Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 250525
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
1025 PM PDT Sun May 24 2020

.Synopsis...
High pressure will continue to build over the region with dangerous
heat forecast for later this week. The hottest days Tuesday through
Thursday with moderate to high Heat Risk expected. A pattern change
is expected on Friday, which will bring cooler temperatures and
chances of widespread mountain showers and possible thunderstorms
over the weekend.

&&

.DISCUSSION...
High pressure will build/amplify over CA this week resulting in a
prolonged period of very hot (if not dangerous heat for sensitive
groups) for interior NorCal. As a result, the excessive heat watch
was upgraded to an Excessive Heat Warning for elevations below 3500
feet over most of our CWA.

Tomorrow, Memorial Day, valley highs are forecast to be in the upper
90s, perhaps even a 100 degrees in a few locations. The foothills
are forecast peak in the mid 80s to mid 90s, while the mountain
highs range from the 70s to mid 80s. While Monday will not be the
hottest day this week, it will be the first day of a prolonged heat
event. If you plan on enjoying the outdoors this holiday, be sure to
practice heat safety and a cold water safety if visiting area
waterways.

Looking forward in the week, heights are forecast to continue
building, allowing for further heating into mid-week. Forecast 850mb
temperatures in the 22-26C range should support the first widespread
triple digits of the season Tuesday and Wednesday. NAEFS shows an
850-mb temperature return interval of 10-30+ years, indicating the
unusual nature of this excessive heat event. Thus, Valley highs will
range from 100 to 104 degrees on Tuesday and peak around 106 degrees
on Wednesday.

Several locations may tie or exceed daily high temperature records,
especially on Wednesday. NBM shows a 96% chance of tying and/or
breaking record maximum temperatures on Wednesday at Sacramento
Executive AP, 95% at Modesto, 88% at Red Bluff and 84% at
Stockton. In addition, little overnight relief is expected, with
morning lows in the mid 60s to mid 70s; locally in the upper 70s.
These overnight lows will run 10 to 20 degrees above seasonal
normals.

With the upcoming heat wave, the general population should take
precautions to prevent heat related illnesses given the potential
for moderate to very high heat risk. Stay hydrated, limit outdoor
activities, and check on sensitive groups.

Area waterways continue to run very cold and fast, and precautions
should be taken if heading to the water.

&&
.EXTENDED DISCUSSION (Thursday THROUGH Sunday)...

A well established omega block remains in place into Thursday and
Friday with anomalous ridging centered over the southwestern
U.S. To the western periphery of this blocking pattern, a rather
deep upper low over the subtropical east Pacific will likely enter
the picture by Friday into next weekend. Until then, blazing heat
will continue to be the norm as widespread low to mid 100s
blankets the Central Valley. As with previous days in this
forecast period, daily records will be challenged across the
region. There will be little escape from the heat in the Delta and
foothills as readings soar into the low 100s while mountain
locales reach the 70s and 80s. Compared to typical late May
climatology, region-wide highs will sit around 17 to 23 degrees
above average. Unfortunately little escape can be found overnight
as lows stay in the upper 60s to low 70s, perhaps even nearing the
mid 70s across the Sierra foothill thermal belts. An exception to
this rule would be the more Delta influenced locations which could
see overnight readings into the low to mid 60s. Overall, Heat Risk
levels remain in the high to very high category over the Central
Valley. This means extended outdoor exposure to these elements
will be dangerous, especially for the vulnerable population.

Aside from Delta influenced locations, Friday has been trending a
bit warmer given the slower approach of the offshore cut-off low.
Triple digit heat should continue in the northern Sacramento
Valley with expected highs between 100 and 103 degrees. The region
spanning the Delta into the greater Sacramento area can expect a
10 degree cool off given the increase in west-southwesterly flow.
Some mountain showers and a few thunderstorms are also possible
given increasing moisture aloft with the offshore low. Increased
cloud cover should also shave off several degrees on temperatures
compared to Thursday.

Heading into next weekend, the influence of the mentioned upper
low should be well underway. Temperatures are likely to be about
20 degrees cooler than where they were earlier in the week. The
overall coverage of shower activity with this upper low will
depend heavily on its strength and timing. Its track is favorable
to draw mid/upper-level moisture northward from the subtropics
toward the West Coast. Have maintained a chance of showers into
the weekend across the mountains while question marks remain over
the lower elevations. Some deterministic models drop precipitation
into the Valley but will await future model runs to address this
issue. ~BRO

&&

.AVIATION...

VFR conditions over interior NorCal continue over the next 24
hours. Northerly wind gusts 10 to 15 knots are possible through
02Z.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Excessive Heat Warning from 11 AM Monday to 7 PM PDT Thursday for
Carquinez Strait and Delta-Central Sacramento Valley-Northern San
Joaquin Valley-Motherlode-Mountains Southwestern Shasta County to
Western Colusa County-Northeast Foothills/Sacramento Valley-Northern
Sacramento Valley-Southern Sacramento Valley.

Excessive Heat Warning from 7 AM to 7 PM PDT Thursday for Shasta
Lake Area / Northern Shasta County.

&&

$$



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