Introduction to QPFIntroduction to QPF

Wes Junker Wes Junker NCEP/HPC (ret.)NCEP/HPC (ret.)

(with minor tweaks/modifications by SMR)(with minor tweaks/modifications by SMR)

The COMETThe COMET®® Program ProgramUniversity Corporation For Atmospheric Research

OutlineOutline

IntroductionIntroductionWhere?Where?When?When?How Much?How Much?Other QPF Considerations Other QPF Considerations SummarySummary

Quantitative Precipitation Forecasts Quantitative Precipitation Forecasts (QPF)(QPF)

Focus of talk is Focus of talk is liquidliquid precipitation precipitationPredicting how much precipitation Predicting how much precipitation

will fall during a specified time will fall during a specified time periodperiod

How hard can How hard can thatthat be? be?

Composing a QPF Composing a QPF isis Difficult Difficult

A forecaster must determine:A forecaster must determine:– Where? Where? (will precipitation fall)(will precipitation fall)

– When? When? (will precipitation fall)(will precipitation fall)

– How Much? How Much? (precipitation will fall)(precipitation will fall)

Requirements:Requirements:– Good analysis and forecasting skillsGood analysis and forecasting skills– Good pattern recognition skillsGood pattern recognition skills– Working knowledge of local climatology and precipitation Working knowledge of local climatology and precipitation

processes at multiple scales processes at multiple scales (synoptic/mesoscale/microscale)(synoptic/mesoscale/microscale)

– Good understanding of numerical forecast models Good understanding of numerical forecast models (strengths/limitations/biases)(strengths/limitations/biases)

Where, When and How Much?Where, When and How Much? Generically, precipitation is produced in regions Generically, precipitation is produced in regions

of combined moisture and liftof combined moisture and lift Precipitation amount is determined by:Precipitation amount is determined by:

–   Moisture availabilityMoisture availability– Precipitation intensity (convective/stratiform/both?)Precipitation intensity (convective/stratiform/both?)

Heaviest precipitation usually occurs in regions Heaviest precipitation usually occurs in regions of high moisture and best lift where the of high moisture and best lift where the atmosphere is most unstableatmosphere is most unstable

Challenge to forecasters: Challenge to forecasters: – find the regions of best moisture, lift and instabilityfind the regions of best moisture, lift and instability– identify how fast the “MLI” regions will moveidentify how fast the “MLI” regions will move

Questions to Ask Questions to Ask When Preparing a QPFWhen Preparing a QPF

1.1. What is the time range and forecast period?What is the time range and forecast period? 2.2. Which NWP model is ‘handling’ the mass/wind Which NWP model is ‘handling’ the mass/wind

fields best?fields best?3.3. Is the present synoptic or mesoscale pattern Is the present synoptic or mesoscale pattern

conducive to heavy precipitation?conducive to heavy precipitation? 4.4. Is a MCS likely to develop? Is a MCS likely to develop?

• Need to forecast initiation, mode, scale, and motionNeed to forecast initiation, mode, scale, and motion• NOTNOT handled well by large-scale NWP models handled well by large-scale NWP models

5.5. What type of precipitation event is likelyWhat type of precipitation event is likely?? Convective?Convective? Stratiform?Stratiform? Combination of both?Combination of both?

Questions to Ask Questions to Ask When Preparing a QPF (cont.)When Preparing a QPF (cont.)

66.. How confident are you in your How confident are you in your forecast?forecast?

If you lack confidence, be conservative!If you lack confidence, be conservative!

Where?Where?

How Large of an Area?How Large of an Area?

To help determine how large an area of To help determine how large an area of rainfall to expect, consider:rainfall to expect, consider:1.1. Scale of the synoptic forcing (size/strength)Scale of the synoptic forcing (size/strength)2.2. Moisture availability, which depends upon:Moisture availability, which depends upon:

absolute/relative ambient moisture (PW/mixing absolute/relative ambient moisture (PW/mixing ratio/RH)ratio/RH)

strength of moisture transport into the areastrength of moisture transport into the area

3.3. Anticipated system movement during its lifetime Anticipated system movement during its lifetime (fast or slow?)(fast or slow?)

