Thursday, September 20, 2018


Previous Forecast
This blog entry serves as a post-mortem for yesterday’s heavy rain event.  Widespread moderate precipitation developed over much of SE Arizona with some areas of extreme rainfall.   One area southern Maricopa and western Pinal were RADAR estimates were near 4”. Other areas were south of Tucson with some gauge reading of 4 inches in Green Valley.  RADAR estimated precipitation with the product below was not very good for that area due to beam blockage as it had only 1.5 inches.

The MRMS Q3 Gauge Corrected RADAR product was a bit better as it had 2.5-3 inches in the area but was still not enough.  I wonder how much much precipitation really fell to the SW of Green Valley? Estimates were not much better out in eastern Arizona as Rainlog sites in Safford reported 2-3 inches while the various RADAR derived precipitation products were around 1/2 that.

The WRF forecasts from the 18 at 12Z varied in their accuracy quite a bit with the WRF-RR being fairly good as it had areas of very heavy precipitation over southern Arizona.  It was a bit too far south with south central Arizona but had the right idea of developing storms there during the early afternoon. The WRF-GFS was fairly good too but didn’t have enough accumulation.  The WRF-NAM didn’t have nearly enough precipitation.
The model runs from the 19th were fairly consistent with their forecast of a heavy rain band across SE Arizona.  Some of the details were wrong as the WRF-RR had too much precipitation and it was too far north as well as having the south-central Arizona activity too far south.  
The WRF-NAM accumulations were better and the band was mostly in the correct location.  It didn’t develop the early afternoon south-central Arizona activity quite far enough but I can’t complain as the weather situation was complex.
The WRF-GFS was also pretty good but might of had a bit too much accumulation around the Tucson area.  It did well in eastern Arizona with locations and accumulations. Overall, I was pleased with the model performance especially the Day 1 forecasts.
As a side note, the 12Z WRF-GFS ran with both the ACM2 (winter) and BouLac (monsoon season) PBL schemes and I thought it would be interesting to see the differences.  The ACM2 WRF-GFS produced too much rain and was a bit too far to the east with the heaviest precipitation.

Wednesday, September 19, 2018


Previous Forecast
Strong storms formed in the Nogales area much as was forecast by the various 12z runs.  Mostly light precipitation continued into the evening over parts of far SE Arizona.

The upper-air pattern is much like yesterday with a west coast trough and an anticyclone over the central part of the CONUS.   Of great interest is the weak upper trough over northern Baja which is taking its sweet time ejecting to the north. WV imagery indicates there is no longer a closed circulation and looks like an open wave as depicted by the 300mb plot below. It should eject over southern Arizona today.   The various 12Z initializations appear to have the major features initialized well.

A large area of deep convection extends from the lower Gulf of California all the way to Sonora and was initialized fairly well by the RR and NAM.  Much of the southern GofC activity is associated with newly minted TD 19-E which is located near Loreto BCS. The RR had the TD initialized quite well and the NAM wasn’t too bad either.

There are widespread clouds over much of the state with the overcast being quite thick in many places.  Fortunately, both the RR and NAM have initialized clouds pretty well. The RR PW initialization was very good.  Even the NAM was pretty good except it was about 5mm too wet in NW Mexico. Overall, for such a complex situation, both initializations look good and model confidence is high.

Day 1
Arizona is back in the soup again as PW over the low deserts ranges from 42-47mm and is even higher to the south at Sonora sites report over 50mm.  Moist advection continues to increase PW during the morning hours as a Gulf Surge continues.

The wettest air is over SE Arizona during the day thus this is the area of greatest potential for very heavy rain and flash flooding.

The late morning forecast vertical profile for Tucson is nearly saturated all the way to the tropopause and contains around 1000 J/kg of CAPE. This profile supports extremely heavy rain.  Not much lift is needed to develop deep convection and as of this writing, storms are already developing over southern Arizona. As Bob Maddox pointed out, the wind profile is also good as there is decent directional sheer.  Mid-level steering is also good as it is southeasterly at 15 knots. Mesoscale lift will be provided by the ejecting shortwave and longwave troughs.

