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FXUS65 KPSR 300540 AFDPSR

Area Forecast Discussion National Weather Service Phoenix AZ 1040 PM MST Wed Nov 29 2023

UPDATE
Updated Aviation section.

SYNOPSIS
A low pressure system will be moving in from the west this evening giving the Arizona high terrain a decent chance for some light precipitation. This system will also bring much cooler weather allowing our afternoon high temperatures to drop at or slightly below normal through the weekend. A secondary system will move through late Thursday into Friday bringing widely scattered precipitation chances mainly across south-central and eastern Arizona. High pressure and warming temperatures return early next week.

DISCUSSION
Upon analysis of IR WV imagery this afternoon, a low pressure system has moved inland over California and will continue to progress southeastward throughout the day. Afternoon high temperatures will be in the low 70s across the lower deserts, which is slightly above normal. Breezy conditions have already developed across southeastern California as a result of the low moving further inland. Breeziness will continue to increase through the day as the low continues to move southeastward with gusts above 30 mph possible late this afternoon and early this evening across the western portions of Imperial County.

The trough will dig across our region late this evening through early Friday morning. Moisture is limited for this first round of precipitation as PWATs will be in the 0.5-0.6” range.
Precipitation chances are best eastward of a Prescott, AZ-Tucson, AZ line with PoPs peaking at 40-55% during the overnight hours.
Areas west of this line should remain dry, although a passing shower cannot be completely ruled out as PoPs are less than 15%.
Due to the lack of moisture, QPF will remain light and be less than 0.10”. The NBM gives the AZ high terrain a 70% chance of getting more than 0.01” of rain, but only a 45% chance of getting more than 0.10” of rain.

A secondary shortwave trough will move in quickly behind as the first one exits the area to the east. This secondary wave will move through Arizona Thursday evening through Friday. The amount of moisture available will be slightly better than with the 1st round, as PWATs will be in the 0.7-0.8” range. This enhanced moisture in conjunction with forcing from 500mb vorticity will allow for better PoPs as well as higher QPF amounts. Activity will spark in Yuma and La Paz Counties Thursday evening and move eastward and into the Phoenix Metro and AZ high terrain late Thursday night and continue into the overnight hours. Activity will dwindle through the day on Friday and should exit the region by late Friday evening. Since there is more moisture and better forcing for this round of precipitation, the QPF totals will range from 0.05-0.20” for Yuma, La Paz, and western Maricopa Counties, 0.2-0.35” for the Phoenix area, and 0.4-0.6” for the higher terrain north and east of Phoenix. Snow levels will dip to around 6000 ft, which will cause mixed precipitation at times in the high terrain in southern Gila County.

These shortwave troughs will not only bring us these precipitation chances, but will also usher in cooler air. Afternoon daily high temperatures will be below normal beginning tomorrow and lasting through the weekend. Although dry conditions return on Saturday, temperatures will remain below normal for the weekend due to being under the influence of broad troughing and lower heights aloft.
Daily afternoon high temperatures, tomorrow through the weekend, will be in the mid-to-upper 60s across the lower deserts and range from the upper 40s to upper 50s across the high terrain. Daily morning lows for this time frame will be at or just slightly above normal and will range from the upper 30s to upper 40s across the lower deserts and be in the 30s for the high terrain. Sunday morning looks to be the coldest morning coming up and some of the more typical coldest lower desert rural areas and even Globe may see their first freeze or near freeze of the season. The NBM gives areas near Miami and Globe a 30% chance of reaching freezing Sunday morning.

Global ensemble models agree that ridging will start to build across the region late Sunday and into Monday and will stick around through at least the middle of next week. This will lead to a gradual warm up with temperatures likely going back above normal as early as Monday. The NBM gives us a 55% chance of the afternoon high temperature going above normal on Monday. By the middle of next week, as the ridge moves overhead, the NBM has temperatures going back into the mid-to-upper 70s.

AVIATION
Updated at 0540Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:

An upper trough moving through the southwestern CONUS will exit Arizona tonight for decreasing cloudiness beginning about 09Z.
Before that, anticipate SCT-BKN decks of cumulus with bases of 8-10kft ASL along with BKN-OVC layers AOA FL150. There is a low probability (10%) of getting a very light shower/sprinkle at any given TAF site tonight before the clearing. Better chances over the higher terrain north and east of the Greater Phoenix area along with more significant ceilings. Clouds start increasing again Thursday afternoon as another system approaches. That one will lead to more widespread ceilings with lower bases Thursday night along with scattered light showers. In fact, there is about a 30% chance of ceilings dipping into the MVFR category after 05Z or so. As for winds, westerly winds have begun to weaken and will continue to do so favoring E/SEly by 12Z if not sooner. Anticipate redevelopment of southwest/westerly winds in the afternoon Thursday.

Southeast California/Southwest Arizona including KIPL and KBLH:

Westerly winds at KIPL have weakened but expect west to continue through the night. Light SW/Wly is favored at KBLH as well.
Westerly winds increase during the afternoon with gusts up to 20-25kts at KIPL (lighter SWly at KBLH. High clouds increase during the day Thursday with lowering ceilings approaching FL100-120 by 00Z Friday. FEW-SCT cumulus of 5-7kft ASL can be anticipated Thursday afternoon. Potential for showers at KBLH late Thursday afternoon/Friday evening is too low to reflect in the TAF at this time.

FIRE WEATHER
A couple of weak weather disturbances are expected to affect the region through Friday with the first one moving through later today and tonight and the second late Thursday into Friday. As a result, moisture will increase over the next couple days pushing MinRHs to above 35% on Thursday and Friday. Rain potential later today will be confined to the Arizona high country, while the second system is likely to bring good chances for wetting rains over much of southern and central Arizona. Expect some breezy conditions mainly across the western districts later today and again areawide on Thursday. For the weekend into the first part of next week, high pressure and drying conditions will take over leading to a return of above normal temperatures by around Monday and MinRHs dropping to between 20-30% by Sunday or Monday.

PSR WATCHES/WARNINGS/ADVISORIES
AZ...None.
CA...None.

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