4.4. Pattern recognitionPattern recognition5.5. Model guidance Model guidance

A good first guess, especially for cool-season eventsA good first guess, especially for cool-season events

Importance of Pattern RecognitionImportance of Pattern Recognition

Pattern recognition is VERY useful to QPFPattern recognition is VERY useful to QPF Patterns vary by: Patterns vary by:

– SeasonSeason– Geographic regionGeographic region– ScaleScale

Patterns can be identified usingPatterns can be identified using– Conventional dataConventional data– Model outputModel output– Satellite imagerySatellite imagery– Radar imageryRadar imagery

Verify patterns by looking at previous events Verify patterns by looking at previous events

Pattern Recognition (cont.)Pattern Recognition (cont.)

Heavy rainfall events can often be Heavy rainfall events can often be identified by their patternsidentified by their patterns– Must understand Must understand whywhy pattern favors pattern favors

heavy precipitation!heavy precipitation! Heavy rainfall events often share Heavy rainfall events often share

certain characteristicscertain characteristics– Even in winter, heavy rain (and snow) Even in winter, heavy rain (and snow)

usually falls in usually falls in mesoscalemesoscale bands bands

Well-established connection between Well-established connection between short-wave troughs and convectionshort-wave troughs and convection– ‘‘Primes’ area for convectionPrimes’ area for convection– However, synoptic-scale UVM rarely lifts However, synoptic-scale UVM rarely lifts

parcels to their LFCs (by itself)parcels to their LFCs (by itself) What does large-scale lift do, then?What does large-scale lift do, then?

– Steepens the lapse rateSteepens the lapse rate– Promotes moisture transportPromotes moisture transport– Weakens convective inhibition Weakens convective inhibition (cap)(cap)

– Changes vertical wind shear Changes vertical wind shear (more for severe wx)(more for severe wx)

Pattern Recognition Process:Pattern Recognition Process:Step 1: Synoptic ScaleStep 1: Synoptic Scale

Do a mesoanalysis of surface Do a mesoanalysis of surface andand upper- upper-air features air features (if time allows)(if time allows)

Perform a multi-sensor analysis to Perform a multi-sensor analysis to identify surface boundaries identify surface boundaries

– Sources of lift and focus of convectionSources of lift and focus of convection

– Fronts, drylines, outflow boundaries, sea breeze Fronts, drylines, outflow boundaries, sea breeze fronts, etc.fronts, etc.

Examine satellite and radar imagery to Examine satellite and radar imagery to determine what is causing any current determine what is causing any current precipitationprecipitation

Pattern Recognition Process:Pattern Recognition Process:Step 2: MesoscaleStep 2: Mesoscale

Look for favorable synoptic and mesoscale heavy Look for favorable synoptic and mesoscale heavy precipitation patterns in NWP output (large-scale precipitation patterns in NWP output (large-scale and mesoscale models)and mesoscale models)

Models can provide decent forecasts of low-level Models can provide decent forecasts of low-level wind and moisture fields for assessing moisture wind and moisture fields for assessing moisture transport (sfc/850/700 hPa)transport (sfc/850/700 hPa)

Model products are useful for assessing areas of Model products are useful for assessing areas of forcing and may be useful for forecasting locations of forcing and may be useful for forecasting locations of larger boundarieslarger boundaries

Models also provide actual QPF numbers, but use Models also provide actual QPF numbers, but use these with caution (esp. during convection)these with caution (esp. during convection)

Remember: be aware of model limitations when Remember: be aware of model limitations when performing these assessments!performing these assessments!

Pattern Recognition Process:Pattern Recognition Process:Step 3: Using NWPStep 3: Using NWP

When?When?

Will Convection Occur?Will Convection Occur?

Answering “When?” often requires determining Answering “When?” often requires determining ifif and and whenwhen convection will occur convection will occur

Convection produces Convection produces mostmost heavy rainfall events heavy rainfall events Three ingredients necessary to initiate deep moist Three ingredients necessary to initiate deep moist

convection (DMC)convection (DMC)1.1. MoistureMoisture2.2. InstabilityInstability3.3. Upward vertical motion (lift)Upward vertical motion (lift)

Large-scale models have limitations forecasting Large-scale models have limitations forecasting convectionconvection

– Parameterization of sub-grid scale processesParameterization of sub-grid scale processes– Some mesoscale models now predict convection explicitlySome mesoscale models now predict convection explicitly

Assessing InstabilityAssessing Instability

Soundings are the best tools to Soundings are the best tools to assess instability and wind profile:assess instability and wind profile:

– CAPE/CIN (don’t forget parcel choice!)CAPE/CIN (don’t forget parcel choice!)– Stability indices Stability indices (LI, K, TT, Showalter)(LI, K, TT, Showalter)

– Depth of moisture Depth of moisture (K index useful)(K index useful)

– Vertical wind profile Vertical wind profile (how much shear?)(how much shear?)

