HEAVY RAINFALL EVENTS EAST OF THE ROCKIES

MOST ARE ASSOCIATED WITH MCCs/MCSs AT NIGHT

OCCUR NEAR THE 500 MB RIDGE POSITION

OCCUR AT THE NOSE OF THE LOW LEVEL WIND MAXIMUM

VERTICAL SHEAR IS WEAK TO MODERATE ALLOWING SLOW MOVEMENT

ABNORMALLY MOIST, PWS USUALLY ARE 1.40” OR HIGHER AND AVERAGE ABOUT 1.62”.

FROM MADDOX ET AL., 1977

MADDOX SYNOPTIC TYPECONSISTENT FEATURES

STRONG 500 MB TROUGH MOVING SLOWLY EASTWARD OR NORTHEASTWARD; WEAK IMPULSE LIFTING OUT AHEAD OF MAIN TROUGH

TRAILING FRONT USUALLY MOVES SLOWLY OR STALLS

HEAVY RAIN USUALLY OCCURS IN WARM MOIST AIR AHEAD OF FRONT, ALONG THE AXIS OF STRONG LOW LEVEL WIND

AVERAGE PWS=1.50 (180% NORMAL), AVERAGE K INDEX=36

COUPLED JETS OFTEN ENHANCE VERTICAL MOTION

MOST COMMON IN THE TRANSITION SEASONS

LCOOL AND DRY COOL AND MOIST

Td70oFTd=60oF

Td=66oF

SLOW MOVINGOR STATIONARYFRONT

130 nm

OLD WARM FRONT

SURFACE 850 MB

Td=10oCTd=12oC Td=14oC

max

AX

IS

130 nm

A POTENT LOW-LEVEL JET IS PRESENT

Synoptic type (low levels)

1) Front is usually almost parallel to the mean flow

2) Note strong confluent flow at 850 mb

streamline

500 mb

SYNOPTIC TYPE(mid and upper levels)

T-Td<6oC

250 mb wind max

Shortwave sometimes

2nd jet streak

500 mb winds

are plotted

PW=1.46(181%)

300 mb

500 mb

SFC

700 mb

200 mb

850 mb

-57

2

8-11

10-38

35

6774

15

K=36

SI=2

6

5

4

3

2

1

0

NU

MB

ER

OF

EV

EN

TS

CAN HAPPEN ANY TIME OF YEAR

MOISTURE FOR SUMMERTIME MCC DEVELOPMENT/ MADDOX TYPE

HEAVY RAIN EVENTS PRECIPITABLE WATER

– 150 % OF NORMAL (> 1.60” EAST, ~ 1.00” WEST)

SURFACE DEWPOINTS (70S IN EAST, 50S IN INTERMOUNTAIN REGION)

850 MB DEWPOINTS (MID TEENS IN EAST)

700 MB DEWPOINTS (6-8oC IN WEST)

DEPTH OF MOISTURE IMPORTANT– IS THERE A CAP PRESENT TO INHIBIT CONVECTION

USE OF MODELS TO IDENTIFY PATTERNS(11/16/87 CASE)

NGM INDICATED A MADDOX SYNOPTIC PATTERN– SLOW MOVING FRONT AND 500 MB TROUGH

– PLENTY OF MOISTURE WAS FORECAST OVER REGION HIGH PWS (1.50”) AND 850 MB DEWPOINTS (>10oC)

– THICKNESS VALUES ACROSS THE SOUTH WERE NOT FORECAST TO RISE (GOOD ADIABATIC COOLING FROM VV)

NGM PROGGED A VERY SLOW MOVING UPPER LOW AND ASSOCIATED SURFACE FRONT

1008

576

1016

1032

1024

1020

570

564

558

NGM 48 HR SFC

1012

552

546

VT 1200Z 16 NOV 1987

LL

NGM 48 HR 500MB

0-48 HR

MOVEMENT

A TYPICAL SYNOPTIC TYPE HEAVY RAINFALL SURFACE AND 500 MB PATTERN

NOTE THE STRONG LOW-LEVEL JET AND HIGH PW VALUES FORECAST NEAR THE COAST

NOTE THAT AT THE TIME THE NGM GENERALLY HAD A LOW BIAS OF 850 MB DEWPOINTS.

20 M/S24 M/S

NGM 48HR 850 WIND VECTORS & ISOTACHS

10oC10oC

250 WIND MAXES PW>1.5

NGM 48HR 850MB DEWPOINTS

HOW DID THE NGM DO ??? The NGM missed the MCS over LA and therefore predicted too much

rain over the Plains.

.5”

1.0”

1.0”

1.0”.5”

.5”

24-48 HR NGM QPF VERIFYING PRECIPITATION

=4.0”

=6.0+”

MODEL FORECAST THE SYNOPTIC PATTERN, THE LOW-LEVEL JET AND

MOISTURE DISTRIBUTION REASONABLY WELL, BUT...

