Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
514 PM 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 SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
THE TEXAS COAST AND SOUTHERN LOUISIANA COAST...

...Texas/Louisiana...
Introduced a Slight Risk across portions of the Texas coast in
response to the latest guidance which brings Beta westward and
slows its forward speed.  With areal average rainfall amounts now
exceeding 5 inches in a few spots, felt it an upgrade was
necessary. 

Most of the rainfall with Beta should be offshore at the beginning
of the forecast period, but increase throughout the period.  The
storm will be slowing as it approaches the TX coast during Day 2. 
Spread in the numerical guidance remains as to when this happens
which has an immediate impact in how much rain will fall and
where,  Still needed to rely on model proxies to emulate the track
and speed of NHC guidance and considerable amount of manual edits
to increase rainfall amounts near the core of the tropical
cyclone.  It may be that depth of the system in a sheared
environment may be impacting how the models are handling Beta and
its rainfall.  Rainfall amounts and associated risks of flooding
are dependent on the exact track and speed of the system.  Changes
in the track of Beta could necessitate changes in QPF and the
Excessive Rainfall Outlook in later forecasts.

Farther east in Louisiana, persistent rainfall has been occurring
where the fetch of deep Gulf Moisture has been interacting with
the remnants of an old cold front in a diffuse pressure trough. 
This will still likely be occurring early on Sunday morning when
the Day 2 period begins...with several inches of rainfall possible
given the deep moisture in place and low/mid level frontogenesis
on the periphery of Beta.  Expanded the Marginal Risk to cover the
broadest rain and introduced a small Slight Risk area south of
I-10 in the region most likely to experience the longest duration
of rainfall based on the latest numerical guidance.

...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.  Numerous high
resolution guidance members continue to show local 3.00/4.00 inch
rainfall amounts, particularly across coastal sections of central
and southeast FL, where the best convergence is expected. No
changes were made to the on-going Marginal Risk area here.

Bann


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

...THERE IS A MODERATE 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 or not the storm
makes landfall over the Middle TX coast. The rainfall forecast,
and the associated Excessive Rainfall Outlook, was based on NHC
guidance bringing the system on-shore early in the Day 3 period
and then takes a northeastward track once inland in response to
the approach of a mid- and upper-level trough from the west. 
Considering that the inland track remains within 20 or 30 miles of
the Middle Texas coastline, the system should still be able to
maintain decent rainfall rates along and north/northeast of its
position.  There are still pieces of guidance which hint that the
system may remain off-shore and only brush the coast, while others
fail to get the system closer enough to produce more than some
moderate rainfall amounts.  The WPC QPF and Excessive Rainfall
Outlook are based on the most recent NHC track for Beta.

Following the most recent NHC track has resulted in a westward
shift of the of the highest WPC QPF rainfall amounts along and
just inland of the Mid and Upper TX coast...and the potential
remains for 3 to 6 inches rainfall during the Day 3
period...resulting in combined Day 2 and Day 3 rainfall amounts to
exceed 10 inches.  As a result, hoisted a Moderate Risk area.

Bann



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