Area Forecast Discussion

SYNOPSIS

Warm and dry into the weekend. Breezy conditions are anticipated for the second half of the week into this weekend, resulting in elevated to near critical fire weather conditions east of Tucson Wednesday through the weekend.

DISCUSSION

The upper pattern is defined by a weak upper-low moving east through central New Mexico and a strong upper-low off the coast of the Pacific Northwest and northern California with a low-center positioned near 40N/130W. Strong agreement with the grand ensemble members of the GEFS, ECMWF and GEPS tracking this upper low into NRN CA by 22/00Z this afternoon, and then across the northern Great Basin Wednesday with a followup shortwave again on Thursday. As this occurs, a strong ridge of high pressure off the coast of British Columbia Canada will force the development of a weak low south of itself near 35N/132W by 24/00Z Thursday afternoon. This results in a mean open trough with westerly zonal flow along its base directed into the Desert Southwest. This orientation is conducive for progressive shortwaves to zip quickly by to our north in the westerly flow aloft. As this pattern sets-up, models suggest a couple of low-to-mid level disturbances move through southern Arizona Wednesday and again Thursday. These features will be dry, but will result in breezy WLY winds across Southeast Arizona both days. Given the breezy westerly winds (20-foot west winds 15 to 20 mph with gusts 30-35 mph) and the dry airmass across the region (minimum RHs in the teens areawide), we can expect to see elevated to near critical fire weather conditions, with the most impactful day being Wednesday. The 21/00Z HREF and REFS both have a probability 20-40 percent for reaching critical fire weather conditions (RH<=15% and 20-foot wind speed=>20 mph) across portions of Fire Weather Zone 152 Wednesday.

The upper-low to our west eventually pushes into southern California Saturday in response to a stronger upper-low undercutting the ridge and moving into its place from the west. This will bring stronger mid-level flow into the area, with breezy to windy southwest to west winds across Southeast Arizona this weekend. Although lower-level moisture will push into the CWA from the west Saturday, areas from Tucson east may be slower to moisten up, resulting in yet another day of near critical fire weather conditions Saturday. Cooler temperatures and deeper lower-level moisture associated with the upper low eventually move into Southeast Arizona Sunday, increasing RHs and reducing the fire weather threat.

AVIATION

Valid through 22/12Z. FEW-SCT mid-level clouds 10-15k ft AGL BKN-OVC high clouds AOA 20k ft AGL through the valid period. SFC winds under 10 kts and terrain driven through 21/18Z, then SWLY/WLY 10-15 kts with occasional gusts to 20-25 kts into the early evening. Otherwise, SFC winds remain under 10 kts and terrain driven. Aviation discussion not updated for TAF amendments.

FIRE WEATHER

Warm and dry into the weekend. Minimum RHs will generally be in the 10-20 percent areawide through Saturday, then increase to 10-25 percent in the valleys and 25-35 percent in mountains Sunday as weather disturbance moves into the area. Passing systems to the north this week will keep Southeast Arizona dry, with breezy west winds the second half of the week into next weekend. There is a 20-40 percent probability that areas Tucson eastward will reach elevated to near critical fire weather conditions Wednesday, with elevated to near critical conditions persisting through the Saturday.

TWC WATCHES / WARNINGS / ADVISORIES

None.