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May 28, 2021 – Dry weather with temperatures near or above average through the rest of the week. Hot conditions with moderate to high heat risk develop late weekend into early next week.

Discussion

The short term forecast remains on track this morning. The upper level trough over the PacNW will continue to lift into Canada today. This will be the start of a warming trend as near zonal flow is expected over NorCal today, with an eastern Pacific high building over the ocean. Models continue to develop a weak shortwave just off the coast of CA late today and into Saturday which will then push east into the southern Great Basin late Saturday into Sunday. The most noticeable feature with this trough will be the potential for thunderstorm activity over the Sierra, primarily from the Lake Tahoe region southward Saturday afternoon and evening. Sounding profiles show long skinny CAPE along with inverted V lower levels, gusty winds with some small hail will be possible with any of the stronger cells that develop. PWATS will also push to around 0.50″ and with little to no wind shear storm motion will be slow and brief heavy downpours can be expected before cells collapse on themselves.

Heading into Sunday, the eastern Pacific High will nose into the West Coast. Heights are forecast to be around 585-589 dm rising to near 590-593 by mid-week. This will bring our first triple digit heat to the region this year, with most Valley locations making ito the upper 90s to near 100 on Sunday. Hot temperatures will continue as we head into Monday as Tuesday. The majority of the Valley will likely be sitting at the 100-108 degree range on Monday, while the foothills and mountains (“coolest” at the higher elevations) soar into the 80s to near 100 degrees. In addition to these hot summer like conditions, overnight lows will be toasty as well. Valley and adjacent foothill locations will only fall into mid 60s to mid 70s. Thus the heat risk category will be high from Sacramento to Redding. An Excessive Heat Watch remains in effect Sunday afternoon and will continue into next week as additional warming is expected.

If you have outdoor plans over the holiday weekend you will want to be prepared for the heat. You can start practicing heat safety now, by looking before you lock your vehicle, wearing sunscreen, and staying hydrated to name a few tips. For more heat safety practices visit weather.gov/safety/heat.

If you’re wondering when does the Valley typically reach 100 degrees, check out these locations and dates.

Sacramento – June 18
Redding AP – June 4
Stockton – June 9
Modesto – June 11

Extended discussion (Tuesday through Friday)

5H Cluster Analysis indicates a positively-tilted ridge will prevail over the Ern Pac into Norcal early next week before becoming more neutrally-tilted as it builds inland by the middle of the week. A +2 to +3 standardized height and temperature anomaly is indicated by the NAEFS from Mon-Wed for the STO forecast area with much of the CWA dominated by a 2+ 850 mbs temp anomaly with temps in the mid 20s Deg C range. Deterministic models forecast 500 mbs heights in the 588-591 dam range.

This will be Norcal’s initial widespread triple digit heat wave of the season with the heat continuing into the middle of the week. Max temps in the 105-110 degree range are forecast across the Nrn portion of the Sac Vly Mon and Tue with Tue likely to be the hottest day.

Not yet convinced on the amount of cooling in the Srn Sac Vly on Wed given the 5H cluster analysis and the NAEFS mean temp anomaly, along with the deterministic models maintaining strong ridging over our area. Modeling does trend to climo sometimes too quickly. However, the Delta (Sea) Breeze may respond to the 2 to 3 days of intense heating in the Valley and may respond to the land-sea temp gradient resulting in a true Sea Breeze. With weakening pressure to the N of our CWA, and the seaward displacement of the Pacific High (ridge), this could be a “Class IIa” Sea Breeze which barely affects max temps at all, especially over areas farther away from the “Delta Influence”.

There is enough evidence to suggest more noticeable cooling on Thu as the ridge axis shifts into the Great Basin as a trof develops off the CA coast along 130W. An Excessive Heat Watch is in effect into Tuesday evening, and a heat product may be needed for the Nrn Sac Vly on Wed with max temps still near/above 105.

BTW most foothills and populated mountain locales will be well into the 80s and 90s during this heat wave. Overnight lows will also be mild/warm especially in the thermal belts where they will only fall into the mid 70s. ECMWF brings stronger cooling to the forecast area by Fri (Cluster 2) while the other clusters still maintain a little stronger ridging with the potential for near to slightly above triple digit heat in the Nrn Sac Vly while onshore flow provides cooling for much of the rest of the area.

Dry forecast for the area during the EFP with a low chance of convection on the fringes of our forecast area on Thu.