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Introduction to QPF RFC/HPC Hydromet 01-2 Presented by Wes Junker Wednesday, 6 December

Introduction to QPF RFC/HPC Hydromet 01-2 Presented by Wes Junker Wednesday, 6 December. Introduction to QPF. Must determine Where When How Much rainfall will occur Must undersand the processes that determine the size, scale and intensity of an area of precipitation

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Introduction to QPF RFC/HPC Hydromet 01-2 Presented by Wes Junker Wednesday, 6 December

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  1. Introduction to QPFRFC/HPC Hydromet 01-2Presented byWes JunkerWednesday, 6 December

  2. Introduction to QPF • Must determine • Where • When • How Much rainfall will occur • Must undersand the processes that determine the size, scale and intensity of an area of precipitation • (synoptic, mesoscale, and even microscale meteorology) • Must ..Possess Good Pattern Recognition Skills and understand what gives the pattern the potential to produce signficant rainfall • Must Possess a working Knowledge of • Local Climatology • Understand numerical models • especially model biases and why they occur • These are gained through experience and research

  3. WHERE, WHEN, AND HOW MUCH WHERE and WHEN does precipitation fall ? Generically, Precipitation is produced in regions of combinedmoisture and lift. How much precipitation will fall ? Is determined by • Available moisture • Intensity of precipitation • Will the precipitation be convective or not? • Areal coverage of precipitaiton • Speed of precipitation area • Existence of training cells • Enhancement by boundaries/topography The heaviest precipitation usually occurs in regions of high moisture and best lift where the atmosphere is most unstable (instability).

  4. QUESTIONS TO ASK WHEN PREPARING A QPF • WHAT IS THE TIME RANGE AND PERIOD OF THE FORECAST? • IS THIS SYNOPTIC OR MESOSCALE PATTERN ONE YOU RECOGNIZE? • WHAT TYPE PRECIPITATION EVENT AM I DEALING WITH, CONVECTIVE OR STRATOFORM? OR SOME COMBINATION OF BOTH? • DOES THIS PATTERN FAVOR HEAVY OR LIGHT RAINFALL • HOW CONFIDENT ARE YOU OF YOUR FORECAST? • IF YOU LACK CONFIDENCE, BE CONSERVATIVE

  5. ALSO ASK • Which model is handling each system best • use a model qpf as a background • then try to figure out what the model is doing correctly and incorrectly • if no model is clear choice use an ensemble approach based on the various models • eta, eta/kf, mm5, avn, gem, nogaps • models do not do a good job predicting heavy rainfall. Use the models mass fields and knowledge of model performance to help forecast the heavier rains.

  6. THE AMOUNT OF RAINFALL THAT FALLS OVER AN AREA DEPENDS ON • SIZE OF THE RAINFALL AREA • THE INTENSITY OF THE RAINFALL WITHIN IT • HOW FAST THESE AREAS MOVE • HOW FAST NEW RAIN BEARING CLOUDS ARE FORMING UPSTREAM (PROPAGATION)

  7. A FEW IDEAS TO HELP DETERMINE HOW BIG AN AREA OF RAINFALL TO FORECAST • THE SIZE IS DEPENDENT ON HOW MUCH MOISTURE IS PRESENT AND ON THE STRENGTH OF THE MOISTURE TRANSPORT • IS DEPENDENT ON BOTH THE ABSOLUTE (PWS, MIXING RATIOS) AND RELATIVE MOISTURE (RH) • THE SIZE IS DEPENDENT ON THE SYSTEMS MOVEMENT • SIZE IS DEPENDENT ON THE SCALE OF THE FORCING • PATTERN RECOGNITION IS ONE OF THE BEST TOOLS TO USE WHEN TRYING TO FORECAST THE SCALE OF THE EVENT. • MODEL GUIDANCE PROVIDES A DECENT FIRST GUESS, ESPECIALLY OF COOL SEASON STRATOFORM EVENTS

