davies 2011 chaser con forecast school slideshare

Storm Chasing Forecast Class

Jon Davies – Trimble/Kansas City MO

Basic Tools for doing Severe Weather Analysis and Forecasts

Surface maps

Radar & Satellite images

Upper air maps

Computer model forecasts

Reading UTC time

Air pressure in millibars(upper right)

(here 1005.1 mb)

Temperature (upper left) (in o F on surface maps,

here 74 o F )

Dew point (lower left)

(a measure of moisture, in o F on surface maps,

here 62 o F )

Wind direction and speed (here from the south-southwest at 15

knots)Line extending outward gives

direction from which wind is blowing,barb gives speed in knots (kts):

full barb = 10 kts; ½ barb = 5 kts(1 knot = 1.15 miles per hour)

Basic surface station weather observation plot

Current weather(middle left)

(here, light rain)

Station location

Boundaries that separate different air masses are called fronts:

A dryline is aboundary betweendry air to the westand moist air to the east.

Moist air(high dew points)

Dry air(low dew points)

dryline

Boundaries can be located on surface weather maps by looking for any one or a combination of the followingorganized along a line or curve:

• Sharp temperature changes over a short distance• Changes in moisture content (dew point)• Changes in wind direction

Wind flow aroundlow pressure iscounterclockwisewith air spiraling inward toward the center.

Wind flow aroundhigh pressure isclockwise with airspreading outwardfrom the center.

Older thunderstorms or clusters of stormscan produce boundaries due to cool air flowing outward from some of the storms.

These outflow boundaries can behave like small cold fronts, warm fronts or stationary fronts.

outflow boundaryfrom storms oversoutheast KS andsouthwest MO

Surface maps

Upper air maps

Reading UTC time

Satellite and radar information can help with weathermap analysis.

Satellite

Surface maps

Upper air maps

Reading UTC time

This 500 mb map (~ 18,000 ft MSL) is the result of information from radiosondes:

Radiosondes released twice per day (6 a.m. CSTand 6 p.m. CST) give us weather observations above the ground.

Radiosonde: a box equipped with weatherinstruments and a radio transmitter attached to a gas-filled balloon.

North Americanradiosonde sites

Weather maps above ground are important becausethey tell us a lot about what will be going on with our weather at the ground.

Changes in the upper pressure and wind patterns and the jet stream help meteorologists to predict significant changes in our weather.

Jet stream

Jet stream (purple)(18,000-30,000 ft MSL)

surface frontsWhere jet stream winds

and upper flow dip south,weather becomes colder

Where jet stream windsand upper flow bulge north,weather becomes warmer

Relationship of jet stream and upper winds to fronts

troughridge

Surface fronts

are usually located

close to and beneath the jet stream !

The jet stream winds generally run along the tightercontours of weather maps at 500 mb (roughly 18,000 ft above sea level) and 300 mb (roughly 30,000 ft above sea level). These contours make troughs and ridges inthe flow aloft that greatly affect our weather at theground.

wind flow and contours at roughly 18,000 ft MSL (500 mb)

general cloudiness & storminess (divergence aloft – winds spreading

apart to create lift)

less clouds & “nicer” weather (convergence aloft- winds pushing

together to create sinking)

Troughs and ridges are waves aloft…

wave wave

Shortwaves (smaller waves of energy in the upper flow)move through the larger troughs and ridges (longwaves), generating more concentrated areas of clouds and storminess within the larger troughs and ridges.

Longwave trough

500 mb~ 18,000 ft MSL

Longwave ridge

Longwave trough

Shortwave

ShortwaveShortwave

Longwave trough

500 mb~ 18,000 ft MSL

Longwave ridge

Longwave trough

Shortwave

risingmotion

(clouds &storms)

risingmotion

(clouds &storms)

Shortwaverising

motion(clouds &storms)

sinkingmotion(better

weather) sinkingmotion(better

weather)

sinking

Note the areas of weather just ahead of these waves aloft – these shortwaves “plow up” the atmosphere, so to speak, to create lift and storminess.

Fronts also tend to move with thestronger wave disturbances aloft in the jet stream.

strong shortwave

surface front surface

dryline

500 mb contoursand surface systems

(a spring storm)

300 mb

500 mb

surface

30,000 ft(9 km MSL)

18,000 ft(5.5 km MSL)

Ground level

warmH H

Upper air maps, different “slices”through the atmosphereat increasingelevations,help meteorologistsunderstand andpredict all thisweather.

