How complex can things get?

I have been tied up with other work this morning and have just now gotten around to looking at this morning's weather charts and model progs out through 48 hours.

This is one of the most complex and difficult situations that I can remember seeing here in the Southwest.

I have taken a very careful look at the NAM and GFS models out through 48-h; I have little confidence in any of the models or their forecasts beyond 48-h. There are strong and competing features in battle with each other during the next 48-h, as Hurricane John moves north at the same time as a very large upper-air blocking pattern sets up over the north-central U.S. While these things happen, a strong Canadian cold front moves rapidly southward down the front range of the Rockies, bringing much rain and moisture in its wake. This front back-doors into the Southwest and brings low-level winds around to the east over the eastern half of Arizona. Finally, low-level moisture surges very rapidly northward after after 24 to 30 hours. The only NWS offices discussing in their FDs the role of the cold front in determining the Holiday weekend weather are ELP and ABQ, but it seems fairly clear that the front is going to affect the evolution of events over a large part of Arizona also.

Brief summary of the models progs at 12, 30, and 48 hours follows:

12 - hours: Both models indicate light northerly winds at 850 and 700 mb. At 500 mb both models have light westerly flow over southern Arizona, with a weak anticylone south of the border. Upper-level winds are westerly, are strong and appear confluent over southern Arizona.

30 - hours: Winds at 850 mb and 700 mb continue northerly over much of southern Arizona. The cold front has pushed into southern New Mexico and is further west in NAM with stronger easterly winds. The NAM has northerly winds south to about Hermosilla; whereas, the GFS indicates the expected Gulf Surge has moved into the lower Colorado River Basin. Both models indicate moisture advection at 700 mb into southeastern Arizona from the east. At 700 to 500 mb there is a pronounced inverted trough swinging around Hurrican John and approaching the New Mexico bootheel. Increasing heights at 700 mb over west Texas act to increase the gradient and southeasterly winds to the northeast of John over much of northwestern Mexico. Upper-level winds continue strong from west but may be slightly difluent over southern Arizona. The NAM is moving John up the east side of Baja and the GFS is moving John up along the west coast of Baja.

48 - hours: At this time the two models have diverged markedly in their forecasts. The NAM has begun moving John westward, probably because of the large block to north and northeast. Moisture has surged rapidly northward as far as southern Nevada. The backdoor front has produced strong easterly winds over southeastern Arizona, but signficant moist advection continues at 700 mb from the cool side of the front. There is a weak 500 mb trough and vort max over southeastern Arizona, but 500 mb winds are strong northerly over most of the state. Upper-level flow appears divergent.

In contrast, the GFS continues moving John northward; the surge has moved far north and into the southern Great Basin. Easterly winds have increased over southeastern Arizona in lower-levels. GFS also has weak trough and vort max at 500 mb over southeastern Arizona. Upper-level flow is weaker and difluent over southeast Arizona.

So what does all this mean? It appears that through 48 hours the most signficant storms will occur Saturday afternoon and night.

Southern New Mexico appears to have most certain chance of widespread significant storms and rains. Southeastern Arizona may have a good storm event, but timing of moisture increases from John and encroachment of front from the east will be critical. If the front comes through too soon, the best activity zone will be shifted to the west and northwest. There is a good liklihood that the influx of low-level moisture will push far enough north that northwestern Arizona and southern Nevada (and nearby California) will experience signficant storms also.

As I said at the start of this ramble, things are really complex and getting the forecast right for tomorrow afternoon and night is a huge challenge! In the longer-term, the GFS continues John northward into the lower Colorado basin, carving out a weak trough to the west of the block. The magnitude of the rain event that the GFS forecasts after 48 hours is HUGE in the lower Colorado River Basin. The NAM takes John to the west and just forecasts a nice pattern for strong storms over the Southwest for several days.