  8. PATTERN RECOGNITION REMAINS IMPORTANT DESPITE EXAMPLES BY RAUBER AND BOSART (1997). THEY NOTED THAT SIMILAR SYNOPTIC PATTERNS CAN PRODUCE VERY DIFFERENT QPFS • HOWEVER, HEAVY RAINFALL EVENTS SHARE CERTAIN CHARACTERISTICS. • EVEN IN WINTER, HEAVY RAIN USUALLY FALLS IN MESOSCALE BANDS • HEAVY RAINFALL EVENTS CAN OFTEN BE IDENTIFIED BY THEIR PATTERNS • BUT YOU NEED TO UNDERSTAND WHAT IT IS ABOUT THE PATTERN THAT FAVORS HEAVY RAINFALL • YOU NEED TO UNDERSTAND THE PHYSICS. • HOWEVER, PATTERNS VARY BY • SEASON, GEOGRAPHIC REGION AND SCALE • PATTERNS ARE IDENTIFIED • BY CONVENTIONAL DATA, MODEL OUTPUT, SATELLITE AND RADAR IMAGERY

  9. START BY LOOKING AT SYNOPTIC SCALE (THE BIG PICTURE) • THERE IS A CLEAR ASSOCIATION BETWEEN SHORT-WAVE TROUGHS AND CONVECTION • THE VERTICAL MOTION ASSOCIATED WITH SYNOPTIC SCALE LIFT DOES NOT TYPICALLY ALLOW PARCELS TO REACH THE LEVEL OF FREE CONVECTION (LFC) • HOWEVER, LARGE SCALE LIFT • STEEPENS LAPSE RATE • PROMOTES MOISTURE TRANSPORT • WEAKENS CAP • AFFECTS VERTICAL SHEAR (more important for severe weather forecasting)

  10. NEXT LOOK FOR MESOSCALE FEATURES • DO A MESOANALYSIS OF SURFACE AND UPPER AIR DATA IF TIME ALLOWS. • LOOK AT SATELLITE AND RADAR AND TRY TO IDENTIFY MESOSCALE FEATURES. ALSO TRY TO DETERMINE WHAT IS CAUSING THE CURRENT PRECIPITATION. • IDENTIFY SURFACE BOUNDARIES • (FRONTS, DRY LINES, OUTFLOW BOUNDARIES, SEA BREEZE FRONTS, LAND USE BOUNDARIES, ETC.

  11. USE MODELS TO IDENTIFY SYNOPTIC AND MESOSCALE PATTERNS THAT ARE FAVORABLE TO HEAVY RAINS • CAN USE THE SURFACE, 850- AND 500-MB PATTERNS TO IDENTIFY MADDOX ET AL. OR OTHER TYPES OF HEAVY RAINFALL EVENTS • ALSO NEED TO LOOK CLOSELY AT MOISTURE, MOISTURE TRANSPORT AND INSTABILITY • MODELS OFTEN PROVIDE DECENT FORECASTS OF LOW-LEVEL WIND AND MOISTURE FIELDS • 850 MOISTURE TRANSPORT (MOISTURE FLUX) • PWS • OUTPUT CAN BE USED TO ASSESS FORCING AND TO FORECAST THE LOCATION OF BOUNDARIES. • HOWEVER, UNDERSTAND THE LIMITATIONS OF THE MODELS AND BEWARE OF MODEL BIASES!!

  12. PRECIPITATION INTENSITY • IS PROPORTIONAL TO THE VERTICAL MOISTURE FLUX INTO THE CLOUD. • THEREFORE, FORECASTS SHOULD START WITH AN ASSESSMENT OF HOW MUCH MOISTURE WILL BE AVAILABLE • NEED TO ESTIMATE WHAT PROPORTION OF THE MOISTURE ENTERING THE CLOUD SYSTEM WILL FALL AS RAIN (THE EFFICIENCY OF THE SYSTEM) • NEED TO ASSESS THE LIFTING • ARE MESOSCALE SOURCES OF LIFT PRESENT? • WILL TERRAIN CONTRIBUTE TO LIFTING • HOW MUCH POTENTIAL BUOYANT ENERGY (PBE) IS PRESENT? PBE GIVES AN IDEA OF THE STRENGTH OF THE UPDRAFT DURING ANY CONVECTION THAT DEVELOPS.