– Equilibrium level Equilibrium level (warm top convection?)(warm top convection?)

CAPECAPE

CAPE is a better indicator of instability than CAPE is a better indicator of instability than any index that only uses mandatory levels any index that only uses mandatory levels (LI/SI/KI/TT)(LI/SI/KI/TT)

Most heavy precipitation events have some Most heavy precipitation events have some instability, but they do not instability, but they do not requirerequire high CAPE high CAPE

– In fact, storms with high precipitation efficiencies In fact, storms with high precipitation efficiencies typically have moderate CAPE valuestypically have moderate CAPE values

– ‘‘Shape’ of the CAPE important in +RA eventsShape’ of the CAPE important in +RA events– ‘‘Skinny’ (distributed) CAPE vs. ‘fat’ CAPE (strong Skinny’ (distributed) CAPE vs. ‘fat’ CAPE (strong

UVM)UVM) Models generally do not forecast CAPE wellModels generally do not forecast CAPE well

CINCIN

The negative area (energy) on the The negative area (energy) on the sounding mainly below the LFC (AKA cap)sounding mainly below the LFC (AKA cap)

Often the key for determining “Often the key for determining “when/ifwhen/if” ” convection will occurconvection will occur

CIN can either:CIN can either:– ‘‘store’ energy, allowing it to be released store’ energy, allowing it to be released

explosively later; orexplosively later; or– inhibit convection completelyinhibit convection completely

Models generally do not predicted CIN Models generally do not predicted CIN wellwell

CIN (cont.)CIN (cont.)

Bluestein and Jain (1985) suggested that slightly Bluestein and Jain (1985) suggested that slightly stronger upstream CIN may lead to backbuilding stronger upstream CIN may lead to backbuilding convectionconvection

Convection will fire in areas of weakest CIN first, Convection will fire in areas of weakest CIN first, followed by areas with slightly higher CIN valuesfollowed by areas with slightly higher CIN values

Changing StabilityChanging Stability

A forecaster needs to anticipate how the A forecaster needs to anticipate how the stability is changingstability is changing

Lapse rate can change due to:Lapse rate can change due to:– Diabatic heating/coolingDiabatic heating/cooling– Lapse rate ‘advection’Lapse rate ‘advection’– Differential temperature advectionDifferential temperature advection– (Differential) vertical motion(Differential) vertical motion

Models often miss stability changesModels often miss stability changes

How Much?How Much?

Rainfall AmountRainfall Amount

Amount of rainAmount of rainfall that falls over an fall that falls over an area depends on three factorsarea depends on three factors::

1.1. INTENSITY of the rainfallINTENSITY of the rainfall

2.2. SIZE of the rainfall areaSIZE of the rainfall area

3.3. PROPAGATIONPROPAGATION of the rainfall areaof the rainfall area

Rainfall Amount (cont.)Rainfall Amount (cont.)

Additional rainfall total Additional rainfall total considerations:considerations:

1.1. How much moisture is available?How much moisture is available?

2.2. Will the precipitation be convective or notWill the precipitation be convective or not??

3.3. Will cells train over the same area or not?Will cells train over the same area or not?

4.4. Will new storms form or continue to form Will new storms form or continue to form upstream? (regeneration)upstream? (regeneration)

5.5. Will a boundary or local topography provide Will a boundary or local topography provide local enhancement?local enhancement?

Precipitation IntensityPrecipitation Intensity

Precipitation intensity is proportional to Precipitation intensity is proportional to the vertical moisture fluxthe vertical moisture flux

Thus, it is important to assess the Thus, it is important to assess the following:following:

– How much moisture will be available (ambient How much moisture will be available (ambient or transported)? or transported)?

– What proportion of the moisture entering the What proportion of the moisture entering the cloud will fall as rain cloud will fall as rain (precipitation efficiency)?(precipitation efficiency)?

