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Showers and weak storms continue to stream northward over Imperial County, California resulting in cloudy skies and much cooler temperatures than originally forecast this afternoon.

Above normal temperatures will continue through the forecast period with Extreme heat Warnings in effect for portions of the area through mid week.

Typical monsoon conditions are expected into next week with daily thunderstorm chances favoring Northern Arizona high terrain and Southeast Arizona and low chances in the South Central Arizona lower deserts.

After a slight drop in temperatures during the first half of the coming work week, high pressure will strengthen over the region once again late in the week, resulting in another period of unseasonably hot temperatures and widespread Major Heat Risk.

Short Term

A couple remnant weak vorticity centers which formed from convective activity over northern Sonora yesterday evening have passed into SoCal from the Gulf of California this morning, forcing showers and several decaying storms ahead of them into the western half of Imperial County into this afternoon. This has resulted in considerable cloud cover and rain cooled air surging through the Imperial Valley, leading to much cooler temperatures than originally forecast. Clearing will occur on the backside of these vorticity centers over the next few hours, resulting in a quick warmup into the triple digits within the Imperial Valley, but highs will likely struggle to reach 105F in the typically hotter spots. Elsewhere across the forecast area, mostly clear skies prevail, and intense surface heating under anticyclonic subsidence will result in highs between 108F-113F for the lower deserts.

Over the next few days, a low grade monsoonal pattern will continue to evolve, with relatively weak easterly to southeasterly flow maintained over the forecast area as a broad subtropical high meanders over Colorado/NM. Near-surface mixing ratios will hover between 9-11 g/kg, with dewpoints in the upper 50s to lower 60s across the lower deserts. Daily thunderstorm development over the Northern/Central AZ high terrain and prominent terrain features of Southeast AZ will continue to occur, with some variation in coverage and intensity day to day, however, prospects for lower desert storms across South-Central AZ are not great. While moisture is marginally supportive of lower desert storms, the threat is highly dependent on a forcing mechanism, such as a strong, organized outflow boundary coming down from the Rim/high terrain east of Phoenix or from Southeast AZ or mesoscale features (e.g., MCVs from prior day convection). Additionally, forecast soundings indicate MUCAPE values mostly staying below 1000 J/kg and a modest cap with convective cap staying mostly <-25 J/kg. Still, a few storms may develop over terrain features of La Paz County and the Table Top area in southern Maricopa County this afternoon and over the next few days.

Dangerously hot temperatures with little overnight relief continues to be the primary hazard as we head into the first half of the coming work week. Ensembles remain in excellent agreement that H5 heights will decrease from RAP analyzed 597-598 dam today into a 594- 596 dam range by Tuesday as the subtropical high weakens somewhat and remains centered solidly to our east. However, the very warm air mass in place will not be dislodged, and thermal profiles do not substantially cool as heights decrease. NAEFS mean 850-700 mb temperatures between the 90th and 98th percentile of climatology will result in daily highs peaking between 108F-114F across the typically hotter lower desert locales, between 2F-8F above daily normals. The decent boundary layer moisture remaining in place as well as debris clouds moving downstream and over the forecast area from high terrain T-storm activity each day will make for less effective radiative cooling overnight, and so forecast lows are also elevated, remaining very warm from the middle 80s to lower 90s. Temperatures drop a few degrees heading into Tuesday/Wednesday, however, some of the hottest lower desert locations remain in the Major HeatRisk category, and so Extreme Heat Warning have been extended for the Phoenix Metro, Yuma area, Lower CO River Valley, and Imperial Valley through Wednesday. More expansive headlines will likely be needed by late week as the subtropical high moves closer and restrengthens.

Long Term

By the latter half of the week, most global ensembles support notable strengthening of the high pressure, with 600dam H5 heights materializing and the center of the anticyclone shifting to over northern AZ/southern UT. The primary impact of this outcome will be another extreme heat event with a return of widespread Major HeatRisk and area of Extreme HeatRisk, beginning as early as Thursday and the heat likely peaking on Friday and Saturday. There is already high enough confidence to warrant extreme heat headlines and the issuance of an Extreme Heat Warning, but it is still being considered whether to just extend the current Extreme Heat Warning through the brief reprieve Tuesday-Wednesday or separate the events. This forecast outcome will also likely add more subsidence over the area and subdue afternoon convection. Global ensembles also show a slow downtrend in precipitable water, falling under 1.5" by Thursday-Friday. So, this is not great for the prospects of seeing storms push into the lower deserts. Latest the blend-of-models forecast keeps slight chance of rain focused in the AZ high terrain. The one potential outcome that could increase chances for storms into the lower deserts however is if the anticyclone can reposition closer to southern NV, in a Maddox Type II position, to provide stronger E-NE winds over the Rim.