This NAM Forecast Is a Work of Art

We rarely see such a beautiful comma cloud and precipitation pattern over the Intermountain West as forecast by the NAM for 0900 UTC (0300 MDT) Saturday.  Simply spectacular!

0600 UTC 14 May 2015 NAM forecast of 500-mb wind (vectors), temperature (red contours every 1ºC)
3-h accumulated precipitation, and outgoing long-wave radiation valid 0900 UTC 16 May 2015

Patterns like this are associated with well developed upper-level troughs, but often such troughs have cloud and precipitation patterns that are broken up by the topography of the west. That's not the case in the NAM forecast above.

The beautiful comma cloud and hook-shaped precipitation pattern is associated with two major airstreams.  The first is sometimes called the TROWAL (TRough Of Warm air ALoft) airstream.  The TROWAL airstream is associated with warm advection and ascends along a tongue of warm air that wraps round the back side of the upper level low, forming the comma.  

The second airstream descends on the back side of the upper-level trough, wraps around the forward side of the upper-level trough, and forms the dry slot.  At upper-levels, this airstream is typically dry because it has descended from the upper-troposphere.  The interlacing of these two three-dimensional airstreams leads to the beautiful comma cloud.

The term dry slot can give the false impression that this is an area of benign weather.  Au contraire in some situations.  Although the dry slot may be cloud free and feature low relative humidity air at upper-levels, at low levels, the air beneath the dryslot can vapor laden.  The airmass beneath the dry slot often features large Convective Available Potential Energy (CAPE), which is jacked up further by solar heating during the day if the dry slot is cloud free.  Being ahead of the upper-level trough, the forward portion of the dry slot is also an area of rising motion.  As a result, convection frequently breaks out within or at the leading edge of the dry slot.  The vertical wind shear in the dry slot region is also sometimes favorable for the development of severe convection.  Even the low-resolution NAM is generating some dry-slot convection in the forecast above.