Transcript QUANTITATIVE PRECIPITATION FORECASTING REQUIRES …

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
Subtle heavy rainfall events (SHARS)
1) Strong vorticity max, usually round shaped
2) Often the vort strengthens (with a shortening half-wavelength)
3) Beware of weak upper lows in a moist environment !!!
4) Seem to occur more in the northeast quadrant of the CONUS
-53
200 MB
10
8
-11
500 MB
7
700 MB
4
4
850 MB
13
2
12
N
10
16
T-Td.6oC
6
500 MB
PW=1.35
RH=84%
SOUNDING
68
SURFACE
65
1008
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 ISOTACHS
850 MB TEMPERATURE ADVECTION
( BLACK DOT IS CENTER OF HEAVIEST RAIN)
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
-8
-6
-4
-2
0
2
4
6
8
850 mb moisture flux (left) and moisture flux divergence (right)
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
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
SOME OF THE HEAVY RAINFALL
PATTERNS USED WHEN
FORECASTING QPF
 MADDOX
FLASH FLOOD TYPES (MADDOX ET AL
1979 AND 1980)
– EASTERN (SYNOPTIC, FRONTAL, MESOSHIGH)
– WESTERN
 SUMMER
- TYPES I, II, IV
 WINTER - TYPE III
– FRONT RANGE EVENTS
MADDOX SYNOPTIC TYPE
CONSISTENT 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
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
Td$70oF
Td=60oF
Td=66oF
130 nm
Td=10oC
Td=12oC
Td=14oC
130 nm
A POTENT LOW-LEVEL JET
IS PRESENT
-57
SYNOPTIC TYPE
(mid and upper levels)
250 mb wind max
500 mb winds
300 mb
-11
8
500 mb
PW=1.46
(181%)
5
3
700 mb
K=36
15
2
850 mb
SI=2
74
67
SFC
O F
55
44
N U M B R
NUMBER OF EVENTS
sometimes
2nd jet streak
-38
10
66
T-Td<6oC
Shortwave
E V E N T S
are plotted
200 mb
33
22
11
00
J
F
M
A
M
J J A
MONTH
S
O
N
D
CAN HAPPEN ANY TIME OF YEAR
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
546
558
1032
552
1008
0-48 HR
MOVEMENT
L L
1024
564
1020
1012 1016
570
576
NGM 48 HR SFC
VT 1200Z 16 NOV 1987
NGM 48 HR 500MB
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.
250
WIND
MAXES
PW>1.5
10oC
24 M/S
20 M/S
NGM 48HR 850 WIND VECTORS & ISOTACHS
10oC
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.
=4.0”
=6.0+”
.5”
1.0”
.5”
.5”
1.0”
1.0”
24-48 HR NGM QPF
VERIFYING PRECIPITATION
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
MADDOX FRONTAL TYPE
COMMON 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.
H
SURFACE FRONTAL
SURFACE MESOHIGH
BUBBLE
HIGH
COOL AND MOIST
Td=60oF
OUTFLOW
BOUNDARY
H
COOL AND MOIST
L
WARM
AND
MOIST
L
WARM
AND
MOIST
Td=70oF
Td=70oF
Td=70oF
Td=60oF
120 nm
Td=70oF
H
Td=60oF
120 nm
Td=60oF
FRONTAL AND MESOHIGH
(850 MB)
Why does the orientation of the low-level jet favor heavy rainfall?
Td=14 oC
Axis of
o
Td=12 oC Td=10 C
120 nm
120 nm
FRONTAL
Td=16
oC
About 60% of mesohigh and frontal type heavy
rainfall events occur near the ridge axis.
MESOHIGH
FRONTAL
MOIST
MOIST
500 mb
500 mb
-57
-56
200 MB
-36
300
MESOHIGH
10
-36
PW=1.64”
-10
200 MB
500
300
15
-10
6
500
7
3
700
17
4
850
(162%)
18
3
1014
E V E N T S
71
66
700
K=39
850
SI=-5
70
65
SFC
MESHOHIGH
20
1013
E V E N T S
6
7
4
FRONTAL
PW=1.60”
(158%)
K=38
SI=-4
SFC
FRONTAL
14
12
O F
O F
N U M B E R
N U M B E R
15
10
10
5
8
6
4
2
0
0
J
F
M
A
M
J
J
MONTH
A
S
O
N
D
J
F
M
A
M
J J
MONTH
A
S
O
N
D
DO THESE 500MB AND SURFACE
PATTERNS LOOK FAMILIAR ???
A STRENGHENING LOW LEVEL JET IS
PRESENT TO TAP MOISTURE.
HOW DO THE 850MB WINDS AND PWS COMPARE TO THE THOSE
CONSISTENT WITH MADDOX FRONTAL TYPE EVENTS?
A surface boundary is located over
Nebraska and a cold pool over Iowa
K INDICES ACROSS EASTERN NEB WERE
FORECAST TO BE IN THE
UPPER 30S TO LOW 40S
WHERE DO YOU THINK CONVECTION MIGHT FORM?
The axis of strongest moisture flux or
transport shifts very slowly eastward
06Z forecast
09Z forecast
Where do you think the MCS should be
located at 06Z and 09Z?
24 HR PRECIPITATION VALID 12 UTC 17 JULY 1996
MESOETA FORECAST
ANALYSIS
6” OR MORE
3” OR MORE
1” OR MORE
Not too shabby for the model, how did you do?
WHAT TYPE IS THIS ????
BOUNDARY LAYER WIND AND TEMPERATURE FORECAST V.T. 00Z 18 JULY
STRONG LOW LEVEL JET IS PRESENT WITH LOTS OF MOISTURE
OOZ 18 JULY FORECASTS OF
250 JET AND DIVERGENCE
BEST LI AND BOUNDARY LAYER WINDS
Where do you think we are going to see heavy rain ???
What kind of maximum amounts could we see ???
Does the model have a handle on where there will
be heavy rain ???
Is the model amount in the ball park ???
REMEMBER TO LOOK FOR LOW-LEVEL
BOUNDARIES.
NOTICE THE THERMAL GRADIENT OVER IL
RUC MOISTURE FLUX FORECAST
V.T. 06Z
RUC SURFACE WINDS, TEMPERATURES
AND MOISTURE CONVERGENCE
HOW DID YOU DO?
MODEL FORECAST
How did you do?
OBSERVED
6” OR MORE
3” OR MORE
1” OR MORE
THE LAKE BREEZE FRONT FOCUSED CONVECTION OVER IL !!!
COMPOSING QPF IN WEST
(PRIMER)

