Tuesday, August 23, 2016
Another nice sunrise this morning - one of many this summer.
Southeastern Arizona totally suppressed with respect to thunderstorms yesterday. Trough in westerlies dominates next several days, with dry and stable conditions southern third of the state.
Note - nice aerial photo of a Phoenix Haboob on Jim Steenburgh's blog this morning - link to right.
Monday, August 22, 2016
Ron Holle shot the above photo of a microburst on Saturday evening (August 20th) a bit before 7:00 pm MST (time corrected). He was looking south from the Desert Museum and estimated surface winds of 40 to 50 mph, based on the blowing dust.
Fair coverage of rainfall yesterday brought thunder and gusty winds of 30+ mph here at house, but only 0.01". The ALERT network reports for 24-hours ending at 7:00 am above indicates 32 sites with rainfall - most of these were in the southern half of the network, where the areal coverage was very high. Radar/gauge rainfall from Maricopa County Flood Control for same period below fits the observations quite well. Plot of CG flashes (from Atmo and Vaisala) is at bottom, showing that the metro area was mostly devoid of storms, except for the cells across I-10 just out west of here.
Slow dry-out continues and WRF models forecast only very isolated mountain storms for southeast Arizona next several days, as Fall-like pattern continues.
Sunday, August 21, 2016
I feel beat up after last 5 marginal storm days here in southeastern Arizona. Yesterday there were more storms around, but also lots of anvil shading with storms hugging higher elevations. The 06z WRF forecasts yesterday both tended toward too much storm activity in Pima County, although the NAM version was definitely more conservative than was the GFS version. The 24-hour rainfall map below (radar estimates adjusted by gauge data) is from Maricopa County Flood Control. It shows that most storm and rainfall activity at lower elevations was out along the Colorado River. There were several severe thunderstorms in central and western Arizona.
Not much has changed today, except that MCS activity has helped spin-up a distinct MCV-type circulation that is moving northwestward over the lower Colorado River Basin. The 06 UTC WRF-NAM forecast of composite radar (2nd below) is valid at 1 pm MST today and is as active as the model forecasts for eastern Pima County.
The dismal large-scale pattern continues, with a westerly trough dominating the upper half of the troposphere over the Southwest - has been like this much of the month and begs the question of when did the deep monsoon pattern vanish? Several 300 mb charts are shown here - top 00 UTC on 9 August; just above 12 UTC this morning; bottom GFS forecast for 00 UTC 28 August. Argh - will a deep monsoonal pattern return at all this summer?
Posted by Bob Maddox at 8:04 AM
Saturday, August 20, 2016
Another nice sunrise thanks to cumulus and some buildups drifting around the morning sky.
The unusual summer 500 mb pattern continues, with an anticyclonic circulation center off in northern California and another somewhere down in central Mexico (12 UTC analysis above from NCAR RAL/RAP). A weak trough line lies across southern Arizona. Certainly not a typical monsoon pattern and one without significant features to track. The stronger northeasterly winds over northern Arizona will try to move storms toward the lower Colorado Basin; a situation favorable for severe thunderstorms out there.
Locally, our morning sounding (skewT plot from SPC) above is more moist and definitely more unstable than was yesterday's. Winds in the lower troposphere are becoming light and variable again, increasing chances for local heavy rains and perhaps wet downbursts. There is also CAPE aloft from about 800 mb up to 600 mb, so earlier thunderstorm activity over the nearby mountains should be likely today.
The 06 UTC WRF forecasts from Atmo have a split personality for this afternoon. The WRF-NAM forecasts only isolated storms over eastern Pima County and keeps stronger storms off to south and southwest. However, the WRF-GFS forecasts a very active day for much of the state (forecast of composite radar echoes below valid at 4:00 pm), with very strong outflow winds over about the southwestern third of Arizona. Appears that today will be much more interesting than was yesterday.
Posted by Bob Maddox at 8:18 AM
Friday, August 19, 2016
Thunderstorms occurred yesterday mostly over western half of Arizona. There were some thunderstorms off to the southwest of the metro area before sunrise this morning, and cool outflows prevailed during my early am walk. Plot of detected CG flashes above is for 24-hours ending at 6:00 am MST this morning (from Atmo and Vaisala).
The larger-scale pattern is very complex this morning. The 500 mb analysis above is from the 12 UTC NAM model. There are two weak troughs affecting Arizona today: one extends southwestward from northern Colorado into the Pacific west of southern Baja; while the other extends westward from the Four Corners across California and well out into the Pacific. Regardless, all of the Southwest is in the westerlies, with little left of a monsoon pattern, except for residual low-level moisture. The short wave across eastern Arizona rapidly weakens and vanishes, while the California trough persists.