So far this morning, the WRF-RR has the best forecast of the showers and storms around the Tucson area.  This area continues to expand and intensify as the morning goes on.

The question isn’t “if”, but “when” and “where” as all the runs over the past 18 hours have very heavy precipitation for parts of SE Arizona.  Both the WRF-GFS (which is now available) and WRF-NAM (below) hold off the heaviest precipitation until the afternoon. As there is already quite a bit of activity, they are probably too slow.

The morning activity didn’t dampen the WRF-RR’s afternoon storms at all.  Many strong echos are present over SE Arizona during the afternoon.

Extremely wet air continues to feed into SE Arizona on 20 knots of southerly flow which enhances the heavy rain and flash flooding risk considerably and contributes to the redevelopment of storms.

Precipitation rates generally decrease into the evening over SE Arizona but areas of heavy precipitation continue.  

This is likely to be a major flash flooding event for many areas of SE Arizona as depicted by the rainfall totally from the various 12Z runs.  There are minor difference between 12Z runs but in general, they are quite consistent with a band of 2-4” precipitation from eastern Pima up to the NE.  Some isolated amounts of 4-6” are possible.

The tropical depression moves NE into Sonora this evening and towards far SE Arizona and SW NM overnight.  

This results in another round of precipitation over far SE Arizona during the early morning hours.  All model runs are nearly identical so confidence is high for the track. Rainfall amounts look to be around .5 to 1.5” from the TD.

I’ll post results from the 15Z WRF-RR on Twitter later.

Tuesday, September 18, 2018


The 500mb high is located over far NW Texas while a weak trough is along the Pacific coast.   A well defined mid/upper inverted trough is located over central Baja and multiple areas of disturbed weather are present in the eastern Pacific so it’s quite a complex situation.    Both the NAM and RR have the Baja low initialized a few hundred km too far to the south and this error results in initialized clouds and showers were too far to the south. Both initializations have a low-level cyclonic circulation the SW of the southern tip of Baja which does verify with morning low-level cloud motions from satellite.  

As seen above, very wet air is moving northward with the yellow areas having greater than 2.0” of PW.  NAM PW initialization wasn’t too bad as most errors were +/- 3mm and the RR was good with mainly minimal errors.  As mentioned above, clouds were initialized too far south plus the coverage was too low too. Model confidence for Day 1 is OK but with the big error in the position of the upper low, Day 2 for Arizona is low.

Day 1
PW has increased over southern Arizona the past 12 hours and is now around 30-34mm.  A shallow surge is underway at Yuma with dew-points around 75F. Moist advection continues during the day which continues to increase PW over much of southern Arizona.

In spite of this moisture increase, CAPE remains too low to support any deep convection except for near the border.  CAPE is very high over far SW Arizona but the atmosphere is strongly capped so deep convection is unlikely there.

Both the Tucson and Phoenix afternoon vertical profiles look similar as there is limited CAPE, a warm layer at 400mb, and a shallow mixed layer with a weak inversion on top.

Both runs develop activity over far SC Arizona and into northern Sonora later this afternoon.  The WRFRR has stronger and more widespread storms and I can’t say which is more accurate.

The WRFRR continues showers and a few storms into the evening hours.

Day 2
By 12Z, the broad west coast trough deepens while the upper low over Baja begins to eject towards Arizona. The big unknown is where exactly it will go as the initialized position may be off.   A strong jet is located on the east side of the trough with Arizona being near the right rear entrance region. The 250mb plot shows considerable divergence over the state from both the large-scale trough and the shortwave ejecting out of NW Mexico.

Combined with the increase in moisture, light showers develop over much of the western ½ of the state during the early morning hours.

