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NWS Technical Weather Discussion


FXUS66 KMTR 172125
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
225 PM PDT Tue Jul 17 2018

.SYNOPSIS...Seasonably warm and dry weather will continue across 
the interior with marine air keeping conditions cooler near the 
coast. Monsoon moisture will move across the region from late 
Wednesday through Friday which may bring a slight chance of high- 
based thunderstorms to the Central Coast on Thursday. Otherwise, 
look for an increase in high level clouds and continued warm 
temperatures through late week.

&&

.DISCUSSION...as of 02:24 PM PDT Tuesday...A strong ridge of high
pressure remains parked over the region this afternoon with the 
interior already warming into the 80s and 90s. Meanwhile, a marine
layer of around 1,500 feet in depth has kept conditions much 
cooler near the coast with 50s to lower 60s being reported where 
cloud cover persists. Expecting little change in the pattern 
during the next 24 hours with low clouds likely to penetrate back 
into the valleys overnight which will linger into Wednesday 
morning. Temperatures aloft are likely to continue to warm and 
result in slightly warmer conditions for the interior by Wednesday
afternoon. Given limited cooling in the hills and hot afternoon 
temperatures, cannot rule out a few locations in the higher 
elevations across the interior reaching moderate to high heat risk
levels. However, the threat will not be widespread and not likely
reach Heat Advisory criteria, especially across any urban 
locations. 

The ridge is then forecast to shift a bit inland to the northeast by 
late Wednesday and Thursday which will allow the flow aloft to 
switch to the southeast. This will bring an increase in monsoon 
moisture back over the region with a slight increase in instability 
as well, especially over the Central Coast on Wednesday. However, 
the moisture is forecast to be greatest offshore and above 500 MB 
which will limit the potential for convection. Regardless, feel 
there is enough confidence to maintain a slight chance of 
thunderstorms over the southern portion of the region on Thursday. 
Any precipitation that does develop will be high-based which will 
likely evaporate as it falls through a much drier air mass above the 
marine inversion. Thus, dry lightning would be most likely with any 
isolated thunderstorms that may develop. Something worth monitoring 
in the coming days. The end result may only be an increase in 
mid/high level clouds region-wide late Wednesday into early Friday. 
This too would likely hold temperatures inland down by a few degrees 
yet also bring more humid conditions to the region through late 
week. 

The upper level flow over the region will gradually shift back to 
the southwest during the upcoming weekend under persistent high 
pressure. This should result in little day-to-day changes with 
afternoon temperatures slightly above seasonal averages, especially 
across the interior. Also, look for some presence of a marine layer 
which will result in cooler conditions at the coast along with late 
night/morning low clouds.

&&

.AVIATION...As of 10:38 AM PDT Tuesday...For 18z TAFs. Terminals
clearing and expecting VFR for much of the afternoon. Gusty to
breezy onshore flow will develop by late this afternoon. Expect
another night of low clouds and some patchy fog pushing back
inland tonight...similar to this morning.  

Overall confidence is medium to high.

Vicinity of KSFO...VFR this afternoon. Breezy onshore winds 
through this evening, sustained 15-20 kt. Cigs will eventually
return overnight through tomorrow morning.

SFO Bridge Approach...similar to KSFO .

Monterey Bay Terminals...VFR through the afternoon. Light to 
moderate west/northwest winds, 10-15 kt. Stratus return again 
early this evening. 

&&

.MARINE...as of 08:47 AM PDT Tuesday...West to northwest flow 
will persist over the coastal waters today. Building high pressure
will lead to slightly stronger winds later today and tonight. 
Winds will be strongest north of Point Reyes. Northwest swell will
increase by this evening and continue through the week. A long- 
period southerly swell is forecast to arrive this weekend.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Arena to Pigeon Pt 10-60 nm

&&

$$

PUBLIC FORECAST: RGass
AVIATION: MM
MARINE: MM

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Prepared by Boulder-Creek.com Weather at: Tue Jul 17 16:30:01 PDT 2018

From the National Weather Service


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Revised: 29 Mar 2008 09:53 -0800 GMT (Pacific)