textproduct: NWS Phoenix

KEY MESSAGES

- A band of light rain will impact portions of the region later today and into Monday before more scattered showers and thunderstorms become possible starting Monday afternoon.

- A Flood Watch has been issued for Monday night and much of Tuesday for higher terrain areas of central and eastern Arizona. Moderate to heavy rainfall may also impact portions of the Arizona lower deserts, but we are expecting it to be much more localized.

- Near to below normal temperatures are expected through the middle part of the week before warming to slightly above normal by next weekend.

SHORT TERM /TODAY THROUGH TUESDAY/

The main forecast concern continues to be the potential for widespread rainfall, particularly from late Monday through Tuesday afternoon. Unfortunately, model guidance is still showing a good deal of uncertainty with the timing and track of the incoming Pacific low and how it will interact with the influx of moisture through Monday. Current satellite imagery shows high level clouds streaming into our region from the fast decaying TC Odalys well to our southwest, while a much better organized TC Polo is slowly drifting northward toward southern Baja. Even the forecast for later today is a bit tricky as the higher level moisture from Odalys will begin to interact with the incoming Pacific trough which is approaching northern California.

We anticipate a continuous feed of higher level moisture from the southwest for the rest of today and likely lasting into early Monday morning. As this occurs, the moisture will gradually shift more into the mid-levels due to top-down saturating of the layers. There is also model evidence of some weak to moderate ascent within the saturated layer prompting showers to begin to develop later this morning. CAMs generally agree on increasing light shower coverage later this morning, but most of this will not reach the surface as the sub-cloud layer is starting out very dry. This band of showers should eventually moisten the lower levels enough by later this afternoon into this evening to allow for some off and on light rain somewhere from southeast California through southwest Arizona into around the Phoenix area. This rain is not likely to amount to much, but most locations within this swath should at least measure a few hundredths to maybe a tenth of an inch. Some guidance shows a very narrow (25-50 miles maybe) of amounts upward of a quarter inch from this evening through tonight, which seems reasonable if it stays over the same area for 6-12 hours. We also can't rule amounts upwards of 0.50" closer to the mountains in northwest Maricopa County. The atmosphere should remain quite stable through Monday morning, so we are at most expecting a few rumbles of thunder.

Model trends over the past 24 hours has shown a little faster progression of the Pacific low as it moves through our region later Monday through Tuesday, as well as slightly less available moisture. This still does not mean heavy rainfall will not happen, it just means it will most likely be more confined to higher terrain areas north and east of Phoenix and a more localized. The center of the low is forecast to move into southern California by Monday evening with increasing forced ascent during the latter half of Monday and continuing through the first half of Tuesday. The strongest forcing is expected just out ahead of the low center, focused over the western deserts Monday afternoon/evening and then progressing through southern and central Arizona Monday night through Tuesday afternoon.

After the initial swath of light rain tapers off at some time Monday afternoon, we are expecting scattered showers and thunderstorms to develop somewhere along the Lower CO River Valley late afternoon before slowly tracking eastward through southwest Arizona in the evening/early overnight hours. A few of these storms may end up being strong with potential for an isolated severe storm too. Additional scattered showers with embedded weaker storms are likely to impact higher terrain areas in Arizona later on Monday before becoming more widespread at some point Monday night and/or Tuesday morning. The timing is still a bit uncertain, but the peak time frame for rainfall for central and eastern Arizona looks to be between 3 AM Tuesday through Tuesday afternoon. A Flood Watch has been issued for the foothills areas of northern Maricopa County, northern and eastern portions of the Phoenix metro and for the higher terrain northeast and east of Phoenix. The Watch will be in effect from midnight Monday night through the first part of Tuesday evening. Guidance generally agrees the low will begin to exit the area to the east Tuesday night with any shower activity winding down from west to east during the evening/early overnight hours.

QPF amounts have gone down some from previous forecasts with rainfall amounts averaging from around 0.1" in the Lower CO River Valley to 0.35-0.75" for the lower deserts of south-central Arizona. Higher amounts are expected within the Watch area, averaging between 1.0-1.25", but localized amounts of 2-3" will be possible. Keep in mind, there is still a good amount of spread among the models for anticipated rainfall amounts and these average expected amounts could still easily go up or down some.

As far as the strong to severe storm threat, the faster timing of the system is leading to less of a severe storm threat. Forecast instability remains the main issue with CAPE not likely getting much above 500 J/kg. However, even with roughly 500 or maybe 750 J/kg, this may be enough to allow for a few stronger and maybe even an isolated severe storm on Tuesday as the low center moves eastward through central Arizona. As of right now, this limited risk of severe storms is likely to stay across far southern into southeast Arizona from late Tuesday morning through Tuesday afternoon.

LONG TERM /WEDNESDAY THROUGH SATURDAY/

Upper level ridging is forecast to quickly surge in from the west on Wednesday leaving clearing skies and lowering surface moisture through Thursday. Temperatures for the coming week will stay somewhat stable across the western lower deserts, but still on average slightly below normal today through Tuesday. Much cooler temperatures are expected for most of Arizona with highs only in the 80s for the Phoenix area Monday-Wednesday. Once the ridging moves in from the west later this week, temperatures will creep upward again with highs topping 100 degrees across the western deserts by Thursday or Friday and potentially for the Phoenix area by next weekend.

AVIATION

Updated at 1000Z.

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

The primary aviation weather concerns during the TAF period will be increasing chances for SHRA conditions and lower than usual confidence on winds due to the potential showers and in the vicinity. Winds will overall follow diurnal trends, however, current guidance indicates a tendency for a more NW/N wind direction this evening and through the overnight hours, and the typical southeasterly nocturnal wind shift may not occur. Gusts into a 15- 25 kt range cannot be ruled out this evening as a result of nearby shower activity. Better chances for SHRA arrive Monday morning, leading to the introduction of the prevailing line at KPHX starting around 12Z. Before Monday morning, rain will be fairly light and isolated in coverage, with mostly virga showers initially. BKN-OVC cloud decks with bases above 10K ft AGL will be common through tonight, with FEW-SCT lower decks becoming introduced Monday morning.

Southeast California/Southwest Arizona including KIPL and KBLH:

The possibility of VCSH/SHRA will be the main aviation concern in the upcoming TAF period. Winds will fluctuate between SE and SW at both terminals through the period, with speeds generally AOB 10 kts. VCSH/SHRA will start to move into KIPL's area later this morning. Confidence in SHRA is low (less than 30%) so that has been left out of the TAF for now. In fact, most rain may fall at virga. At KBLH, any rain in the morning and afternoon is expected to remain to the south and east of the terminal with better chances for VCSH/SHRA at the terminal arriving after 00Z. BKN/OVC high clouds will continue with FEW-SCT mid level clouds moving in later this morning.

FIRE WEATHER

Moisture will increase across the area today and Monday with light rain developing this evening followed by locally moderate to heavy rainfall with thunderstorm chances later Monday into Tuesday. The bulk of the expected rainfall will favor the eastern districts. MinRHs today will dip to 20-30% this afternoon before improving to 40-60% over much of the area for Monday and Tuesday. Winds through Monday will generally follow the typical diurnal tendencies, but with a tendency to favor the south. A front moving through the area on Tuesday will turn winds out of the west northwest with gusty winds of 20-30 mph possible across the western districts. Drier conditions and near to slightly above normal temperatures are forecast for later this week.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Flood Watch from late Monday night through Tuesday evening for AZZ534-537-541-542-545-547-549-552-555>558-560>563.

CA...None.