Scientific Forecaster Discussion
fxus66 kmtr 200555
Area forecast discussion
National Weather Service San Francisco Bay area
1055 PM PDT Thu Jul 19 2018
Synopsis...monsoon moisture is expected to spread from south to
north across the region late tonight and Friday, resulting in a
slight chance of high-based showers and thunderstorms. Meanwhile,
seasonably warm daytime temperatures will persist region-wide
with marine air keeping conditions cooler near the coast. A more
robust warming trend is then possible next week, especially for
inland areas, as high pressure builds over the region.
Discussion...as of 8:55 PM PDT Thursday...as has been the case
the past several days, today was another warm day inland, with
temperatures in the inland valleys from 5 to 10 degrees above
normal. Meanwhile, a persistent marine layer with a depth of about
1500 feet, along with continued onshore flow, kept coastal
temperatures near seasonal averages. No significant changes to
this temperature pattern are expected through the weekend and into
early next week.
Primary forecast concern in the short-term is monsoon moisture
that continues to flow up from the southeast. Moisture today was
primary confined to our east and southwest, with the bulk of the
monsoon moisture over the southern Sierra and well offshore.
Models generally agree that another pulse of mid/upper level
moisture and instability will move from southeast to northwest
across our area from late tonight through Friday. Based on the 00z
NAM, this incoming moisture/instability will not spread north as
quickly as previously forecast. Therefore, a forecast update
earlier this evening included confining slight shower/thunderstorm
chances only to areas south of Monterey Bay late tonight, to
areas south of San Jose on Friday morning, and to areas south of
Santa Rosa on Friday afternoon. Also, both the NAM and GFS agree
that convective precipitation potential will be highest across the
south and become less likely farther to the north.
An increase in mid/upper level clouds tomorrow will likely result
in slight cooling in most places. At the same time, look for this
increased moisture to result in warmer nights, particularly on
Drier southwest flow aloft will develop by the weekend, which will
bring an end to precipitation chances.
A large upper high that dominates the the southwestern United
States is currently centered near the New Mexico/Texas border.
Models agree that this high will slowly retrograde over the next
week and be centered over Southern California by a week from
today. This will likely mean warmer temperatures inland by the
middle of next week. But onshore flow is projected to persist
through the week, keeping coastal areas mild.
Aviation...as of 10:50 PM PDT Thursday...for 06z tafs...ceiling
onset occurred in the early evening at the Monterey Bay
terminals, while VFR conditions continue at all San Francisco Bay
area terminals. Marine stratus is moving into the coastal valleys
a bit slower than yesterday evening, but still think ceiling
development likely at all terminals but ksjc and klvk during the
overnight hours. Clearing Friday morning looks to occur by
midmorning around sf Bay and late morning Monterey Bay. The marine
layer depth is remaining pretty steady and shallow, with latest
coastal profiler data and ksql sodar indicating an inversion
height in the 900 to 1300 foot range. Some upper level moisture
and instability is projected to spread over the area from the southeast
on Friday and result in a slight chance of thunderstorms.
Vicinity of ksfo...low-to-moderate confidence in projected late-
night ceiling development. Higher confidence in VFR conditions
prevailing after about 16z Friday.
Sfo bridge approach...similar to sfo.
Monterey Bay terminals...high confidence ceilings will continue
through at least midmorning on Friday, but lower confidence in
details of height and visibility variation.
Marine...as of 08:57 am PDT Thursday...light to moderate winds
will once again prevail over the coastal waters. Winds will be
locally stronger north of Point Reyes. Winds along the coast will
turn southerly on Friday as low pressure develops off the Southern
California coast. Monsoonal moisture from the southeast will leave a
very slight chance of thunderstorms Thursday and Friday. A
southerly swell will arrive Saturday night and continue through
early next week.
... Arena to Pigeon pt 10-60 nm
Public forecast: dykema
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