Experimental Lightning Forecasts
Developed by Dr. Phillip Bothwell
Storm Prediction Center
[phillip.bothwell@noaa.gov]
Experimental Lightning Forecasts
Background ~ Technique was developed by SPC to meet
a stated WR requirement.
Goal ~ Provide fire weather forecasters with improved
guidance for forecasting lightning activity levels [LALs]
Science ~ Forecasts are based on the operational NAM
through 84 hours.
– Forecasts are at 6 hour intervals out to 4 days
Output  Maps of…
– Lightning Strike Probabilities
– Dry Thunderstorm Potential Index [DTPI]
Experimental Lightning Forecasts
Lightning Strike Probabilities
Probabilities range from 0 to 100 %
– Values at 1, 5, 10, 40, 70, and 90 percent intervals are
mapped
Forecasts are for two intensity levels
– Probability of *1 or more Cloud-to-Ground [CG] flashes
Lightning activity at this level is usually DRY
– DRY  equal to or less than 0.10 in of rain
– Probability of *100 or more CG flashes
Lightning activity at this level is usually wet and can be associated
with flooding rains
[* “Number” of CG flashes per 40 km box in a three hour period]
Experimental Lightning Forecasts
Dry Thunderstorm Potential Index  DTPI
DTPI is an Index, not a percentage
Values range from 0 to over 100
– “0” value indicates the lowest potential for dry lightning
– “100” or more value corresponds to the highest potential for Dry
Thunderstorms [DRY: rainfall = or < 0.10 in]
Only values of 50 or higher are plotted
– Contours are at 50, 75 and 100
DTPI and Lightning Strike Probability are calculated
independently
– However, DTPI is really only valid when there is a probability of a
lightning strike.
Experimental Lightning Forecasts
http://www.spc.noaa.gov/exper/fcstfirewxltg/loopmain.html
~ This is the
SPC Lightning
Forecast
website
~ Simply page
down to access
the forecast for
the SW US
Experimental Lightning Forecasts
http://www.spc.noaa.gov/exper/fcstfirewxltg/loopmain.html
~ These are the
forecasts for the
SW US.
~ Simply click
on the day of
forecast and on
the number of
CG flashes you
want to look at.
Experimental Lightning Forecasts
This is an
example of the
guidance product.
Note that most
of the area has
less than a 10%
probability of 1 or
more CG flashes
This example is the partial Day 4, 15 hour
Probability of 1 or more CG Flashes forecast
Experimental
Lightning
Forecasts
Note the three areas with
Probabilities of 1 or more
flashes > 40%
[A, B, and C]
These areas appear the
same, but are very different
For “A” DTPI is > 50
For “B” DTPI is < 50
For “C” DTPI is
mixed, but mostly >50
It can be a real “eye test”
to read these graphics.
Experimental Lightning Forecasts
The SPC Lightning Guidance is a good aid for
the Fire Weather Forecaster.
However, to be more useful, there needs to be
some correlation between the Probability of
Lightning & DTPI provided by guidance, and the
Lightning Activity Levels [LALs] that we forecast.
The following page is a first attempt to arrive at
this correlation.
Experimental
Lightning
Forecasts
This table is a first
attempt to relate LAL
to the DTPI and
Lightning Potential
Forecasts provided
by SPC.
 Only time and
experience will tell if
these relationships
are useful.
Experimental
Lightning
Forecasts
Let’s apply the table of
experimental relationships
between DTPI and LAL to
our previous three points:
A, B, and C to determine
reasonable values of LAL
For “A” DTPI is > 50 and
LAL = 2
For “B” DTPI is < 50 and
LAL = 3
For “C” DTPI is mixed,
but mostly >50, so LAL = 2
Experimental Lightning Forecasts
This slide shows
another example of
how the DTPI Table
could be applied to
determine areas of
various LAL values
The LAL=2 areas
have DTPI >50 with a
Chance of 1 or more
>10%, but less than
70%
The LAL=6 area has
DTPI ~ 100 with a
Chance of 1 or more
>70%.
Experimental Lightning Forecasts
Errors are possible in the
“guidance.” This is one…
The expanded view shows a
line of 100 DTPI cutting right
through an area of >70%
probability of 100 or more CG
flashes
This is inconsistent since a
lightning probability this high
would be associated with wet
thunderstorms and
flooding…not dry lightning.
The programmer said it was
due to a rapid change in the
moisture field which the
lightning probability calculation
responded to quicker than the
DTPI calculation.
Experimental Lightning Forecasts
Another source of error is due to problems with
the NAM.
According to the developer…
“If the lower moisture/CAPE forecast fields are too high,
especially in steeply sloping terrain (as in the West), the
Perfect Prognosis Forecast may produce probabilities
that will not verify (likely an over-forecast). At this time, it
appears a solution to this moisture problem within the
NAM may not tested be until the fall.”
Dr Phillip Bothwell
Experimental Lightning Forecasts
Summary
– The SPC Experimental Lightning Forecasts
provide valuable guidance on…
The Probability of Lightning at two thresholds
– 1 or more CG Flashes
– 100 or more CG Flashes
The potential [DTPI] that the lightning will be DRY
– Assuming the Probability of Lightning is non-Zero
– However, a direct relationship to LAL has yet
to be perfected.