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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0054Z Sep 19, 2020)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
854 PM EDT Fri Sep 18 2020

Day 1
Valid 01Z Sat Sep 19 2020 - 12Z Sat Sep 19 2020

...A MARGINAL RISK OF EXCESSIVE RAINFALL REMAINS FOR PORTIONS OF
WESTERN OREGON AND SOUTHWEST WASHINGTON...

...Western Oregon and Southwest Washington from the Klamath
Mountains into the Eastern Willamette Valley, Cascades, and
Western Columbia River Gorge...
Deep low pressure will linger over northern OR tonight as the
attendant trough swings into the northern Great Basin Friday. 
Difluent flow east of this system and daytime heating allowed for
thunderstorms in and near the terrain of the Pacific Northwest
this afternoon.  Isolated hourly rainfall rates of 0.5 to 1.0 inch
within the strongest cells remain possible for the next several
hours before waning tonight per the latest high-res CAM guidance. 
These short term rainfall rates, where realized, would likely lead
to localized flash flooding and debris flows in the vicinity of
recently burned areas around the Archie Creek, Holiday Farm,
Beachie Creek, Lionshead, and Riverside fires in OR, as well as
the Big Hollow fire in WA.

Roth/Hurley/Jackson


Day 2
Valid 12Z Sat Sep 19 2020 - 12Z Sun Sep 20 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE
NORTHERN ROCKIES...AS WELL AS PORTIONS OF THE EAST COAST OF
FLORIDA...

...2130 UTC Excessive Rainfall Discussion...

...Northern Rockies...
How the models handled the large scale synoptic pattern changed
little in the 18/12Z cycle...both in terms of mass fields and
precipitation.  As a result, no changes were needed to the
previously-issued Marginal Risk area.

...East Coast of Florida...
A few high-resolution guidance members, led by the NAM NEST, show
a blossoming of 1 to 3 inch isohyets clustered along the east
coast of the Florida peninsula early in the period.  Still think 2
inch per hour rainfall rate are attainable and that isolated
rainfall amounts of 2 to 4 inches are possible where cells train
or if there are repeated rounds of convection impacting the same
location.  Only change was to expand the boundary of the Marginal
Risk area a bit.

Bann



...0830 UTC Excessive Rainfall Outlook Discussion...

...Northern Rockies...
Deep moisture from the eastern Pacific and marginal instability in
place will fuel low topped convection capable of producing heavy
to locally excessive rainfall across portions of the Northern
Rockies during Day 2. Ahead of a negatively tilted tracking from
the Pacific Northwest into the Northern Rockies, the mid level
flow transports 0.75/1.00 inch precipitable water air (which is
between two and three standard deviations above the mean) across
ID/MT/WY ahead of a frontal boundary snaking along the terrain.
Model soundings showed 500/1000 J/KG of MUCAPE in place, and the
combination of moisture and instability should support at least
scattered low topped storms between 19/20z and 21/03z from west to
east.

Difluence ahead of the mid level trough could allow storms to
become organized for a time from eastern ID across southwest MT
into northern WY, though it is unclear just how widespread the
convection becomes. Deep moisture in the column could certainly
support storms with hourly rainfall rates of 0.50 inches (which is
shown in most of the 00z high resolution guidance) in these
locations during the convective window mentioned above. As the mid
level trough approaches, storm motions could drop below 10 knots,
especially over MT/WY before 20/00z. Any short term training or
cell mergers could allow for local rainfall amounts approaching
1.50 inches.

Three hour flash flood guidance values are generally between
1.00/1.50 inches, and given the moisture in the column, these
amounts are attainable. Based on that, a Marginal Risk was placed
over a larger portion of the Northern Rockies for Day 2.


...East coast of Florida...
Deep moisture and instability along and behind a front crossing
the FL Peninsula could support convection in the lower level
northeast flow that produces heavy to locally excessive rainfall
across portions of coastal northeast and central FL during Day 2.
Strong surface high pressure over the Northeast will push a
surface front across the northern and central portion of the
peninsula. Along and just behind the front, a low level east to
northeast flow funnels 2.00/2.25 inch precipitable water air
across the region, with the low level convergent flow providing a
focus for development of convection in the 1000/2000 J/KG of
SBCAPE in the column (per model soundings).

