Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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697 FXUS65 KPSR 221011 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 311 AM MST Sat Jun 22 2024 .SYNOPSIS... Thunderstorm chances and coverage will continue to increase this weekend, with today anticipated to be the more active across south-central Arizona relative to yesterday. Daily thunderstorm chances will continue through early next week, but become more confined to the higher terrain areas. Temperatures will continue to run several degrees above normal, although the warming trend through the middle of next week is becoming more muted, resulting in mostly Moderate HeatRisk. && .DISCUSSION... The upper level pattern continues to usher in mostly higher clouds early this morning from the remnants of former tropical cyclone Alberto, as the general center of this feature moves offshore of the far southern Baja Peninsula early this morning. A strong ridging feature centered over the Lower Mississippi River Valley is the main source of steering flow, which is bringing in these increased moisture levels and clouds into southern to southeastern Arizona. The latest MRMS radar indicates areas of light showers/virga across southern Arizona, with even a few embedded convective showers that are propagating into the southwestern Yuma and southern Maricopa Counties. Upon further model analysis, there is a weak inverted trough circulating around the ridge feature at the 700-500 mb levels, resulting in the initiation of these showers. Hi-res models suggest these light showers/isolated convection to persist into the morning hours and continue to stream further northward, ahead of more robust convection that is already evident on MRMS that is currently centered over the northeast reaches of the Gulf of California. These current events, along with PWAT`s continuing to increase upwards of 1.60"-1.90" are anticipated to result in a more active day across the region. While cloud cover may inhibit the thermodynamic potential (HREF ensemble means generally 500-1000 J/kg) a bit, the aforementioned shortwave should aid in overcoming this deficit. Additionally, steering flow will be much weaker due to the stronger influence of the ridge to the east, resulting in more favorable outflow propagation into the lower deserts this afternoon/evening. As such, hi-res models show a decent signal of storms that initiate in the higher terrain early this afternoon, with chances for thunderstorms increasing going into this evening across the lower deserts of south-central Arizona, marking the first legitimate chances to see thunderstorm activity across the metro for the monsoon. Chances for showers and storms are forecasted around 40-60% (20-30%) for the higher terrain (lower deserts) of southcentral AZ this afternoon and evening. While thunderstorms may be nice to see, the usual threats/impacts will continue to be a possibility across south-central and evening into western/southwestern AZ today and into tonight. HREF probabilities of 35 mph wind gusts around 50-70% across the Phoenix metro and reaching further westward into Yuma and La Paz Counties at around 30-50% going into the overnight hours. Thus, the severe potential (55+ mph) is seemingly lower relative to the past few days, which shouldn`t come as too much of a surprise due to the higher moisture levels and the resultant DCAPE`s around 1000-1200 J/kg. With these storms/outflows, blowing dust will continue to be a threat/impact as the outflows roll in from the north and (especially) east/southeast. It should be worth mentioning as well, that with higher moisture levels and weak steering flow, any localized downpours could cause some localized flood potential for today (some hi-res hourly QPF output upwards of 0.75"-1.00"). Looking past today, Sunday is looking like a less active day, as seen in the extended hi-res guidance. While another weak vort max looks to rotate around the ridge once again, steering flow looks less favorable as model soundings show more southerly to southwesterly mean flow in the low to mid-levels, which would limit chances to mostly higher terrain. As mentioned previously, despite the well above normal moisture levels over the weekend, the convective coverage on Sunday will be dependent on how the activity pans out today, as it is not that common to get widespread activity on back to back days. However, similar threats of locally heavy downpours and gusty winds will continue to be a threat for any storms that develop on Sunday as well. Looking towards next week, the ridging over central CONUS will slowly retrograde further westward, resulting in mid-level heights to build into the 594-597 dam range by the middle of the week. As such, temperatures near normal over the weekend will warm several degrees going through early and into the middle of next week. However, one noticeable difference with the latest ensemble suite is that there is seemingly slight changes in ridge positioning, resulting in slightly cooler temperatures. Thus, the current forecast is calling for primarily Moderate HeatRisk across the region, with NBM probabilities showing lower likelihoods of excessive heat (i.e. Major to Extreme HeatRisk) developing during this period. Looking past the middle of the week, ensemble clustering showing longwave troughing moving into the Pacific Northwest, resulting a pretty noticeable weakening of the ridging entrenched across the Desert Southwest. However, there are timing and depth differences, resulting in increasing uncertainties in temperature trends across the region going into late next week. Elevated moisture levels will continue through next week, with daily thunderstorm chances anticipated to continue as well. However, as PWAT`s wane back below 1.50" by the middle of the week, chances for any lower desert activity drops off precipitously past Monday (current forecast of 10-20%). Unless the ridge positioning and steering flow are ideal on any given day, convective activity will be become more confined to the higher terrain areas going through the end of the week. && .AVIATION...Updated at 0545Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: Storm activity over south-central AZ has pretty much ended. However, an upper level disturbance tracking westward across far northwest Mexico and brushing southern AZ may kick off some very weak showers/sprinkles late tonight into Saturday morning. Probability of SHRA/VCSH with that disturbance at any given TAF site too low to reflect in the TAFs at this time. But, anticipate an uptrend in mid-tropospheric cloudiness though ceilings will still be AOA FL120 if not FL150. Anticipate some thinning of clouds during the afternoon. Southeasterly breeziness will weaken and become light east/southeast or variable late tonight and continue that way through the morning. High resolution models have been depicting the development of storm activity over higher terrain just north and east of the Greater Phoenix area in the afternoon and in turn strong outflows overspreading the Valley floor roughly between 00Z-03Z. Probability of TSRA at the TAF sites is too low at any given TAF site this time to reflect in the TAFs. But, would anticipate KSDL and KDVT to have the best chance of a direct hit and/or VCTS. Southeast California/Southwest Arizona including KIPL and KBLH: Skies at the TAF sites will be mostly clear except for some minor mid and high clouds Saturday. Winds at KIPL will continue to be light before southeasterly develops near sunrise. SEly winds strengthen by midday (gusts 20-25kt). At KBLH, expect a familiar change from southwesterly early in the TAF period to nocturnal drainage winds (NWly) late tonight. Expect an uptrend in southerly breeziness (gusts to 25kts) starting by late morning. && .FIRE WEATHER... Moisture will continue to increase across the region through the weekend, bringing daily chances for afternoon thunderstorms, with the best chances expected across the higher terrain areas of south-central AZ. Best chances for thunderstorms across the lower deserts of south-central to western/southwestern AZ will be today. MinRH values this afternoon will range between 35-45% across the far eastern districts to between 15-25% across the western districts. MinRH values will remain elevated going into early next week before slowly drying into the middle of the week. Winds will follow their typical daily tendencies, with gusty outflows from any thunderstorm activity anticipated over the next several days. High pressure is expected to strengthen over the region by early to middle of next week leading to even hotter temperatures along with slightly drier conditions. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...Young AVIATION...AJ FIRE WEATHER...Young/Lojero