Summary
A low pressure system in northeast Arizona is moving north, bringing potential storms to northern Arizona and western New Mexico, while the Tropical Upper Tropospheric Trough (TUTT) near Baja may interact with outlfow from Hurricane Fausto. Tropical Storm Bertha is encountering upper-level winds, but could still bring heavy rain to southeast Texas.
Monsoon moisture is affecting much of the Southwest, with flash flood watches in western New Mexico extending north into Utah and Colorado. Forecast soundings indicate isolated afternoon and evening storms near Arizona’s mountains, with subsidence in the wake of the low into northeast Arizona limiting storm development elsewhere.
Tomorrow is expected to be similar, though slightly more active as subsidence weakens. Looking ahead, an increase in moisture and thunderstorm activity is possible over the weekend, but forecasts remain uncertain due to changing weather patterns and the potential influence of Hurricane Fausto.
Yesterday’s Weather
The thunderstorms were confined over and near the mountains for both Phoenix and Tucson. Storm motion was guided by the upper low, with storms moving north in western New Mexico, west in northern Arizona, and then southwest/south along I-17. In southern Arizona, storms either formed and dissipated in place or, due to weak outflows, spread from mountain to mountain. Fewer storms occurred in eastern New Mexico and western Arizona, as shown by the lack of lightning strikes.
In the Tucson area, storms formed on the Catalina and Rincon Mountains, then moved around the cities’ edge from the southeast, then south, then west, as shown on the radar animation below (forecast left, observation right).
Locally heavy rain occurred mainly on the mountain tops, where 24-hour totals are >1″. Vail and along I-10 east of Vail also received heavy rain >0.5″.
The low pressure in eastern Arizona forced storms to move towards Phoenix from the north along I-17 before they dissapted before midnight. Some of the storms moved into the valley and north and east of the 101 all the way down to Gilbert.
Below is the HRRR-UA WRF 12Z 24-hour accumulated precipitation forecast (left) and corresponding observed precipitation from MRMS (right). The forecast was generally accurate. The light morning showers west of Sells to Yuma along the International border were a little overdone in the forecast, and no rain fell in western Arizona, as indicated by the observed data.
Analysis
Remnant circulation from Tropical Storm Elida is now caught in the jet stream and is no longer influential. The low in northeast Arizona this morning is continuing to move north and may affect storms in northern Arizona and western New Mexico this afternoon/evening. The Tropical Upper Tropospheric Trough (TUTT) off the coast of Baja is moving north and may get tied into the upper-level outflow from Hurricane Fausto. Tropical Storm Bertha is being affected by strong upper-level winds as it moves slowly west. It has the potential to produce heavy rain in southeast Texas once it moves onshore.
A few showers are northeast of Phoenix this morning, but they will weaken as they move further south and east. The flash flood watch in western New Mexico extends northward into parts of Utah, Colorado, Idaho, and Montana as monsoon moisture has moved into this area. Nearly all of Arizona is above 60°F, whereas northern New Mexico and the Colorado Plateau are in the 50s.
The HRRR-UA WRF 12Z initialization (left) and observed sounding (right) for Phoenix at 5 am is below. Both the forecast initialization and observations are in good agreement, so the forecasts today should be good. Preciptatable water remains high, > 49 mm.
At 250mb, the monsoon ridge is along the Texas/Oklahoma border having moved a little further south since yesterday; the ridge is poised to move back to the west as the upper low over northeast Arizona moves north.
A trough over Arizona and Utah is between the two high-pressure centers at 500 mb. The trough is weakening, allowing the highs to merge into one. Temperatures are colder aloft in northeast Arizona and eastern Utah and this will aid in the development of thunderstorms with heavy rain.
At 700mb, the low in Arizona is moving to the northwest and slowly weakening as it moves northward. Ahead of it’s path will be a focus of storms today, but mostly in northeast Arizona, Utah, and Colorado. In about 24 hours, it will dissipate and the monsoon circulation will reestablish itself across New Mexico and Arizona.
Today
Both the Phoenix and Tucson forecast sounding at 4:30 pm have enough Convective Available Potential Energy (CAPE) and low Convective Inhibition (CIN) to support isolated afternoon and evening thunderstorms near the mountains.
The HRRR-UA WRF 12Z radar reflectivity forecast for this afternoon shows only isolated storms in southern Arizona near the mountains. In the wake of the upper low in northeast Arizona, subsidence (sinking air) will occur and suppress thunderstorms. The storms in northern Arizona will move to the northeast out of the region.
Tomorrow
Tomorrow, will be somewhat of a repeat of today with a few more storms developing as upper-level subsidence weakens.
Outlook
An increase in moisture is expected Saturday from a possible gulf surge while an easterly wave approaches Arizona from the east. Forecasts for a gulf surge and thunderstorms three days in advance is risky, since uncertainty is high that far; it may not occur at all or be weaker. Circulation in the wake of Hurricane Fausto could be a contributing factor, so we’ll be monitoring its track for the next few days.
The global model ensemble forecasts show an increase in activity, but a day later than the the regional forecasts. In a general, an uptick in thunderstorms over the weekend is likely.
Discussion written by Pat Holbrook and Patrick Bunn