East slopes of mountain range type events

Orographic lifting may trigger convective storms along the slopes of mountains given the correct flow regime (Maddox et al. 1998).  Four such examples have been studied extensively, the Big Thompson (Colorado,  the Rapid City (South Dakota), the Cheyenne (Wyoming) and Madison County (Virginia) flash floods (Maddox et al. 1978, Chappell and Rodgers 1988).   The upper-level pattern was very similar for all four events.  A full latitude trough was located along or just off the west coast of the U.S.  A much smaller scale shortwave was noted to be lifting northward on the east side of trough during each case. 

From Pontrelli et al. 1999

The figures from an article by Pontrelli et al (1999) below show the 500-hPA patterns associated with the four flash floods.  The dashed line is the weak shortwave and the red line is the axis of the upper level ridge.  Note that latter has a negative tilt in each case.  The negative tilting 500h ridge was associated helped induce the development of a surface high that supplied low-level easterly flow into the mountains during each case.  North of the  low moving or stationary front,  each also had a ribbon of moist air where there also was an easterly low level jet.  The shaded area on the  figures below show the ribbons of high dewpoint air at the onset of each storm.

All four events were characterized by having low cloud bases relative to the terrain and low centroid tops to the convection with most of the condensate forming and falling below the freezing level.  The mid– and upper-level flow during each of the events was relatively light (Pontrelli et al 1999). 

From Pontrelli et al. 1999

Pontrelli and his colleagues offered an 1800 UTC sounding for IAD during the Madison county event.  The sounding illustrates how deep the moisture was across the region.  In all likelihood,  the relative humidity in the layer below 800-hPa on the IAD sounding would have higher relative humidity with less of a dewpoint depression than farther to the northwest where the cooler air would have been deeper.  Such a modification to the IAD sounding would suggest a very low LCL and cloud base.  The 1745 UTC reflectivity cross section of the Madison county storm clearly shows a low centroid of high reflectivity

The deep moisture and warm layer on the sounding also have another important implication.  The most efficient was to product downdrafts is through evaporation and melting.  When most of the condensate is occurring in a very moisture atmosphere with a deep warm layer that is above freezing,  the potential of cooling trough evaporation and melting are minimized which limits downdrafts.  A weaker cold pool leads to less movement to the boundary giving a system more potential for new cells to form in place. 

  The idealized 500-hPA and surface patterns below show the typical threat area relative to the mountains and the typical look of the synoptic pattern.   Note the similarity with Maddox type I western events.













In Summary, the following synoptic and mesoscale characteristic are common for warm season extreme rainfall events along the front range of north-south mountain ranges.


1) A weak 500-hPa shortwave swings northward or northeastward towards the threat area.

2) Light southeast to south-southeast winds are present in the mid-and upper levels.

3) A slow moving or stationary front lying just to the south of the threat area.

4) High moisture content though a deep layer

5) West and south of the threat area, afternoon heating in combination with cold advection to the east of the threat area helps intensify thickness and pressure gradients

6) A narrow band of moist and unstable air located just north of the front.  A mechanism to provide a cap that limits development until the air is lifted by terrain.

7) The convection and heavy rains usually forms to the east of the highest terrain.

8) Convection may slowly move but keeps reforming on the terrain along the western flank of the convection. 



From Pontrelli et al. 1999

From Pontrelli et al. 1999

More that 600 mm of rainfall were estimated during the Madison County rainfall event.  250 mm were reported by several observers in a 2 hour period attesting to the intense rainfall rates that occurred. 

There are a few differences differences between the events along the front range of the Rocky mountain and similar events along the east slopes of the Appalachians is average precipitable water associated with the events.  The figure at the left provides the average profile of the moisture and instability associated with the Rocky mountain events.    The precipitable water for the western events is usually significantly lower than during Appalachian events.  The scale of the events is often smaller.  However,  in precipitable water values are typically much higher than normal during these west coast events. 

Top number is temperature in oC except at the surface and the bottom number is the dewpoint depression

Figures adapted from Chappell COMET notes

The figures above show the seasonal frequency of east slope events along the front range of the Rocky mountains and the local time of occurrence.  In the west these events are strongly tied to the monsoon and typically occur during the afternoon and evening. Along the Appalachians such events have been observed from June-early November