– How strong is the UVM?How strong is the UVM?– How much instability is present (if any)?How much instability is present (if any)?

Precipitation EfficiencyPrecipitation Efficiency

A (sometimes significant) portion of the water A (sometimes significant) portion of the water vapor that enters a storm system passes through vapor that enters a storm system passes through without condensing (although most does without condensing (although most does condense)condense)

Of the water vapor that does condense:Of the water vapor that does condense:– Some will evaporateSome will evaporate– Some will fall out as precipitationSome will fall out as precipitation– Some will be carried away by the cloudsSome will be carried away by the clouds

Two inhibitors of precipitation production:Two inhibitors of precipitation production:– A dry layer in the storm system, andA dry layer in the storm system, and– Strong vertical wind shearStrong vertical wind shear

1.1. A deep warm layer A deep warm layer – Rainfall Rainfall intensity will be greater if depth from the LCL to intensity will be greater if depth from the LCL to

the 0the 0°°C isotherm is ~3-4 km (greater in SE U.S.)C isotherm is ~3-4 km (greater in SE U.S.)– Generally, low cloud base events are more efficient Generally, low cloud base events are more efficient

precipitators (higher RH)precipitators (higher RH)

2.2. Increased residence time in clouds Increased residence time in clouds – Enhances collision-coalescence processEnhances collision-coalescence process– Strong UVMStrong UVM

3.3. A broad spectrum of cloud droplet sizesA broad spectrum of cloud droplet sizes– Also enhances the collision-coalescence processAlso enhances the collision-coalescence process– Occurs when air masses have long over-ocean trajectoriesOccurs when air masses have long over-ocean trajectories

4.4. Weak to moderate shear Weak to moderate shear

Precipitation Efficiency: Precipitation Efficiency: Favorable FactorsFavorable Factors

System MovementSystem Movement

Slow-moving systems usually produce the Slow-moving systems usually produce the heaviest rainfall (no way!)heaviest rainfall (no way!)

For very short-term (0-6 h) forecasts, For very short-term (0-6 h) forecasts, extrapolation based on radar and satellite extrapolation based on radar and satellite provide primary guidanceprovide primary guidance

For longer-term forecasts, models provide For longer-term forecasts, models provide decent guidancedecent guidance

– Some NWP models have ‘spin-up’ issues, so Some NWP models have ‘spin-up’ issues, so better in long termbetter in long term

– But you still need to consider model But you still need to consider model characteristics, limitations, and biasescharacteristics, limitations, and biases

Problems with NWPProblems with NWP

Initialization and QC smoothes data fields Initialization and QC smoothes data fields (important details can be lost)(important details can be lost)

Lack of data over oceans/MexicoLack of data over oceans/Mexico Terrain is (over)simplifiedTerrain is (over)simplified Subgrid-scale processes are often Subgrid-scale processes are often

parameterizedparameterized– convection*, boundary-layer processes, cloud convection*, boundary-layer processes, cloud

microphysics, radiation, etc.microphysics, radiation, etc.– *some models can now represent/forecast *some models can now represent/forecast

convection explicitlyconvection explicitly

Problems with NWP (cont.)Problems with NWP (cont.)

Atmospheric processes are nonlinearAtmospheric processes are nonlinear– Small changes in initial conditions can lead to large Small changes in initial conditions can lead to large

forecast variations (uncertainty)forecast variations (uncertainty)– Basis for ensemble forecasting (probabilistic)Basis for ensemble forecasting (probabilistic)– Not the perfect solutionNot the perfect solution

What does a CPS do when it is ‘turned on?’ What does a CPS do when it is ‘turned on?’ – It changes vertical stability profileIt changes vertical stability profile– It generates and redistributes heatIt generates and redistributes heat– It redistributes momentumIt redistributes momentum– It makes clouds/precipIt makes clouds/precip

Is CPS physically ‘accurate?’ Good question…Is CPS physically ‘accurate?’ Good question…

CPS IssuesCPS Issues

Several types with own strengths/weaknessesSeveral types with own strengths/weaknesses– Kain-FritschKain-Fritsch– Betts-Miller-JanicBetts-Miller-Janic– GrellGrell

If the CPS is not vigorous enough:If the CPS is not vigorous enough:– Model will generate too much explicit (grid-scale) Model will generate too much explicit (grid-scale)

precipitationprecipitation– This will lead to erroneous latent heat feedbackThis will lead to erroneous latent heat feedback

Most CPSs handle outflow poorlyMost CPSs handle outflow poorly– Model therefore cannot forecast propagation wellModel therefore cannot forecast propagation well– Explicit convection representation can help somewhatExplicit convection representation can help somewhat

An Example of a CPS ProblemAn Example of a CPS Problem

Eta 6-h forecast of convective precipitation

(< 1 inch)

Eta 6-h forecast of grid-scale precipitation

(~5 inches!)

