180
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.
&&

$$