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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0835Z Sep 15, 2019)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
435 AM EDT Sun Sep 15 2019

Day 1
Valid 12Z Sun Sep 15 2019 - 12Z Mon Sep 16 2019

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE SOUTHWEST...

...Southwest...
Potent shortwave/MCV lifting through the Southwest around the
western periphery of a mid-level ridge will provide robust ascent
for convection on Sunday. This feature is progged to move out of
Mexico shortly and then lift steadily north to north-northeast
thereafter. Precipitable water values (PWs) across the Southwest
and southern Rockies should rise to ~1.25", supporting the heavy
rainfall threat on Sunday within terrain.  The mean 850-400 hPa
wind is forecast to be generally 5-10 kts, significantly less than
the expected 700 hPa wind (up to 30 kts), so storms should be
efficient from a rainfall perspective, supporting hourly totals up
to 2"/hr should enough instability build.  There is some concern
near and ahead of this feature that cloud cover may hinder
convective development due to later initiation, though areas just
in its wake should see instability build.  This is echoed by SREF
mean ML CAPE remaining 500-1000 J/kg at most. However, the
anomalous column moisture should still permit excessive rain rates
once thunderstorms develop, and support the ongoing small SLGT
risk in southeast AZ and adjacent southwest NM.  A MRGL risk
surrounds this slight risk, and extends northward into southern CO
where instability is more lacking and 14-day rainfall departures
are less conducive for flash flooding. 


...Northwest...
A modest Atmospheric River will advect onshore ahead of a
deepening trough and surface cold front early Sunday. The
strongest overlap of forcing through upper diffluence within the
right rear quadrant of an increasingly poleward oriented jet
streak and invading upper trough/cooling aloft to produce ascent
early on, which will exist concurrently with the highest PWs of
1-1.25", more than 2 standard deviations above the climatological
mean.  The combination of this moisture and expected instability,
with MU CAPE roughly near 250 J/kg along the coast, is sufficient
to support  hourly rain totals of 0.5"+/hr, especially into the
terrain of the Coastal Ranges of Oregon. The time for such heavy
rainfall should be short, so the risk area is capped at marginal,
but isolated flash flooding will be possible as training develops
in response to 0-6km mean wind becoming progressively more
oriented parallel to the approaching cold front.


...North-central Illinois & vicinity...
Convection currently gathering near the IL/WI/IA border is
expected to shift southeast into IL this morning with heavy
rainfall.  The low level jet is forecast to slowly weaken through
the morning and shift ahead of a cold front dropping south across
the Upper MS Valley.  With upstream MU CAPE remaining 2000+ J/kg,
hourly rain totals to 1.5" and local amounts to 3" possible from
north-central IL into northwest IN through roughly 16z. 
 
Three hour flash flood guidance values are roughly 2" in three
hours for much of the area, with locations on the northern fringe
most susceptible due to recent heavy rains.  The rainfall amounts
forecast would be enough to pose a low end flash flood during the
morning hours. A marginal risk remains here to cover the threat. 

Roth


Day 2
Valid 12Z Mon Sep 16 2019 - 12Z Tue Sep 17 2019

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE FOUR CORNERS AND THE TEXAS COAST...

...Four Corners...
A potent shortwave will be lifting east-northeast early on Monday
around the periphery of a mid-level ridge to the east. Through the
first half of day 2, this feature will become absorbed by the
westerlies, causing the best lift to wane as some NVA develops in
its wake. PWATs will remain quite high across the region, 0-75" -
1.25", which is nearly 2 standard deviations above the
climatological mean. While this moisture could support heavy
rainfall across much of the Southwest, instability will be modest,
<500 J/kg, and forcing will be mostly missing during the time of
peak heating. Some enhanced ascent will approach late Monday as a
trough digs into the Great Basin, but this will likely occur too
late to overlap the best instability, and the enhanced belt of
westerlies ahead of the height falls suggests any convection that
develops during the evening will move quickly in AZ/UT. For this
reason have trimmed the MRGL to just the terrain of NM/CO where at
least a brief period of instability/forcing overlaps, and
convection can persist with heavy rainfall to produce an isolated
flash flooding threat.