Tornadic supercells possibleS or SE surface winds & sizable CAPE (instability)

(warm air & large dew points)

Divergence

Winds veer (become more westerly) and

increase with height

Jet stream anddivergence aloft

(winds spreading apart)with

upper trough / wave

Meteorologicalsetting forpossible

supercells andtornadoes

Ground level

MSL(5.5 km)

MSL(3.0 km)

MSL(1.5 km)

MSL(9.0 km)

Ground level

divergence

We’ll lookmainly atthese levelsaloft.

MSL(5.5 km)

MSL(3.0 km)

MSL(1.5 km)

MSL(9.0 km)

‘upper levels’

‘mid levels’

‘low levels’

Ground level

divergence

Notice that the “mb” numbers

(850 – 700 – 500 - 300) get smaller with height, just the

opposite of elevation.

MSL(5.5 km)

MSL(3.0 km)

MSL(1.5 km)

MSL(9.0 km)

‘upper levels’

‘mid levels’

‘low levels’

~ 18,000 ft / 5.5 km MSLEta/NAM/WRF model forecast

from UCAR site

SurfaceEta/NAM/WRF model forecast

from UCAR site

Surface maps

Upper air maps

Reading UTC time

Computer model forecasts use current data observations to initialize numerical models that approximate the atmosphere’s behavior. These models are then “run” forward in time using acomputer to solve physical equations to make a forecast.

GFS (Global Forecast System): goes out to 16 days (usually displayed only out to 7 or 10 days), run 4 times per day (the 12z morning & 00z evening runs are best!)

NAM (North American Mesoscale): goes out to 3.5 days (84 hours), run 4 times per day (again, the 12z morning & 00z evening runs are best!) (also called the WRF or Eta)

RUC (Rapid Update Cycle): goes out to 18 hours (usually displayed only out to 12 hours), run hourly.

(The NAM & RUC have higher resolution and more detail)

Here are the main computer models that are run and maintained by the National Weather Service (National Centers for Environmental Prediction, or NCEP) and used by most forecasters:

The GFS is used to look farther ahead (out to 2 weeks),but it is very unreliable beyond 5-7 days. The early panelscan also be used as a comparison to the NAM.

The NAM/WRF/Eta is used to look out to 3 days or so, andis more detailed than the GFS. The panels out to 12 hourscan also be used as a comparison to the RUC.

The RUC is used the day of an impending event and isupdated frequently with available current observations foreven more detail.

You can access output from these models at many internet sites…

The UCAR site above has been around for a long time: www.rap.ucar.edu/weather

RUC NAM(Eta/WRF) GFS

Some other sites with computer model forecast output:

Earl Barker’s site: www.wxcaster.com/conus_0012_us_models.htm (NAM/WRF & GFS)www.wxcaster.com/conus_offhr_models.htm (RUC)

College of DuPage:weather.cod.edu/forecast (RUC, NAM/WRF, & GFS)

TwisterData:www.twisterdata.com (RUC, NAM/WRF, & GFS)

Unisys Weather:weather.unisys.com (RUC, NAM/WRF & GFS)

Example of computer model output: 12 hr forecast NAM model CAPE (instability)

UCAR site: www.rap.ucar.edu/weather/model

Example of computer model output: 12 hr forecast RUC model 3 hr accumulated precipitation

Earl Barker’s site: www.wxcaster.com/conus_offhr_models.htm (RUC)

Example of computer model output: day 2 through day 10 forecast GFS 10 day 500 mb contours (colors) and surface isobars (black)

Unisys site: weather.unisys.com

Example of computer model output: 180 hr forecast GFS 6 hr accumulation precipitation forecast at 7.5 days

TwisterData site: www.twisterdata.com

Some problems with computer forecast models:

• Accurate information to initialize (start up) computer models is sparse in many locations, such as over oceans.

• Models, at best, are only an approximation and estimation of the atmosphere. They make assumptions that may not apply well in many situations.

• As models go farther out in time, errors amplify to make large inaccuracies the longer they run.

Part of the job of meteorologists making forecasts is to use their knowledge and experience to determine when model guidance is reasonably accurate and useful, and when it is not.

Accepting computer model forecast outputwithout question and thought is not really weather forecasting.

True weather forecasting requires experience!

However, one can make a quick “forecast guess” for a tornado chase by looking at only a few panels of model output. Just understand that is only a crude guide based on a computer’s “opinion”, rather than a true forecast.