DID NOT FORECAST THE MESOSCALE SURFACE BOUNDARY THAT FOCUSED THE RAINFALL

MISSED THE PRECIPITATION MAX ACROSS LA AND AR (AND OVERPREDICTED THE RAINFALL OVER THE PLAINS)

FORECASTER WAS ABLE TO MODIFY THE PRECIPITATION FORECAST BASED ON PATTERN RECOGNITION AND KNOWLEDGE OF NGM BIAS

STILL…the NGM and ETA DID predicted characteristics consistent

with a synoptic type flood.

A SLOW MOVING FRONT AND 500MB TROUGH

PWS IN EXCCESS OF 1.50”

A STRONG LOW-LEVEL JET

THICKNESS VALUES ACROSS THE SOUTH WEREN’T FCAST TO RISE (SUGGESTING STRONG VERTICAL MOTION WOULD BE PRESENT)

Notice how these also satisfy the Moisture, Lift, Instability criteria

MADDOX FRONTAL TYPECOMMON FEATURES

MOST COMMON DURING SUMMER AND AT NIGHT

USUALLY ASSOCIATED WITH E-W FRONTAL BAND

WARM MOIST AIR OVERRUNS FRONT – Therefore, heaviest rain is on the COOL SIDE of the front.

Frontal and Mesohigh Type (surface)

Outflow boundary or front provides focus for lifting. The area at highest risk for heavy rainfall is in red.

L

Td=70oF

H

HTd=70oFTd=60oF

Td=60oF

H

BUBBLE HIGH

OUTFLOW BOUNDARY

L

COOL AND MOIST

WARM AND MOIST

Td=70oFTd=70oF Td=60oF

Td=60oF

SURFACE MESOHIGH

120 nm

WARM AND MOIST

COOL AND MOIST

120 nm

SURFACE FRONTAL

Td=14 oC

FRONTAL AND MESOHIGH (850 MB)

Why does the orientation of the low-level jet favor heavy rainfall?

FRONTAL Td=16 oC

Axi

s of

Max

Win

ds

120 nm

Td=10 oC

T d=1

4 o C

Td=12 oC

Max

Win

ds

Axi

s of

120 nm

About 60% of mesohigh and frontal type heavy rainfall events occur near the ridge axis.

500 mb

MESOHIGH FRONTAL

500 mb

MOIST

MOIST

0

5

10

15

20

NU

MB

ER

OF

EV

EN

TS

J F M A M J J A S O N DMONTH

MESHOHIGH

0

2

4

6

8

10

12

14

NU

MB

ER

OF

EV

EN

TS

J F M A M J J A S O N DMONTH

FRONTAL

200 MB

500

850

700

300

SFC

-56

-36

6-10

17

3

4

7

1013

6570

PW=1.60”

(158%)

K=38

SI=-4

15 FRONTAL

200 MB

300

500

700

850

SFC1014

-36

10

-10

-57

674

183

7166

PW=1.64”

(162%)

K=39

SI=-5

MESOHIGH

Investigation of the MCSs during the Great Flood of 1993

MCSs were investigated for June-Sept 93’.

All 2, 3, 4 and 5” areas were measured for each MCS identified

Systems were categorized based on the size of the 3” coverage

The largest scale-heaviest events were compared with smaller scale events that produced less rain.

COMPOSITES OF THE 12 LARGEST SCALE HEAVY RAINFALL EVENTS

CENTERED ON THE RAINFALL MAXIMUM

USED RDAS GRIDDED FIELDS INTERPROLATED TO 2 DEGREE LATITUDE GRID

THE HEAVIEST RAIN OCCURS AT THE NOSE OF THE LOW-LEVEL JET IN/OR NEAR THE STRONGEST WARM ADVECTION

1

850MB WIND DIRECTION (ARROWS) AND ISOTACHS850 MB TEMPERATURE ADVECTION

( BLACK DOT IS CENTER OF HEAVIEST RAIN)

1918

0 2-2

2

4

4

6

6

8

8

0

-2

-4

-6

-4-8

-6-8

10

9

8

7

6

17

850 mb moisture flux (left) and moisture flux divergence (right)

Note that the heaviest rain occurred southeast of the strongest 850 mb moisture convergence.

THE HEAVIEST RAIN USUALLY OCCURS TO THE NORTHEAST OF THE THETA-E RIDGE, NEAR BUT JUST

SOUTH OF THE MAXIMUM IN THETA-E ADVECTION

THETA-E Ve

INVESTIGATION SUMMARY

Most of the MCSs formed to the north or northeast of the strongest 850 mb winds and moisture flux.

Most occurred in an area of 850 mb warm and theta-e advection

Most occurred on the southern edge of the 250 mb divergence