Friday, September 07, 2007

Henriette and Today's Weather

Rain from Henriette

Here at the house we received only 0.03" of rain on the afternoon of the 5th (Wednesday) but very heavy storms occurred in the vicinity (see Photo). We experienced considerable lightning and thunder and gusty outflow winds but little rain. Then, during the early morning hours of Thursday from about 5 am to 9 am there were several moderate showers with no lightning or thunder here. These produced an additional 0.41" of rainfall. Henriette has left behind a very wet low-level atmosphere and the forecast for today is of interest.

Today's weather

There is a large pool of very moist air lingering over much of Arizona this morning, along with considerable low and middle level cloudiness.

Winds in the lower half of the troposphere remain L/V while upper-level winds and shear are quite strong (this due to strong height gradients between the anticyclone to the southeast and a trough off the Pacific coast). The upper-levels appear to be divergent over southeastern Arizona.

The morning TWC sounding indicates substantial CAPE is present below 350 mb. The questions for the day are: how believable is the TWC sounding, and how will drying during the day affect the CAPE? In the morning soundings in the layer between 400 and 300 mb there is a pronounced inversion between the moist lower atmosphere and the dry upper-levels. At TWC the inversion is about 7C warmer than at any other upper-air site in the Southwest. To me, the TWC sounding appears suspect between 520 to 280 mb. My guess is that the elevated inversion is not as strong as indicated here, and that it will not strengthen due to the dry flow aloft, so that there is at least moderate CAPE present.

I expect that strong storms are likely this afternoon over the southeastern third of Arizona, especially where the clouds break and there is decent solar heating. The storms should have the potential to produce strong, moist downbursts and some hail, especially at higher elevations.

All this assumes that I have been able to properly unravel the uncertainties in the upper-air data.

Wednesday, September 05, 2007

Forecast Remains Difficult

See yesterday's message about tough forecasts!

Today we have moisture and hopefully CAPE will remain through the day -we are getting some mid-level warming now. Upper-level winds are difluent between the hurricane and the system to NW. Winds are very light in lowest half of troposphere - again. Low-level winds become southwesterly during the day and there is more IPW to the west. So, there should be storms around this afternoon with the potential for heavy rains.

Tomorrow is much tougher and it's possible that it will be an all or nothing situation here in the Tucson area. Who remembers Nora?

NAM forecasts a decent rainfall event, but keeps the really heavy rain with the core of what was Henriette. It is of note that there were soundings this morning at La Paz and Guaymas for the first time in ages, so the model actually had some observations in its initialization.

Negatives tomorrow are that NAM forecasts winds to become strong downslope below 500 mb into the local area. This is coupled with what appears to be fairly strong confluence aloft. The GFS members forecastsignificant rain event also but, of course, local terrain really isn't in that model.

If the remains of Henriette move northward to the west of the NHC and NAM tracks, get out the sandbags!

So, I conclude that, with so many uncertainties in the next 24 hours, the only thing to do is to watch the observations closely and see what actually transpires during the afternoon and night.

Tuesday, September 04, 2007

Henriette Leads to Rapidly Changing Conditions over Southern Arizona

A Very Difficult Forecast Situation

The last two days have been extremely suppressed and very dry in southern Arizona, with dewpoints yesterday falling into the upper 30s. This was the third dry-down of the summer that came in on easterly to southeasterly winds.

This morning a number of things have changed. There has been a northward surge of low-level moisture into southwest and south-central Arizona during the late night and early morning hours. This was likely triggered by strong winds to the northeast of Hurricane Henriette. At 7 am the moisture had increased dramatically at Yuma, Sells, and Sasabe. The VAD Doppler radar data at Yuma indicate that this surge is about 3,000 ft deep. It may provide enough destabilization for at least mountain storms over the southern third of the state this afternoon.

The track of Henriette will then be the key determining factor for the weather over southeastern Arizona. The models now are in fair agreement that Henriette should move into the southern GoC. Tomorrow there will be a strong isallobaric wind blowing down the GoC and that may slosh today's low-level moisture back toward the south. Then, the main issue becomes how far north the deep moisture and precipitation fields associated with the weakening storm will come.

It appears that the current trough in the northwest will move rapidly over the top of the middle-level anticyclone leaving some remnants of Henriette blocked and decaying over northern Sonora and the GoC. This would allow the moisture associated with the storm to move directly into at least southeastern Arizona and give the potential for a significant rain event.

Monday, September 03, 2007

Yuma Storm on Saturday

I have checked the Yuma newspaper this morning and there are two stories online that describe the intensity of the severe thunderstorm that struck Yuma Saturday evening. There are numerous damage photos available - and perhaps even a video (I couldn't get it to play though).

Sunday, September 02, 2007

A Big Storm Day in Southern Arizona and California

Yesterday afternoon , Saturday September 1st, turned out to be a big storm and severe storm day across southern Arizona and into southern California. There were several small tornadoes in California, and events in Arizona were mostly from hybrid downbursts, although there was one hail report. Most notable was a measured gust to 84 mph out at Yuma, Arizona. I have not yet chased down many photos, but will try to post some damage shots tomorrow.

Strong, drying easterly winds today have led to mostly suppressed conditions across southern Arizona, with significant storm activity focused in southern California.

Data from TWC yesterday provide a good illustration of how difficult it is to use RRS soundings with suspect data in making a forecast.

