Patterns and Probabilities of Heavy Rainfall in Forecasting

 
Life on the edge
Patterns and Probabilities of
heavy rainfall
 
Richard H. Grumm
National Weather Service Office
State College PA 16803
 
Introduction
 
There are known patterns for heavy rainfall
 
There are known models to predict heavy rainfall
 
There is uncertainty associated with forecasting
heavy rainfall
 
There are ensembles and probabilities to deal with
uncertainty
 
Sometimes
 
Your in the sweet spot with a high probability
and a good high probability outcome. Life is
relatively easy.
 
Other times, your on the edge, life is tough,
you find yourself running from the cops and
being chased by thugs, well not exactly but the
forecast is tough on the edge.
 
Look at the sharp edge in this
precipitation shield
 
Real edges of precipitation shields can be
real tight
 
Models often Predicted these edges
 
Ensembles Predict sharp edges
Why?
 
Why Sharp Edges?
 
The real atmosphere has these features they are
real.
Our models are getting better
They deal better with hydrometeors so spill out QPF is
reduced
They can really predicted the synoptic and finer
models, mesoscale features quite well
Finer scale models produce better rain shadow and
terrain effects.
Thus they are simulating reality better
The edges vary by cycle
 still uncertainty
 
A few points about edges
 
Life is easier inside the edges
More confident in the higher probability outcome
 
Beware hubris as uncertainty 
 forecast length
 
The high probability and edges move about more at
longer ranges
Shorter ranges one can get real confident
 
What is long range or short range?
“Luke feel the uncertainty go with the uncertainty”
 
Good question each system has it’s own unique
predictability horizon.
Some events are more predictable than others
Success with one event does NOT translate to success
with the next event.
Beware along the edges
Patience is your ally
Never rush to warn on the edge new forecasts will
come in with new edges
Remember on the edges 
 
Patience is your ally
 
Forecasting Heavy Rainfall
eastern US bias here
 
Know the patterns which produce heavy rainfall
Synoptic Type event
Frontal event
And more troublesome mesoscale events.
 
Match the pattern with the EFS probabilities
The EFS and models should predict the pattern
And produce realistic rainfall patterns
But the location and details will vary and not be correct!
 
Be mindful of the uncertainty
The details will vary with forecast length and
Model/EFS resolution
More uncertainty the more patience is required
 
The Synoptic Pattern
A strong southerly flow
V-wind anomalies ahead of a generally N-S frontal
zone
Plumes of high PW air in close proximity to the v-
wind anomalies
Produce most of the big 4+ inch rainfall events
Well predicted pattern in the NCEP Models
The details may vary see an ensemble near you!
 
SREF has more details
sharper edges too
!
 
The rainfall
Sharp edges for the heavy rainfall
 
Classic Synoptic Example
 
19 January 1996 QPE
A strong easterly flow
Negative u-wind anomalies on the cool side of an
E-W frontal zone
Plumes of high PW to east
Produce many 1-4 inch rainfall events
Well predicted pattern in the NCEP Models
The details may vary see an ensemble near you!
The Frontal Pattern
 
The QPE
Lots of terrain influenced details
 
Forecasts had sharp edges
 
Downscaled model runs showed
00Z 11 March Terrain effects and lots of “edges”
 
12Z 11 March Downscaled GFS
details changed
Big Event from the Past
sharp edges too!
The worse case events
A synoptic that transitions to a frontal event
You can get really big rainfall amounts
29-30 March 2010
The synoptic with Tropical wave
Deadly rainfall amounts
Think June 2006!
 
Those Pesky edges
Try Wisk the edge remover
 
All these events had sharp edges
 
So did the forecasts
 
Life along the edges is hard
Easy in the high probability areas
Heard on the edges
 shorter forecast ranges give
better probabilities and confidence
 
More Pesky edges for a record event
 
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There is uncertainty associated with ALL forecasting
Heavy rainfall
Heavy snow
Severe weather you name it….
 
The models and EFS can predict the patterns for event types
But the details are elusive often elusive
There is uncertainty and there will always be uncertainty
The patterns produce high probability areas
 confident forecasts
The patterns can produce sharp edges
 uncertain forecasts
 
Embrace the uncertainty along the edge 
 be patient
Shorter lead-time warnings take patience and courage
Avoid hubris and avoid that Titanic unsinkable sense of being.
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Known patterns and models exist for heavy rainfall forecasting, but uncertainty remains. Ensembles and probabilities help manage this uncertainty. Sharp edges in precipitation shields are key, with models improving to anticipate these features more accurately. Understanding the dynamics of edges can enhance forecast confidence and decision-making, balancing the challenges of uncertainty and probability in meteorological predictions.

