Met Office College – Course Notes
Precipitation Forecasting
Contents
1
Introduction
2
Frontal Precipitation
3
Non-frontal precipitation
3.1
3.2
4
Instability precipitation
4.1
4.2
5
Organised precipitation
Precipitation from stratiform cloud
Showers
Cumulonimbus and thunderstorms
Type of precipitation
5.1
5.2
5.3
5.4
Hail
Drizzle
Rain or snow?
Freezing precipitation
6
Quantity of precipitation
7
Snow Predictors
7.1
7.2
7.3
7.4
7.5
8
Boyden’s technique using 1000 - 850 Thickness
Height of wet bulb zero degree isotherm above ground level
Hand's rule
Height of zero degree isotherm above ground level
Screen wet-bulb temperature technique (Lumb)
Further Reading
 Crown Copyright. Permission to quote from this document must be obtained from The Principal, Met
Office College.
Page 1 of 16
Last saved date: 15 February 2016
File: MS-TRAIN-COLLEGE-WORK-d:\533546845.doc
Met Office College
1 Introduction
We have seen previously the mechanisms, which produce
precipitation. In this section we will look at the problem
of forecasting precipitation both from layer and convective
cloud.
Forecasting precipitation requires the consideration of
many aspects:
1. Type, e.g. drizzle, rain, hail or snow.
2. Area of coverage, i.e. local or widespread.
3. Intensity i.e. slight, moderate or heavy.
4. Length of time precipitation will last.
5. Total quantity of precipitation.
We can split the problem into three main parts:1. Frontal precipitation
2. Non-frontal precipitation
3. Instability precipitation
2 Frontal Precipitation
Synoptic chart - Study the chart and try to identify the
fronts. In the case of an active front this may be
relatively simple; inactive fronts are more difficult to
position, especially over sea areas. In both cases the
satellite pictures are very useful. Colour in the
precipitation areas in green.
Nimbus - Can be used effectively to track areas of
precipitation by overlaying the weather elements on the
satellite products. This will give an idea of which cloud
produces what precipitation. Then overlaying the model wind
fields for 700 hPa gives an idea of advection. Use the
radar option to track elements, and hence find the speed of
movement. Beware of development, dissipation and orographic
effects when timing rain bands.
History - Look at earlier charts and see how the front has
moved and also how the precipitation area has grown or
diminished. Draw up continuity charts of the movement of
the fronts and associated rain areas. Look at the
isallobaric pattern, which will show whether the parent
depression is deepening or filling. Usually if the
depression is deepening the front is becoming more active,
and vice versa.
Page 2 of 16
Last Saved Date : 15 February 2016
File: MS-TRAIN-COLLEGE-WORK-d:\533546845.doc
Precipitation Forecasting
Tephigram - A study of the tephigram will help in the
positioning of the front (T analysis). Consider how moist
or dry the ascents are. Are there regions of instability
which may lead to heavy rain?
Upper air - A useful guide to precipitation forecasting is
the 700 hPa wind. The precipitation area will often move in
the direction and at the speed of the 700 hPa wind. Are there any
upper air troughs or jet streams present? A front will become more
active if an upper air trough such as one at 300 hPa over-runs a surface
front.
3 Non-frontal precipitation
Assuming an area of non-frontal precipitation can be tied
in with a significant feature then, because we have a good
idea of the development of these features, we should be
able to make a reasonable forecast for changes in the
precipitation area.
For example, if we forecast a feature to become more
intense, say as a upper trough sharpens, then we would
expect the precipitation to become heavier due to increased
vertical motion. Similarly if we expected a polar low to
track across land then we would expect the precipitation to
gradually die out as the system loses its heat source.
Areas of non-frontal precipitation which are not readily
associated with any significant feature are more difficult
to handle.
Causes of non-frontal precipitation can be divided into two
main categories:1. Those connected with organised features
2. Those not connected with organised features
3.1
Organised precipitation
Surface The main surface features are troughs and polar
lows. These tend to be less conservative than fronts. The
main method used is to move the rain area associated with
troughs with the 700 hPa wind. The rain area associated
with the polar low tends to move more erratically and is
best tracked using continuity or as a rough guide move with
the 850 hPa wind.
—
Upper air Cold pools and troughs in the thickness pattern
and troughs in the contour pattern are useful indications
of the probability of instability rain, showers or
thunderstorm. Both in summer and winter these features are
—
Page 3 of 16
Last Saved Date : 15 February 2016
File: MS-TRAIN-COLLEGE-WORK-d:\533546845.doc
Met Office College
responsible for mesoscale differences in precipitation
distribution.
