textproduct: NWS Phoenix

UPDATE

Updated 18Z Aviation Discussion.

KEY MESSAGES

- A pair of weather systems will produce unsettled conditions today and tomorrow with a couple rounds of showers and breezy to locally windy conditions.

- Below normal temperatures are expected through the end of the workweek before rebounding this weekend as high pressure makes a return.

-Dry conditions return region wide on Friday and continue into the first half of next week.

SHORT TERM /Today through Thursday/

An unsettled weather pattern sets up today and will continue through Thursday as two weather systems move through the region resulting in a couple rounds of rain, cooler temperatures, and breezy to locally windy conditions across the region. Taking a look at upper level water vapor satellite imagery early this morning, we can see the two features that will result in our unsettled weather over the next couple of days. The first one is a closed low that is currently centered off the northern coast of the Baja. The second one is a trough digging off the coast of the Pacific NW.

The aforementioned closed low will be moving through our region today and will quickly advect into the Plains overnight tonight and tomorrow. The closed low will move northeastward through the day, weaken as it does, eventually becoming an open wave, with the center eventually moving through SE AZ by this evening. This low will bring a decent amount of moisture and forcing (both from the associated vorticity center and favorable position of the upper- level jet) along with isentropic ascent, resulting in scattered to widespread rain across much of the CWA, SE CA will most likely only get some isolated to scattered light showers. Current PWATs are in the 0.4- 0.7" range across the CWA, and will increase to 0.8-0.9" (around 200% of normal) as the low moves through. The scattered to widespread showers look to move into SW AZ by the early to mid morning timeframe, and then into south-central AZ (including the Phoenix Metro) by the late morning hours. With the low not exiting the region until late tonight/the overnight hours, off and on showers are expected through the afternoon and evening. As for SE CA, the best time to see activity out there will be from the mid morning through the mid afternoon. With this round of showers there is little to no instability, so no thunderstorms are expected. Additionally, with the lack of instability and only moderate moisture rain rates will not be impressive, so despite the prolonged period of rain chances QPF totals will remain on the lighter side. QPF totals are as follows: 0.00-0.15" for SE CA, 0.05-0.30" for SW and south-central AZ, and 0.30-0.75" for the foothills and higher terrain areas.

A break in activity is expected during the overnight and morning hours on Thursday before the second system arrives. The aforementioned second system, trough currently off the coast of the Pacific NW, looks to move onshore Wednesday night into Thursday and dive south/southeastward, arriving in our CWA by Thursday afternoon. This system will push a cold front through our region. This front coupled with the residual moisture from the first system (PWATs of 0.5-0.6") will result in isolated to scattered showers along and ahead of the front. Additionally, a few isolated thunderstorms will be possible, as MUCAPE values will be around 100-300 J/kg. This activity is expected to remain confined to south-central AZ, with the best chances across the higher terrain areas north and east of Phoenix. QPF values with this system are: around 0.00-0.10" isolated 0.25" across the lower deserts of south-central AZ, and 0.25-0.50" isolated 1.00" across the higher terrain areas. Also, with this system coming from the NW it will be a colder system, with snow levels falling to around 5,000 ft, resulting in accumulating snowfall above 5,000-6,000 feet. Additionally, with the marginally unstable conditions and lowering freezing levels, some graupel will be possible. Also, given the strong forcing, steep cold front, and deepening system juxtaposed with this shallow convection, these showers may also tap higher momentum towards the surface in the form of gusts 30+ mph.

Another consequence of a stronger vorticity center and sharper negative tilt orientation is more intense upstream pressure rises and ability to vertically mix stronger winds behind the frontal passage. The trend in model output strongly supports a pattern of rapidly intensifying (40-50kt) post frontal H9-H8 winds surging down the lower Colorado River valley late Thursday afternoon, peaking Thursday night with added thermal influences. Have started to increase wind speeds during this time frame over the mandated NBM which seems to not be capturing the impacts of such a pattern common during the winter. Some wind products may be needed, however, the highest gusts, and those meeting Wind Advisory criteria, are mostly confined to the ridgetops in SE CA.

With multiple weather systems moving through the region along with the associated rain chances and cloud cover, temperatures will cool to below normal. Afternoon high temperatures are forecasted to be in the upper 50s to mid 60s across the majority of the lower deserts (mid to upper 60s across western Imperial County and the Yuma area) and in the upper 40s to mid 50s across the higher terrain.