Does this look reasonable?

So what happened here?So what happened here?

CPS was not vigorous enoughCPS was not vigorous enough– Not enough convective precipitation Not enough convective precipitation

generatedgenerated– Momentum/moisture/heat not redistributed Momentum/moisture/heat not redistributed

properlyproperly

As a result, the pressure and wind fields As a result, the pressure and wind fields were erroneouswere erroneous

This lead to anomalously strong grid-scale This lead to anomalously strong grid-scale UVM/precipitationUVM/precipitation

Echo Training or RegenerationEcho Training or Regeneration

Factors that can lead to echo training or Factors that can lead to echo training or regeneration:regeneration:

– Slow-moving low-level boundary or frontSlow-moving low-level boundary or front– Quasi-stationary LLJQuasi-stationary LLJ– Quasi-stationary area of upper-level Quasi-stationary area of upper-level

divergencedivergence– Low-level boundary almost parallel to the Low-level boundary almost parallel to the

mean flow (source of moisture mean flow (source of moisture convergence)convergence)

– Lack of strong vertical wind shearLack of strong vertical wind shear

Cell Movement and PropagationCell Movement and Propagation System movement depends on cell System movement depends on cell

movement movement andand propagation propagation

Individual convective cells usually move Individual convective cells usually move at around 90% of the mean wind with a at around 90% of the mean wind with a slight deviation to the rightslight deviation to the right

Propagation is affected by how fast new Propagation is affected by how fast new cells form along some flank of the systemcells form along some flank of the system

MCS PropagationMCS Propagation

Early Stage of MCSEarly Stage of MCS– Individual cells move approximately with 850-300 hPa mean windIndividual cells move approximately with 850-300 hPa mean wind– Much like ordinary thunderstormsMuch like ordinary thunderstorms

Mature Stage of MCSMature Stage of MCS– Preferred cell motion slightly to right of mean windPreferred cell motion slightly to right of mean wind– Active portion of MCS moves slightly to right of mean flowActive portion of MCS moves slightly to right of mean flow

MCS Movement and PropagationMCS Movement and Propagation

MCS MCS movementmovement is dependent upon: is dependent upon:1.1. 850-300 hPa mean flow (Corfidi 1994); and 850-300 hPa mean flow (Corfidi 1994); and 2.2. Rate/location at which new cells are Rate/location at which new cells are

growing (propagation)growing (propagation) MCS MCS propagationpropagation is dependent upon: is dependent upon:

– Location of the most unstable airLocation of the most unstable air– Axis/orientation/relative speed of the LLJAxis/orientation/relative speed of the LLJ

Stronger LLJ Stronger LLJ greater deviant motion greater deviant motion

– Cold pool strengthCold pool strength– Location of strongest LL moisture Location of strongest LL moisture

convergence convergence

MCS PropagationMCS Propagation

MCS Propagation and MCS Propagation and Thickness PatternsThickness Patterns

MCCs often track along the 1000-500 MCCs often track along the 1000-500 hPa (or 850-300 hPa) thickness contours hPa (or 850-300 hPa) thickness contours

The amount of moisture needed to The amount of moisture needed to produce a large MCS/MCC appears to produce a large MCS/MCC appears to be dependent on 1000-500 hPa be dependent on 1000-500 hPa thickness and precipitable waterthickness and precipitable water– Look for preferred thickness values and high Look for preferred thickness values and high

PWs/% of normal valuesPWs/% of normal values Watch for MCC development and heavy Watch for MCC development and heavy

rain in areas of rain in areas of diffluent thicknessdiffluent thickness

MCS Propagation: Stationary CaseMCS Propagation: Stationary Case

What features do you see that support a heavy What features do you see that support a heavy precipitationprecipitation event? event?

