1. 452 Precipitation

2. Prob. Of Precip.– Cool Season (0000/1200 UTC Cycles Combined)

4. Orographic precipitation enhancement
The basic idea is simple. The details of the dynamics and microphysics are more complicated. Flow over topography is a special case of dynamics that has to be considered separately or in addition to the other processes producing lifting and condensation. The pattern of fallout of the orographic precipitation is sensitive to the microphysical processes. How fast can the precip fall out?

6. Annual Precipitation

7. SW Olympic Slopes-Hoh Rain Forest: 150-170 inches yr-1

8. Sequim: roughly 15 inches per year.

9. Orographic Enhancement of Precipitation
Upslope site
(elev. 1067m)
Valley site
Precipitation Rate (mm/hr)
Distance (km)

10. Rainshadows Shift with Approaching Flow Directions and Depend on Stability
As the flow approaching a barrier changes direction, so does the orientation of the rain shadow.
Less rainshadow during warm frontal period, more post-frontal.

11. Rainshadow

14. Rainshadow in westerly flow

15. Rainshadow Amplitude and Structure Depends on Synoptic Situation and Stability
Less rainshadow during warm frontal and pre-occlusion periods, when deep and generally stable precipitation. Also winds tend to more southerly (terrain parallel) during this period.
More rainshadow and less lee precipitation post-frontally, when flow is less stable and more westerly The former produces more shallow windward convection and greater westerly flow increases low-level upslope on windward side and drying downslope on leeward side.

16. Pre-occlusion, less profound rainshadow (15 UTC 23rd)

17. Moist warm southerly flow at 850 mb

18. Visible sat photo…clouds over region, with some suggestion of rain/cloudshadowing

20. Progress in Precipitation Prediction in Terrain
NWP can handle terrain impacts on precipitation fairly well if sufficient resolution and state-of-the-art microphysics
But there are still some issues (such as too much snow aloft over barriers)

21. NGM, 80 km, 1995

22. NGM, 1995

23. 2001: Eta Model, 22 km

24. 36-km
12-km

25. NWS WRF-NMM (12-km)

26. 2007-2008 12-km UW MM5 Real-time 12-km WRF-ARW and WRF-NMM are similar
December 3, 2007
0000 UTC Initial
12-h forecast
3-hr precip.

27.
4-km MM5
Real-time

32. Smaller Scale Terrain Modulates
Precipitation
10-km

33. 12-km
4-km

35. Small-Scale Spatial Gradients in Climatological Precipitation on the Olympic Peninsula
Alison M. Anders, Gerard H. Roe, Dale R. Durran, and Justin R. Minder
Journal of Hydrometeorology
Volume 8, Issue 5 (October 2007) pp. 1068–1081

36. Annual Climatologies of MM5 4-km domain

37. Verification of Small-Scale Orographic Effects

38. Convective Precipitation
Until recently, subject approaches have dominated.
Both subjective and modeling approaches more more skillful with stratiform.
Convection is least skillful precipitation in virtually all models
Need at least 2-4 km resolution to get most convection even half right.
If grid spacing is more coarse must have cumulus parameterization—SAS is used for MM5/WRF.
High resolution models can potentially give a heads up type of precipitation a day ahead (squall line, supercell, etc.)

39. Real-time WRF 4 km BAMEX Forecast
Valid 6/10/03 12Z
4 km BAMEX forecast 36 h Reflectivity
4 km BAMEX forecast 12 h Reflectivity
Composite NEXRAD Radar

40. Composite NEXRAD Radar
Real-time 12 h WRF Reflectivity Forecast
Valid 6/10/03 12Z
4 km BAMEX forecast
10 km BAMEX forecast
22 km CONUS forecast
Composite NEXRAD Radar

41. Hurricane Isabel Reflectivity at Landfall
18 Sep Z
Radar Composite
41 h forecast from 4 km WRF

42. Diurnal Precipitation Variation During the Warm Season

45. Atmospheric Rivers
Meridional flow of moisture is often limited to relatively narrow currents of moisture and usually warm temperatures.

46. Most West Coast heavy precip events are associated with “atmospheric rivers”, a.k.a. the “Pineapple Express”
A relatively narrow current of warm, moist air from the subtropics…often starting near or just north of Hawaii.

