Wednesday, July 15, 2009


I overlooked the importance of the upper-level trough off the end of Baja and Pat Holbrook pointed this out to me an e-mail at 8:45 am yesterday, as per:
Bob, A couple of notes…the upper low off the southern tip of Baja (north of the developing tropical system) should make it favorable again today for strong convection in the sierra madre’s like we saw yesterday…some pushing close to our border...but more importantly moving out away from the mountains into the Hermosillo area. The low turns the corner around the ridge tomorrow which should help out AZ on Wednesday. Satellite precipitable water shows the surge moisture lurking just south of the border and with tonight’s convection we should have a much deeper push into AZ…pressure difference between MHO and Yuma is around 4mb now and should increase some tonight. Pat
A spectacular MCS developed over Sonora during the evening (see photo above) as the upper trough moved northward. This MCS has pushed outflows and deeper moisture over most of the southern part of state, while in far west Yuma Td has gradually increased through the night. So, what a difference in things today!

At sunrise sky was heavily overcast with thick cirrus from the decaying MCS and a brisk southeast breeze was blowing. Temperature here at house was 81 at 6 am. There continue to be some light midlevel showers spreading north across this part of the state. Other features of note this morning:

Depression Five-E looks more like a TS or hurricane in the morning satellite images.

Upper-level trough is now about half way up Baja.

Precipitable water amounts have increased about ¼ of an inch since yesterday afternoon, and are around an inch and a half over southern Arizona. Most stations in southeast Arizona had outflows during the night form the south gusting 40 to 50 mph.

It appears there may be a circulation and vorticity center left behind by the MCS – it appears to be south of Nogales and moving to the north-northwest. This feature could play an important role in this afternoon’s events. It has certainly destroyed the accuracy of the NAM initialization this morning, which is extremely bad over the Southwest!

Observed 850 mb Tds are still only around 10C at Tucson, Yuma, and Phoenix this morning. But south winds of 15 to 20 kts at all three sites should help further increase low-level moisture during the day. Surface dewpoints are currently mostly in mid-to-upper 50s, except for Yuma which has increased to 70F.

Mexican soundings have reappeared this morning, but probably not in time to get into the NAM initialization.

The Tucson sounding’s winds are very chaotic this morning. The 500 mb wind is from the north because the MCS circulation has popped off a small anticyclone to our southwest – this should dissipate as the MCV moves by and winds at 500 mb will probably return to be southerly, but light, this afternoon.

Not clear how much sunshine we’ll get to fight the low-level cooling from the MCS outflows. Thus, not obvious yet how much CAPE there may be at low elevations this afternoon and evening. But, storms will pose a threat both for heavy rain and wet microbursts.

Should be an interesting day, but with all the features at play it’s a real can of worms. I did note that the Tucson NWS early morning forecast just defaulted to climo of 30% for all 14 forecast periods through next Tuesday. No further comment.

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