Note E-W frontal zoneand high PWs

Note southerly 35-40 kt windsand high instability

This MCS remained stationary for about 9 hours!This MCS remained stationary for about 9 hours!

MCS Propagation: Stationary Case MCS Propagation: Stationary Case Satellite ImagerySatellite Imagery

MCS Propagation: MCS Propagation: Importance of Shape and MotionImportance of Shape and Motion

• Vector C represents motion of storm complex • Note the difference in the rain received at circled

point!

Other QPF Other QPF ConsiderationsConsiderations

Short-range ForecastsShort-range Forecasts

Short-term (0-6 h) QPF Short-term (0-6 h) QPF forecasts rely forecasts rely primarily on current observations primarily on current observations and and trendstrends

Radar and satellite imagery provide Radar and satellite imagery provide excellent information on intensity, size, excellent information on intensity, size, and propagation of precipitation systemsand propagation of precipitation systems

Still have to consider non-linear Still have to consider non-linear changes, such as new cells forming changes, such as new cells forming upstreamupstream

Using Radar Imagery Using Radar Imagery

Modern radars provide estimates of Modern radars provide estimates of rainfall rates and accumulationsrainfall rates and accumulations– High temporal resolutionHigh temporal resolution– Precipitation estimates in between rain Precipitation estimates in between rain

gauges and observation pointsgauges and observation points– NotNot a perfect system a perfect system

Using Radar Imagery (cont.)Using Radar Imagery (cont.)

Animating imagery Animating imagery is useful for:is useful for:– Determining Determining

cell/system cell/system movementmovement

– Determining if/where Determining if/where cells are training cells are training

Discussion QuestionDiscussion Question How do the following radar ‘issues’ act as How do the following radar ‘issues’ act as

limitations that can adversely impact your limitations that can adversely impact your decision making during a heavy decision making during a heavy precipitation event?precipitation event?– Elevation angle:Elevation angle:

beam can overshoot maximum reflectivity beam can overshoot maximum reflectivity compare to composite reflectivity to account for this errorcompare to composite reflectivity to account for this error

– Terrain:Terrain: beam blockagebeam blockage

– Melting ice:Melting ice: bright-banding and hail contaminationbright-banding and hail contamination

– Z-R relationship variations/applicability:Z-R relationship variations/applicability: precipitation character/time of year/geographyprecipitation character/time of year/geography

Using Satellite Imagery Using Satellite Imagery

Modern satellites provide:Modern satellites provide:– Means to monitor pre-storm environment Means to monitor pre-storm environment

(e.g., cloudy vs. clear, stable vs. unstable)(e.g., cloudy vs. clear, stable vs. unstable)– Measurements of environmental Measurements of environmental

moisture/stability variables (with moisture/stability variables (with somesome success)success)

– Precipitation estimates for areas in Precipitation estimates for areas in between rain gauges and lacking radar between rain gauges and lacking radar informationinformation

– Cloud-top temperature informationCloud-top temperature information

Using Satellite Imagery (cont.) Using Satellite Imagery (cont.)

Animating imagery Animating imagery is useful for:is useful for:– Locating forcing Locating forcing

features (short features (short waves, jet streaks, waves, jet streaks, etc.)etc.)

– Assessing moisture Assessing moisture transport transport

– Determining Determining cell/system cell/system movementmovement

– Monitoring system Monitoring system growth and decaygrowth and decay

Jet Streak ImpactsJet Streak Impacts

Jet streaks have been associated Jet streaks have been associated with:with:– Variations in strength of the LLJVariations in strength of the LLJ– Cyclogenesis and major snowstormsCyclogenesis and major snowstorms– FrontogenesisFrontogenesis

Remember velocity and curvature Remember velocity and curvature changes in the upper-level jet are changes in the upper-level jet are both important both important

Jet Streaks and CyclogenesisJet Streaks and Cyclogenesis

Most cyclones to the lee of N-S mountain Most cyclones to the lee of N-S mountain ranges form under the left exit region of a jet ranges form under the left exit region of a jet streakstreak

The LLJ is enhanced due to the isallobaric The LLJ is enhanced due to the isallobaric winds associated with the pressure fallswinds associated with the pressure falls

The low-level winds also strengthen in The low-level winds also strengthen in response to the increase in pressure gradientresponse to the increase in pressure gradient

Differential temperature and moisture Differential temperature and moisture advection destabilize the air mass advection destabilize the air mass

Example from Summer 1993Example from Summer 1993

The heaviest rainfall was usually located at the southern edge of the divergence

(revealing the sloping nature of the DTC’s ascending branch).

