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
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