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FXUS66 KSTO 312233 AFDSTO

Area Forecast Discussion National Weather Service Sacramento CA 333 PM PDT Fri May 31 2024

SYNOPSIS
Slight cooldown Saturday through Monday before the return of triple digit heat in the Valley Tuesday through Friday. Hottest temperatures expected Wednesday. Slight chance of showers for Shasta and Tehama counties Monday afternoon.

DISCUSSION

Discussion at a Glance - Less hot Saturday-Monday with increased onshore flow and Delta Breeze.
- Areas of Major HeatRisk Tuesday for the Valley, becoming widespread Wednesday & Thursday for the Valley & Foothills with widespread triple digit high temperatures. Prepare now for elevated daytime highs and overnight lows and reconsider outdoor activities during the hottest part of the day (3:00-7:00 PM).
- Moderate HeatRisk next Friday, with a few isolated spots of Major HeatRisk in the Valley; Some triple digits in the northern Sacramento Valley (areas north and including Chico)
with cooler low to mid 90s in the Delta and Sacramento metro area as the Delta Breeze lowers temperatures.

Short Term Discussion

Today was a warm day, with high pressure moving over the area. Latest temperatures are 2 to 6 degrees higher than the same time yesterday.
Skies are mostly clear, except for over the higher mountains, where some fair weather afternoon cumulus have developed. Breezy northerly winds died down this morning. Some light onshore flow has begun in the Delta, with a Delta Breeze spreading inland bringing cooling into influenced areas.

Saturday, weak upper level troughing moving through the Pacific Northwest will help reduce high temperatures, as the axis of the trough will slide through NorCal. High temperatures for the weekend will be in the upper 80s to lower 90s for the Valley, and cooler mid 80s for the Delta. Breezy winds will be possible in the Delta, with wind gust speeds 15 to 20 mph. Further cooling is expected Monday, when another Pacific Northwest shortwave trough will eject from a closed low in the Gulf of Alaska and will brush through the area, with a westerly, onshore flow pattern. There is a slight chance (20-40%) of showers Monday for Shasta and Tehama counties, with highest probabilities in the mountains north of Redding, though there is a small chance of showers extending into the far northern Sacramento Valley. There is decent (100-200%)
Precipitable Water (PW) with this trough, however most of the Valley will remain dry as the trough will stay well north of our area. The trough will quickly vacate the area, and upper level ridging will begin to build in the eastern Pacific and spread eastward. Locally breezy conditions may exist for the Delta and Valley as the trough leaves the area, with wind gusts of 20-25 mph possible.

.EXTENDED DISCUSSION (Tuesday THROUGH Friday)...Tuesday will see upper level ridging moving inland, increasing heights and temperatures around the region. Daytime high temperatures in the upper 90s for the Valley, with widespread areas of Major HeatRisk in the Valley and extending into the foothills and portions of the Delta. National Blend of Models (NBM) probabilities of reaching 100 for the area are around 50-75% on Tuesday. Peak temperatures are expected Wednesday with the ridge firmly over the area, with hot temperatures continuing Thursday. An Excessive Heat Watch has been issued from 11 am Tuesday to 8 pm Thursday. The Delta Breeze could bring some slightly less hot weather to influenced areas, and how strong this effect will be should become more clear over the next few days.

Major HeatRisk affects everyone without effective cooling and/or adequate hydration. Consider canceling outdoor activities during the heat of the day. Clusters show there is some uncertainty between ensembles over the strength and location of the ridge late in the week. As a result the NBM has a large spread between the 25th and the 75th percentiles, which means a large range of possible high temperatures for late in the week and into the following weekend. Currently widespread Moderate HeatRisk is expected for Friday, with isolated Major HeatRisk for the Valley/foothills.

AVIATION
VFR conditions expected over the next 24 hours.
Sustained surface winds under 12 kts except winds 15 to 25kts in the Delta after 00Z this evening.

STO WATCHES/WARNINGS/ADVISORIES
Excessive Heat Watch from Tuesday morning through Thursday evening for Carquinez Strait and Delta-Central Sacramento Valley- Motherlode-Mountains Southwestern Shasta County to Western Colusa County-Northeast Foothills/Sacramento Valley-Northern Sacramento Valley-Northern San Joaquin Valley-Shasta Lake Area / Northern Shasta County-Southern Sacramento Valley.

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