Surface maps

Upper air maps

Reading UTC time

Then, subtract the proper number of hours for your time zone from the UTC time to get your local time in a24-hr or “military” time system.

UTC is Universal Time Coordinated, the same asGreenwich Mean Time (GMT) or Zulu time (Z).

To convert from UTC time to local time:

First, learn to think in a 24-hr time system (“military time”):

Pacific Time Zone:

Subtract 8 hoursfrom the UTC timeto get PST time.

Subtract 7 hoursfrom the UTC timeto get PDT time.

Mountain Time Zone:

Subtract 7 hoursfrom the UTC timeto get MST time.

Subtract 6 hoursfrom the UTC timeto get MDT time.

Central Time Zone:

Subtract 6 hoursfrom the UTC timeto get CST time.

Subtract 5 hoursfrom the UTC timeto get CDT time.

Eastern Time Zone:

Subtract 5 hoursfrom the UTC timeto get EST time.

Subtract 4 hoursfrom the UTC timeto get EDT time.

ST =Standardtime

DT =Daylighttime

Other important information:

• If the UTC time is given as 4 digits, put a decimal at 2 places: 1325 UTC = 13.25 when doing the conversion 0045 UTC = 00.45 when doing the conversion

• If the UTC time is given as 2 digits, it is a whole number(no decimal needed): 03 UTC = 03.00, or 3 when doing the conversion 11 UTC = 1100, or 11 when doing the conversion • If the UTC time is less than the number of hours you’resubtracting for your time zone, add 24 to the UTC time: 0243 UTC (02.43) is less than any of the hours one would subtract in any U.S. time zone, so add 24: 2.43 + 24 = 26.43 when doing the conversion

Examples (refer to charts & guidelines on prior 3 pages)

•Convert 2115 UTC to Central Standard Time (CST):

21.15 – 6 = 15.15 (1515 hours, or 3:15 pm CST)

•Convert 0700 UTC to Mountain Daylight Time (MDT):

07.00 – 6 = 01.00 (0100 hours, or 1:00 am MDT)

•Convert 0245 UTC to Central Daylight Time (CST):

02.45 is less than the 5 hours one would subtract todo the conversion in the Central Time Zone, so add 24:02.45 + 24 = 26.45 then do the conversion,26.45 - 5 = 21.45 (2145 hours, or 9:45 pm CDT)

0000 UTC = 6:00 p.m. CST = 7:00 p.m. CDT0300 UTC = 9:00 p.m. CST = 10:00 p.m. CDT0600 UTC = midnight CST = 1:00 a.m. CDT0900 UTC = 3:00 a.m. CST = 4:00 a.m. CDT1200 UTC = 6:00 a.m. CST = 7:00 a.m. CDT1500 UTC = 9:00 a.m. CST = 10:00 a.m. CDT1800 UTC = noon CST = 1:00 p.m. CDT2100 UTC = 3:00 p.m. CST = 4:00 p.m. CDT

Table converting Universal Time (UTC) to Central Time (CST or CDT):

Or, if all this seems too complicated, you can memorize the time conversions for your

specific time zone, and work from there:

Also, remember that dates change backwardsa day when doing evening and early nighttime UTC time conversions.

Above, 0000 UTC 5 May 2007 is actually 7:00 pm CDT 4 May 2007and, 00 UTC 21 February 2011 is actually 6:00 pm CST 20 February 2011

Basic Requirements for Severe Thunderstorms

Moisture and instability

Significant wind shear

Forcing and upward motion

Basic Requirements for Severe Thunderstorms (Strong Convection)

Moisture and instability- seasonably large dew points- moisture and warmth for CAPE (instability)

Significant wind shear - strong winds aloft for supercells- winds veering and increasing with height

in lower-levels for possible tornadoes

Forcing and upward motion- waves of energy aloft (divergence)

- surface boundaries for convergence and lift- daytime heating can help- absence of an inhibiting capping inversion

Basic Requirements for Strong Convection

Moisture and instability:

- seasonably large dew points

* 50’s F may be enough in early spring* 60’s and 70’s F are better

- moisture and warmth for CAPE (instability)

* monitor computer model forecasts of CAPE* monitor current estimates of CAPE

(CAPE)convectiveavailablepotentialenergy

Yellow area is where lifted low-level

air parcels arewarmer than their

environment --a “positive” area

Large CAPE means the

atmosphereis very

unstable.