In winter they are often responsible for significant
variations in local accumulations of snowfall. The most
vigorous convection associated with a thermal pool or
trough will often give precipitation in areas which under
normal streamline flow would not receive any. An obvious
example of this would be well inland or to the lee of high
ground, where organised features may bring significant
precipitation because they maintain vertical momentum
better than non-organised systems.
In summer the effect is similar but the most vigorous
activity will occur overland (not at sea as in winter)
where there is maximum heating. Any showers will be
enhanced by upper troughs or cold pools due to increased
instability and may be the mechanism which provides the
trigger for thunderstorms.
Upper troughs or cold pools may give rise to shower
activity outside the period that daytime heating would
suggest.
3.2
Precipitation from stratiform cloud
Precipitation produced from stratiform cloud which is nonfrontal e.g. drizzle or light rain, is related to the
microphysics of the cloud rather than the broader dynamics
involved with the previous sections. We need to consider
the temperature and depth of layered cloud to see if the
cloud droplets will grow sufficiently large to fall to the
ground without evaporating.
Drizzle is often associated with haar/sea fret and affects
similar areas, i.e. east and north coasts, and is cleared
inland by radiation and near the coasts by advection.
Drizzle tends to be more frequent during the latter part of
the night and early morning as the ST/SC thickens due to
nocturnal cooling. For drizzle to reach the ground it must
fall from ST cloud with its base less than 1000 ft. The
dew-point depression in the air below the cloud should be
less than 20C, otherwise the drizzle will evaporate before
reaching the ground. Heavier drizzle is mainly associated
with orographic cloud.
Sporadic light rain can often be associated with thicker
layers of SC in the circulation of an anticyclone. As SC is
notoriously difficult to forecast, the forecasting of this
type of rain is also difficult. If the high pressure builds
Page 4 of 16
Last Saved Date : 15 February 2016
File: MS-TRAIN-COLLEGE-WORK-d:\533546845.doc
Precipitation Forecasting
enough then the SC will become thinner and the rain will
cease.
The satellite imagery on NIMBUS can help to identify areas
of colder ST/SC tops where rain or drizzle is more likely.
Again the overlay of actual weather on these satellites can
be useful.
4 Instability precipitation
Synoptic chart - Study the charts and decide whether
showers and thunderstorm are already occurring. Are they
associated with any topographical feature (e.g. over the
sea, land, mountains, etc)? Are they associated with any
isobaric pattern (e.g. troughs, thundery lows, etc.)?
NIMBUS - The satellite facility can be used to see the
spatial distribution of showers. By using the temperature
scale an estimate of the cloud tops temperatures can be
found hence from an adjacent tephigram an estimate of the
cloud heights can be made.
History - Study yesterday’s charts. Did showers or
thunderstorm develop in the sea air mass and if so at what
time?
Tephigrams - Choose a representative ascent. Is it
unstable? Is it moist or dry? In general the higher the
moisture content, the more likely it is that convection
will be vigorous with showers or thunderstorm. Find the
temperature required to release instability. Using the
heating curve this will give you a good idea when showers
will occur. If the ascent is very unstable then the larger
CU will build up quite rapidly and the showers will start
earlier.
Possible changes - We are interested in whether the ascent
will become more unstable, less unstable, moister or drier.
4.1
Showers
The distinction between the conditions which give rise to
showers, and those which do not, is a fine one. There is no
technique which enables a forecaster to say exactly when
any specific place will have a shower.
As a summary of shower formation we can say that showers
are likely if the cloud is greater than 5000 ft in depth
with cloud top temperatures colder than -120C.
From the study of radar imagery has shown that showers are
not always randomly distributed. As an example the North
Channel between Scotland and Northern Ireland can produce
semi-permanent shower patterns due to the long warm and
Page 5 of 16
Last Saved Date : 15 February 2016
File: MS-TRAIN-COLLEGE-WORK-d:\533546845.doc
Met Office College
moist sea track. This has the effect of producing
significant rain/snow falls in parts of Wales and Cornwall
while the majority of the country is cloud free.
Significant wind shears can deplete large CU of their
rainfall, allowing only lighter precipitation to reach the
ground.
More detail is given in the Convective Cloud Forecasting
notes.
