Threat of Strong Thunderstorms Tuesday

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Quick post (I can’t seem to find enough time on my hands as of late for a full post) on the thunderstorm threat looming on Tuesday, March 21st.

Mother nature will be celebrating spring on Tuesday with fireworks of her own (potentially) across northern Tuesday as a trough swings a cold front through overnight Monday/early Tuesday, becoming negatively tilted through the day Tuesday as a stronger jet streak rounds the base of the trough helping it get oriented further and further eastward from its origin in the Gulf of Alaska. By mid-morning Tuesday, the back edge of the cloud shield/primary cold front will swing through the central valley allowing some post-frontal clearing by late morning — but the frontal band of showers itself could be convective in nature, with some embedded thunderstorms possible as it moves through the valley between 8am and 11am west to east, respectively. Precipitation from this band of showers will help saturate the low levels in the morning, creating a pooling effect boosting surface dew points into the low to mid-50s across the Sacramento valley, with localized pockets of dews in the low 60s in the San Joaquin valley.

High resolution NAM’s forecast surface temperatures, valid at 2pm Tuesday. Yellow & orange suggests temperatures in the 60s and 70s — mostly 60s in the central valley.

This amount of low-level moisture combined with expected breaks in cloud cover behind the front should promote some (relatively) warm surface temperatures, too, which when warmed beneath the cold air aloft thanks to the trough overhead will boost instability bigtime. High resolution (and even medium resolution) models suggest CAPE (an instability parameter that stands for convective available potential energy) values anywhere from 500 – 1000 j/kg, that 1,000 being on the high end of the spectrum by California’s standards. Some models even suggest values in excess of that in parts of the valley, but have fluctuated a bit. Overall, models have been consistent showing a potential band of strong to severe storms in the early afternoon from around the Sacramento area southward to perhaps Fresno, with the maximized threat from around Merced to Elk Grove.

0z NAM’s forecast CAPE, 500mb heights, and surface wind (vectors/arrows), valid at 2pm Tuesday. Green areas depict CAPE values in excess of 750 j/kg, with deeper greens suggesting 1000+ j/kg (notable in central Sacramento valley).

Additionally, through the day, a stronger (50 – 80 knot) mid/upper-level jet streak will also round the base of the trough out of the southwest, while a strong low-level jet backed out of the southeast in the interior Sacramento & San Joaquin valleys will create favorable shear for rotating thunderstorms across much of the central valley. It’s where the best lift and instability also happen to appear where the thunderstorm risk gets maximized. Right now, the best lift appears to be in the southern Sacramento valley into the central and northern San Joaquin valley. However, in the north, less storm coverage could allow one or two more discrete storms to develop. If anything were to develop north of Sacramento — less surrounding storm interference could prove to be a good thing.

Threats with strong storms Tuesday will range from large hail to perhaps a couple funnel clouds or a brief tornado — with strong/damaging wind also possible given a 35 – 45 knot low-level jet available for tall/strong storms to utilize. The best severe threat, as mentioned will likely be from Sacramento south, however on an isolated basis it could still exist in the north.

Storm chasers: if you live south of Sacramento, Merced would be my target, while if you live north of Sacramento, somewhere near the Sutter Buttes could be a good start point.

The northern/central Sacramento valley may have a hard time getting thunderstorms developing — however if they do, they’d likely be more discrete and have good potential if something does go up. The San Joaquin valley has both good shear & instability with all high resolution models showing lots of potential strong storms, however outflow could become an issue if too many storms go up.

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