HEAVY RAINFALL EVENTS EAST OF THE ROCKIES

1. 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

2. 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

3. Synoptic type (low levels) 1) Front is usually almost parallel to the mean flow 2)Note strong confluent flow at 850 mb SURFACE 850 MB L COOL AND DRY COOL AND MOIST SLOW MOVING OR STATIONARY FRONT OLD WARM FRONT max AXIS Td=10oC Td=66oF Td$70oF Td=12oC Td=14oC Td=60oF 130 nm 130 nm A POTENT LOW-LEVEL JET IS PRESENT

4. -57 SYNOPTIC TYPE(mid and upper levels) 200 mb -38 10 300 mb -11 PW=1.46 8 (181%) 500 mb 5 3 700 mb K=36 250 mb wind max 500 mb winds are plotted 15 850 mb SI=2 2 74 SFC 67 6 T-Td<6oC 500 mb 5 streamline NUMBER OF EVENTS 4 3 2 Shortwave 1 sometimes 2nd jet streak 0 CAN HAPPEN ANY TIME OF YEAR

5. 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

6. 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)

7. NGM PROGGED A VERY SLOW MOVING UPPER LOW AND ASSOCIATED SURFACE FRONT 546 0-48 HR MOVEMENT 558 L L 1032 552 1008 1024 564 1020 1016 1012 570 576 NGM 48 HR 500MB VT 1200Z 16 NOV 1987 NGM 48 HR SFC A TYPICAL SYNOPTIC TYPE HEAVY RAINFALL SURFACE AND 500 MB PATTERN

8. NOTE THE STRONG LOW-LEVEL JET AND HIGH PW VALUES FORECAST NEAR THE COASTNOTE THAT AT THE TIME THE NGM GENERALLY HAD A LOW BIAS OF 850 MB DEWPOINTS. 250 WIND MAXES PW>1.5 10oC 10oC 20 M/S 24 M/S NGM 48HR 850 WIND VECTORS & ISOTACHS NGM 48HR 850MB DEWPOINTS

9. HOW DID THE NGM DO ???The NGM missed the MCS over LA and therefore predicted too much rain over the Plains. =4.0” =6.0+” .5” .5” 1.0” .5” 1.0” 1.0” 24-48 HR NGM QPF VERIFYING PRECIPITATION

10. 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

11. 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

12. 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.

13. SURFACE MESOHIGH OUTFLOW BOUNDARY BUBBLE HIGH COOL AND MOIST H L WARM AND MOIST Td=70oF Td=60oF Td=70oF 120 nm Td=60oF Frontal and Mesohigh Type(surface)Outflow boundary or front provides focus for lifting. The area at highest risk for heavy rainfall is in red. H SURFACE FRONTAL Td=60oF COOL AND MOIST L WARM AND MOIST H Td=70oF Td=70oF Td=60oF 120 nm

14. FRONTAL AND MESOHIGH (850 MB)Why does the orientation of the low-level jet favor heavy rainfall? Td=14 oC Max Winds Td=10 oC Td=12 oC Max Winds Axis of Td=14 oC Axis of 120 nm 120 nm Td=16 oC FRONTAL

15. About 60% of mesohigh and frontal type heavy rainfall events occur near the ridge axis. MESOHIGH FRONTAL MOIST MOIST 500 mb 500 mb

16. -57 200 MB MESOHIGH -36 300 10 PW=1.64” -10 500 (162%) 6 7 700 4 K=39 18 850 SI=-5 3 1014 71 SFC 66 -56 200 MB -36 300 FRONTAL 15 -10 PW=1.60” 6 500 (158%) 7 700 3 K=38 17 850 SI=-4 4 1013 SFC 70 65

17. 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.

18. COMPOSITES OF THE 12 LARGEST SCALE HEAVY RAINFALL EVENTS • CENTERED ON THE RAINFALL MAXIMUM • USED RDAS GRIDDED FIELDS INTERPROLATED TO 2 DEGREE LATITUDE GRID

19. 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 ISOTACHS 850 MB TEMPERATURE ADVECTION ( BLACK DOT IS CENTER OF HEAVIEST RAIN)

20. Note that the heaviest rain occurred southeast of the strongest 850 mb moisture convergence. 8 6 7 4 6 8 2 10 0 17 -2 9 19 18 -4 -6 -8 0 -4 -6 -2 2 4 -8 6 8 850 mb moisture flux (left) and moisture flux divergence (right)

21. 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 VCL1e

22. 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