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Excessive Rainfall Discussion
(Caution: Version displayed is not the latest version. - Issued 2237Z Sep 19, 2020)
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
637 PM EDT Sat Sep 19 2020

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


...Southeast Louisiana...
Rainfall related to Tropical Storm Beta continues across the
region, with recent radar estimates of 0.5-1" an hour and amounts
of 3-6" having already fallen (though surface observations
indicate that radar estimates might be a little high).  As the
low-level ridge slowly weakens from here on out and after the
mid-level ridge moves across AR around 06z into the TN valley, the
guidance suggests that the front over the northeast side of Beta
would progress northward, which increases the threat of heavy
rainfall in and near the southeast LA coast particularly towards
12z.  The 18z HREF probabilities of 3" are in the 30-40% range
right along the Louisiana coast.  Considering the amounts that
have fallen thus far, thought it prudent to add a Marginal Risk

...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.  Winds at 850 mb
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 18z HREF
probability of 5+ inches in 12 hours is 30-40 percent in and near
Boulevard County from 00z-12z.

...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.


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


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

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.


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


...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.  In terms of guidance spread, there are still pieces of
guidance which hint that the system may remain off-shore and only
brush the coast (perhaps headed back out to sea rather than
paralleling the coast once it begins to move again), while other
pieces of guidance 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...which has resulted in a westward shift
of the of the highest WPC rainfall amounts along and just inland
of the Mid-Texas coast compared with the previous set of WPC
guidance.  Highest rainfall amounts in excess of 5 inches are
possible 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.  Any shift in guidance, either with respect
to the track or the timing of the system, could result in
considerable changes in the rainfall forecast or the risk of
excessive rainfall.  Always refer to the National Hurricane Center
for the latest information.


Day 1 threat area:
Day 2 threat area:
Day 3 threat area: