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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2129Z Jul 21, 2018)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
529 PM EDT Sat Jul 21 2018

Day 1
Valid 2059Z Sat Jul 21 2018 - 12Z Sun Jul 22 2018

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ASSOCIATED WITH A
COASTAL LOW AFFECTING THE MID ATLANTIC...

...Coastal Mid Atlantic...
21z Update:
An area of low pressure is well defined on visible satellite near
Norfolk, VA. A broad area of steady rainfall extends northward
ahead of this low. The heaviest rains are occurring just to the
north and northwest of the low within a strengthening deformation
axis. The low is forecast to turn more northerly and eventually
northwesterly with time as it moves into eastern PA overnight. The
heaviest rainfall will likely continue to be along and just
northwest of the track in the aforementioned deformation axis.
Recent RAP runs show an area of 500-1000 J/KG of MUCAPE developing
within the developing dry slot of the system. On the nose of this
instability axis, would anticipate we see some locally enhanced
rates develop into the overnight hours. While the forecast amounts
may be a bit overdone, the general idea shown by recent HRRR runs,
the 12z ARW and 12z ECMWF seems reasonable...with the heaviest
rains occurring where this weak instability is advected into the
slow moving deformation axis. This brings the threat of locally
intense rates into portions of northeast VA into MD...with much of
this area already having seen heavy rains through the day. Thus
would think that flooding concerns will increase across this area
into the evening hours...with at least a localized flash flood
threat as well. The Slight Risk was shifted a bit west across
northern VA to cover this threat.

Total rainfall magnitudes should not be as high east of the low
from eastern PA into NJ. These areas have generally seen lighter
rainfall through the daytime hours. While it does appear that
convective elements with higher rates will become a bit more
common overnight...these cells should be quick moving off to the
north (unlike further southwest within the deformation axis). Thus
would anticipate the flash flood risk here to be isolated and
mainly be confined to more susceptible urban locations. Thus opted
to trim back the northeastward extent of the Slight Risk and just
carry a Marginal.

Chenard

...Lower Great Lakes---mid to Upper OH Valley---Central to
Southern Appalachians into the Southeast...
Slight Risk of excessive rainfall is maintained over portions of
the Ohio Valley where FFG has been severely compromised by heavy
rain that fell Friday. Although less organized today, there should
be scattered, relatively slow moving showers and thunderstorms
today beneath the cold core upper low.  Marginal Risk extends up
the inverted trough axis into Lower Michigan, and extends down
into Alabama / Georgia. The environment farther south is marked by
greater instability ahead of a cold front that is secondary to the
coastal front along the Atlantic. Northwest flow would generally
yield progressive convection this evening, but enhanced WNW inflow
 at 850 mb could support brief local episodes of training to raise
event total rainfall enough to support Marginal Risk probabilities.

...Southwest...
Another day of widely scattered convection likely Saturday
afternoon into Saturday evening across the Southwest. There
appears to be very little net change in the morning RAOBs from
Friday to Saturday, with the exception of steeper mid level lapse
rates at Desert Rock / Vegas. Would expect a similar day to what
occurred on Friday, with the majority of diurnal convection
sticking to higher terrain. There will be subtle height rises
today, however, so the overall character of the convection may be
even a little less robust / organized. A potential organizing
influence, though, could be an MCV associated with the small
outflow dominated MCS crossing the AZ desert areas this morning.
As the MCV rounds up the western side of the ridge into the
AZ/NV/UT portion of the Colorado River basin there may be some
enhancement to convective coverage / organization this afternoon.
Per usual, short term rain rates could pose a flash flood threat,
although there is no particularly well defined precipitation
signal in the models.

...High Plains...
Noting the Flash Flood Watch in the Black Hills, and the generally
wet summer that has occurred in the northern High Plains up into
eastern MT and western ND, we introduced a Marginal Risk of
excessive rainfall - and used the SPC Slight Risk of severe as a
guide for placement. The hi-res model QPF signal from the 12Z
cycle does include some locally hefty storm scale amounts here, on
the order of 2-plus inches. Bunkers supercell motion for
right-movers is only 10 knots per the NAM/GFS this
afternoon/evening.

Burke/Oravec


Day 2
Valid 12Z Sun Jul 22 2018 - 12Z Mon Jul 23 2018

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE NORTHEAST...

...Ohio Valley/Mid-Atlantic and Northeast...
Significant moisture will be in place over the Mid-Atlantic and
New England at the beginning of the Day 2 period (12Z Sunday) as
the remnants of a compact mid level trough/low and associated
surface reflection lift up across southeast NY and adjacent areas
of southeast New England. Southerly 850 mb flow of 30 to 40 kts
interacting with a frontal zone, coupled with orographic
forcing/upslope flow and a tropical airmass (via precipitable
water values of over 2 inches) will support areas of organized and
locally organized heavy rainfall this period.The heaviest amounts
are expected to be across the southeast facing slopes of the
Poconos, Catskills, Berkshires and also possibly the Green and
White mountains of VT/NH. Locally as much as 2 to 3+ inches of
rain may occur given the robust upslope flow and highly efficient
rainfall processes that should be involved for enhanced rainfall
rates. Consequently, a Slight Risk of excessive rainfall has been
expanded to include these areas. Elsewhere across the OH Valley
and adjacent areas of the Mid-Atlantic, there will be scattered
areas of heavy showers and thunderstorms which will be driven
mainly by the proximity of the deep layer closed low over the OH
Valley, as well as local orographics and diurnal heating, but the
convection should tend to be more disorganized across these areas
which will mitigate the excessive rainfall threat a bit. A
Marginal Risk remains in place to cover these areas.

