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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2015Z Sep 13, 2019)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 PM EDT Fri Sep 13 2019

Day 1
Valid 16Z Fri Sep 13 2019 - 12Z Sat Sep 14 2019

...MARGINAL RISKS OF EXCESSIVE RAINFALL EXIST OVER PORTIONS OF THE
GREAT LAKES, UPPER OHIO VALLEY, SOUTHERN ROCKIES, & SOUTHERN
APPALACHIANS...

...16Z Update...
No real change in thinking. Hi-res guidance remained consistent in
the 12Z cycle, still highlighting locally heavy downpours over
Northern Ohio, the southern Appalachians, southern NM/AZ this
afternoon. Flash flooding in Ohio and adjacent states will be
dependent on storm scale interactions, brief training, and whether
cells can form ahead of the more strongly forced cold frontal
environment to offer targets for cell mergers. In the Desert
Southwest deep layer forcing is minimal, and model QPF signal
somewhat underwhelming, but the Marginal Risk covers the region of
greatest predicted CAPE which also overlaps with mountainous /
flashy terrain. -Burke

...Previous Discussion...
...Great Lakes/Upper OH Valley...
A swath of high precipitable water (1.75 inches) will be drawn
northward in a 30 to 50 knot low-level jet, ahead of and into a
low pressure center tracking toward Ontario.  Some of the early
morning convection currently moving into the Lower Peninsula of MI
and northern IL should continue marching eastward into the morning
hours into OH.  An axis of 1000-1500 J/KG of ML CAPE is forecast
to peak just ahead of the front.  Convection is expected to spread
into western Pennsylvania and New York with time.  With the
available moisture nearly double that of climatology, the
environment could support hourly rates of up to 2". Three hour
flash flood guidance values are generally between 2-3" across much
of the region, as it has been fairly dry over the past 14 days. 
But, southwest PA is fairly rugged and sensitive to heavy rains. 
The lower end of the flash flood guidance could be realized where
short term training occurs, so the Marginal Risk was left in
place. 

...Southern Rockies/Southeast AZ...
Gulf moisture will be transported through Texas into portions of
New Mexico on Friday, sneaking into southeast AZ with time as a
low-level easterly wave escorts the moisture into the area. The PW
values within the low-level jet will be 1.25-1.5", which is nearly
two standard deviations above normal. Also during this time
1000-2000 J/KG of ML CAPE in place as the moisture arrives, and
the combination of moisture and instability should be sufficient
to support at least scattered storms, tied initially to the
terrain. The speed of individual storms will likely be slow due to
having mid-level ridging in place across the Southwest states;
which will be conducive for cell mergers and short term training,
especially over the terrain. 
 
The increased moisture could support hourly rainfall rates in
excess of 0.50", with local rainfall amounts over an inch.  Three
hour flash flood guidance values are as low as 1" over portions of
south central NM, and these amounts could be approached where cell
mergers or training occurs. 

...Southern Appalachians...
A southward moving front will bring precipitable water values up
to 1.75-2" and upslope flow into the Piedmont of the southern
Appalachians with ML CAPEs of 1000-2000 J/kg expected during the
afternoon.  The non ECMWF guidance was agreeable here.  Pulse
convection is the expectation, with hourly rain totals to 2" and
local amounts to 4".  While the region has been bone dry over the
past couple weeks, steep topography, particularly in southwest NC,
could be problematic with such heavy rains.

Roth


Day 2
Valid 12Z Sat Sep 14 2019 - 12Z Sun Sep 15 2019

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ALONG THE
ATLANTIC COAST OF FLORIDA...

...Florida...
20z update...The NHC track, along with much of the 12z model
guidance, suggests that the Tropical Depression Nine tracks a bit
further east that previous guidance. This is, of course, on
development during Day 1, so there could still be some wiggle room
with respect to how far west the rain shield with this system
comes west. Based on the nudge to the east, rainfall amounts were
shaved across the FL East coast into coast GA, though heavier
amounts are possible in banding as the storm parallels the coast.

The nudge to the east also required small changes to the Day 2
Excessive Rainfall Outlook (ERO). After collaborating with WFO
MFL, the southeast coast of FL was removed from the Slight Risk.
Otherwise, no significant changes were made to the ERO.