..Texas Coast...
An area of disturbed weather in the Gulf of Mexico will drift
westward towards Texas on Monday, but guidance still varies
considerably into how much this system will organize, and what it
does as it lifts onshore. The consensus, as weak as the agreement
is, appears to be a modest system which will weaken quickly
beneath a mid-level ridge to the north. Despite that, the 700mb
trough should advect onshore bringing increasing PWATs and
instability into Texas. PWATs near 2.25" are forecast to approach
the coast, but low-level inflow modeled by the 850mb wind is weak.
This suggests that the heaviest rainfall, with rates potentially
reaching 2"/hr, will be confined to the immediate coast, and this
is echoed by much of the available guidance. Do not anticipate
that moisture will penetrate too far inland on Monday due to the
weak low-level flow, so the MRGL risk is confined to a narrow
stripe along the entire Texas coast where frictional convergence
could enhance the otherwise modest ascent to produce rainfall
which could approach the 3-hr FFG of 2-3".

Weiss

Day 3
Valid 12Z Tue Sep 17 2019 - 12Z Wed Sep 18 2019

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL IN THE PACIFIC
NORTHWEST...THE TEXAS COAST...AND EASTERN MONTANA...

...Pacific Northwest...
A quick but robust atmospheric river will lift onshore the Pacific
Northwest Tuesday, bringing a round of heavy rainfall from the
extreme northern California coast through Vancouver Island. While
the duration of this event may be short, the combination of PWATs
over 1" and robust 850mb flow to 50 kts will drive intense moist
advection, focused most strongly onto the Oregon coast. While this
setup supports heavy rainfall, and model consensus indicates a
high probability for 2-3", highest within the coastal ranges,
lacking instability should limit rain rates to less than 0.5"/hr
across most of the region. This is expected to reduce the
potential for flash flooding, although isolated instances will be
possible in the terrain, across burn scars, or where rainfall
departures have been excessive (as much as 200% of normal in parts
of Oregon). After coordination with the WFOs, capped the risk at
MRGL due to the short temporal duration of highest IVT and
expected rain rates supporting more beneficial rainfall than that
which is capable of flash flooding.


...Texas Coast...
The same area of disturbed weather from Monday will advect further
to the west Tuesday bringing higher PWAT and better instability
into Texas. There still remains considerable spread in the
guidance envelope as to how this will develop/evolve, but the
better chance for heavy rainfall appears Tuesday as there is a
longer duration of easterly fetch onshore to provide higher MLCape
and PWATs over 2 inches. Rainfall probabilities from the ECENS
show a high chance for 1" along the coast, with low probabilities
for up to 3", and this matches well the bias-corrected in-house
WPC forecasts which suggests the heaviest convective rainfall will
be focused along the immediate coast. This makes sense as
frictional convergence within the area of highest instability
should produce the greatest rain rates, 2"/hr at times, right
along the coast, with decreasing intensity inland. Still have
drawn the MRGL risk a bit further inland as the 700mb wave moves
westward which should allow for some instability/PWAT combination
to bleed inland towards the hill country Wednesday.


...Montana...
An intense shortwave and vorticity lobe will rotate through the
base of an amplified trough lifting onto the Pacific Northwest
coast and atop the Northern Rockies Tuesday aftn/eve. This will
combine with enhanced upper diffluence and increasing moist
advection as deep-layer SW flow brings higher PWATs into the
region. Thunderstorms that develop should lift quickly to the
northeast as noted by forecast 0-6km mean wind over 20 kts, but
unidirectional shear parallel to the surface front could allow for
some training with rain rates of 1"/hr possible. Some storm
organization is also possible as bulk shear intensifies ahead of
the potent shortwave, and the ECENS probabilities indicate the
potential for up to 3" of rainfall across portions of eastern
Montana. This could lead to flash flooding as soils are extremely
saturated across this area due to 14-day rainfall departures as
much as 600% of normal, and 3-hr FFG is less than 1" across much
of the MRGL risk area.


Weiss


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