2 Chapter 13 reviewA persistence forecast is a prediction that future weather will be the same as the present weather, whereas a climatological forecast is based on the climatology of a particular region. A steady-state forecast assumes the weather systems will continue to move in the same direction and at the same speed as they have been moving so far.Weather forecasts for up to a few hours are called very short-range forecasts; those that range from about 6 hours to a few days are called short-range forecasts; medium-range forecasts extend from about 3-5 days into the future, whereas long-range forecasts extend beyond, to about 8.5 days.Seasonal outlooks provide an overview of how temperature and precipitation patterns may compare with normal conditions.For a forecast to show skill, it must be better than a persistence forecast or a climatological forecast.Predicting the movement of weather systemsSteady-state methodLows (Highs): towards the greatest pressure drop (rise)In the direction of the winds aloft (5500 m)
4 ThunderstormsStorm with lightning and thunder. Also heavy precipitation, sometimes hail, gusty surface winds.Favorable conditions:Unstable atmosphereStrong convection (convective storm)Buoyant force – the force on a less-denseobject immersed in a denser environmentTrigger for the convection:Weather fronts - fast uplift of warm airUnequal surface heatingSurface convergence and divergence aloftTopographic barriersArrival of cold air aloftEureka!
5 Ordinary (Cell) Thunderstorms Also: Air mass thunderstorms, pop-up thunderstorms.Typically they form where the surface converging winds have no significant change of either strength or direction with height (limited wind shear). What goes up comes back down in the same place.Typical summer afternoon thunderstormsLast no more than an hourExtend no more than a kilometerRarely produce strong winds or large hail
6 Stages of Development - I Cumulus (growth) stage:Updraft: warm and moist air is uplifted, expands, condenses and forms towering clouds.Latent heat is released -the temperature in the cloud is higher than outside the cloud, air keeps rising.The updrafts are strong so that the cloud droplets remain suspended in the cloud.Usually no precipitation and lightning during this stage.
7 Stages of Development - II Mature stage:Precipitation startsEntrainment: dry air from around the cloud is drawn into the cloud. Some of the cloud droplets evaporate and chill the air.Downdraft: the cold and heavy air is sinkingThe updraft and the downdraft form a storm cell.Gust front: the boundary between the cold and the warm air at the surfaceLightning and thunder
8 Stages of Development - III Dissipating stage:the updraft weakensthe gust fronts move awaythe precipitation is lightthe downdrafts dominate: cut off the fuel supply of the storm
9 Multicell StormsThe downdraft from the dissipating storm fuels the formation of next storm cell.
11 Severe Thunderstorms At least one of the following: Large hail precipitationstrong wind guststornadoes..The longer a storm lasts the grater the chance for it to become a severe storm. Often forms along a weather front.They typically form if strong vertical wind shear (large changes of the wind with height) is present. The warm air updraft is not suppressed by the downdraft and the precipitation.
18 Supercell thunderstorms Supercell storm: consists of a single violently rotating updraftFavorable conditions: when the speed and direction of the winds aloft change with heightformation of a rotating updraftUpdraft and downdraft do not cross -> the storm lives on for hoursOften produce large hail, damaging surface winds, tornadoesA supercell thunderstormwith a tornado sweeps over TexasSkip the discussion aroundFigs and 14.7
19 Squall Lines and MCCsSquall line: A line system of thunderstorms. It shows as a line of storms on the radar images.It forms along or in front of an advancing cold front.Pre-frontal squall lines may be due to gravity wavesMesoscale convective systems: a large circular cluster of storms
20 Thunderstorm Movement The storms typically move in the direction of the winds aloft (middle troposphere).Some move at 30 deg to the right of the winds aloft. (Effect of surface friction).In general the multicell storms move in the direction where the humid and unstable conditions in the atmosphere prevail.The squall line storms move in the direction of the front movement.
21 FloodingFlash floods: floods which rise rapidly with little or no advance warningConditions: the influx of water into an area is more than the amount of water that can drain away from the region.The ground is saturated from a prolonged rainy seasonVery strong precipitation over a short time periodNot so strong precipitation but for an extended time- storms reoccur at the same location (training).Stationary weather fronts can result in a series of thunderstorms over the same region.
23 Thunderstorm Climatology Most thunderstorms occur in FloridaUnstable atmospheric conditions (warm and moist surface air) prevail throughout the year.Summertime afternoon air mass thunderstorms.Most severe thunderstorms occur in the Great Planes.The warm (unstable) air is dry and shallow and the ice crystals do not have time to melt.Stormy days / yearDays with hail / year