LONG TERM /Friday through Tuesday/

Ensembles are in pretty good agreement with the overall synoptic pattern for this weekend and heading into next week. The trough that will bring us rain chances on Thursday will quickly advect into the Plains Friday into Saturday. This will result in strong subsidence and an abundance of dry air over our region. Ensembles show PWATs dropping to 0.1-0.3" by Friday. On Friday we will still be under the influence of some lower heights aloft, with H5 heights in the 558- 567 dm range, the wide range is due to the trough, and associated tight pressure gradient, exiting our region through the day. As a result, high temperatures on Friday will be below normal, with highs in the upper 50s and low 60s across the lower deserts and in the upper 40s to low 50s across the higher terrain. With the trough in the Plains on Saturday, H5 heights will rise to a 573-576 dm range. As a result temperatures will warm on Saturday, with afternoon high temperatures in the low to mid 60s across the lower deserts and in the 50s across the higher terrain. Additionally on both Friday and Saturday morning, the morning low temperatures will be some of the coldest so far this winter. With the trough quickly exiting the region, the clouds associated with the trough will also quickly exit our region. This will result in clear skies overnight and maximizing radiational cooling. So, frost/freeze impacts may be an issue for rural valley locations and freeze products may be needed. Low temperatures both days are currently forecasted to dip into the low to mid 30s across rural lower desert locations, mid 30s to low 40s across more rural lower desert communities, and in the upper 20s to low 30s across higher terrain areas.

A blocking pattern will develop this weekend with high pressure returning to Western CONUS, and centered over northern CA. This pattern will continue through at least the beginning of next week, with the high pressure strengthening through the first half of next week. H5 heights will be in the 576-579 dm range Sunday- Tuesday. With the high pressure strengthening through the beginning of next week, temperatures will gradually warm as a result. On Sunday an Monday temperatures are forecasted to be in the upper 60s to low 70s across the lower deserts and in the upper 50s to low 60s across the higher terrain. Then on Tuesday, afternoon high temperatures will warm to the low to mid 70s across the lower deserts and in the low to mid 60s across the higher terrain. Additionally morning low temperatures will also warm, going from the upper 30s to low 40s (low to mid 30s) across the lower deserts (higher terrain) on Sunday to the low to upper 40s (mid to upper 30s) across the lower deserts (higher terrain) on Tuesday. Additionally, with high pressure building back into the region, dry conditions return region wide on Friday and continue through at least the first half of next week.

AVIATION

Updated at 1755Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The main aviation weather concerns throughout the TAF period will be SHRA and MVFR CIGs. Wind speeds will remain aob 8 kts through Thursday morning. -SHRA activity is gradually spreading through the area from the south, however, based on the latest radar trends and model guidance it appears the -SHRA will begin across the terminals by 21z. Winds through the early afternoon will maintain a SE-SSE before switching to the S-SSW by around 21z, when the -SHRA begins. At around the same timeframe, CIGs will be lowering to as low as 3-4 kft with visibilities reducing to MVFR conditions in any heavier pockets of rain. Winds by early this evening will then make a shift out of the west and will stay out of the west through the rest of the period. The most widespread -SHRA will exit to the east by around 02-03z, with VCSH remaining through 06-07z. CIGs, however, despite the -SHRA ending, will remain in the 3-5 kft range through Thursday afternoon. Another round of scattered SHRA with isolated TS is likely Thursday afternoon accompanied by gusty winds.

Southeast California/Southwest Arizona including KIPL and KBLH: The main aviation weather concern through the TAF period will be some reduced CIGs and VCSH through late this afternoon. At KIPL, winds will remain light and variable through late this afternoon before switching out of the west afterwards. At KBLH, winds will be out of the northwest through this afternoon before shifting out of the southwest by this evening followed by light and variable winds overnight. Wind speeds at both sites will remain light aob 7 kts through Thursday morning, before speeds pick up by late Thursday morning out of the northwest. CIGs between 5-7 kft as well as VCSH, especially at KIPL, will continue through late this afternoon before clearing skies take place by early this evening.

FIRE WEATHER

Moisture levels will remain elevated through Thursday with wetting rainfall chances heightened across eastern districts as a pair of weather disturbances pass through the region. Much drier conditions will return on Friday and continue into next week. MinRHs today will be in the 50-70% region wide with excellent overnight recovery. Tomorrow, minRHs will be in a 50-80% range across the central and eastern districts and will be in the 20-40% range across the western districts, with excellent overnight recovery in the eastern districts and good overnight recovery in the western districts. By Friday, minRH will fall into a 15-35% range, with good overnight recovery, region wide and will continue through the first half of next week. Strong northerly wind gusts in excess of 30 mph will likely surge into the lower Colorado River valley Thursday afternoon through Friday morning yielding an elevated fire danger. Elsewhere, gusts should be somewhat weaker with lesser impacts before returning to more seasonal levels over the weekend.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...None. CA...None.