TWO STRIKES AGAINST YOU
– 1. Tracking features in the mid and low levels is not a trivial task


Discerning fronts and mid level shortwaves can be a nightmare in complex
terrain.
Pressure reduction over the mountains is REAL sloppy
– 2. Models have a hard time accurately simulating mt meteorology.

DOES LOOKING FOR MLI WORK (trying to answer questions of Where,
When and How Much) ?
– Sure…but moisture is limited
– Lift can be annoyingly abundant (upslope)
– Instability is also abundant in the spring-summer (terrain acts daily as an
elevated heat source)
– So in some cases you’d end up tracing the terrain for best MLI
– In actuality….QPF over the mountains is such a small scale…assessing
MLI isn’t accurate enough.
COMPOSING QPF IN WEST
So where can we start ???

Find regions of Best Upslope and Downslope in mid and low levels.
– 1. Use mean layer winds (800-500mb) are good

Assess moisture
– 1. Use mean layers

You get lots of high (aoa 9Kft) clouds lots of the time…but it seems you need
upslope in a dry environment to help moisten up the low levels OR you need
decent pre-existing low level moisture (moderate to high dew points).
– 2. Use PW analysis
– 3. Use WV imagery

Assess MLI and Synoptic Scale Lift
– 1. Will this enhance the upslope (or overcome the down-slope) &
vice/versa.

There ain’t nothing like experience !
– This will help develop pattern recognition
QPF IN WEST
(INCLUDING THE FRONT RANGE OF THE ROCKIES)

CHARACTERISTICS DIFFER BY SEASON
– SCALE OF THE EVENTS IS VERY SMALL vs WINTER.

SCALE IS TIED TO FORCING AND RH (local upslope and available
moisture)
– SUMMER: FORCING DRIVEN PRIMARILY BY CONVECTION AND LOCAL
OROGRAPHIC LIFT
– WINTER: FORCING DRIVEN PRIMARILY BY OROGRAPHY (ENHANCED
BY SYNOPTIC SCALE LIFT)

REQUIRES RIGOROUS USE OF SAT IMAGERY (RELY HEAVILY ON
WV LOOPS FOR DISCERNING SHORT WAVES)