4.2
Cumulonimbus and thunderstorms
Cumulonimbus(CB) are important because we associate
thunderstorms, hail and flash floods with larger CB, and
heavy rain even with smaller CB. These are easy to track
using radar and satellite but remember about daughter cell
formation when tracking larger storms. The large amounts of
heavy localised precipitation can cause most havoc when
falling on very dry ground which is unable to absorb the
larger quantities of water. The water quickly accumulated
in rivers and roads producing the summer flash floods which
can destroy bridges and roads on occasions. The Environment
Agency and other customers are interested in the amounts of
precipitation falling over a short period. Some local
accumulations can be in excess of 100 mm in a few hours.
For the development of thunderstorm, we need a large depth
of convection with a large amount of moisture. The
temperature of the top of the cloud should be colder than 200C.
Look out for reports of heavy rain, SFLOC activity or large
echoes on the radar.
Note that CB and thunderstorms are often embedded within
fronts, especially cold fronts, and in these circumstances
can be very difficult to spot.
Polar lows in winter, heat lows and convergence lines in
summer are other favoured mechanisms for CB and
thunderstorm formation.
5 Type of precipitation
So far we have only discussed precipitation in general, but
it is necessary to distinguish between the various types of
precipitation.
5.1
Hail
Hail is usually forecast whenever large CB are possible.
The temperature of the top of the cloud should be -200C or
colder. Little conclusive work has yet been done on hail in
Page 6 of 16
Last Saved Date : 15 February 2016
File: MS-TRAIN-COLLEGE-WORK-d:\533546845.doc
Precipitation Forecasting
this country but the following technique has been
suggested:forecast hail if the parcel method of convection gives
cloud tops of 15 000 ft or more and the path and
environment curves are at least 40C apart
A similar method, based on the parcel curve, is:1. On a representative sounding, construct the parcel
curve.
2. At the point where this curve reaches -200C, measure the
difference between this temperature and the environment
temperature.
3. If this difference is equal to or greater than 50C,
forecast hail.
If this difference lies between 50C and 2.50C, forecast
soft hail or rain.
If this difference is less than 2.50C, forecast rain.
For large hail then a ‘steady state’ storm is required
where the hail can circulate within the CB for a sufficient
time to allow the hail to increase in size.
Page 7 of 16
Last Saved Date : 15 February 2016
File: MS-TRAIN-COLLEGE-WORK-d:\533546845.doc
Met Office College
Figure 1.
5.2
Conditions required for hail.
Drizzle
When drizzle forms, the rarity of ice crystals in cloud
with cloud-top temperatures greater than -50C indicates
that the coalescence mechanism is predominantly
responsible. The minimum depth of cloud for producing
drizzle is around 2000 ft (this also implies that in a
stratus situation if it begins to drizzle then the cloud
will probably be at least 2000 ft thick!).
The main problem in winter is determining if the
precipitation will reach the ground as rain, snow or rain
and snow mixed. The temperatures in the lowest part of the
atmosphere are the most critical in determining what will
fall at the surface hence most of the snow predictors we
use are based on the temperatures in the lowest 100 hPa.
Wet bulb temperature is an important feature to watch
because when rain commences the dry bulb temperature is
reduced by evaporative cooling to the wet bulb, often
within a period of one or two hours.
Page 8 of 16
Last Saved Date : 15 February 2016
File: MS-TRAIN-COLLEGE-WORK-d:\533546845.doc
Precipitation Forecasting
5.3
Rain or snow?
Whether precipitation will fall as rain or snow can be
difficult to forecast. There are many factors to consider.
Site - Higher inland sites are more prone to snow than low
coastal sites.
Snow cover - If snow is already lying and quite extensive
then precipitation is more likely to fall as snow since the
lowest layers of the atmosphere will be cooler. This
implies that if there is extensive snow cover snow can be
forecast with a higher 1000-850 hPa thickness value than
would be expected.
Wind direction - If on a coastal site the winds are
blowing from the sea this effectively warms the lowest part
of the atmosphere. Most of the precipitation is rain or
rain and snow mixed, but if the wind changes slightly, and
has a cooler land track, especially when snow is already
lying inland, then the precipitation can readily change to
snow.
If the situation arises where there is a cold easterly wind
at low-level undercutting the warmer moister air then the
potential for large amounts of snow increases, as the lowlevel thickness values fall.