Previous forecast...
Tropical Depression Nine will organize near the Bahamas and
attempt to lift north/northwest beneath a mid-level ridge. The 00Z
suite of both the NCEP and Non-NCEP models have shifted eastward
this morning, and a blend of continuity, ECMWF/UKMET/NMMB produce
a track that is closest to the 03Z NHC advisory positions for TD9
on day 2. While the eastern shift has produced a decrease in QPF,
especially across northern Florida, there remains enough
uncertainty that the SLGT risk was left generally unchanged
despite somewhat lower QPF. Aiding in this decision to make only
minor changes to the ERO is that a plume of tropical moisture,
PWATS 2.25" or more, will pool east of FL and advect towards the
Atlantic coast on easterly low-level flow of around 20 kts. This
suggests that despite a more eastward solution now showing up in
guidance, enough low-level moist advection will allow for tropical
downpours across the Atlantic Coast of Florida, highest along the
immediate coast where frictional convergence may aid in ascent.
Intensifying upper diffluence between a subtly retreating ridge to
the northeast and upper low across the Gulf of Mexico will aid in
ascent as well, maintaining the potential for heavy rainfall of
1-2" with isolated higher amounts regardless of where TD9
eventually tracks. Should the eastward shift in guidance, the
heaviest rain may be confined offshore, which may necessitate a
lowering of the SLGT risk, but for now the onshore flow looks to
support heavy rainfall regardless of the exact track of TD9.


...Pacific Northwest...
20z update...No changes were made to the Marginal Risk across WA,
as precipitable water values rise to between two and three
standard deviations above the mean, and IVT values increase to
near 750 kg/m/s between 15/06z and 15/12z.

Previous discussion...
An early season Atmospheric River is progged to lift onshore
beneath a 110+kt jet streak rotating into British
Columbia/Washington State late Saturday night. GFS progs for the
AR suggest a high probability for IVT > 500 kg/m/s moving into
Washington, with PWATs exceeding 1.25" on 30 kts of 850mb flow.
This robust moist advection will be wrung out via ascent within
the RRQ of the pacific jet streak, combined with height falls and
surface convergence along/ahead of a mid-level trough driving a
cold front onshore. Additionally, mean MUCape values are forecast
to rise above 250 J/kg, with some spread indicating the potential
for more than 500 J/kg Saturday evening. This will support rain
rates which at times may exceed 0.5"/hr. As flow briefly becomes
parallel to the front, especially into the Olympic Range of
Washington, training and upslope enhancement could produce
isolated instances of flash flooding across the terrain. After
coordination with SEW/PQR, a MRGL risk has been added despite the
expected short temporal duration of favorable dynamics for
excessive rain rates.


...Desert Southwest...
20z update...After collaborating with WFOs FGZ/PSR, the Marginal
Risk was extended further west in AZ to accommodate the westward
extent of the instability and moisture. Otherwise, no changes were
needed to the QPF and ERO for Day 2.

Previous discussion...
A return of Monsoonal moisture will spread across portions of
Arizona and New Mexico Saturday as a mid-level ridge subtly
retreats to the east. Flow around the periphery of this ridge will
become S/SE, advecting anomalous PWAT into the region of +1 to
+1.5 standard deviations above the climatological mean. At the
same time, a weak shortwave is progged to rotate atop the region
as energy rotating around the ridge to the east, which combined
with enhanced upper diffluence and robust low-level instability
will support scattered thunderstorms across the region. MUCape as
high as 2000 J/kg will support rain rates which may reach 1"/hr,
which could exceed the 1-hr FFG which is as low as 0.75" across
portions of AZ, and as low as 1" in NM. 0-6km mean wind of just
5-10 kts suggests storm motions will be slow, allowing for cell
mergers and brief training which could support isolated flash
flooding.


...Upper Mississippi Valley/Upper Great Lakes...
Moisture and instability focusing on a frontal boundary extending
from SD across IA into southern WI could be sufficient to support
storms capable of producing heavy to locally excessive rainfall
over areas with lower flash flood guidance.

Convection is expected to develop between 15/18z and 16/00z across
portions of IA, northern IL and far southern WI on the northern
edge of an impressive CAPE gradient (where MLCAPE values top 3000
J/KG). Initially, the storms are expected to be scattered along
and ahead of the front, and in the presence of strong instability,
should be outflow driven early. The frontal boundary becomes quasi
stationary after 15/00z, as the fast mid level flow becomes more
parallel to it. Subtle short wave energy in the flow crosses the
region after that time, and ahead of the short wave, an increasing
low level jet transports 1.75 inch precipitable water air (which
is about two standard deviations above the mean) along the front,
mainly after 15/06z.

Ahead of the short wave, the propagation vectors eventually
becomes better aligned with the 850-300 mb mean layer flow, which
should promote training along and south of the front. Though 12z
model guidance QPF amounts are fairly modest, the ingredients
appear to be in place for locally heavy rainfall. Three hour flash
flood guidance values across portions of northeast IA/northern IL
and far southern WI are as low as 1.00 inches, due to heavy rains
earlier this week. While it remains to be seen how widespread the
convection becomes, the potential for training over areas with
compromised soils merits a Marginal Risk for Day 2 in the
abovementioned areas.

Weiss/Hayes

Day 3
Valid 12Z Sun Sep 15 2019 - 12Z Mon Sep 16 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG THE ATLANTIC
COAST FROM CENTRAL FLORIDA THROUGH NORTHEAST SOUTH CAROLINA...