In my previous post I noted that the 12 UTC (1 Sept. 2007) sounding at TWC was suspect and that it was difficult to estimate what the CAPE might actually be in the afternoon. I guessed that a well-mixed afternoon BL might have a mixing ratio of ~9 g/kg. But I also mentioned that 1 g/kg either way would have substantial implications for the evolution of afternoon storms. The TWC sounding at 00 UTC (2 Sept. 2007) apparently entered a storm updraft - see Figure 1. The data are mostly bad or suspect, except for the temperature trace from the surface to about 650 mb. However, the updraft data from 500 to a bit above 400 mb have theta w values of 24 to 25C. This implies that the BL air at cloud base had a mixing ratio of 10.5 to 11.5 g/kg.

My morning estimate, using the sounding with the suspect Td trace, was clearly way too low. It is fairly obvious that moisture advection, given the winds below 650 mb for the morning and evening soundings, was not a major player during the day. Figure 2 shows the morning sounding with lines added showing q = 10 g/kg and theta w = 23.5C. The Sippican hygrister appears not to have responded well at all in the residual layer and indicated conditions that were too dry.

It is likely that the sounding problems yesterday contributed to forecasts that significantly under-estimated the extent of the severe storm outbreak. And so it goes in the new era of NWS RRS soundings - a serious crap shoot for forecasters that sometimes has public safety implications.

Saturday, September 01, 2007

Weather Discussion 1 Sept. 2007

There have been many interesting changes from yesterday morning to this morning.

Atmosphere has moistened up some wrt IPW, but the soundings are very hard to interpret.

Last evening's TWC sounding (see Fig. 1) had a dry spike off the surface and then seemed to indicate a BL with a bit less than 8 g/kg and IPW of 25.8 mm (this was extremely dry wrt the GPS value). The evening sounding appeared to have no CAPE - but storms developed to the south of the airport around 0200Z and TUS carried a brief tstm (nice lightning visible from here with this cell).

This morning's sounding (see Fig. 2) has IPW of 37.5 mm, quite a large increase of almost 12 mm in 12 hours according to the soundings, but GPS data (see Fig. 3 from FSL) show a 12 hour increase of only about 2.5 mm. So, once again, the first forecasting dilemma for today is that of trying to figure out what the local thermodynamic conditions actually are.

The morning sounding has NO residual BL in moisture but a very deep residual BL in temperature. I'm going to guess that it's there (a residual BL in moisture) and that the mixing ratio we're heading toward is about 9 g/km. This would give a decent amount of CAPE this afternoon. But if I'm off by either +/- ~1 g/kg, the difference goes from a marginal day with mostly mountain storms to a very good day at low elevations with moderate CAPE available for storms. The deep BL and high cloud bases leads to either the potential for dry microbursts, or to severe hybrid downbursts at lower elevations - all depending upon what the afternoon BL actually looks like.

Am I frustrated trying to use bad sounding data to make a forecast? VERY!

Other interesting aspects of today's forecast:

Surface pressures are down 2 to 3 mb in south-central AZ compared to 24 hours ago.

An upper-level S/W trough is swinging southward across New Mexico, giving much of AZ flow aloft that is quite difluent.

Easterly low-level winds are still present but have diminished quite a bit. It will be important for the evolution of storms this afternoon if WNW diurnal winds can develop - the lower deserts to west and northwest are more moist than the local area.

Steering level winds are around 10 to 15 kt from the north, but anvil level winds are also northerly at about 20 kts. Thus, unless storms can propagate rapidly southward, anvil cloud will spread out ahead of the cells - not favorable for storms at lower elevations.

Thus, it's a very difficult call for lower elevations. Mountain and higher elevation areas should be very active. Will significant storms occur at lower elevations of southeast AZ? It's probably a coin toss, and we'll just have to see what kind of event unfolds this afternoon and evening.

As for TS/possible Hurricane Henriette, the NAM and the GFS have much different scenarios predicted for the storm. The NAM takes the storm fairly quickly into the area around the tip of Baja. This would not trigger a surge of low-level moisture into Arizona until the storm remnants actually moved to the north end of GoC or northern Sonora. The GFS takes the storm northward considerably further to the west, along a track that would trigger a significant surge. The GFS then brings the storm remnants right into southern AZ for a big rain event. However, it appears that only the operational GFS has this solution, and all the other GFS ensemble member forecasts from 0600 UTC hardly indicated that Henrietta exists. So, there are more coin tosses in the long-term outlook also.

August Weather Summary

A quick summary of what August brought here at the house.

Rainfall - 2.21" (less than half of July's 4.94")

Measurable precip - 8 of 31 days

Trace or more - 13 of 31 days

Thunderstorms in general area - 19 of 31 days

Severe thunderstorms in Tucson area occurred on: 10th, 14th, and 24th

Heavy tstm rains on the 4th produced severe flash flooding and several fatalities in Sabino Canyon.

All in all an active month, but much less so than was July.

The WRF run yesterday indicated that we might see a considerable increase in storm activity today. So, I'm about to see what the morning observations and products are saying. I will note that at sunrise today there were patches ofmiddle cloud floating around in all quadrants with some ACC present toward the south. This is much different than yesterday's sunrise, when there were only some wispy cirrus and lenticular clouds.