Sunrise 5:15 AM   Sunset 7:27 PM Moonrise 2:25 AM   Moonset 3:37 PM

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FXUS65 KTWC 022210 AFDTWC

Area Forecast Discussion National Weather Service Tucson AZ 310 PM MST Sun Jun 2 2024

SYNOPSIS
Temperatures will remain above normal this week, with dangerously hot temperatures likely Thursday. Sunny skies and afternoon breeziness can be expected over the next few days. A weather system moving into the area will bring a chance of thunderstorms Friday and Saturday. These storms will produce little if any rainfall, but will be capable of producing gusty and erratic outflow winds.

DISCUSSION
Current pattern defined by a broad ridge of high pressure over Mexico with a weak dry trough of low pressure moving through the Great Basin. The southern end of the trough axis moving through Arizona today has kept afternoon temperatures 1-3 degrees cooler than this time yesterday, but still hot with valleys across Southeast Arizona still expected to top out in the upper 90s to near 100 degrees. This trough is also responsible for breezy conditions this afternoon southeast of Tucson, with a few spots gusting up to 30 mph (below critical fire weather thresholds).
Expect Monday to be more of the same as the overall upper pattern remains similar.

A weak upper-low seen on satellite imagery developing near off the coast of southern California (28.7N/127.0W) this afternoon will be impactful to our neck of the woods later this week. Before this happens through, we will see the impacts of a longwave amplified ridge developing across the western states, including Arizona, this week. Ridge axis slowly translates east, over Arizona Wednesday and into New Mexico Thursday. Highest thicknesses values expected Thursday afternoon, with both 02/12Z operational runs of the GFS and ECMWF suggesting 850-700mb thicknesses over Tucson of 1690-1700m. These values would correspond to a high temperature at KTUS between 107-111 degrees.
The NBM 25th/75th percentile for Tucson on Thursday afternoon is 106/110 with the operational NBM 109 degrees. These values are slightly warmer than previous runs, but I am confident it wont get much warmer as additional amplification of the ridge will be difficult given its orientation. In any event, an Excessive Heat Watch is in effect for the valleys of Southeast Arizona Thursday.
It is possible that we will need to extend the excessive heat watch into Friday as it too has been trending warmer with every run. After coordination with surrounding offices, we have decided to wait because it is possible that more significant mainly mid and high cloud cover Friday afternoon will help moderate temps.

Focus then shifts back to that developing upper low. This feature slowly digs south to off the Baja Peninsula and then northeast to the Baja Spur Friday. This will result in decent diffluence/ synoptic lift over Southeast Arizona Friday and Saturday. Limited moisture will initially move into the area within the mid-to-upper levels. This will result in significant virga across the majority of forecast area Friday afternoon and evening, moistening the atmosphere from the top-down Friday night. Expect mainly dry thunderstorms Friday with light rain possible more on Saturday.
The problem is that the sub-cloud layer will remain very dry, so the potential will exist both days for strong and erratic outflow winds.

The ensemble members are all over the place with how the upper low ejects next weekend. The GEFS members typically track it north along the Colorado River with a more progressive ECMWF bringing it through southern New Mexico. The positive impact of this system will be to cool temperatures back down from the excessive heat on Thursday to near normal levels Saturday and Sunday.

AVIATION
Valid through 04/00Z.
SKC with a few high clouds through the period. SFC winds SWLY 12-17 kts with gusts to 30 kts into the early evening, before gradually diminishing to less than 10 kts overnight. Wind Saturday will be again be SWLY 12-17 with gusts to 30 kts. Aviation discussion not updated for TAF amendments.

FIRE WEATHER
Dry conditions and high temperatures 2 to 5 degrees above normal the next few days, becoming 5 to 8 degrees above normal the second half of the week. Dangerous heat is anticipated across the valleys of Southeast Arizona Thursday and possibly again Friday. Minimum relative humidity values 5-10% in the lower elevations 8-15 percent in the mountains through the week, along with poor overnight recoveries between 20-30%.

Afternoon winds should be breezy this afternoon and again Monday with southwest winds 15-20 mph with gusts to around 30 mph (more likely south to southeast of Tucson). Otherwise, an approaching weather system will result in the potential for dry thunderstorms with gusty outflow winds Friday and Saturday.

TWC WATCHES/WARNINGS/ADVISORIES

Excessive Heat Watch from Thursday morning through Thursday evening for AZZ501>509.

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