textproduct: NWS Phoenix

KEY MESSAGES

- Today will mark the start of a short drying trend with a return to overall sunny skies and temperatures holding around, to slightly above normal through the weekend.

- Starting Sunday, moisture will surge back into the region with cloudy skies and eventual chances for light rain later in the day.

- Monday and Tuesday may bring good chances for rain and possibly some strong thunderstorms. Heavy rainfall will also be possible, particularly across the Arizona high terrain. - Temperatures will cool to below normal during the first half of next week with highs from the mid eighties to the mid nineties.

SHORT TERM /TODAY THROUGH SATURDAY/

A shortwave trough with a well-defined vort center currently moving northeastward through south-central Arizona brought showers and thunderstorms since last evening, but it has finally coming to an end. As the disturbance continues to move to the northeast this morning, we may see some lingering showers over the eastern Arizona high terrain before coming to an end near or just after sunrise. Drier air will then gradually move into the region from the southwest later today through Saturday resulting in overall clear skies. Expect plenty of low level moisture to stick around through Saturday morning before the drier air finally mixes down to the surface Saturday afternoon. Temperatures will also warm up a bit with the lack of clouds, pushing daytime highs back into normal range for the Phoenix area to slightly above normal across the western deserts.

LONG TERM /SUNDAY THROUGH THURSDAY/

A significant weather event could be shaping up for next Monday into Tuesday as models are beginning to come to a better consensus for a seasonably strong upper level trough moving through our region. The biggest uncertainty still involves how much moisture will be available when the trough does move through, but it is looking more optimistic compared to last night.

This next event is expected to start with strong upper level moisture advection from the southwest which will actually come from the remnants of the decaying tropical system Odalys. Odalys is forecast to drift northward over the next day or two before getting absorbed into the westerlies as a trough digs southeastward off the coast of California. We can anticipate plenty of higher level moisture and clouds overspreading our area beginning Saturday evening and continuing through much of Sunday. This moisture fetch is likely to be very consistent for 18-24 hours with gradual top-down moistening of the atmosphere. By Sunday mid to late morning, some areas may start to see some sprinkles, or even very light rain, but no measurable rainfall. It would not be surprising to see a fairly narrow band of light rain developing by Sunday evening somewhere across the area, but amounts are expected to remain very light.

Also starting Sunday, Hurricane Polo is forecast to approach southern Baja before eventually tracking to the northeast across Baja and the Gulf of California most likely later on Monday. The latest NHC track of Polo keeps it well to our south as it is expected to fall apart quickly as it moves onshore into southern Sonora Mexico. We are not worried about any direct impacts from Polo, but it is becoming increasingly more likely it will help direct additional moisture northward into our region, specifically in the low to mid levels. This moisture will be necessary for any decent rainfall to occur early next week, so any big deviations in the forecast moisture will affect our rain chances for Monday and Tuesday.

Although the details are expected to change, forecast confidence for a decent weather/rain event continues to increase. As the bulk of the upper level moisture from the remnants of Odalys mostly pushes into New Mexico later on Sunday, we should begin to see moisture stream out of the south southwest from Polo by Monday morning. The best moisture is expected to bypass our region to the southeast, but we still should see good moisture working into most of Arizona early on Monday, right ahead of the approaching upper level trough. Ensemble mean PWATs of 200% of normal are shown for the eastern 2/3rds of Arizona on Monday with the bulk of this moisture falling in the low and mid levels, just where it would be needed for a decent rain event.

The upper level trough will play a very important role early next week as it looks likely to provide strong forcing on Monday and Tuesday. Upper level ascent is expected to be quite good, along with very good 30-45 kts of southwesterly upslope forcing within the mid levels. It is still too early to have a great idea on the details of the evolution of the rainfall, but it could start in earnest by Monday morning. NBM PoPs increase areawide by Monday morning with 30-50% chances across the western deserts to 50-75% from Phoenix and north and east of Phoenix. These PoPs basically hold through Monday night before gradually tapering off from west to east on Tuesday. We may see fairly steady upslope showers and isolated thunderstorms across south-central and eastern Arizona Monday through early Tuesday.

We are currently looking at three possible scenarios for early next week. First, and the most likely outcome (~60% chance), is for a modestly strong upper level trough to push through Monday into Tuesday with very adequate, but not tropical level moisture getting pulled into our area right ahead of the passing trough. This scenario would support widespread rainfall amounts of 0.10-0.30" over the lower deserts to 0.5-1.0" over the high terrain north and east of Phoenix. This should also involve a period of strong thunderstorms due to quite good wind shear. Second, (~20% chance) would keep the best moisture to our south and/or a much weaker and faster trough. In this scenario, we would expect much more scattered rainfall with lower desert amounts under a 0.25" and higher terrain amounts more like 0.25-0.75". At the other end of the spectrum (~20% chance) would leave us with the upper end of potential moisture combining with a stronger and slower trough. This would bring a longer rain event with lower desert rain amounts averaging from 0.2-0.5" over the western deserts to 0.75-1.50" for the south-central Arizona lower deserts. As much as 2-3" would also be very possible over the high terrain north and east of Phoenix. This stronger event is also likely to involve a period of strong to severe storms.

It will be another day or two until we have a good idea on the expected impacts for this upcoming event, but as stated above there is at least some possibility of a high impact event. We will continue to monitor the model trends and provide the latest updates with this upcoming weather event.

Temperatures are expected to take a big dip early next week with lower desert highs falling to as low as the mid 80s in the Phoenix area to the low to mid 90s across the western deserts. Once the weather system ejects to the east by next Wednesday, we should fall back into a drier and warmer pattern.

AVIATION

Updated at 0537Z.

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

Isolated to scattered showers and storms should mostly focus around eastern portions of Phoenix tonight, while slowly moving ENE. Activity should then come to an end by 12Z Friday morning. A light W-N wind is favored away from the activity and more variable to light E-SE in the vicinity of the activity, but overall wind direction forecast confidence is low. Once activity moves east of all terminals a prevailing light E wind should develop at all terminals. The rest of Friday will feature more typical diurnal tendencies with an afternoon W-SW shift. Wind speeds through the day Friday should remain light (aob 8 kt). Brief CIGs down to 6-7K ft AGL are possible in SHRA/TSRA, with mid and high CIGS more common through the night. FEW to SCT cumulus around 7K ft AGL is expected Friday afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH:

No aviation weather impact are expected during the next 24 hours. Both terminals will experience light winds throughout the TAF period with periods of calm and light VRB. Wind directions will mostly favor a southeasterly component at KIPL and SSW-SSE at KBLH. FEW to SCT skies will be present over the region through early tonight, followed by clearer skies heading into Friday. High cirrus will then spread back over the area Friday evening.

FIRE WEATHER

Lingering morning showers across eastern Arizona as well as a slight chance of an isolated shower or weak storm in the afternoon are expected today. Much drier air will settle into the region later today and last through Saturday with temperatures returning to around normal. MinRHs today will bottom out between 25-45% across the eastern districts and 15-25% across the western districts before dropping further to 15-25% in most areas on Saturday. Winds will generally follow the typical diurnal tendencies with limited to no afternoon gusts.

PSR WATCHES/WARNINGS/ADVISORIES

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