Another difficult forecast day

A look at the observations this morning, as well as the NAM and GFS, indicates another difficult forecast situation for the Borderlands. The following is my brief discussion of the situation.

First, the middle and upper-level wave spinning over the northern Plains has a weak trough hanging back, east-to-west, over the western U.S. The models appear to move this feature southward across at least eastern Arizona tomorrow. There is currently cold advection with the S/W, and this would be quite favorable, especially given the very nice vertical wind shear profiles that develop during next 24-hours, for strong storms, if there low-level moisture and CAPE in place. There also appears to be an upper-level moisture plume from what was Ileana approaching the Southwest.

A gradual advection of low-level moisture is occurring across southern New Mexico and into southeastern Arizona - this is due to the back-door cold front that has coming down the Plains as it tries to push westward across the Continental Divide.

Now to the south. No observational data wrt to upper-air soundings, as per the last 7 weeks. The NAM initializes a strong inverted trough over the southern Gulf of California at 500 and 700 mb - the 700 mb trough appears to brush across southern Arizona tomorrow afternoon. This occurs about the same time the 500-300 mb weak S/W is positioned over eastern Arizona. Thus, the question is: how much of this is real and how much is happening only in the NAM world?

Satellite loops indicate that a pronounced wave is moving westward across the lower Gulf of California. So, the moisture surge predicted by the NAM may indeed be getting underway. Surface obs indicate very high dewpoints along the GoC and south winds at LaPaz and Loretto. The U 0f A GPS precipitable water time series page shows that a huge increase in precipitable water has occurred at Hermosilla during past nine hours, i.e., since midnight. Values there are almost 50 mm this morning.

So it appears that there is probably at least a 50/50 chance of everything coming together over next 24 to 36 hours to support a new round of summer thunderstorms over parts of southern Arizona. The dilemma here is that very dry air will be lurking very near. The key is likely to be tied to whether or not a GoC surge actually occurs by tomorrow morning.

Who said that summer forecasting is easy?