Moistureprofile

Temperatureprofile

SkewT-log p diagram – a thermodynamic diagram

Forcing and upward motion - waves of energy aloft (divergence) - surface boundaries for convergence and lift

- daytime heating can help- absence of an inhibiting capping inversion

- strong winds aloft for supercells

* at 500 mb, winds at least 30-35 kts, preferably 40-50 kts or more

* 0-6 km bulk wind shear at least 30-35 kts, preferably 40-50 kts or more

- winds veering and increasing with height in lower-levels for possible tornadoes

* monitor low level jet at 850 mb* sizable 0-1 km storm-relative helicity (SRH):

desired values will vary (100-300+ m2/s2), depending on amount of availableCAPE * 0-1 km SRH < 100 m2/s2 is poor.

Wind at 6 km above ground (near 500 mb) (from SSW at 20 kts)

Wind at ground level (from SSE at 15 kts)

0-6 km bulk shear isthe straight-line vector

difference between 2 vectors:the surface wind, and the

wind at 6 km above ground.

Wind at 6 km above ground (near 500 mb)(from SW at 47 kts) Wind at ground level

(from SSE at 15 kts)

Here, the 0-6 km bulk shear is only 7 kts (very poor)

Here, the 0-6 km bulk shear is around 40 kts (very good

for supporting supercells)

- strong winds aloft for supercells

* at 500 mb, winds at least 30-35 kts, preferably 40-50 kts or more

* 0-6 km bulk wind shear at least 30-35 kts, preferably 40-50 kts or more

- winds veering and increasing with height in lower-levels for possible tornadoes

* monitor low level jet at 850 mb* sizable 0-1 km storm-relative helicity (SRH):

desired values will vary (100 - 300+ m2/s2), depending on amount of available CAPE * 0-1 km SRH < 100 m2/s2 is poor

Tornadoes develop from low-level mesocyclones (strong rotation near the ground) in supercells.

Strong wind shear is needed in the supercell environment near the ground to generate low-level mesocyclones.

0-1 km SRH is a measureof low-level wind shear --

it measures the areaunder the wind profile “curve”

below 1 km... The larger the curve, the larger

the low-level wind shear.

Winds at increasing 200 m intervals above ground

up to 1 km (1000 m)(winds increase in speed &

become more southwesterlywith height)

Wind at ground level (from SE at 15 kts)

Wind at 1 km above ground (just below 850 mb) (from SW at 35 kts)

Here, the 0-1 km SRH is around 210 m2/s2 (good enough

when combined with2000-2500 J/kg of CAPE or more

to support low-levelmeoscyclones &

possible tornadoes)

This red line is a hodograph

Good low-level wind profile for

supercell tornadoes

0-1 km SRH is a measureof low-level wind shear --

it measures the areaunder the wind profile “curve”

below 1 km... The larger the curve, the larger

the low-level wind shear.

Winds at increasing 200 mintervals don’t increase

in speed with height, and thewind profile is “small”

with not much of a “curve”

Wind at ground level (from SE at 15 kts)

This red line is a hodograph

Here, the 0-1 km SRH is onlyaround 70 m2/s2 (very poor

for supporting low-levelmeoscyclones &

tornadoes)

Poor low-level wind profile for

supercell tornadoes

Wind at 1 km above ground (just below 850 mb) (from SSW at 19 kts)

5/13/09 NW Missouri(non-tornadic supercell)

5/13/09 NC Missouri(EF2 tornadic supercell)

0-1 km SRH from contrasting hodographs

0-1 SRH = 70 m2/s2 0-1 SRH = 430 m2/s2

poor very good

Tilting and stretching of horizontal vorticity (low-level wind shear):

Low-level mesocyclones, possible tornadoes?

Combinations of CAPE and SRH are important

for this process.

Energy-Helicity Index (EHI)shows combinations of

CAPE and SRH.

Forcing and upward motion - waves of energy aloft (divergence) - surface boundaries for convergence and lift

- waves of energy aloft (divergence) - surface boundaries for convergence and lift

* Check 500 mb forecasts for waves, jet stream branches and divergence.

* Check surface map forecasts and evolving surface maps for areas of wind and moisture

convergence, also for heat/thermal axes.* Check 700 mb forecasts for temperature

pattern (capping issues).* Check model precipitation forecasts for initiation of convection.

