Wednesday, September 25, 2019


Previous Forecast
Overall, WRF forecasts were mostly accurate as supercell thunderstorms developed in western and central Arizona. The MRMS website is not available this morning, so I can't say exactly how the precipitation forecasts verified.  I can say that the WRFRR was too wet for the Tucson area, as it had isolated 4-6 inch amounts over far eastern Pima County.  The maximum I could find was 3.5" in Vail.  Generally, the valley received less than an inch.  The WRFGFS was the most accurate for Pima County.

The cut off low is located over the far northern Gulf of California. It is responsible for ongoing showers/storms and thick clouds over southern Arizona and into northern Sonora.  Getting these features initialized correctly is going to be important as heating will make a big impact on forecast accuracy.  Unfortunately, the 12Z NAM was very poor as it had no activity and clear skies over southern Arizona.  The 12Z RR wasn't much better. 6Z runs had some activity this morning, but it was too far to the west.  The 15Z WRFRR was much better and had deep convection and clouds initialized farther east and more or less in the right locations.  The PW initialization was also good, so it's the favored run, by far, for today.

Day 1
PW as increased since yesterday and it back into the 30-35mm range as moisture from Mario was advected into the state overnight.  PW is forecast to remain about the same into the early afternoon as the southeasterly flow continues over much of the southern 1/2 of the state.

CAPE was only a few hundred J/kg at Yuma at 12Z.  It doesn't get much better by this afternoon except for areas near the Mexican border.  This is quite a bit different from earlier forecasts which had moderate CAPE as far north as Coconino County.

Afternoon storms are restricted to mainly southwestern Arizona.

The cut-off low moves a little northwest and by early evening. Note the front side speed max which may mean it will start to move out to the north over the next day.

This speed max can also be seen at 250mb.  This jet is in a favorable location to help organize storms for central Arizona, as long as there is any CAPE available.

CAPE does increase over central Arizona somewhat this evening.

Storms form by early evening over Pima county and move rapidly north.

Phoenix does appear to have sufficient CAPE to support evening activity as well as a deeply mixed PBL.  The surface to 400mb vertical wind profile does exhibit some directional and speed shear, so storms could be organized.

Scattered storms, some strong, move through the Phoenix area this evening.

The 15Z WRFRRx is now available it has more clouds around during the day, including the Phoenix area.  It doesn't develop storms until a bit later in the evening, and not as widespread.

Storms rotate around the upper low and move into western Arizona by late in the evening.

CAPE is sufficient to continue the risk of thunderstorms into the early morning hours from western Pima and all the way up to the western Rim.

Day 2
The low moves slowly north and is south of Vegas by 18Z tomorrow.

CAPE continues to be sufficient for scattered thunderstorms over much of the western 1/3 of the state.

Monday, September 23, 2019

20190923 Day 2

Day 2
The storms just keep coming.  As mentioned in Day 1, the focus shifts to southern Arizona overnight and into the morning hours, as the low continues to move south overnight.
CAPE becomes very high over southern Arizona and will support strong to maybe even severe storms.  CAPE is also more than sufficient to support continuing storms for Maricopa County too.

Storms are again moving into the Phoenix area during the early morning hours and beginning to develop over southeastern Arizona.

A surface low begins to form over the far northern Gulf of California which increases low-level moisture advection over far southern Arizona. 

Widespread storms continue to redevelop over many areas of southeastern Arizona around sunrise.

Moist and unstable air continues to feed into southern Arizona during the morning hours fueling thunderstorm redevelopment.  Note the strength of the surface low; I've never seen one this strong before.

Thunderstorms continue throughout the morning.  All the model runs are similar to this outcome, so I'm fairly confident it will happen.

The differences are in the amount of precipitation.  The WRFRR series have the most and if these verify, this is going to be a major flood event for parts of southeastern Arizona, including Tucson.  Here are the four runs from this morning:

Storms generally dissipate during the afternoon hours.  Some activity might be possible under the cold-core cutoff over southwestern Arizona as depicted by the WRFNAM.


Previous Forecast
A flash flood event is occurring in eastern Maricopa County.  Some locations have had over 4 inches of rain.
The 15Z Q3 product is about the same as the observed data.
The 15Z WRFRR and dev runs were quite active and predicted a lot of rain, but were just south, in Pinal County.

As discussed yesterday, the morning activity was probably enhanced by the jet streak dynamics.  Flagstaff had 90 knots at 300mb.  Pretty good for early fall.  On the backside of the trough, winds are even stronger as Reno reported 115 knots.   This backside jet will continue to dig the trough to the south.
Winds at 500mb are also strong with 60 knots reported.   There is also a pronounced temperature gradient with -4 at Tucson and -16C at Reno which results in some cold air advection over southern Nevada and into western Arizona.  Initializations of the upper-air features are accurate.

It was a complicated situation at 12Z, as there were ongoing showers and storms, but the RR and NAM did a pretty good job.  The 6Z WRFGFS didn't have enough activity forecast for 12-15Z so won't be of much use.  NAM and RR PW initialization were too moist in northern Sonora and southern Arizona which is a little worrisome as the errors are around 5-8mm. 

Day 1
Supercell storms are already underway over western Arizona as of 16:30 and SPC has western Arizona in a "Enhanced" risk.  I don't know if that's ever happened before!  By the looks of the Yuma sounding, I can see why.