250 mb Isotachs250 mb Isotachs Divergence Divergence

Significance of the Low-Level JetSignificance of the Low-Level Jet Speed convergence maximized at the nose of the LLJSpeed convergence maximized at the nose of the LLJ Confluent low-level flow is often present along the Confluent low-level flow is often present along the

axis of the LLJaxis of the LLJ Strength of vertical moisture flux related to the Strength of vertical moisture flux related to the

strength of the LLJstrength of the LLJ Differential moisture and temperature advection can Differential moisture and temperature advection can

lead to rapid destabilization lead to rapid destabilization A quasi-stationary LLJ supports the cell regeneration A quasi-stationary LLJ supports the cell regeneration

and/or echo trainingand/or echo training The LLJ is often located on southwest/western flank The LLJ is often located on southwest/western flank

of a backward-propagating MCS of a backward-propagating MCS

Subtle Heavy Rainfall Signatures Subtle Heavy Rainfall Signatures (SHARS)(SHARS)

Plan View Vertical Profile

• Note strong vorticity max• Comma-shaped cloud signature• Slow-moving• U/L lows in moist environments

• Note significant veering• Weak speed shear• High absolute/relative moisture

•low DD values•high RH/PW values

Heavy Rainfall in Tropical SystemsHeavy Rainfall in Tropical Systems

• South of ~35South of ~35oo latitude latitude– Typically from slow movement/terrain influenceTypically from slow movement/terrain influence– Typically associated with eyewall convection Typically associated with eyewall convection

(especially where wind is perpendicular to coastline) (especially where wind is perpendicular to coastline) and feeder bandsand feeder bands

– Often located along the region of max inflow just to Often located along the region of max inflow just to the east of the centerthe east of the center

• North of ~35North of ~35oo latitude and Western U.S. latitude and Western U.S.– Often associated with mid-latitude interaction (most Often associated with mid-latitude interaction (most

intense rainfall north & west of track)intense rainfall north & west of track)– Terrain can provide focusTerrain can provide focus

More on Rainfall with Tropical SystemsMore on Rainfall with Tropical Systems

Max rainfall=100/storm speed (rule of thumb, doesn’t always Max rainfall=100/storm speed (rule of thumb, doesn’t always work)work)

Amount of pre-existing moisture important in governing rainfall Amount of pre-existing moisture important in governing rainfall potentialpotential

As storm decays, the heaviest precipitation often shifts to the As storm decays, the heaviest precipitation often shifts to the northwest side of storm, especially if it interacts with westerliesnorthwest side of storm, especially if it interacts with westerlies

Watch for nighttime "core" rains near center (may be Watch for nighttime "core" rains near center (may be deceptively inactive during the day)deceptively inactive during the day)

Tropical moisture associated with storm sometimes interacts Tropical moisture associated with storm sometimes interacts with fronts north and east of the system (even if the system is with fronts north and east of the system (even if the system is hundreds of km away)hundreds of km away)

Pacific systems moving northeastward from Mexico can cause Pacific systems moving northeastward from Mexico can cause heavy rains well ahead of the center (can focus on a front in the heavy rains well ahead of the center (can focus on a front in the Southern Plains)Southern Plains)

+RA Forecasting Rules of Thumb 1+RA Forecasting Rules of Thumb 1

Max rainfall often occurs where center of Max rainfall often occurs where center of strongest inflow intersects a boundarystrongest inflow intersects a boundary

Max rainfall often occurs to NE of Max rainfall often occurs to NE of ee ridge ridge – LL moisture convergence centerLL moisture convergence center– Best LL thermal forcingBest LL thermal forcing

Summertime +RA often forms along outflow Summertime +RA often forms along outflow boundaries south of warm frontsboundaries south of warm fronts

Inverted isobars (i.e. inverted trough) along a Inverted isobars (i.e. inverted trough) along a front can indicate +RA potential front can indicate +RA potential – Usually associated with low/mid-level WAA Usually associated with low/mid-level WAA – Lower pressure upstreamLower pressure upstream