Divergence ahead of strong 500 mb trough --spreading of winds causes upward motion to trigger storms

cooling withheight

warming with height

capping inversion

moistureprofile

temperatureprofile

SkewT-log p diagram – a thermodynamic diagram

CAPE above capping inversioncan’t be realized

capping inversionis near 700 mb

March approx > 5-6o C

April approx > 7-8o C

May approx > 9-11o C

June approx > 12-13o C

August approx > 12o C

September approx > 9-11o C

October approx > 7-8o C

November approx > 5-6o C

700 mb temperature estimations of areas that are “capped” (too warm aloft for thunderstorms):

Spring Fall

This chart doesn’t work well in the western High Plains due to elevation (e.g., eastern NM, eastern CO, far western NE, etc.).

Capping not an issue in this case (far south of target area)

Storms initiating?

Model shows storms initiating

Real observations…

tornadic cell (EF2)

tornadic cell (EF1)

Again, analyze for these ingredients:

Moisture and instability

A word about storm motion…

It’s important to get an idea of which directionand how fast mature storms and supercellswill be moving on a given day. That way you can intercept more effectively, and also planquickly to get out of the way when you have to.

You can use the 500 mb winds to make avery rough storm motion estimate (about 1/2 to2/3rds the 500 mb wind speed and slightly to the right of the direction).

You can also find more specific storm motion estimates on some computer model sites, such as Twisterdata.

Monitoring and adjusting your target during a storm chase

Next case makes a good example of:

Sig Tor Parameter (STP) incorporates

CAPE, SRH, deep shear,& LCL height intoone parameter

Target?

Front farther south than model forecasts

Tornadic storm (EF3),much farther south than original targetnear front

Non-tornadic storms well north of surface stationary front

SPC mesoanalysishttp://www.spc.noaa.gov

Near real-time estimation from SPC site…

EF3 tornado near Hill City KS

Here’s another case comparing model output,This time starting a little farther out in time (48 hrs), and then moving on up to making target adjustments during the day of the actual chase…

48-hr “radar echo forecast” from NAM model (Earl Barker’s NAM site)

From this and surface map forecasts, warm front does not move as far northas on earlier 48-hr forecasts

12-hr “radar echo forecast” from NAM model (Earl Barker’s NAM site)

Target a little farther south?

Tornadic storm

Tornadic storms

March approx > 5-6o C

April approx > 7-8o C

May approx > 9-11o C

June approx > 12-13o C

August approx > 12o C

September approx > 9-11o C

October approx > 7-8o C

November approx > 5-6o C

700 mb temperature estimations of areas that are “capped” (too warm aloft for thunderstorms):

Spring Fall

This chart doesn’t work well in the western High Plains due to elevation (e.g., eastern NM, eastern CO, far western NE, etc.).

Note “cap” estimate from chart –storms did not build much south of OK/TX border

Potpourri comments and impressions about how computer models handle some things:

• The GFS seems to handle cut-off lows better thanthe NAM up to around 24 hours before an event.

• I used to think that NAM precipitation forecastswere more accurate than those from the RUC, but in the last couple years they seem to be more equal, and the RUC may even be a little bit better.

• The NAM does not resolve/depict low-level windshear and SRH as well as the RUC. As a result,SRH values (and composite parameters that include SRH) on the RUC tend to be somewhat more reliable.

• Be careful processing information in the field… it’s easy to make knee jerk reactions to incoming news (e.g., where other chasers are located) without considering data in the context of your original forecast!

• Be careful about stationary fronts and the temperature contrast across them. Backed winds (increased shear?) north of the front may be tempting, but the air may too cool for tornadoes. Be suspicious of stationary boundaries where the temperature drops 10-12o F or more in 30-40 miles distance going from warm to cool side (usually S to N).

Some disjointed other thoughts…

• On N-S “outbreak” days with strong winds aloft that move storms along fast, position yourself at least 50 miles east of where you think the firing line will be, and don’t close too fast on the developing storms. Give things time to shape up so you can pick a good cell.

• If there’s a line or cluster of closely spaced cells, the south end cell, or a cell with a sizable open space to it’s south, often has the best potential for tornadoes.

• Double check the severe potential close to home before heading out on a long chase trip… there could be a chase opportunity nearby that’s easy to overlook.

Many storm chasers who are good forecastersoften have an intuitive sense about where to target after looking over data.

If you feel you have that, try not to let “too much knowledge” cause you to over think chase target decisions.

Try to stay in touch with your “gut feelings”, and usethe knowledge you learn to supplement and confirmyour intuitive sense about potential chase settings..

May 11, 1970

Target?

May 22, 2004

Target?

Too much capping south?

May 24, 2004

Target?Target?

Too much capping south?

When forecasting, do what works for you,and have fun with it !!!

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