The remnants of Tropical Storm Iselle are taking an unusual pathway into the western US interior and will be visiting northern Utah this weekend.
Yesterday morning (1200 UTC 26 August), Iselle's remnants were to the west of the Baja Peninsula. The color contours below are precipitable water in millimeters with warmer colors indicating higher values. Precipitable water is a measure of the total water vapor in the atmosphere expressed as the depth of water if all of that water vapor were to condense and fall out. Iselle's remnants were associated with a maximum in precipitable water above 65 mm.
Iselle's remnants moved northwestward yesterday and last night. As this happened, they began to interact with the southwest flow ahead of an upper-level trough off the Pacific Coast, resulting in a plume of high precipitable water movign northeastward toward California.
That plume spreads eastward and ultimately serves as the locus for precipitation development over Nevada tomorrow afternoon (0000 UTC 29 August). A relatively strong front also forms in that area during the day tomorrow due in part to daytime surface heating in southern Nevada, which is more cloud free, and northern Nevada which is cloudier and experiencing cooling beneath the precipitation. This leads to a 700-mb temperature difference across Nevada of almost 10°C at 700 mb.
A look at the ECMWF AIFS forecast shows it is much more bullish on precipitation Saturday morning. The plot below (upper right) is a 6-hour accumulation whereas the one above for the GFS is a 3-hour accumulation, but still, the precipitation is clearly heavier and more widespread.