  13. MORE ON PRECIPITATION EFFICIENCY OF A SYSTEM • SOME OF INFLOWING WATER VAPOR PASSES THROUGH THE SYSTEM WITHOUT CONDENSING • OF THE VAPOR THAT CONDENSES • SOME EVAPORATES • SOME FALLS AS PRECIPITATION • SOME IS CARRIED AWAY AS CLOUDS (PERHAPS EVAPORATING SOMEWHERE ELSE) • a dry layer inhibits precipitation production • strong shear also not condusive to efficient precipitation production FROM DOSWELL NOTES, 1995

  14. PRECIPITATION EFFICIENCY FACTORS(The cloud physics) Warm rain processes are more efficient • WANT A DEEP WARM LAYER • RAINFALL INTENSITY WILL BE GREATER IF DEPTH OF WARM LAYER FROM LCL TO 0oC ISOTHERM IS 3-4 KM. • LOW CLOUD BASE (USUALLY OCCURS WITH HIGHER RELATIVE HUMIDITIES) • COLLISION-COALESCENCE PROCESSES ARE ENHANCED BY INCREASED RESIDENCE TIME IN CLOUD • WANT A BROAD SPECTRUM OF CLOUD DROPLET SIZES • THIS IS PRESENT WHEN AIRMASSES HAVE HAD LONG TRAJECTORIES OVER OCEANS. • WEAK TO MODERATE SHEAR

  15. An inch an hour rainfall rates peak in June (left), Frequency (events/year) or 1 in. h-1 or larger rainfall totals for July objectively analyzed to a regular grid form the HPD stations. Contour intervals of 0.1, 0.2, 0.25,0.33,.0.5,0.66,0.75, and 1.0 events year -1 From Brooks and Stensrud 2000 MWR

  16. NOTE LOGARITHMIC DECAY OF RAINFALL RATES From Brooks and Stensrud 2000 MWR Shadyside Oh, Fort Collins Co, and Madison County VA all had extreme rainfall events. How do I tell when rainfall rates will be 1/hr verus 4 or 5 inchers per hour???

  17. Will convection occur • convection produces most heavy rainfall events • there are three ingredients needed for deep moist convection • moisture • instability • upward motion

  18. ASSESSING INSTABILITY • SOUNDINGS ARE BEST TOOL, LOOK FOR • DEPTH OF MOISTURE • VERTICAL WIND PROFILE • CAPE AND CIN • EQUILIBRIUM LEVEL (WARM TOP CONVECTION) • STABILITY INDICES (LIFTED, K, TOTALS, SHOWALTER) • K INDICES ARE A GOOD INDICATOR OF THE DEPTH OF THE MOISTURE

  19. ANTICIPATE HOW THE STABILITY IS CHANGING • THE LAPSE RATE CAN BE CHANGED BY • DIABATIC HEATING • ADVECTION OF A DIFFERENT LAPSE RATE INTO THE AREA • DIFFERENTIAL ADVECTION OF TEMPERATURE • VERTICAL MOTION/DIFFERENTIAL VERTICAL MOTION

  20. CAPE • THE POSITIVE AREA OF THE SOUNDING BETWEEN THE LFC AND EQUILIBRIUM LEVEL • THEORETICAL MAXIMUM VALUE OF UPDRAFTS WITHIN A STORM =(2CAPE)1/2 • CAPE IS A BETTER INDICATOR OF INSTABILITY THAN ANY INDEX THAT USES ONLY MANDATORY LEVELS • WHILE INSTABILITY IS PRESENT WITH ALMOST ALL HEAVY RAINFALL EVENTS, HIGH CAPES ARE NOT NEEDED FOR HEAVY RAINS • STORMS HAVING MODERATE CAPE ARE USUALLY MORE EFFICIENT • MODELS OFTEN DON’T FORECAST CAPE WELL

  21. TO RELEASE CONVECTIVE AVAILABLE POTENTIAL ENERGY • SYNOPTIC SCALE FORCING DOES NOT ACT QUICKLY ENOUGH TO BREAK A CAP. • BUT DOES ACT TO MOISTEN THE AIRMASS AND WEAKEN THE CAP • YOU NEED MESOSCALE SOURCE OF LIFTING TO REACH LEVEL OF FREE CONVECTION. • LOW-LEVEL BOUNDARIES, FRONTS • LOW LEVEL CONVERGENCE • TRY TO FIND BOUNDARIES IN TEMPERATURE, DEWPOINT, THETA-E AND WIND FIELDS