RELY HEAVILY ON WV AND PW IMAGES FOR MOISTURE SURGES
SOME CLIMATOLOGY
LOCATIONS OF LARGEST 13 MCS/MCC SYSTEMS USED TO
PREPARE 4 TYPES OF ATMOSPHERIC COMPOSITES
LOCATION OF
COMPOSITE CASES
2
4 44
14
1
1
2 2
2
3 22
1
3 1 32 2
2121 2 1
211
3
4 44
211211
31
2
2
LEGEND
SYNOPTIC TYPE
#EVENTS
(45)
2
1
BLOCKING ANTICYCLONE
15
2
DEFORMATION ZONE
17
3
SHORTWAVE TROUGH IN
6
NORTHWEST FLOW
SHORTWAVE TROUGH IN ZONAL
4
7
FLOW
FROM CHAPPELL
COMET NOTES
SOME CLIMATOLOGY
FR E Q U E N C Y O F O C C U R R E N C E
FREQUENCY OF FLASH FLOODING OR 2”/24HR RAINFALL FOR 137
EVENTS IN WEST
NOTE THE HIGH FREQUENCY IN LATE JULY AND AUGUST
60
P E RCE NT O F CA S E S
35
30
50
25
40
20
30
15
20
10
10
5
0
J
F
M
A
M
J
J
A
S
O
N
SEMI-MONTHLY TIME OF YEAR
D
0
N-4PM
4-8PM
8PM-M
M-4AM
4-8AM
8-AM-N
LOCAL TIME OF OCCURRENCE
FROM CHAPPELL COMET NOTES
N U M B E R OF C A S E S
INTERMOUNTAIN PLATEAU (83 EVENTS)
25
20
15
EVENTS IN INTERMOUNTAIN
REGION ALSO HAVE A
DISTINCT MAXIMUM DURING
THE 6-HR PERIOD BETWEEN
2 PM AND 8 PM LOCAL
DAYLIGHT TIME.
10
5
0
J
F M A M J J A S O N D
SEMI-MONTH TIME OF YEAR
OCCUR MOSTLY IN AUGUST
AND SEPTEMBER INTO
EARLY OCTOBER.
FRONT RANGE EVENTS (49)
80
N U M B E R OF C A S E S
P E R C E NT OF C AS E S
The vast majority of front range events occur
during the late July and early August,
60
40
20
0
8AM-2PM
2PM-8PM
8PM-2AM
LOCAL TIME OF DAY
2AM-8AM
DIVIDING INTO HALF-MONTH PERIODS
20
15
10
5
0
J F M A M J J A S O N D
SEMI-MONTHLY TIME OF YEAR DISTRIBUTION
and occur during the late afternoon and
early evening hours (2-8 PM)
FROM CHAPPELL COMET NOTES
HEAVY RAIN EVENTS ALONG THE FRONT RANGE
BIG THOMPSON, FORT COLLINS, CHEYENNE, MADISON COUNTY (VA)
•A SLOW MOVING FRONT IS LOCATED UP JUST SOUTH OF THE AREA
•WINDS ALOFT ARE LIGHT AND SOUTHEASTERLY
•A LARGE AMPLITUDE NEGATIVE-TILTED UPPER RIDGE AXIS LIES
NORTH AND EAST OF THE AREA
•A WEAK SHORTWAVE ROTATES NORTHWARD TOWARDS THE AREA
RESULTING IN WEAK PVA
FROM MADDOX ET AL., 1977
* A NARROW BAND OF MOIST UNCONDITIONALLY UNSTABLE AIR JUST
BEHIND THE FRONT
* THE MOISTURE USUALLY EXTENDS TROUGH A DEEP LAYER (TO 300 MB)
PWS 150-200% OR NORMAL
* OROGRAPHIC LIFT PROVIDES MECHANISM TO RELEASE INSTABILITY
300
-31
9
500
-6
8
300
12
5
SFC
79
63
PW=1.18”
(181% OF
NORMAL)
1015.3
Anytime surface dewpoints
exceed 50 degrees F (especially
in excess of 60F) and extend
westward to the front range…the
region should be monitored very
closely for potential heavy rain/ff
…particularly when the low level
winds are upslope (east or
southeasterly)
RH=66% (SFC-500
MB)
AVERAGE SOUNDING FOR
LI=-5
K=37
BLOCKING
ANTICYCLONE
FROM CHAPPELL COMET NOTES
Cells develop east of highest terrain
* Cells then move slowly north and northwest
* Redevelopment occurs on SE or S flank