Very wet tropical air is over the state by morning as the surge continues.  There is a lot of uncertainty as the WRFNAM has more moisture but less CAPE tomorrow morning.

This results in widespread showers with some thunderstorms during the late morning hours.

In spite of less CAPE, the WRFNAM also develops scattered showers and storms during the late morning over southern Arizona.  One big problem will be that the morning clouds and showers are likely to reduce heating and result in only limited afternoon activity for some locations.

The WRFRR continues to have around 1000 to 1500 J/kg of CAPE over much of the state during the afternoon hours which results in widespread strong storms.

The WRFNAM forecast looks very similar with regards to coverage but with lower CAPE, strong storms are not as widespread and no strong storms over central Arizona where widespread clouds and showers are present much of the day.

The WRFNAM decreases activity into the evening hours but the WRFRR redevelops strong storms over southern Arizona.

This results in some locations receiving extremely heavy rain with amounts over 3 inches.  WRFRR 30 hour total precipitation.

WRFNAM 30 hour total precipitation

The WRFGFS is now far enough along that I can talk about it.  In general, it’s more like the WRFNAM as it only has mainly light precipitation in many areas of central Arizona but does develop stronger storms during the late afternoon for areas of eastern Arizona.  

Monday, September 3, 2018


Previous Forecast
It certainly has been a busy couple of days.  Yesterday’s activity was centered over SC Arizona along with a few severe storms east of Phoenix.  

All the runs had the right idea but were off with the details.  The 12 and 15Z WRFRR looked the best as they developed widespread activity over the Tucson area during the afternoon and then redeveloped storms during the late evening.  They were a little slow with the second batch though.

It’s a complicated pattern as a broad trough continues over the SW with a shortwave over eastern Arizona up towards the 4 corners and another weaker over southern NV.

259mb still has a jet streak over California with the exit region over Arizona.   

Most of the state is clear except for the far eastern parts where some cloud and showers continue but these are moving eastward into NM will be no factor.  The models initialized clear skies, correctly, across the state. PW was initialized well and overall, both the NAM and RR look good. Note that most of the day 2 forecast was using only a single 12Z run as I have somewhere I need to be this morning and couldn't wait so day 2 confidence is low.

Day 1
After 2 days of widespread activity, the atmosphere is quite worked over and both Tucson and Phoenix have almost no CAPE.   PW has slowly decrease too and is only in the 25-30mm range over the lower deserts. All of this is much different from the forecast yesterday where moisture continued and CAPE was high.

CAPE is low to moderate over much of the state and looks to be insufficient for much activity over the lower deserts.

It doesn't get much better during the day and by late afternoon Tucson only has around 300 J/kg.  Note that the surface based CAPE is too high which happens sometimes. The wind profile has a bit of directional shear but not as much as the past few days.  Just looking at the model Skewt-T, I’d say this will be a down day for the deserts.

Phoenix looks no better.

It looks like only the higher elevations will see any activity today.  

Day 2
Moisture is forecast to increase over southern Arizona tomorrow so it should be a more active day.

Along with the returning moisture, CAPE increase over far southern Arizona so it looks like that area will have the most activity.

The SW trough is still hanging around and is cut off from the main branch but is not quite a cutoff low with a closed circulation but it sure is acting like it.  The center is located around Vegas and has much of Arizona in westerly flow along with fairly coo air at the mid-levels.

The Tucson forecast Skew-T looks much improved as CAPE is around 1000 to 1200 J/kg along with a deeply mixed PBL.  The wind profile does have some directional shear favorable for some storm organization. So, Tucson could see storm activity tomorrow.

Scattered storms form over southern Arizona during the early afternoon.

It is possible that in or around Phoenix could see some activity later tomorrow afternoon or evening as CAPE is around 1200 J/kg and the PBL is mixed deeply.  However, there is an inversion on top of the PBL that is going to need some lift to break which looks unlikely as no storm activity is forecast for the higher elevations around Phoenix.