The morning sounding plot below from SPC shows the westerlies now prevailing through the entire troposphere, in a sounding with minimal CAPE. The situation would be very unstable with an increase in PW from the west. Clearly a Fall-type transition situation. The 06 UTC WRF forecasts differ, with the NAM version again keeping afternoon storms out to the west, while the GFS version forecasts more widespread storms affecting all of Pima County.
Meanwhile, Tropical Storm Kay has developed south of Baja, as per the morning graphic above from NHC. Kay is forecast (below from NHC) to track southwest of southern Baja, along a path that would be good for GoC surge initiation, if the storm were larger or stronger. So, it appears to be a situation that's unclear, but worth monitoring.
I looked at the 06 UTC WRF forecasts of PW on the 5.4 km grid and found that the two models have quite different forecasts for the track of Kay. Both forecasts shown are valid at 11:00 am on August 22nd - WRF-NAM top and GFS version below.The storm is located much closer to Baja in the NAM version forecast, while the GFS version is more similar to the NHC forecast. A puzzling difference, since the NAM is essentially embedded within the GFS. My guess is that the NAM version is reflecting the TS location within the 00 UTC GFS forecast, while the GFS version reflects the new 06 UTC GFS forecast. The difference leads the NAM version to forecast a significant GoC surge, while the moisture intrusion is not nearly so strong on the GFS version.
Posted by Bob Maddox at 8:23 AM
Thursday, August 18, 2016
During the late night and early morning hours there was considerable thunderstorm activity from western Pima County northward to regions well west of Phoenix - plot of detected CG flashes above from Atmo and Vaisala for 24-hours ending at 6:00 am MST this morning. These storms were well predicted by yesterday morning's WRF forecasts - see Mike Leuthold's early afternoon discussion at:
This morning low-level moisture has increased back to around 1.25 inches, as higher PW to the west has inched eastward. The blended PW analysis above is from CIRA at Colorado State University and is for 11 UTC. Driest air continues over far northwest and southeast Arizona.
This morning's skewT plot of the TWC sounding data (above from SPC) is somewhat similar to Tuesday morning's. The upper half of the troposphere remains in a westerly flow regime, with features of interest residing in the lower half, where the wind field remains weak and chopped up. There are three elevated mixed layers (EMLs) present from just above 500 mb down to just above the surface. The lowest of these has considerable CAPE, but the two old boundary layers above are hostile and stable, meaning that considerable forcing would be needed to produce storms in eastern Pima County.
The forecast of PW below is from the 06 UTC WRF-NAM and is valid at 5:00 pm this afternoon, when the model predicts a strong east-to-west moisture gradient across Pima County. The evolution of storms this afternoon will depend very much on where storms first develop and how their outflows evolve. My first guess, based mostly on the morning sounding, is that storms may be most likely in our area during the night and early morning hours. Best strategy is to check the 12 UTC WRF forecasts and watch for Mike's discussion later today.
Posted by Bob Maddox at 6:49 AM
Wednesday, August 17, 2016
The blended PW analysis above (for 12 UTC from CIRA at Colorado State University) indicates PW has fallen to around an inch over southeast Arizona, as drier air encroached from the east to southeast during the night. The morning skewT plot below for TWC (from SPC) fits with the CIRA analysis, as PW is at 1.02 inches and sounding is quite stable. Both WRF models run at 06 UTC forecast a down day for southeast Arizona, with no thunderstorm activity.
Posted by Bob Maddox at 7:59 AM
Even though the situation was marginal yesterday, the nasty capping inversion moved away and some storms developed at low elevations late in the day. View above at 5:52 pm MST shows Cbs and developing towers to the north. My photo below at 7:17 pm shows rain shaft to the north-northwest of here.
Radar at 8:04 pm and CG flash density for 12-hours ending at 7:00 am this morning both show the storm activity focused over Catalinas and northwest metro area.
The ALERT rainfall for 24-hours ending 7:00 am this morning (above) indicates 24 sites with measurable amounts (0.04" or greater). The airport had a 0.01" and DM had a Trace. Radar indicates that heaviest rainfall may have occurred in the data void north of downtown toward Limberlost, since heavy radar echoes were nearly stationary over that area. Here at the house we had several periods after 6:45 pm with gusty winds of 30 to 40 mph, a bright lightning show, and then a brief period of heavy rain just after dark - total here was 0.32".
Finally, the observed 00 UTC sounding from TWC is shown below - compare to the forecast sounding from the midnight WRF-NAM shown in previous post, which was quite accurate.
Posted by Bob Maddox at 7:31 AM