* Heaviest rain falls over a very small area
H
LOW LEVEL
JET AND MOISTURE
TONGUE
THREAT AREA
L
500 MB
TROF
L
L
LOW LEVEL
Td$65o
F
IDEALIZED
SURFACE PATTERN
THERMAL AXIS
ADOPTED FROM MADDOX ET AL., 1977
PREDICT WHERE THE HEAVIEST RAIN WILL FALL
ETA MODEL FORECAST OF SFC AND 1000-500 MB THICKNESS.
H
H
30
24
24
20
11L
H
16
L
60 F+
20
70oF+
dewpoints
L
o
70oF+
16
dewpoints
12 HR V.T. 00Z
dewpoints
24 HR V.T. 12Z
ETA 500MB FORECASTS
NOTE THE TILT OF THE UPPER RIDGE AXIS.
L
12 HR V.T. 00Z
24 HR V.T. 12Z
OBSERVED MAPS VALID 00Z
71
.. 169
58 7
68 171 71
51
48
62
72 173 55
54
54
50 0
65
56
64
46
SURFACE
78
61 207
71
64
179
74
77 205
5
65 13
60
20
193
82 193
73
68 17364 13 67
63 72 185 76 19382 190
70 82
63 13 66
70
72 169
..
83 178
5
63
65
16
82
70
82
148
85 131
63
66
Td$15o
C
850 MB
500 MB
FROM THESE MAPS WHAT CAN BE INFERRED ABOUT THE
PRECIPITATION EFFICIENCY OF ANY CELLS THAT FORM?
MODEL 12-36 HR QPF
$
AVN
$.01”
$.50”
$1.0”
$.2.0”
$.3.0”
NGM
ETA
WHICH MODEL DO YOU THINK HAS THE BEST
FORECAST OF THE SCALE OF THE 2.00” OR
GREATER AMOUNTS? WHAT ABOUT THE
LOCATION OF THE MAXIMUM RAINFALL?
VERIFICATION
8-10” OF RAIN ON
FORT COLLINS &
NEARBY FOOTHILLS
4 MILES AWAY
ONLY .83”
OBSERVED
$.50”
$1.0”
295 HOUSES OR MOBILE
HOMES DESTROYED, 4 KILLED
$2.0”
.
$3.0” A LARGER MCS MOVED
SOUTHEASTWARD AWAY FROM
A SMALLER SCALE QUASISTATIONARY CONVECTIVE
STORM.
VERIFYING ANALYSIS VALID
12Z JULY 29
Note the barren dry
expanse known as
southeast Wyoming
Southwest, blocking high type heavy rainfall event.
Note that the high center shifts east of 4-corners. This allows moisture to be pulled
northward from the Gulf of CA.
582
10
L
576
14
312
315
582
10
588
08
12
TROF
H
TROF
L
10
500 HEIGHTS AND
VORTICITY
08
06
315
0oC
H 2oC
-2oC
06
588
8o
L
6oC
315
4oC
Td
700 MB COMPOSITE
Note there is often a upper low to the southwest with a vorticity lobe extending
towards the threat area. These features often show up best on water vapor imagery.
And at the surface..
1020
1024
1016
1016
300
500
H
50o
50
o
1024
1020
SURFACE
COMPOSITE 1016
L
55o
60o
1012
1008 65o+
700
SFC
-31
13
PW=1.32”
-6
6
RH=58%
12
6
(133% OR
NORMAL)
LI=-2
KI=37
89
63
NOTE WEAK MEAN FLOW AND RELATIVELY DEEP MOISTURE. SCALE
OF THE EVENT IS SMALL.
DEFORMATION TYPE
STRONG UPPER LEVEL DIVERGENCE IS PRESENT, OFTEN ENHANCED BY
ENTRANCE REGION OF UPPER LEVEL JET STREAK
570
12
10
L
576
582
L
588
309
-8o
312
-6o
315
-2o
0
2o
318
594
04
-4o
4o
L
-2o
10
08
500 HEIGHTS AND
VORTICITY
0
2o
700 HEIGHTS AND
DEWPOINTS
An old front/surface boundary and
weak mean flow are usually present
1016
1012 1010
1010
300
1012
50o
500
50o
700
1016
60o
1012
65o
60o
1012
1008
65o
1008
SFC
-33
9
PW=1.15”
-8
5
10
8
81
59
(160% OF
NORMAL)
RH=66%
LI=-2
KI=35
1010.7
WINTER QPF IN THE WEST