...Southeast Coast...
20z update...The most recent NHC track for Tropical Depression
Nine, as well as much as the 12z guidance, suggests that slightly
eastward track with the system. This would suggest that the bulk
of the tropical moisture remains with the cyclone. However, the
12z GFS suggests that frontogenetic forcing centered on a low
level boundary across NC this afternoon could take advantage of
moisture siphoned from the cycle could form convection over or
near the coast from coastal GA into southeast NC. Interaction with
short wave energy across the northern Mid Atlantic could have some
influence on the placement of the highest rainfall, so the WPC QPF
and Excessive Rainfall were based only partially on this solution.

The bottom line is that the QPF across portions of central FL into
coastal GA were trimmed back based on the further east track.
Across the SC coast, QPF amounts were shaved as well, but this may
be the most uncertain part of the rainfall forecast. The changes
required removing the MLB and TBW CWAs from the Slight Risk. In
addition, there was no change to the Slight Risk along the SC
coast (per collaboration with WFO CHS), though 14 day rainfall
amounts here are as high as 300+ percent of normal.

Previous discussion...
Tropical Depression Nine may become a Tropical Storm by day 3
/Sunday/ while it lifts slowly northward in the vicinity of the
Florida coast. While guidance continues to feature considerable
spread in the track forecasts, by day 3, a blend of continuity,
ECMWF/ECENS/GFS yield a track that is closest to the NHC 03Z
advisory positions. While the GFS remains on the western edge of
the guidance envelope, there has been a notable eastward shift in
both the NCEP and Non-NCEP suite at 00Z/13. Have kept close to the
NHC track, but note that if this eastward shift continues QPF
could become much less and may no longer necessitate a SLGT risk
on day 3.

However, robust moist advection on 30 kts E/SE flow from 2.25"
PWATs pooled near the center of TD9 still could produce heavy
rainfall, especially along the immediate coast from northern Fl
into central SC. It is in this region where WPC QPF was adjusted
upward slightly, while most of the rest of the area featured a
downward trend. The reason for this subtle increase is for the
aforementioned robust moist advection, combined with signals for
improving upper level outflow and channeled diffluence to drive
tropical rain showers onto the coast. This could occur regardless
of the eventual track of TD9, but would be enhanced the closer TD9
gets to the coast. After coordination with the Southeast coastal
offices, a SLGT risk was extended to the NC/SC border, and a sharp
NW to SE gradient in QPF is preferred due to the potential for TD9
to curve sharply eastward as it becomes blocked by upper level
ridging to the north. Confidence is less than average at this
time, and updates are likely as the NHC track shifts and models
reach a more agreed upon consensus. At this time however, there is
potential for 1-2" of rain due to hourly rain rates in excess of
1"/hr, with isolated higher amounts possible, which could produce
localized flash flooding necessitating the SLGT risk.


...Southwest...
20z update...
Returning moisture and lift associated with the short wave/MCV
tracking out of northern Mexico into eastern AZ/western NM could
allow for convection to become more widespread than Day 2 across
much of AZ/NM into southwest CO. However, instability could end up
being the limiting factor for an enhanced flash flood threat here.
Multi layered synoptic layered cloudiness may prevent the airmass
from recovering until afternoon. Most of the 12z guidance showed
this possibility, with MLCAPE values topping out between 500-100
J/KG.

After collaborating with WFOs TWC/PSR/FGZ/ABQ/GJT/PUB, it was
decided that the cloud cover may hamper a more robust flash flood
threat, so the area was not upgraded to a Slight Risk with this
forecast package. If it appears as though instability will be more
of a factor, a Slight Risk could be needed in later forecasts.

Previous discussion...
Monsoon flow will persist into day 3 as a mid-level ridge centered
near Arkansas spins in place, allowing for moisture and energy to
rotate around its periphery into the Southwest. On day 3 /Sunday/
there is a decent signal for a strong shortwave to lift out of
Mexico northward towards the Colorado Rockies, combined with moist
advection of PWATs as high as 1.5" pooled near the Gulf of
California. A modest trough noted at 850mb via subtly cooler
temperatures suggests lapse rates will increase Sunday, supporting
more widespread thunderstorm coverage than on day 2. Beneath this
feature, storm motions may slow to near 0 as shown by 0-6km mean
wind forecasts, suggesting thunderstorms with heavy rainfall may
become stationary, or feature cell mergers, to persist excessive
rain rates across the region. FFG across the area is quite low,
less than 1"/1hr across most of the MRGL risk area, and with rain
rates potentially exceeding that, flash flooding could be a
problem Sunday. The guidance still features some uncertainty into
how widespread convection will become, but there is potential that
if the signal becomes more robust a SLGT risk may be needed for
portions of the area.


Weiss/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