Warm front approaching - This scenario is one which causes
many problems for forecasters because of the potential for
large accumulations of snow. If the air at low levels ahead
of the front is very cold (e.g. after good radiation
conditions) then precipitation can start of as snow before
turning to rain as the warm air approaches. The situations
which cause problems is when the warm from becomes slow
moving or stationary to the south of you and your station
remain in the cold air. In this circumstance most of the
frontal precipitation tends to be slight but if a shallow
wave develops upstream then the potential for significant
accumulations of snow in enhanced.
Guidelines given in the Handbook of Weather Forecasting,
paragraph 19.7.6.3, suggest that if the warm air mass is
tropical maritime, with wet-bulb temperatures around +100C
then in the cold air snow rarely falls within 50 miles of
the front. On the other hand, if the warm air mass is polar
maritime, with a wet bulb temperature of the order of 4.50C
then in the cold air snow may extend right up to the front.
Polar Lows - They tend to be very important features in the
northern parts of Scotland and Ireland due to the
significant snow falls associated with them. They can be
identified from satellite and tracked before coming into
radar range. Don’t ignore ships with high winds and weather
Page 9 of 16
Last Saved Date : 15 February 2016
File: MS-TRAIN-COLLEGE-WORK-d:\533546845.doc
Met Office College
which are at odds with the chart - they may be right!
Remember the model cannot handle these small scale features
well.
Snow showers - One significant feature of winter snow
showers in a westerly is the great depth of instability
giving large CB which produce heavy and sometimes prolonged
precipitation. With frequent heavy showers precipitation
can turn to snow with higher initial temperatures than from
frontal precipitation - see table 1.
Initial wet bulb temperature
Type of precipitation
To which snow will
descend
To which snow is
unlikely
Prolonged frontal
precipitation

+2.0 C

+2.5 C
Extensive areas of
moderate or heavy
instability precipitation

+3.0 C

+3.5 C
Table 1. The relationship between downward penetration of
snow beneath the 0C level and initial wet bulb
temperature, from Handbook of Weather Forecasting,
paragraph 19.7.61f
Drifting or blowing snow - Can be a serious aviation
hazard. It can also be a more widespread problem bringing
communications to a stand still with large snow drifts and
terrible visibility.
With air temperatures less than zero and dry snow we need:

wind <12 kn for no significant drifting

wind 12-16 kn for slight to moderate accumulations wind
>17 kn severe drifting or blowing snow
There are several snow predictors available to the
forecaster and these are shown in the Annex. One of the
more reliable methods is Boyden’s adjusted 1000-850 hPa
thickness method and as with most methods relates the mean
temperature for the lower part of the atmosphere to the
risk of snow. The 1000-500 hPa thickness is unreliable
because it can be affected by warm or cold air aloft which
has little influence to the precipitation near the surface.
Page 10 of 16
Last Saved Date : 15 February 2016
File: MS-TRAIN-COLLEGE-WORK-d:\533546845.doc
Precipitation Forecasting
5.4
Freezing precipitation
Freezing precipitation can be in the form of rain or
drizzle. Freezing rain is caused when solid precipitation
falls through a layer with positive air temperatures and
melts before falling through a shallow sub-zero layer near
the surface. When the rain hits the cold ground it spreads
out before freezing forming a sheet of glazed ice. This is
a very dangerous phenomena and can cause many accidents on
the roads, with pedestrians and is a severe hazard for
aircraft operations. A typical ascent for freezing rain is
shown in Figure 2. This ascent is most often found in a
situation where the surface has been intensely cooled due
to radiation (or cold low level advection) with an
approaching warm front. As the front approaches the
precipitation falls as freezing rain before the ground
warms to positive temperatures due to the warm airmass.
Freezing drizzle normally occurs from a similar process to
that of freezing rain but it is possible to get freezing
drizzle from a complete sub-zero ascent. If no ice
particles are present in the stratiform cloud, which could
easily be the case with temperatures just below zero, then
coalescence can still occur with supercooled droplets which
then freeze on reaching the ground.
Page 11 of 16
Last Saved Date : 15 February 2016
File: MS-TRAIN-COLLEGE-WORK-d:\533546845.doc
Met Office College
Figure 1. The typical ascent for freezing rain.
