Holiday Weekend Miscellany

The first day of July dawned with perfectly clear skies over Arizona. The push of dry air yesterday has dropped PW down to around an inch at the southeast corner of the state, and to about only a third of an inch out at Yuma. The subtropical, mT air has been pushed to the south over the GoC. The flow patterns are quite chopped up this morning over the Southwest, but are forecast to quickly recover to a typical, hot summer pattern.

-----------------------------------------------------------

The big fly in the ointment this morning is that most soundings over Mexico are missing, thus, the models are initializing on their previous forecasts. Not a particularly good situation, since all the important features are mostly south of the border this morning. The NAM 700 mb analysis at 1200 UTC (below) has an inverted trough, north of what was Arlene, stretching north from central Mexico toward El Paso. It appears that this trough will bring substantial cooling across the south half of GoC, triggering a northward push of low-level, moist air. However, the midnight run of the Atmo WRF-GFS model, initialized last night, indicates that the first push of increased PW over southern Arizona will come in from east of the Continental Divide, as the inverted trough pushes westward across Mexico

Shown below are three WRF-GFS PW graphics from the midnight forecast run. The first shows the initial PW field (with surface winds), reflecting the strong dryout of yesterday.

The plot below shows the forecast PW at 5 am tomorrow (Saturday July 2nd) - note that this push of increased PW has come in from the east to southeast. The north end of the GoC remains relatively dry, with northerly surface winds.

The final PW plot is for 5 pm on Sunday, July 3rd. A strong push of moisture has come north up the GoC, and dry air is now coming back into southern Arizona from New Mexico. The WRF tends to keep the Gulf Surge moisture out in the western half of the state, and to push it north up the Colorado River Basin.

So, there are considerable advective changes forecast to occur rapidly over the weekend. Tucson reaches a PW value of around 35 mm by Sunday afternoon, but Yuma and Puerto Penasco are both near 50 mm at that time.

--------------------------------------------------------------

The WRF forecast of thunderstorms and precipitation tends to keep storms and rainfall from the south on out far to the west of Tucson - the WRF forecast is only for a Trace of rain to occur at TUS all the way through Sunday midnight. I see that the new morning run of the NAM indicates a similar precipitation pattern.

--------------------------------------------------------------

The morning NAM forecasts excellent, strong easterly steering winds at 700 and 500 mb over southeastern Arizona on Sunday afternoon, but dry advection is also occurring by that time. Additionally, 500 mb temperatures are coldest today (around -8C) but warm through the period to around -5/6C on Sunday - not a good trend either.

--------------------------------------------------------------

So lots of pluses and also minuses for the weekend. Once storms get going again, each day strongly impacts how the next evolves, and we'll just have to watch the weather day-by-day through the Fourth!