texas
coast. Weak northerly flow will drive these clusters, or MCS,
southward towards the coast as a weak vort swings southeastward
around the expansive mid-level ridge to the east. This convection
will occur in an environment ripe for heavy rainfall due to PWATs
over 2 inches and MUCape 2000-3000 J/kg. Storm motions may be slow
before better organization leads to an outflow dominated system
which would likely drop faster to the S/SW into the 850mb moist
inflow. While there remains considerable uncertainty into the
positioning and strength of these features, there is enough
support in a favorable environment to introduce a MRGL risk as
rain rates may approach 2"/hr at times, and there are low
probabilities among the various ensembles for 3" of rainfall.
Despite the high FFG, excessive rain rates, especially in any
urban areas, could produce flash flooding and the MRGL risk was
coordinated with the local WFOs.


Weiss

Day 3
Valid 12Z Wed Jul 31 2019 - 12Z Thu Aug 01 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE FOUR
CORNERS AREA...THE CENTRAL PLAINS AND A PORTION OF THE NORTHEAST...

...Southwest and the Southern Rockies...
19Z Update: Mid-level ridge responsible for lifting monsoonal
moisture into the Southwest will drift slowly westward Wednesday
into Thursday. The flow on the backside of this ridge will persist
the moist advection into the region, with PWATs remaining at or
above 2 inches across Southern AZ, the source region for moisture
across most of the MRGL risk area. Mid-level RH improves across
the area through the day, with the highest RH coincident with peak
heating and the best instability. There may be a convectively
enhanced shortwave/MCV riding around the ridge, but otherwise the
large scale ascent wanes Wednesday as the RRQ of the upper jet
streak shifts away to the northeast. Hourly rain rates near 1"/hr
and slowing storm motions in the large scale weakly forced
environment could support flash flooding, but at this time it
appears to localized due to uncertainty in forcing to raise the
risk above MRGL despite the favorable thermodynamics.

Previous Discussion:
Monsoonal flow affects a larger area across the Four Corners area
during Day 3, as the mid level ridging remains more or less in
place. The low to mid level south to southeast 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), and surface dew
points are expected to rise to near 70 degrees.

Convection could be ongoing across portions of southwest AZ at the
start of Day 3, as there is some indication of short wave energy
(or perhaps an MCV?) tracking from Mexico into this area early in
the period. Outside of this, forcing could be lacking across
AZ/southeast CA/far southern NV 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. Since it it not clear
from this distance where the main threat will materialize, a
Marginal Risk was placed here for Day 3. Ingredients for flash
flooding will be in place, with a focus being the missing element.
If forcing becomes more evident, a Slight Risk could be needed in
later forecasts.

As the deeper moisture rounds the ridge across UT into CO, the
threat for flash flooding is expected to increase. Like AZ, the
flash flood threat is tied to some kind of focus. There is some
hint that weak short wave energy rides around the ridge with the
deeper moisture, but from this distance, it is not clear just how
the forcing might unfold. Instability and deep moisture support at
least scattered convection across the higher terrain (including
the Wasatch and Uinta Mountains in UT, and the San Juan Mountains
in southwest CO).

Hourly rainfall rates could approach an inch in the rich moisture
plume, and the slow cell motions could support cell mergers or
short term training. A Marginal Risk was placed over these areas
for Day 3.


...Central Plains/Mid Mississippi Valley...
19Z Update: An MCS will likely dig southward along the periphery
of the monsoon ridge to the west, into a LLJ moist inflow reaching
30-40 kts. The trend with the guidance this afternoon has been for
a subtly more wet solution, as well as a bit further east track of
the MCS. With exceedingly high PWATs over 2 inches, and antecedent
conditions supportive of flash flooding due to 14-day rainfall
more than 200% of normal, flash flooding is possible. Storm
motions which may be quick to the south should limit a more
significant flash flooding threat, so the risk was left at MRGL.

Previous Discussion:
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/MS during the
second half of Day 3. 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.

There was a multi model signal for strong 700/500 mb moisture
transport to become focused here between 01/00z and 01/06z, which
could support the development of a convective cluster or small MCS
(an idea which is supported by 00z GFS simulated IR imagery) over
southeast NE in this time frame. The cluster or MCS then tracks
across northeast KS into far western MO by 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.

During this time, training in the deep airmass could support
hourly rainfall amounts between 1.50/2.00 inches, especially where
training occurs (an idea supported by the 00z NAM). 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. At this point, there seems to be
a flash flood threat, but there is still some spread on where this
might occur. Based on the above, a Marginal Risk was placed over
this area for Day 3.


...Northeast...
19Z Update: An upper trough driving a cold front across the
northeast will interact with favorable thermodynamics to produce
heavy rainfall across the northeast. Pre-frontal SW flow will
advect anomalous moisture ahead of the front with PWATs reaching
1.5 standard deviations above the climatological mean while warm
sector instability will be favorable for convection. There is also
likely to be large scale ascent due to low-level convergence both
along the front and likely within a pre-frontal trough, as well as
broad mid and upper diffluence in response to the large scale
trough axis and upper jet moving eastward. Training of storms due
to boundary parallel flow could support several rounds of heavy
rainfall, which could lead to flash flooding where FFG is as low
as 1"/1hr or 1.5"/3hrs, focused in the PHL-NYC corridor.

Previous Discussion:
A long wave trough crossing the Northeast during Day 3 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 more than just pulse convection. 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), particularly over Northern New
England.

However, lower flash flood guidance over portions of eastern PA
into northern NJ could result in these areas being vulnerable to a
low end flash flood threat as well. Based on the above, a Marginal
Risk was stretched from northern NJ across much of interior New
England for Day 3.

Hayes/Weiss


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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