OVERALL SCALE IS LARGER THAN IN SUMMER

SCALE AND POSITION OF HEAVIEST RAIN IS OFTEN
CLOSELY TIED TO OROGRAPHY

SATELLITE IMAGERY, SSMI AND GOES PW PRODUCTS
ARE GREAT TOOLS

3 INCH OR GREATER AMOUNTS IN THE CASCADE AND
SIERRA RANGES ARE COMMON WHEN AXIS OF DEEP
MOISTURE IS QUASI-STATIONARY
WINTER QPF IN THE WEST
(continued)

USUALLY THERE IS A SIGNIFICANT RAIN SHADOW TO THE LEE OF
THE MOUNTAINS.

MOUNTAIN MAPPER AND THE RHEA OROGRAPHIC TECHNIQUE
CAN COMBINE TO PROVIDE A VERY GOOD FIRST GUESS.

WHEN MOISTURE IS UNUSUALLY DEEP RAIN, HEAVY RAIN WILL
FALL ON THE LEEWARD SIDE OF THE SIERRA RANGE IN
EASTERN NV AND WESTERN CA

SIGNIFICANT SKILL IN 24 HOURS FORECASTS, SKILL IS
CONSIDERABLY LOWER FOR 6 HOURLY FORECASTS
MADDOX ET AL. WINTER
TYPE III PATTERN

MOST COMMON IN WINTER AND EARLY SPRING

STRONG UPPER TROF SWEEPING INTO WEST COAST FROM
PACIFIC

THERE IS USUALLY A BLOCK OVER ALASKA OR THE GULF OF
ALASKA

LOOK FOR TROPICAL CONNECTIONS

THE STRONGER THE LOW LEVEL WINDS NORMAL TO THE
MOUNTAINS, THE HEAVIER THE RAIN

HEAVY PRECIPITATION ENDS AS UPPER TROF AXIS NEARS
COAST.
Maddox et al., Type III
200
-53
-45
300
PW=.72”
500
-19
-12
700
-2
-6
SFC
58
50
(159% OF
NORMAL)
KI=27
1011
Type III events typically occur in southern California
A block forms over Alaska and Gulf of Alaska and the westerlies
break through to its south
A number of shortwaves then affect the state and produce a prolonged period of heavy rains.
TYPE III CASE STUDY
VALID AT BEGINNING OF PERIOD
MSL, THICKNESS AND 850 WINDS
PRECIPITABLE WATER (INCHES) AND 850
MB WINDS
NOTE: 1) A LONG FETCH OF DEEP MOISTURE
2) A BARRIER JET AND STRONG SOUTHERLY FLOW UP THE
SACREMENTO VALLEY. THIS JET HELPS PRODUCE HEAVY
RAINS NEAR SHASTA
OVERLAYING MODEL OUTPUT WITH SSMI IMAGERY CAN
GIVE YOU A GOOD IDEA OF THE MOISTURE THAT WILL BE
FEEDING INTO THE WEST COAST.
NOTE THE TROPICAL CONNECTION
AND PLUME OF PWS ABOVE 1.00”
THE MODEL’S TERRAIN IS AVERAGED OVER THE
GRID BOX SO THE SLOPE OF THE TERRAIN IS
USUALLY NOT STEEP ENOUGH
THIS CAUSES THE VERTICAL MOTION FIELD TO BE
SHIFTED AWAY FROM THE MOUNTAINS
MODEL LIMITATIONS FOR WINTER
QPFS OVER COMPLEX TERRAIN
BECAUSE OF THE SIMPLIFIED
MICROPYSICS AND INADEQUATE
RESOLUTION OF
MOUNTAINS...MODELS USUALLY:
1) PREDICT PRECIPITATION
TOO FAR WEST AWAY FROM
MOUNTAIN PEAKS
2) DO NOT ALLOW ENOUGH
PRECIPITATION ON THE
IMMEDIATE DOWNWIND SIDE
OF MOUNTAIN RANGES
SUMMARY

DURING SUMMER OVER THE INTERMOUTAIN REGION
ANDMOST OF THE WEST, QPF IS ALMOST
IMPOSSIBLE BECAUSE
– THE MODELS DO A BAD JOB HANDLING THE MOISTURE
– THE SCALE OF EVENTS IS USUALLY SO SMALL

DURING WINTER, THE SKILL OF QPF IS HIGHER OVER
NORTHERN CA, WESTERN OR AND WESTERN WA
THAN ANYWHERE ELSE IN THE COUNTRY
– BECAUSE THE FOCUS FOR MOST OF THE LIFTING IS
STATIONARY (THE MOUNTAINS).
– HOWEVER TIMING OF WHICH 6 HOUR PERIOD WILL RECEIVE
THE HEAVIEST RAINFALL IS STILL DIFFICULT.