Preview of NWS' New Version of Forecast
This preview is not operational and should not be used for support decisions.

Area Forecast Discussion (AFD)

Issued by Sacramento, CA (STO)

                            
000
FXUS66 KSTO 152150
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
250 PM PDT Sun Sep 15 2019


.SYNOPSIS...
Pattern change will lead to significantly cooler temperatures and
a chance of showers and thunderstorms late this evening into
Monday. Another system could bring additional showers by midweek.
Dry weather and warm temperatures return late this week.

&&

.DISCUSSION...
An upper level low pressure system is approaching the West Coast
this afternoon, bringing cooler temperatures to interior NorCal.
High temperatures today will peak around 5 to 10 degrees cooler 
than yesterday with Valley temperatures in the mid to upper 80s
and mountain temperatures in the 60s and 70s. Winds have begun to
increase this afternoon with gusts up to 20 to 25 mph, locally 
higher over the Delta region, observed across the Valley and 
Sierra.

Southwesterly winds will increase in advance of the trough. Gusts
up to 30-40 mph will be possible near the Sierra crest this 
afternoon and Monday. This could result in a brief period of 
enhanced fire weather conditions across the Sierra today, though 
the expected precipitation should help mitigate fire weather 
concerns. 

Deterministic and ensemble guidance indicate rain showers
spreading into Shasta County and the Coastal Range late this 
evening, then reaching the I-80 corridor around mid morning Monday
into the early afternoon hours. Forecast soundings indicate the 
potential for isolated thunderstorms on Monday, generally from 
Sacramento northward including the Valley and higher terrain. At 
this point, the most likely timeframe for convection appears to be
late morning into early evening. Forecast rainfall totals are 
around one tenth to half an inch, and up to one inch over the 
mountains. Locally higher amounts are possible if storms develop. 
Snow levels should remain relatively high as bulk of precipitation
moves in, but could briefly lower to around 6500 ft in 
convection. Coolest temperature readings of the week are expected 
to occur on Monday, with daytime highs as much as 10 to 25 below 
average.

The upper-level trough is forecast to shift east of the state by
early Tuesday, ending precipitation threat across interior NorCal.
However, this dry period should be short-lived as another system
takes aim at the West Coast by midweek. At this point, it appears
that precipitation will approach the northwestern portions of the
forecast area by late Tuesday night/early Wednesday, with best
chances over the foothills/mountains. In addition, guidance
suggests the potential for thunderstorm development at least over
higher terrain Wednesday afternoon. Below normal temperatures 
will continue on Wednesday, with Valley highs in the mid 70s to 
low 80s.

&&

.EXTENDED DISCUSSION (Thursday THROUGH Sunday)...
On Thursday morning, an upper trough will be in the process of 
exiting the region while it reaches the Desert Southwest by the 
evening hours. Some lingering mountain showers are possible 
although these should become more scattered in nature by later in 
the day. Deep-layered northerly flow will follow the trough 
passage with such conditions continuing throughout Friday. A brief
period of height rises are likely for Saturday although some 
ensemble members show a trough nearing the West Coast late next 
weekend. Ensemble means remain at odds so will maintain a warming 
trend in the forecast. From Thursday to Sunday, highs will likely 
rise by around 10 degrees which takes Valley temperatures to near 
90 degrees. ~BRO

&&

.AVIATION...
VFR conditions next 24 hours except local MVFR possible vicinity 
showers northern mountains after 06Z Monday. Local surface wind 
gusts 15-25 kts Central Valley, especially 22Z-03Z. Areas 
southwesterly surface wind gusts 25-35 kts higher elevations 
northern Sierra Nevada after 18Z. &&

.STO WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$