47. Associated with extraordinarily narrow filaments of moisture
Precipitable Water
From Mike Warner

48. A Recent Devastating Pineapple Express: November 6-7, 2006

49. November 6-9,
2006
Dark Green: about 20 inches

50. We know quite a bit about atmospheric rivers and heavy NW precipitation events, although there are still gaps in our knowledge

51. Synoptic Set-Up for Top Fifty Events at Forks Courtesy of Michael Warner

52. Precipitable Water

53. 500 mb height

54. SLP

55. 850 mb Temp

56. Extreme Precipitation Events
The current of warm, moist air associated with atmospheric rivers are found in the warm sector, parallel, near, and in front of the cold front.
Thus, atmospheric rivers are closely associated with the jet core and the region of large baroclinicity.

57. Orographic Enhancement
Upslope flow greatly increases precipitation rates on terrain.
Thus, wind speed and angle of attach can greatly modify the extreme nature of the precipitation.

58. Predicting Clouds Zero-order approach: use relative humidity
70% mb RH often corresponds with thicker, middle-level clouds that have a serious impact on radiation.
700 and 850 mb RH is also used by some.

64. Direct Use of Model Clouds
All modern models predict clouds, specifically mixing ratios of cloud liquid water and cloud ice.
The quality varies…and keep in mind there are serious deficiencies of even the best microphysical schemes.
And problems with other physical parameterizations: boundary layer turbulence or radiation can also mess up model clouds.

65. Direct Use of Model Clouds
Most problematic: stratus, stratocumulus, and FOG.
Remember, some models have spin-up issues: precipitation and clouds improve during the first 3-9 hours. Particularly true of UW WRF which is now cold-started (no clouds at initialization!)

69. Imperfect Predictions

71. Cloud/Precip Forecasting Strategy
Very short-term (0-3 hr): temporal extrapolation (informed by human judgement) is HARD to beat. Nowcasting.
Use radar and satellite animation. Models generally not that useful. Radar apps now available on smartphones to do this (e.g. Dark Sky)
New data assimilation/modeling systems: e.g., EnKF, RUC/RR/HRRR may be viable soon and eventually will take this on successfully.

72. Strategy
Short-term (2-6 hr): Satellite extrapolation becomes central. Model becomes more dominant at the end. Rapid refresh modeling systems.
Daily (6-24 h): Satellite extrapolation and model, weighing model more at longer range.
Remember model spin up issues.

74. Strategy
A wise forecaster ALWAYS evaluates model’s initial moisture fields—often a failure mode and a good indicator of potential model failures.
How? Compare initial model fields and RH to satellite imagery.

75. 700
850

76. Strategy Know regional precip/cloud climatology
Diurnal and geographic features tend to be very repeatable, particularly during the warm season
Example: front range convection

77. Human Forecaster Issues
Precipitation is a parameter where in general forecasters add the least skill compared to objective guidance.
Psychological issues for high and low precip probabilities

78. Human Versus Objective Skill for Precipitation Forecasts: NWS Offices Around the US

79. Brier Scores for Precipitation for all stations for the entire study period.

80. Brier Score for all stations, 1 August 2003 – 1 August 2004
Brier Score for all stations, 1 August 2003 – 1 August day smoothing is performed on the data.

81. Precipitation
Brier Score for all stations, 1 August 2003 – 1 August 2004, sorted by geographic region.

82. Reliability diagrams for period 1 (a), period 2 (b), period 3 (c) and period 4 (d).

83. Rain Psychology
More than a day out….Human Forecasters Tend to Overpredict 10-30%
--exaggerating the threat of rain when it is not likely.
“Its probably not going to rain…but I will throw in 10-30% anyway to cover myself”
And underpredict rain when it is fairly definite. “Well it looks like it will rain…but I am unsure…so I will knock down the probabilities to 60-80% to cover myself”

84. Humans Help with Heavier Precipitation

92. The End

94. Bias scores for the (a) and (b) 4-km model simulations for 1400 UTC 13 Dec 2001 through 0800 UTC 14 Dec 2001.

98. Puget Sound Convergence Zone

99. Flow over terrain can be highly 3D and complex