  22. IMPORTANCE OF CINTHE NEGATIVE AREA OF THE SOUNDING • EARLY IN DAY SOUNDING OFTEN HAS INVERSION • WHEN A STEEP LAPSE RATE IS LOCATED ABOVE INVERSION, YOU HAVE CLASSIC LOAD GUN SOUNDING • THE CAP HELPS STORE ENERGY LEADING TO HIGHER POTENTIAL BUOYANT ENERGY LATER IN THE DAY OR EVENING • BLUESTEIN AND JAIN (1985) HAVE SUGGESTED THAT SLIGHTLY STRONGER CIN UPSTREAM MIGHT SOMETIMES LEAD TO BACKBUILDING CONVECTION

  23. MOVEMENT OF THE SYSTEM • SLOW MOVING SYSTEMS ARE USUALLY THE HEAVIEST RAINFALL PRODUCERS • AT SHORTER TIME RANGES-EXTRAPOLATION BASED ON RADAR AND SATELLITE PROVIDES PRIMARY GUIDANCE • AT LONGER RANGES, MODELS PROVIDE DECENT GUIDANCE • YOU STILL NEED TO TAKE INTO ACCOUNT MODEL CHARACTERISTICS AND BIASES. • AT ALL TIME RANGES, YOU MUST ANTICIPATE WHEN NEW ACTIVITY MAY FORM UPSTREAM

  24. Movement of a system is dependent on cell movement and propagation Actual cell motion • Individual convective cells usually move at around 90% of the mean wind with a slight deviation to the right • Propagation is dependent on how fast new cells form along some flank of the system 850-300 mb mean wind Propagation, the rate new cells are forming upstream System movement

  25. PROPOGATION IS ALSO DEPENDENT ON • OUTFLOW • EVAPORATIONAL COOLING RELATED TO THE ENVIRONMENTAL HUMIDITY • GUST FRONT SPEED RELATED TO TEMPERATURE DEFICIT BETWEEN OUTFLOW AND AIR AROUND IT. • HON-HYDROSTATIC PRESSURE GRADIENTS • INTERACTION OF UPDRAFT WITH ENVIRONMENTAL WIND • STORM RELATIVE WINDS • DETERMINES WHERE LOW LEVEL CONVERGENCE WILL BE LOCATED.

  26. Schematic representing the affect the shape and movement of a system has on the rainfall at a particular point. The shaded colors on the system represent the radar echoes. You live at the blue dot RAINFALL RATE RAINFALL RATE RAINFALL RATE RAINFALL RATE TIME TIME TIME TIME From Doswell et al., 1996 (Weather & Forecasting, 11, 560-581)

  27. When the moisture convergence is aligned with the 850-300 mb mean flow, a sizeable area of 3” precipitation is more likely. THE Y-AXIS REPRESENTS THE LENGTH OF THE -2X10-7 S-1 OR GREATER MOISTURE FLUX CONVERGENCE MEASURED UPSTREAM ALONG A LINE DEFINED BY THE MEAN FLOW. a 3” area is 3600 sq. nm less likely area of 3” more likely (inches)

  28. During maturity, cells apparently move towards the right. The active part of an MCC moves to the right of the mean flow. 360 360 Right of mean flow r=.62 330 330 300 300 Observed cell direction during MCC maturity Mean 850-300 mb wind direction 270 270 240 240 210 210 r=.83 Right of mean flow 180 180 330 390 360 300 270 240 210 180 360 330 300 180 240 270 210 Observed direction of movement of the most active part of MCS Mean 850-300 mb wind direction From Corfidi

  29. 300 240 Direction of low-level jet 180 120 60 r=.65 180 420 480 300 360 240 Direction of MBE propagation The direction of the MBE (the most active part of the MCS) is dependent on the direction of the low-level jet (Corfidi et al., 1997) and on the position of the most moist and unstable air relative to the MCS. The direction of propagation is in the opposite direction of the low-level jet. This may be why MCCs tend to track to the right of the mean wind. Systems with propagation vectors between 0-120 degrees have been plotted between 360 and 480 degrees From Corfidi

