Subtropics to pay a visit

A remarkable week is on tap as a series of storms will pummel the western United States and bring the warmth and wet to Utah.  Winter should persist until Friday, when the first surge of warm air and moisture moves into Utah late in the day.

We will have it easy compared to California, which will be seeing heavy rain associated with two atmospheric rivers that are expected to impinge upon the Sierra Nevada the next few days.  An atmospheric river is a narrow plume of moisture that is typically connected to the subtropics or tropics and extends into the mid latitudes.  The GFS precipitable water and sea level pressure forecast for 1800 UTC 18 Dec (Saturday) produces a marvelous atmospheric river (black arrow) that extends from a region of subtropical moisture just north of Hawaii into California.

72-h GFS forecast of precipitable water (color fill) and
sea level pressure valid 1800 UTC 18 Dec.

The atmospheric rivers are especially dramatic if you examine the GFS forecast loop.  You can see how they are extracted from the reservoir of moisture over the tropics and subtropics.

What I find remarkable about these rivers is how much moisture they lose over the Sierra Nevada.  Note how the PW decreases markedly across that barrier, which reflects water vapor loss to precipitation.  If we could just remove the Sierra, Utah would be a much wetter place!