  • Heavy Rainfall
  • Forecasting
  • Patterns
  • Probabilities
  • Meteorology

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  1. Life on the edge Patterns and Probabilities of heavy rainfall Richard H. Grumm National Weather Service Office State College PA 16803

  2. Introduction There are known patterns for heavy rainfall There are known models to predict heavy rainfall There is uncertainty associated with forecasting heavy rainfall There are ensembles and probabilities to deal with uncertainty

  3. Sometimes Your in the sweet spot with a high probability and a good high probability outcome. Life is relatively easy. Other times, your on the edge, life is tough, you find yourself running from the cops and being chased by thugs, well not exactly but the forecast is tough on the edge.

  4. Look at the sharp edge in this precipitation shield

  5. Real edges of precipitation shields can be real tight

  6. Models often Predicted these edges

  7. Ensembles Predict sharp edges Why?

  8. Why Sharp Edges? The real atmosphere has these features they are real. Our models are getting better They deal better with hydrometeors so spill out QPF is reduced They can really predicted the synoptic and finer models, mesoscale features quite well Finer scale models produce better rain shadow and terrain effects. Thus they are simulating reality better The edges vary by cycle still uncertainty

  9. A few points about edges Life is easier inside the edges More confident in the higher probability outcome Beware hubris as uncertainty forecast length The high probability and edges move about more at longer ranges Shorter ranges one can get real confident

  10. What is long range or short range? Luke feel the uncertainty go with the uncertainty Good question each system has it s own unique predictability horizon. Some events are more predictable than others Success with one event does NOT translate to success with the next event. Beware along the edges Patience is your ally Never rush to warn on the edge new forecasts will come in with new edges Remember on the edges Patience is your ally

  11. Forecasting Heavy Rainfall eastern US bias here Know the patterns which produce heavy rainfall Synoptic Type event Frontal event And more troublesome mesoscale events. Match the pattern with the EFS probabilities The EFS and models should predict the pattern And produce realistic rainfall patterns But the location and details will vary and not be correct! Be mindful of the uncertainty The details will vary with forecast length and Model/EFS resolution More uncertainty the more patience is required

  12. The Synoptic Pattern A strong southerly flow V-wind anomalies ahead of a generally N-S frontal zone Plumes of high PW air in close proximity to the v- wind anomalies Produce most of the big 4+ inch rainfall events Well predicted pattern in the NCEP Models The details may vary see an ensemble near you!

  13. SREF has more detailssharper edges too!

  14. The rainfall Sharp edges for the heavy rainfall

  15. Classic Synoptic Example

  16. 19 January 1996 QPE

  17. The Frontal Pattern A strong easterly flow Negative u-wind anomalies on the cool side of an E-W frontal zone Plumes of high PW to east Produce many 1-4 inch rainfall events Well predicted pattern in the NCEP Models The details may vary see an ensemble near you!

  18. The QPE Lots of terrain influenced details

  19. Forecasts had sharp edges

  20. Downscaled model runs showed 00Z 11 March Terrain effects and lots of edges

  21. 12Z 11 March Downscaled GFS details changed

  22. Big Event from the Past sharp edges too!

  23. The worse case events A synoptic that transitions to a frontal event You can get really big rainfall amounts 29-30 March 2010 The synoptic with Tropical wave Deadly rainfall amounts Think June 2006!

  24. Those Pesky edges Try Wisk the edge remover All these events had sharp edges So did the forecasts Life along the edges is hard Easy in the high probability areas Heard on the edges shorter forecast ranges give better probabilities and confidence

  25. More Pesky edges for a record event

  26. Review embrace the uncertainty and patience is your ally There is uncertainty associated with ALL forecasting Heavy rainfall Heavy snow Severe weather you name it . The models and EFS can predict the patterns for event types But the details are elusive often elusive There is uncertainty and there will always be uncertainty The patterns produce high probability areas confident forecasts The patterns can produce sharp edges uncertain forecasts Embrace the uncertainty along the edge Shorter lead-time warnings take patience and courage Avoid hubris and avoid that Titanic unsinkable sense of being. be patient

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