Page 12 of 16
Last Saved Date : 15 February 2016
File: MS-TRAIN-COLLEGE-WORK-d:\533546845.doc
Precipitation Forecasting
6 Quantity of precipitation
The public often want to know how much rain they are likely
to get, rather than just whether it is going to rain. To
aid you in determining this, some stations broadcast
reports of the amount of rainfall that has fallen in the
past hour. These messages are of the form SREW iiiRRR,
where RRR is the rainfall in tenths of a millimetre. These
are readily available on NIMBUS, so by continuity you can
get a rough idea of the totals expected in your area,
bearing in mind the effects of orography on the
precipitation pattern.
For short-term forecasting of precipitation, rainfall radar
displays and UKMO’s NIMROD system are the best guide in
timing the onset and cessation of rain and for assessing
the movement and intensity of rain areas. Development of
showers and systems can also be monitored in this way.
7 Snow Predictors
7.1
Boyden’s technique using 1000 - 850 Thickness
The 1000–850 hPa thickness needs adjustment using the
following formula:
C= A + (H1000 - HGR)/30
Where:
C
= corrected value for 1000 - 850 hPa thickness
A
= actual 1000 - 850 hPa thickness (gpm)
(H1000) = the 1000 hPa height
(HGR) = height of station or ground above sea level
Using the corrected values of 1000–850 hPa thickness the
probability of snow is as follows:
Percentage probability of snow
Adjusted value of
1000–850 Technique hPa
thickness (gpm)
90%
70%
50%
30%
10%
1281
1290
1293
1298
1303
Figure 3 incorporates these corrections and it illustrates
how a snow probability may be estimated.
Page 13 of 16
Last Saved Date : 15 February 2016
File: MS-TRAIN-COLLEGE-WORK-d:\533546845.doc
Met Office College
Figure 3. Forecasting the probability of precipitation as snow. How to find the
probability of snow at a station with an elevation of 750 ft (230 m), a surface pressure of
990 hPa and a 1000–850 hPa thickness of 1295 gpm.
7.2
Height of wet bulb zero degree isotherm above
ground level
The height of the 0 °C wet-bulb temperature additionally
takes into account latent cooling effects. Beware of cold
surface air undercutting warm; Hand uses mean temperature
of the lowest 100 hPa.
Height of 0 °C wet-bulb
temperature
Form of precipitation
Below 1000 ft
Mostly snow; only light or
occasional precipitation
Page 14 of 16
Last Saved Date : 15 February 2016
File: MS-TRAIN-COLLEGE-WORK-d:\533546845.doc
Precipitation Forecasting
falls as rain.
7.3
1000–2000 ft
Persistent rain readily
turns to snow
2000–3000 ft
Mostly rain; snow unlikely
3000 ft or over
Almost always rain; snow
rare
Hand's rule
Use of mean temperature of lowest 100 hPa above ground to
predict type of precipitation at the surface. In heavy and
persistent precipitation lowest layers will be further
cooled by latent heat of evaporation.
7.4
Mean temperature (°C) in
lowest 100 hPa above surface
Precipitation type usually
reaching surface
< -1.5
snow
-1.5 to 0.5
sleet
> 0.5
rain
Height of zero degree isotherm above ground
level
o
Height of 0 C isotherm
agl (hPa)
Probability of snow
7.5
12
25
35
45
61
90%
70%
50%
30%
10%
Screen wet-bulb temperature technique (Lumb)
For an exposed station at height H (in hundreds of metres)
in central and western regions of the UK under moderate
easterly, or stronger, winds:
1.
for elevations up to 170 m:
Page 15 of 16
Last Saved Date : 15 February 2016
File: MS-TRAIN-COLLEGE-WORK-d:\533546845.doc
Met Office College

Rain turns to melting snow if: Tw < (2.1 - 0.6H) °C
 Rain turns to lying snow if:
Tw < (0.6H) °C
where Tw is surface wet-bulb temperature when
precipitation begins.
2. for elevations 170 to 350 m:

Snow probable if: Tw < (2.1 - 0.6H) °C
3. If Tw > 2.5 °C rain is more likely than sleet,
irrespective of elevation.
Winds should be at least moderate with a good cover of low
or medium level cloud.
Figure 2. Diagrammatic form of Lumb’s method for snow
prediction.
8 Further Reading
For further details on this topic the best reference source
is Met 0875, Handbook of Weather Forecasting, Chapter 19.
Section 19.7 deals specifically with the forecasting of
precipitation.
Page 16 of 16
Last Saved Date : 15 February 2016
File: MS-TRAIN-COLLEGE-WORK-d:\533546845.doc