Generally between 19/15z and 20/03z, waves of convection in the
low level flow are expected to come ashore across northeast and
central FL. Given the depth of the moisture in the column, hourly
rainfall rates could exceed 2.00 inches, and much of the high
resolution guidance (led by the 00z NAM CONUS Nest and the 00z WRF
ARW), shows this potential in the abovementioned time frame. Local
3.00/4.00 inch rainfall amounts are possible where short term
training occurs. Since this is a flow regime that can support
heavy rainfall along the FL coast, a Marginal Risk was placed here
for Day 2 (which was collaborated with WFOs JAX/MLB)

Hayes

Day 3
Valid 12Z Sun Sep 20 2020 - 12Z Mon Sep 21 2020

...THERE IS MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE
TEXAS COAST...AS WELL AS PORTIONS OF THE EAST COAST OF FLORIDA...

...2130 UTC Excessive Rainfall Outlook Discussion...
Heavy to excessive rainfall should be approaching the coast as
Tropical Storm Beta meanders over the western Gulf of Mexico.  The
spread of model solutions remains large...but the ECMWF was west
of NHC guidance and the GFS was closer to the track.  There was a
subtle increase in forward speed from earlier NHC guidance, but
the heaviest rainfall should still remain off shore through
21/12Z.  Even so, rainfall amounts of 2 to 4 inches are possible
immediately along the South Texas coast closest to the circulation
of the storm's core...along with several inches farther up the
coast in a region of on-shore flow of air that had a long
trajectory over the Gulf.  At this point, will maintain the
Marginal Risk area with the expectation that heavier rainfall
rates/amounts will be leading to an increasing risk of excessive
rainfall beyond the end of the Day 3 outlook period. Refer to the
National Hurricane Center for latest information.

Bann



...0830 UTC Excessive Rainfall Outlook Discussion...

...Texas coast...
Deep moisture in the easterly flow north of Tropical Depression
Twenty Two spreads over much of the TX coast during Day 3. Where
the deep moisture and instability overlap, storms could produce
heavy to locally excessive rainfall, even a distance north of the
low level center. At this point, there is a fair amount of spread
concerning how far inland convection gets, and this has a large
impact on not only the QPF but also the flash flood potential.

Tropical Depression Twenty Two meanders toward the far south TX
coast during the end of the period. There is a fair amount of
spread concerning how close the system gets to the coast before
21/12z. Following the most recent NHC track information, the
western edge of the system (and the convection in the outer bands)
are expected to brush the region. Given the depth of the moisture
in the column (with precipitable water values near 2.50 inches),
hourly rainfall rates in the banding could exceed 2.00 inches.

The main forecast issue is how close to the system gets to the
coast by the end of Day 3. Should it remain far enough offshore,
the instability gradient along coast could prevent the banding on
the western side of the storm from getting too far inland. This
can happen with slow moving storms, and this system is expected to
be moving slowly. This could prevent the storms from getting much
past the immediate coast, limiting the areal extent of flash
flooding. If the storm can get closer to the coast, the
instability become less of an issue. Due primarily to the
uncertainty of the track of the depression, a Marginal Risk was
placed here for Day 3 (which was collaborated with WFOS BRO/CRP).
Should the depression get closer than the NHC track, a Slight Risk
could be needed here in later forecasts.

For the Middle and possibly Upper TX coast, the deep easterly flow
transports 2.25+ inch precipitable water across the region.
However, as is often the case in tropical air masses, it is not
clear just how far inland the instability extends. Much of the
regional/global guidance across this area shows the instability
gradient reaching from the western Gulf of Mexico to along the
coast, and not much further inland. If this occurs, it would limit
the extent of coverage of convection moving inland, and greatly
affect the QPF amounts here. For now, a Marginal Risk was placed
here because of the depth of the moisture in the column, but
forecast confidence is below average because of the uncertainty
concerning the placement of the instability.


...East coast of Florida...
Deep moisture and instability in a low level east northeast flow
should support low topped convection that produces to heavy to
locally excessive rainfall across portions of the FL east coast
during Day 3. Like Day 3, the combination of 2.00/2.25 inch
precipitable water air and 1000/2000 J/KG of SBCAPE is expected to
result in clusters of storms producing hourly rainfall rates of
2.00+ inches as they come ashore, especially where training
occurs.

The storms are expected where the low level convergent flow
provides focus, and the activity is expected south of where storms
occur during Day 2. Though not explicitly depicted in the
regional/global guidance, local 3.00/4.00 inch rainfall amounts
are possible, particularly across the central portion of the east
coast of FL, where the best convergence is expected. Once again,
this is a flow that supports heavy rainfall across the FL east
coast, so a Marginal Risk was placed here, despite the modest WPC
and model QPF.

Hayes


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