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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1539Z Sep 19, 2020)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1139 AM EDT Sat Sep 19 2020

Day 1
Valid 16Z Sat Sep 19 2020 - 12Z Sun Sep 20 2020

...A MARGINAL RISK OF EXCESSIVE RAINFALL IS IN PLACE FOR PORTIONS
OF THE NORTHERN ROCKIES AND THE NORTHEASTERN FLORIDA PENINSULA...

1600 UTC update

Only some minor changes made to the marginal risk area across
northeast Florida.  The marginal risk was extended approximately
30 nm farther to the west to cover the spread in the new hi res
guidance.

Oravec

0900 UTC discussion

...Northeastern Florida Peninsula...
Strengthening low level flow from the northeast in the wake of a
slowly sagging cold front through the Florida Peninsula will
support locally heavy/excessive rainfall along portions of the
coast between to a point just south of Melbourne. From just prior
to 18Z to at least 06Z, 850 mb winds are forecast to increase into
the 20-30 kt range, roughly perpendicular to the coast.
Precipitable water values above 2 inches and forecast CAPE in
excess of 1000 J/kg just inland of the coast will be in place
during the afternoon/evening hours along with forecast cell
motions of less than 10 kt. Frictional convergence near the coast
are expected to develop slow moving cores of heavy rain with
potential rates of 2-3 in/hr, sagging slowly southward during the
day as high pressure builds south, pushing the better moisture
axis southward as well. The hi-res models from 00Z showed very
heavy rain with 24 hour totals of 3-6 inches, and maximum totals
near 10 inches in the 00Z NSSL. 10 inches is probably overdone,
but the HREF probability of 5+ inches in 24 hours is 80-90 percent
over Volusia County. After coordination with the JAX and MLB WFOs,
a Marginal Risk was maintained across this region of Florida.

...Northern Rockies...
As a seasonably strong mid-level trough tracks across the interior
northwestern U.S. today, anomalous moisture will move across ID,
MT and northwestern WY. 00Z GFS forecast precipitable water values
approach 1 inch across MT by 00Z/20, which translates into +1.5 to
+2.5 standardized anomalies. The orientation of the upper trough
should become increasingly negative tilt through 00Z with
dynamically induced upper level divergence and diffluence
overspreading the northern Rockies. Instability is forecast by a
majority of the 00Z models to remain the 500-1000 J/kg range, but
will locally be a bit higher just ahead of the associated cold
front and beneath the core of the upper trough, which will support
thunderstorms with heavy rainfall rates. Storms will be
progressive for the most part with the frontal progression, but
short term training with rainfall rates of 0.5 up to 1 in/hr will
be possible where instability maximizes. These values would be in
excess of area flash flood guidance values, therefore, WPC
maintained the Marginal Risk across portions of the northern
Rockies through 12Z Sunday.

Otto


Day 2
Valid 12Z Sun Sep 20 2020 - 12Z Mon Sep 21 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF
THE TEXAS COAST INTO SOUTHWEST LOUISIANA...AS WELL AS A PORTION OF
THE EAST COAST OF FLORIDA...

...Texas/Louisiana...
The flash flood threat across coastal sections of TX into
southwest LA is tied directly to Beta. There is a fair amount of
spread concerning the movements of Beta within the high
resolution/regional/global model guidance, as there is spread with
respect to how a mid level trough to the west of the system
evolves. The WPC QPF and Excessive Rainfall Outlook were based on
the most recent NHC track for the storm, and the 00z GFS/NAM/UKMET
were used as a proxy for the NHC track.

Beta slows as it approaches the TX coast during Day 2. The overall
model trend has been to take the system to the right and slow it
down, as the developing mid and upper level trough to its west
attempt to push the system to the east and northeast. As mentioned
earlier, there is spread concerning how quickly this occurs, and
this has a direct effect on the extent of the QPF moving across
coastal sections of TX into southwest LA. Based on the NHC track
for Beta, it appears as though far south TX coast ultimately be on
the drier side of the system and QPF amounts here were dropped
compared to previous model solutions.

As can often happen with slow moving tropical systems, the
instability gradient that sets up across the coast could determine
just how far inland the outer bands from Beta extend. Most of the
00z guidance suggests that the best instability remains offshore,
which can occur in this setup. Based on this approach, the highest
rainfall amounts where placed near the coast. With precipitable
water values in excess of 2.25 inches, hourly rainfall rates could
exceed 2.00 inches with the convection in the outer bands of Beta.
Given the uncertainty not only of the track of Beta but the
thermodynamic environment ahead of it, a Marginal Risk was
extended across much of the TX coast into southwest coastal LA. Of
course, changes in the track of Beta could necessitate changes in
QPF and the Excessive Rainfall Outlook in later forecasts.


...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 2. 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. Much of the high resolution guidance (including
the 00z NAM CONUS Nest/00z WRF ARW) showed local 3.00/4.00 inch
rainfall amounts, particularly across coastal sections of central
and southeast FL, where the best convergence is expected. Once
again, this flow regime supports heavy rainfall across the FL east
coast, so a Marginal Risk was placed here, despite the modest WPC
QPF values. 

Hayes

Day 3
Valid 12Z Mon Sep 21 2020 - 12Z Tue Sep 22 2020

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE MIDDLE
AND UPPER TEXAS COAST...

...Texas and Louisiana...
The biggest forecast issue with the QPF and Excessive Rainfall for
Day 3 lies with the track of Beta, and whether the storm makes
landfall over the Middle TX coast. Much of the 00z guidance is
hinting at a possible brush with this part of the coast at some
point on Day 3, though there remains a fair amount of spread
concerning where and when this might occur. The WPC QPF and
Excessive Rainfall Outlook are based on the most recent NHC track
for Beta.

As Beta approaches the Middle TX coast on Day 3, it slows as a mid
and upper level trough to the west attempts to push the system to
the east northeast. The 00z NAM/ECMWF/CMC bring the system ashore
between CRP and VCT before 22/00z, while the 00z GFS and the 00z
UKMET do not have a landfall before 22/12z. The differences arise
from each member of the 00z model suite handle the mid and upper
level trough to the west of the system, which affects how the
system steered. The overall model trend has been to nudge the
system to the northeast at the trough to the west strengthens, but
given the model spread with respect to how the trough evolves,
forecast confidence in QPF amounts and placement in below average.

Following the most recent NHC track would keep the highest
rainfall amounts along and just offshore of the Mid and Upper TX
coast. Hourly rainfall rates could exceed 2.00 inches on the
western side of the circulation, with local 5.00+ inch amounts
possible along the coast, even without a landfall. Based on this
approach, and after collaborating with WFOs BRO/CRP/HGX, a Slight
Risk was extended across this area for Day 3. Of course, should
the NHC track come back to the left with Beta, higher rainfall
amounts are possible across the Mid and Upper TX coast, which
could extend further inland. Should this occur, a Moderate Risk
could be needed here in later forecasts.

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