  30. THE PROPAGATION OF A CONVECTIVE SYSTEM IS DEPENDENT ON THE LOCATION OF: 1) THE MOST UNSTABLE AIR, 2) THE AXIS AND ORIENTATION OF THE LOW-LEVEL JET, AND 3) THE LOCATION OF THE STRONGEST LOW-LEVEL MOISTURE CONVERGENCE 1. FORWARD DIRECTION OF PROPAGATION MCS AXIS OF LOW-LEVEL JET UNSTABLE AIR 1000-500 THICKNESS 2. BACKWARD N E W ADOPTED FROM JIANG AND SCOFIELD, 1987 S UNSTABLE AIR

  31. THICKNESS CONSIDERATIONS • MCCS OFTEN TRACK ALONG THE 1000-500 MB THICKNESS LINES • THE AMOUNT OF MOISTURE NEEDED TO PRODUCE A LARGER SCALE MCS OR MCC APPEARS TO BE DEPENDENT ON THE 1000-500 THICKNESS AND THE OBSERVED PW (Relative humidity) • RAINFALL OFTEN OCCURS ALONG A FAVORED THICKNESS CHANNEL • WATCH FOR MCC DEVELOPMENT AND HEAVY RAIN IN AREAS OF DIFLUENT THICKNESS

  32. Area with most unstable Lifted Indices shaded. 35 TO 40 kt winds are feeding across KS into NE An almost e-w frontal band with PWS 1.80” or higher (shaded) An example of a quasi-stationary convective system The most unstable air is usually found upstream of the initial convection during backbuilding or quasi-stationary convective events JUNKER AND SCNEIDER, 1997, NAT. WEA. DIGEST, ,21, 5-17

  33. 1)mean windsthat are directed slightlyaway from the front, Factors favorable to quasi-stationary convection 2) a low-level 1e ridge to west, and 3) the location of the strongest moisture convergence west of the initial convection 00Z 00Z 1000-850 mb layer mean moisture flux (vectors)moisture flux magnitude (dashed) and moisture flux divergence (-4 x10-7s-1 are shaded), the red dot represents the location where convection started 850-300 mb mean winds, 982 mb equivalent potential temperature (dashed) and msl pressure (solid) JUNKER AND SCNEIDER, 1997, NAT. WEA. DIGEST, ,21, 5-17

  34. 21Z 00Z 03Z 06Z MOISTURE CONVERGENCE STRENGTHENS OVER EASTERN NE AS PRESSURES FALL IN RESPONSE TO THE APPROACH OF A WEAK SURFACE WAVE MSL PRESSURE (THICK SOLID), MOISTURE CONVERGENCE (HIGHEST VALUES SHADED), RED DOT IS WHERE INITIAL CELL FORMED THE WIND AND MOISTURE CONVERGENCE FIELDS CAN CHANGE RAPIDLY AS A RESULT OF PRESSURE RISES OR FALLS. THE CORFIDI VECTOR METHOD MAY NOT CATCH RAPID CHANGES IN THE WIND FIELD.

  35. 1st cell 00Z 21Z New cells form upstream Merger 02Z 06Z DURING THE 1993 DSM FLASH FLOOD, THE CONVECTIVE SYSTEM REMAINED STATIONARY FOR ABOUT 9 HOURS, WHY? Accumulated precipitation from the storm

  36. FACTORS THAT LEAD TO TRAINING OR REGENERATION OF CONVECTION • A SLOW MOVING LOW-LEVEL BOUNDARY OR FRONT • A QUASI-STATIONARY LOW-LEVEL JET • A QUASI-STATIONARY AREA OF UPPER-LEVEL DIVERGENCE • A LOW-LEVEL BOUNDARY (MOISTURE CONVERGENCE) ALMOST PARALLEL TO THE MEAN FLOW • LACK OF STRONG VERTICAL SHEAR

  37. Even if you know an MCS will form and know how it will move, it is extremely difficult to predict where 3 inches or more of rain will fall From Kane et al., 1987* Cluster around propagation axis the probability of 1 mm of rain is 100% but for 75 mm drops to 10% (red area)

  38. The probability of 1” in 6 hours (heavy rainfall) is low (from Charba 1985). How do you predict 4 inches in 24 hours?

  39. Forecasts were best with large storms. Studies were made of the relationship of various contour intervals to other ones. Larue and Younkin, Mon. Wea. Rev. (1963) Found that most of the volume for a typical large volume storm was due to coverage of rainfall amounts that were 1 inch or less less

  40. PATTERN RECOGNITION, IS THIS A MADDOX FRONTAL TYPE EVENT? WARM FRONT? WHAT IS THE SIGNIFICANCE OF THE 500H SHORTWAVE APPROACHING THE RIDGE AXIS?

