Monday, September 24, 2018

Something To Watch Already

So much for yesterday's post that there'd not be much to watch, except the thermometer, for rest of September. Skies were overcast with middle cloud at sunrise and temperatures were very warm (78 F here at 7:00 am MST) and dewpoints had jumped into 60s. Very muggy walking compared to last couple of days.

The PW jumped last evening across much of southern Arizona. Although the 500 mb trough (above) is over southern California this morning, below 600 mb it has led to a distinct trough over GoC, which has kicked higher PWs back into our area.

The skew-T plot for 12 UTC TWC upper-air data (below- both from SPC) has PW of 1.32" - which is up half an inch from late yesterday afternoon. Although lower levels have potential CAPE, a very nasty, capping inversion persists at 550 mb (500 temperature is a very warm -3 C). Key question of the day is whether or not some middle-level cooling will swing around the bottom of the trough (coupled with how deep the surface-based BL will grow)?

The available WRF forecasts go as per: the 06 UTC NAM version forecasts some isolated, light sprinkle showers around; the GFS version indicates showers and perhaps a couple of thundersotrms for central Pima County and isolated sprinkle showers for our area; and 12 UTC WRF-RR forecasts isolated showers and storms for eastern Pima County. The forecast soundings appear marginal for thunderstorms because the inversion aloft remains pronounced, although models forecast it to lift up to 500 mb, or bit above. Model forecasts tend to smooth vertical structures, and it appears that the capping layer could remain quite significant through the day.

However, something to watch already.

Strange images this morning:

Above is 7:00 am visible satellite image. While it appears to be stratus or fog, the area from southeast corner of state north-northwestward to Grand Canyon is uniform middle-cloud overcast, from surface observations. I thought that the clear gap in northeast was over the Little Colorado River, but Winslow obs indicate overcast conditions.

Below is view north from Kitt Peak at 8:00 am - this looks to be moderate buildups above a middle cloud layer - with dirty boundary layer and cloud shadows beneath.

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