[Dave W]

UKWW SEVERE WEATHER WATCHES AND WARNINGS


Defintion: severe weather is defined as any weather phenomenon or combination of phenomena that pose a significant risk of directly and/or indirectly causing one or more the following:

* Human casualties (indirectly = increased risk of RTAs for example)

* Damage to buildings and infrastructure

* Significant disruption to the supply of basic utilities, namely water, electricity and telephone

* Significant disruption to land-based travel, domestic, industrial and commercial activities


ADVANCED AND FLASH WARNINGS: DESCRIPTIONS

UKWW Advanced Watches and Warnings:

These are issued 36 hours* prior to an event's expected arrival. The minimum confidence level at which we issue it is 40%.
(*This is not always humanly possible of course but we try to keep to it!)

Two sub-categories:

* UKWW Weather Watch: see LH column of criteria (below).

* UKWW Weather Warning see RH column of criteria (below).


UKWW Flash Warnings:

* Issued if weather event is actually occuring or about to occur within the next few hrs. This may be for areas not included in earlier Warnings, or for upgrades from Watches to Warnings.  

UKWW - Watches and Warnings: Issuance Criteria
Severe Weather Type	Criteria: Weather Watch	Criteria: Weather Warning
Rainfall	25 mm in 0-6 hours or 30 mm in 0-12 hours or 40 mm in 0-24 hours or 60mm in 0-36 hours or 75mm in 0-48 hours	25 mm in 0-3 hours or 40 mm in 0-6 hours or 50 mm in 0-12 hours or 60 mm in 0-24 hour or 80mm in 0-36 hours or 100mm in 0-48 hours
Temperature	Freezes: Widespread low level daytime 2m air temperatures below 3C Heatwaves: Widespread 2m air temperatures in excess of +32C	Freezes: widespread low level daytime 2m air temperatures below 0C Heatwaves: widespread 2m air temperatures in excess of +34C
Snowfall	Snowfall rate, below 600m, of 2cm per hour or more. Snow accumulation, below 600m, of 3 cm or more during a single snowfall event. (Note that if snow looks to be restricted to "high ground" it is likely to affect roads between 300 and 600m above sea level.)	Snowfall rate, below 600m, of 4cm per hour or more. Snow accumulation, below 600m, of 6cm or more during a single snowfall event. (Note that if snow looks to be restricted to "high ground" it is likely to affect roads between 300 and 600m above sea level.)
Blizzards	Blizzards are Warning-only	Snow falling in a wind greater than 35 knots (40mph) below the 600m contour or gusting to over 40knts (45mph), with attendant whiteout reducing visibility to less than 50-200m and high drifting of falling snow. Snow not falling but high drifting of unconsolidated fallen snow - can sometimes be a major problem e.g. along sections of the A9 in Highland Scotland.
Freezing Rain / Ice	Not Applicable	Any freezing rain below the 600m contour - the slightest coating can cause multiple accidents and a rapidly-deteriorating situation.
Fog and Freezing Fog	Widespread long lasting (freezing) fog or fog patches with visibility less than 200m.	Widespread long lasting (freezing) fog or fog patches with visibility less than 100m.
Smog	Widespread dense smog, smoke, dust or poisonous chemical fumes with visibility less than 200m.	Widespread dense smog, smoke, dust or poisonous chemical fumes with visibility less than 100m.
Convective Storms	* Straightline wind gusts of 50 knots (60 mph) or more. * We will mention hail, deadly C-G lightning in text where applicable (NOTE: we use Rainfall Criteria for rain amounts)	* Hail with a diameter of 15mm or more. * Straightline wind gusts of 60 knots (70mph) or more. * Tornadoes - * C-G lightning   (NOTE: we use Rainfall Criteria for rain amounts) (NOTE: Typically these are issued as Flash Warnings where severe storms are being tracked)
Gales	Widespread (away from N and NW Scotland) of Straightline winds and/or widespread gusts, on land below 600m*, as follows: Gales: Mean windspeed of 35 knots (40mph) or more, gusting to 50 knots (60mph) or more *Note that the 600m contour includes all high-level trunk-routes such as the Trans-Pennine ones - high winds over land above that altitude are more of a hillgoer's concern.	Widespread Straightline winds and/or widespread gusts, on land below 600m*, as follows: Severe Gales: Mean windspeed of 45 knots (50mph) or more, gusting to 60 knots (70mph) or more Storm force winds**: Mean windspeed of 50 knots (60mph) or more, gusting to 60-70 knots (70-80mph) or more *Note that the 600m contour includes all high-level trunk-routes such as the Trans-Pennine ones, -high winds over land above that altitude are more of a hillgoer's concern.

UKww Warnings Team, revised June 2011