Strong low-level southerly flow continues into the early afternoon, advecting moist are into much of Arizona.

Extremely high CAPE is forecast for western Arizona for the late morning hours and into the early afternoon.  CAPE also increases over central Arizona as moisture increases.

Both the WRFRR and WRFNAM expand the coverage of the storms into the afternoon hours.  Storms generally remain north of I-10.  With a strong southerly flow feeding new deep convection, some rainfall amount could be extreme.

Note that the below graphic includes the morning activity over eastern Maricopa.

By late afternoon, the upper low is located over Vegas and puts northern and central Arizona in an area of strong upper divergence.  This will result in continued organized strong to severe storms over that area.

There is a lot of noise in the deterministic deep convection forecasts, so it's a bit hard to figure out when and where.  However, some of the runs consolidate convection into a broken line with embedded supercells.  This line moves eastward and impacts the Phoenix area by late afternoon.  Forecast reflectivities are extremely high, so some storms could produce large and damaging hail.

The late afternoon forecast Skew-T for Phoenix indicates very high CAPE, over 2000 J/kg.  Directional shear is minimal, but speed shear is good.  Probably no tornadoes, but as mentioned above, large hail is likely with some storms.  I do have to admit that I'm somewhat out of my element here as Arizona sees so few of these sorts of events.  I can't remember the date, but 10-15 years back, Phoenix had an extreme hail event and this kind of reminds me of that.
I'm seeing some severe winds, most likely from wet microbursts, for the Phoenix area too. 

This evening, the low is just to the SW of Vegas and seems to be digging even more to the west than was previously forecast.   It is also now nearly cut off from the main branch of the westerlies.  With this more westerly track, southern Arizona is in for a lot more activity over the next few days than was previously forecast.  I'll cover that in a second discussion.

The threat for strong to severe storms continues for much of southern Arizona into the evening hours as CAPE continues to be moderate to high.

It looks like Tucson will start getting some activity as CAPE is very high and mid-level temperatures have cooled.  Like Phoenix, directional shear is poor, but speed shear is good, resulting in some organization to the storms.

Of special concern is west of Phoenix where most runs are redeveloping storms.  A major flash flood event is possible.  Storms continue to display supercell characteristics into the late evening hours, so the threat for large hail, isolated severe winds, and very heavy rain continue.

Sunday, September 22, 2019


Remnants of Lorena are located just north of Isla Tiberon or west of Hermosillo.   Some deep convection is present in the northeastern quadrant.  Mario is still a tropical storm but devoid much deep convection and is located just southwest of Cabo San Lucas.  All morning initializations have these features initialized well.  The NAM has issues with southern NM initializing too wet, and the GFS has a web bias at a few sites in eastern Arizona and northwestern Sonora.  The 12 and 15Z WRFRR appear to have the best initializations.

Day 1-2
The big day is almost upon us.  This is one of the more unusual situations that I've seen for transition season as I can't recall ever having a cutoff low that drops down and stalls over Arizona when there is this much moisture available.  The 300mb map has a strong 125+ jet off the NW CONUS that will result in a trough digging down towards Arizona by tomorrow. Even then, there is still a >60 knot jet on the backside of the 500mb low which will result in further digging.

What about moisture?  By late afternoon, moisture moves into the far southern parts of the state.  There is plenty more in Mexico!  Mario moves slowly to the northwest keeping southerly winds going over the southern Gulf of California, and moisture moving northward.  CAPE is limited, so little or no activity is expected this afternoon and into the evening.

CAPE increases significantly during the early morning hours over south-central Arizona in some of the forecasts.  This results in a few storms developing around sunrise.  The 15Z WRFRR has some quite strong storms.

This region is in the favorable right entrance region of a 250mb jet streak, which produces upper divergence/PVA resulting in rising motion, more or less in the area outlined.

Morning storms could be very strong and exhibit organization and supercell characteristics as directional and speed shear are excellent.

CAPE increases over western Arizona to extreme values of over 3000 J/kg. 
With the approaching height falls, cold air advection, and PVA, western Arizona is primed for a major severe weather event.  Strong to severe storms develop during the late morning to early afternoon hours.

Wow!  Impressive CAPE and veering winds with height.  There is pronounced inversion at the top of the mixed layer, but cold air advection should erode that away for areas north of Yuma.  Large hail and even some tornadoes are possible with this kind of wind profile.

At this point, the various model forecasts go off all over the place.    Moisture continues to surge northward into the early afternoon.

Generally, strong to severe storms are possible anywhere over the state tomorrow afternoon except for southern Arizona.  Here's a sample of the various model forecasts:

By late afternoon, the low still exhibits strong digging characteristics with > 50-knot winds on the backside.

Southern Arizona stays capped until evening.  CAPE increased to around 1300 J/kg and the cap weakens as the trough approaches, so some storms are possible during the evening hours.

CAPE continues to be moderate to high during the evening hours, so the potential for strong to severe storms continues.  It's not possible accurately say where or when.

Day 3
There is still some disagreement between the WRF and GFS/ECMWF in regards to the location of the cutoff low.  The WRF has it over far south-central Arizona, while the ECMWF and GFS are over southwestern Arizona.  With the low farther to the west, eastern Arizona is likely to be more active than the WRF is saying for day 3.