...Central/Eastern Colorado...
Maintained the Marginal Risk previously issued over portions of CO
where the operational models were showing a signal that mid and
upper level moisture will interact with a front approaching from
the northwest/north during the day on Sunday. The operational
models were a bit more aggressive with respect to areal coverage
of rainfall here, although there has been a shift more out onto
the immediate High Plains to the east of the Front Range. Not
confident enough that there will be sufficient instability ahead
of the front to support organized widespread rainfall rates
capable of producing organized excessive rainfall threat, but a
non-zero flash flood threat exists nevertheless. Once again, the
greatest potential for excessive rainfall will be located over
recent burn-scars.

...Central Sierra-Nevada into central Nevada...
The global models show a sufficient model signal for slow-moving
convection, and especially over the high terrain of the
Sierra-Nevada that a Marginal Risk area was introduced here.
Widespread heavy rainfall is not anticipated, but the feeling is
that with relatively anomalous moisture (precipitable water values
of 2 to 3 standard deviations above normal), there will likely be
some concerns for isolated flash flooding and especially near any
burn scar areas.

...Big Bend of Florida...
The models show highly confluent and very moist flow focusing into
the Gulf Coast of FL, and especially near the Big Bend area
through Sunday and into Sunday night. The best instability will
likely remain offshore, but the instability axis should nose
inland a bit which coupled with height falls/forcing associated
with the digging OH/TN Valley closed low should favor some
concentrated heavy rainfall. FFG values are high, but given a
signal for locally 3 to 5+ inches of rain, some flooding concerns
will exist, and thus a Marginal Risk area has been introduced here.

Orrison


Day 3
Valid 12Z Mon Jul 23 2018 - 12Z Tue Jul 24 2018

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL APPALACHIANS...MID-ATLANTIC AND THE CENTRAL ROCKIES...

...Eastern U.S...
The mid and upper-trough over the OH/TN Valley region on Sunday
will be digging own across the interior of the Southeast on Monday
and becoming nearly stationary by Tuesday. As this occurs, the
models show a rather notable westward expansion of the subtropical
ridge offshore the East Coast and into especially areas of the
Northeast. This will allow for some retrogressive flow across the
Mid-Atlantic and New England that will afford a westward expansion
of the deep layer tropical moisture plume and offshore frontal
zone which will be losing its identity. The guidance favors a
strengthening of very moist low/mid level south to southeast flow
up across the Mid-Atlantic coastal plain, eastern slopes of the
Central Appalachians including the Blue Ridge and adjacent
Piedmont areas, as well as portions of southeast NY and and
southern New England. The thinking is that with a deep tropical
column, diurnal heating/instability, orographic ascent and
interaction with smaller scale mesoscale boundaries, there will
likely be a rather widespread area of heavy showers and
thunderstorms. Rainfall rates are likely to be impressively high,
2 to 3 inches/hr in some cases, given very efficient rainfall
processes. The well-established deep layer flow from the
south-southeast will favor some convective bands that could easily
train over the same area also become focused over areas of high
terrain near the Blue Ridge. Given the heavy rainfall threat to
the Mid-Atlantic and portions of New England through Sunday, the
soil conditions are likely to be compromised to the level of being
nearly saturated by the Monday and Tuesday timeframe. This will
lead to lower FFG values, and with the additional heavy rainfall
threat, flash flooding is expected to become a more notable
concern. For this period, some of the heaviest rains will likely
set up across areas of northwest NC and stretching north up across
the Blue Ridge from southwest VA northeast up across the WV/MD
panhandles and eastern PA. Portions of northwest NJ and southeast
NY will need to be closely monitored as well. Additional rainfall
amounts of 3 to 4+ inches will be possible. As a result a Slight
Risk has been introduced across these areas. Meanwhile, the
guidance is also suggesting areas of southeast NC may see some
focused areas of very heavy convective rainfall with some
potential for south to north training of showers and
thunderstorms. Relatively enhanced low level convergence is
expected here and they will be in the same deep layer tropical
moisture plume. Locally several inches of rain will be possible
here as well, and thus a Slight Risk has been added here as well.

...Central Rockies...
A plume of deep moisture, characterized by precipitable water
values of nearly 1.5 inches, will extend to the northwest of the
Gulf of Mexico towards the eastern High Plains of CO/NM, including
the Front Range. The moisture will be focusing along a frontal
zone draping down across the region and becoming anchored up over
the adjacent high terrain. The precipitable water values will be
as much as 2 standard deviations above normal and this will lead
to more efficient convective cells for heavier rainfall rates.
Convergence near the front and modest shortwave energy rounding
the Southwest U.S. ridge axis will work in tandem with the
moisture, diurnal heating and orographics for rather focused areas
of heavy showers and thunderstorms. The latest model consensus
appears a bit underdone given the moisture anomalies and localized
forcing, and thus a Slight Risk has been introduced to account for
flash flooding concerns. As much as 1 to 2+ inches of rain can be
expected this period. The main threat area will tend to be down
the CO Front Range and across the Sangre De Cristo range of
northern NM. Will need to also monitor the San Juan mountains of
southwest CO. The more sensitive areas will again include any burn
scar areas.

...Sierra-Nevada into central/southern Nevada...
Monsoonal moisture will again tend to be focused to some degree up
across the higher terrain of the central Sierra-Nevada and
adjacent areas of central/southern NV. Slow cell motions and heavy
rainfall rates will favor some additional concerns for isolated
flash flooding. The local burn scar areas will again be a more
sensitive area to monitor. A Marginal Risk area has been
introduced to monitor this region.

Orrison



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