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FXUS66 KSTO 132124 AFDSTO

Area Forecast Discussion National Weather Service Sacramento CA 224 PM PDT Mon May 13 2024


SYNOPSIS
Unseasonably warm weather will result in areas of moderate heat risk through midweek. A few isolated, late day showers or thunderstorms will be possible over the Sierra through Tuesday.
Otherwise, mostly dry and breezy conditions look to accompany this warm stretch.

.DISCUSSION (Today through Thursday)...

Dry with warm temperatures will continue today, running only slightly lower than yesterdays highs. The ridge of high pressure currently off the CA/OR coast will continue to amplify and shift eastward through mid week. This will lead to increased heights/northerly flow through mid week across NorCal, which will help further dry and heat up the region. A weak closed upper-low off the SoCal coast will slowly drift eastward through the week, keeping onshore flow in the Delta region and limiting peak heating aloft for the San Joaquin Valley at least through midweek.

Hottest and driest conditions are expected in the central Sacramento Valley where temperatures have between a 25-70% of reaching 100 degrees and minimum RHs are less than 20%. These conditions coupled with some breezy northerly winds with gusts around 20-25 mph Wednesday, will lead to locally elevated fire weather conditions. Fuels are currently below or near average ERC levels and fire managers reported fine fuels are starting to cure, but are overall patchy with some green areas. Even so, it's that time of year and this pattern will help continue to cure those fine fuels through midweek.

Flow through the Delta will help to keep temperatures cooler further south, with forecast highs remaining in the low to mid 90s. Not a lot of change is expected on Thursday with maybe slight cooling for areas with a Delta influence as the Delta breeze is expected to increase starting late Wednesday/Wednesday night.
Moderate heat risk is expected Tuesday through Thursday for most of the central valley with the exception of areas with a Delta influence that will have a minor heat risk.

Given the warm unstable air aloft, instability is fairly high across the Sierra each day, but modest capping inversions which will help limit pop-up convection along the crest today. Where the cap (CIN) is weakest the daytime heating will likely overcome the cap and allow for isolated showers/thunderstorms the next couple of days. Best chances today will be along the Sierra crest mainly south of Highway 50. For tomorrow, the capping inversion will be modest/marginal tomorrow (CIN of -10-20 J/kg) with 1000+ J/kg CAPE, so some hires models have the instability overcoming the CIN with more isolated showers and thunderstorms. Even so, there is no significant forcing mechanism other than the lift provided by the mountains themselves. Given this, confidence on coverage tomorrow on the Sierra is medium with the uncertainty of whether enough lift and instability will overcome the capping inversion. If storms do develop, they will be slow moving and fairly short lived as the drift south to southwest given the flow aloft is weak north/northeast flow. As such, the main impacts will be brief heavy rain, lightning, gusty winds and potentially small hail with the stronger updrafts. Drier weather is currently forecast for Wednesday and Thursday.

//Peters

.EXTENDED DISCUSSION (Friday THROUGH Monday)...

Ensembles and clusters indicate an area of ridging over the Pacific Northwest is being pushed westward keeping northwest flow aloft Friday into Saturday. Dry weather persists and high temperatures will remain above normal for this time of year, but the weakening of the ridge and intrusion of a cutoff low to our south off the coast of Southern California will promote a cooling trend that continues through the weekend. Over the weekend, this cutoff low will transport enough moisture over the Sierra to bring the potential for isolated showers/thunderstorms. Currently the National Blend of Models (NBM) indicates a 15-20% chance of isolated mountain thunderstorms along the Sierra crest Saturday- Monday.

AVIATION

VFR conditions over interior NorCal over the next 24 hours with a 15 to 25% chance of isolated mountain thunderstorms south of US 50 between 19Z-04Z. Surface winds will remain below 12 kts.
Through the Delta, southwesterly winds sustained 15-20kts with gusts 20-25 kts between 00Z-14Z Tuesday. The threat for mountain thunderstorms returns tomorrow afternoon/evening.

STO WATCHES/WARNINGS/ADVISORIES
None.

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