  41. BOUNDARY LAYER WIND AND TEMPERATURE FORECAST V.T. 00Z 18 JULY THE WHITE LINE INDICATES A THERMAL BOUNDARY THAT SHOWS UP IN THE FORECAST

  42. A STRONG LOW LEVEL JET IS PRESENT WITH LOTS OF MOISTURE DO YOU THINK THERE WILL BE A SIZEABLE 3” AREA? IS IT TIME TO CALL EMERGENCY MANAGERS? FOR WHICH STATE? MINNESOTA? WISCONSIN? IOWA? ILLINOIS?

  43. OOZ 18 JULY FORECASTS OF 250 JET AND DIVERGENCE BEST LI AND BOUNDARY LAYER WINDS A SHORTWAVE AND JET STREAK IS APPROACHING THE RIDGE. UNSTABLE LIS ALONG THE SURFACE BOUNDARY

  44. IS THIS A GOOD QPF? DO YOU THINK THE RAINFALL IS ORIENTED CORRECTLY WOULD YOU PREDICT MORE RAINFALL THAN 2 .5” FOR A MAX? WHAT DO YOU THINK ABOUT THE PLACEMENT OF THE RAINFALL?

  45. REMEMBER TO LOOK FOR LOW-LEVEL BOUNDARIES. NOTICE THE THERMAL GRADIENT OVER IL RUC SURFACE WINDS, TEMPERATURES AND MOISTURE CONVERGENCE RUC MOISTURE FLUX FORECAST V.T. 06Z THE LAKE BREEZE FRONT FOCUSSED CONVECTION OVER ILLINOIS

  46. HOW DID YOU DO? THIS IS FAIRLY TYPICAL OF OUR HANDLING OF MCCS. WE OFTEN KNOW WHEN ONE WILL FORM BUT USUALLY MISS THE EXACT LOCATION OF THE HEAVIEST RAINFALL. MODEL FORECAST OBSERVED 6” OR MORE 3” OR MORE 1” OR MORE

  47. SHORT RANGE (0-6 HR) FORECASTS • RELY PRIMARILY ON CURRENT OBSERVATIONS AND TRENDS • NEXRAD AND SATELLITE IMAGERY ARE GREAT TOOLS PROVIDING INFORMATION ON THE , SIZE AND INTENSITY AND MOVEMENT OF PRECIPITATION SYSTEMS • HAVE TO KNOW LIMITATIONS OF OBSERVING SYSTEMS • STILL HAVE TO ANTICIPATE NON-LINEAR CHANGES • NEW CELLS FORMING UPSTREAM

  48. RADAR IS A GREAT TOOL FOR MAKING SHORT RANGE FORECASTS • NEXRAD SUPPLIES ESTIMATES OF RAINFALL RATES, ACCUMULATIONS • HIGH TEMPORAL AND SPACIAL RESOLUTION • RADAR SUPPLIES ESTIMATES BETWEEN RAIN GUAGES. • YOU CAN LOOP IMAGES TO SEE • CELL/SYSTEM MOVEMENT • WHETHER CELLS ARE TRAINING • DESPITE STRENGTHS, KNOW LIMITATIONS

  49. LIMITATIONS OF THE THE NEXRAD ESTIMATES • BEAM MAY OVERSHOOT MAXIMUM REFLECTIVITY • BEAM BLOCKAGE • BRIGHT BANDING AND HAIL CONTAMINATION • THE MAXIMUM THRESHOLD REFLECTIVITY (USUALLY 53 dBZ) • VARIATION OF Z-R RELATIONSHIPS • DEPENDENT ON DROPLET SIZE AND DISTRIBUTION • BEWARE OF TROPICAL AIRMASSES

  50. HOW THE UPPER LEVEL JET AFFECTS WEATHER SYSTEMS • JET STEAKS HAVE BEEN ASSOCIATED WITH • VARIATIONS IN STRENGTH OF THE LOW-LEVEL JET • CYCLOGENESIS AND MAJOR SNOWSTORMS • FRONTOGENESIS • REMEMBER CURVATURE AND CHANGES IN THE WIND SPEED ARE BOTH IMPORTANT

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