+RA Forecasting Rules of Thumb 2+RA Forecasting Rules of Thumb 2

+RA often falls in areas of thickness diffluence +RA often falls in areas of thickness diffluence (often implies exit region of ULJ)(often implies exit region of ULJ)

Beware of thickness contours that remain Beware of thickness contours that remain stationary or sink southward in LL southerly flowstationary or sink southward in LL southerly flow– Forms via adiabatic cooling from UVMForms via adiabatic cooling from UVM– Works during cool and warm seasonsWorks during cool and warm seasons

MCSs/MCCs often follow 1000-500 hPa thickness MCSs/MCCs often follow 1000-500 hPa thickness contours (or just to the right of them)contours (or just to the right of them)

MCCs often form under UL ridge axesMCCs often form under UL ridge axes– Weak inertial stability and WAAWeak inertial stability and WAA– Beware of ULJs moving into ridge crestsBeware of ULJs moving into ridge crests

+RA Forecasting Rules of Thumb 3+RA Forecasting Rules of Thumb 3

+RA possible behind weak mid-level vort +RA possible behind weak mid-level vort max/near vort minmax/near vort min– IfIf LL thermal ridging/MCON are present LL thermal ridging/MCON are present– +RA possible despite lack of mid/upper-level +RA possible despite lack of mid/upper-level

supportsupport Watch for favorable ULJ structuresWatch for favorable ULJ structures

– Cyclonically-curved left exit regionsCyclonically-curved left exit regions– Anticyclonically-curved right entrance regionsAnticyclonically-curved right entrance regions

K indices good measures of deep moistureK indices good measures of deep moisture– KI > 35KI > 35C indicate high +RA potentialC indicate high +RA potential– Look for ‘high’ values even during cool seasonLook for ‘high’ values even during cool season

+RA Forecasting Rules of Thumb 4+RA Forecasting Rules of Thumb 4

Max rainfall from tropical systems found in Max rainfall from tropical systems found in center core at night, due to organized center core at night, due to organized moisture convergencemoisture convergence

Watch for tropical connections in WV imageryWatch for tropical connections in WV imagery– Increases precip efficiency via enhanced CC Increases precip efficiency via enhanced CC

processesprocesses– Reduces need for moistening of mid/upper levelsReduces need for moistening of mid/upper levels

Subtle heavy rainfall signatures (SHARS)Subtle heavy rainfall signatures (SHARS)– Slow-moving comma-shaped clouds in satelliteSlow-moving comma-shaped clouds in satellite– Warm-topped convection (> -58Warm-topped convection (> -58C)C)

+RA Forecasting Rules of Thumb 5+RA Forecasting Rules of Thumb 5

Fast-moving systems and/or strong height Fast-moving systems and/or strong height falls not conducive to +RA falls not conducive to +RA – Larger area of moderate RA (1-2”)Larger area of moderate RA (1-2”)– Slow-moving/weak falls better for +RASlow-moving/weak falls better for +RA

Large-scale NWP models usually place axis Large-scale NWP models usually place axis of +RA too far northof +RA too far north– Poor handling of outflow boundaries and other Poor handling of outflow boundaries and other

mesoscale features/detailsmesoscale features/details– Better with synoptic-scale pattern, LLJ, and Better with synoptic-scale pattern, LLJ, and

moisture distributionmoisture distribution– NotNot applicable to mesoscale models applicable to mesoscale models

SummarySummary

SummarySummary

Composing a QPF is challengingComposing a QPF is challenging It requires answering three difficult It requires answering three difficult

questions:questions:– When (or if) precipitation will fall?When (or if) precipitation will fall?– Where will precipitation fall?Where will precipitation fall?– How much precipitation will fall?How much precipitation will fall?

Summary (cont.)Summary (cont.)

Meteorologists must use their analysis and Meteorologists must use their analysis and forecasting skills, along with their local forecasting skills, along with their local climatology knowledge to:climatology knowledge to:– Recognize scenarios known to create Recognize scenarios known to create regions regions

of instability, moisture, of instability, moisture, andand lift lift– Apply their knowledge of cloud and Apply their knowledge of cloud and

precipitation processesprecipitation processes– Use observations from multiple platforms and Use observations from multiple platforms and

NWP forecast products intelligently (with NWP forecast products intelligently (with limitations in mind) to monitor and anticipate limitations in mind) to monitor and anticipate system evolutionsystem evolution