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Area forecast discussion
National Weather Service San Francisco Bay area
445 am PDT Tue Oct 22 2019

Synopsis...a warming and drying trend will continue through
the week as high pressure strengthens over California. Offshore
flow will continue through midweek and strengthen Wednesday night
into Thursday morning. This will result in warm temperatures
reaching all the way to the coast, with the warmest temperatures
expected on Thursday. Locally gusty offshore winds are likely at
times in the hills, especially in the North Bay hills on Wednesday
night. In the longer range, cooling is forecast during the
upcoming weekend, although another round of dry offshore winds
is possible by late in the weekend and into early next week.

&&

Discussion...as of 04:05 am PDT Tuesday...the overall impacts
message over the next few days has not changed much with this
forecast update. Simply put, ridge of high pressure will bring a
warming and drying trend to the region, but most importantly
elevate fire weather concerns to critical levels with gusty
offshore flow.

Current longwave pattern at 500mb shows a solid ridge of high
pressure anchored over the epac. The ridge is still on track to
gradually build and drift eastward over the next two days. At the
surface, high pressure will also build across the pacnw and
eventually the Great Basin. This pattern began to take shape on
Monday with temperatures well above normal for this time of year.
Temperatures were so warm that a few stations set daily record
maximum temperatures for Oct 21. Additionally, the sfo-wmc did
briefly ramp up to 8.4 mb, which led to gusty winds in the
hills early Monday night. Mt Saint Helena maxed out at close to
50 mph. Winds have diminished slightly, but still remain breezy to
gusty in the hills. It also GOES with out saying, these stronger
NE winds area rather dry with relative humidity in the 30-40%
range. These conditions are only setting the stage for a more
critical timeframe Wednesday night with stronger winds and lower
relative humidity, more on that later.

Based on what happened yesterday, building ridge, persistent
offshore flow, model trends - bumped up Max temperatures for
Tuesday. Highs will generally be in the 70s and mid 80s at the
coast and 80s to lower 90s inland - or about 10-15 degrees above
normal. Daytime temperatures will hold steady on Wednesday before
rebounding on Thursday with the strongest offshore flow.
Temperatures on Thursday will likely be the warmest of the week
with highs in the 80s at the coast and upper 80s to mid 90s
interior. Friday was low confidence on Max temps, but latest
guidance suggest another day of hot weather with lingering
offshore flow. Cooling trend finally develop over the weekend and
more so next week. Thermal belts will be in full effect the next
few nights with overnight lows mild in the hills due to offshore
flow.

Speaking of offshore flow there will be some Ebb and flow of the
offshore pressure gradient strength the next few days. None the
less, the period of strongest offshore flow in the short term is still
focused on Wednesday night into Thursday morning. Models have been
rather consistent with hitting the North Bay mts with the highest
winds. Local gusts up to 60 mph still seem plausible. East Bay
hills will see weaker winds, but a gust up to 40 mph still seems
like a possibility. The same can be said for the Santa Cruz mts
with gusts up to 30 mph. The set up definitely keeps the wind
threat more elevated in the hills and less in the valleys, but the
occasional gust in the valley could happen. Winds ease through the
day on Thursday and more so on Friday as the offshore flow
gradient weakens. The weakening gradient is a result of shifting
high pressure and a trough digging into the pac northwest Friday.

For several days now another offshore flow wind event has been
advertised on Sunday. The details, timing and strength, have
varied from model to model and run to run. However, one thing that
has stayed the same is another round of offshore flow. Latest 00z
models are still very robust with some suggesting the Sunday
offshore flow set up could be stronger than Wednesday night. For
now, will have another strong offshore flow event, focused on the
North Bay again, but not going overboard on the potential just
yet.

For specific fire weather details see below.

&&

Aviation...as of 4:45 am PDT Tuesday...light north to
northeasterly flow will maintain predominantly clear skies over
area terminals through the forecast period. Slightly stronger
winds are expected through the afternoon and evening hours.

Vicinity of ksfo...VFR.

Sfo bridge approach...similar to ksfo.

Monterey Bay terminals...VFR.

&&

Fire weather...as of 4:05 am PDT Tuesday...fire weather forecast
remains on track. Near critical fire weather conditions will
exist between now and early Wednesday with periods of gusty
offshore winds, lowering relative humidity and warming
temperatures. By Wednesday afternoon however the the offshore
gradient begins to increase resulting in increasing winds across
the North Bay mountains. Winds will gradually increase over the
East Bay hills and Santa Cruz Mountains Wednesday night and early
Thursday morning. As the winds increase, relative humidity will
gradually decrease with poor humidity recovery Wednesday night.
Therefore the Fire Weather Watch remains valid initially over the
North Bay mountains Wednesday afternoon and then East Bay
hills/Santa Cruz mts Wednesday night. Relative humidity really
plummets during the day on Thursday despite some weakening winds.
Given very low relative humidity and another day of hot temperatures decided to
keep the watch through peaking heating on Thursday. After
coordinating with neighboring forecast offices will let day shift
convert the watch to a red flag warning after morning coordination
calls have been completed with fire agencies and fire partners.
Do want to emphasize this is not an extreme event. It is fall and
the Bay area typically sees offshore flow events like this.

Now for round two - as mentioned above models have been suggesting
another and potentially stronger offshore flow event over the
weekend. Peak for the second would likely be Saturday night and
Sunday morning. If the forecast stays on track additional fire
watches/warnings will likely be issued. It's just one model run,
but if the 00z GFS would verify the sfo-wmc gradient is greater
than 20 mb. Needless to say stay tuned...

&&

Marine...as of 2:52 am PDT Tuesday...high pressure ridging over
the Pacific northwest will produce moderate northerly winds across
the coastal waters through midweek. Strongest winds are expected
across the northern outer waters. As high pressure builds in over
the Great Basin during the second half of the week, a thermal
trough will develop along the coast. This will result in light
offshore flow across the coastal waters. A moderate northwest
swell will continue through midweek.

&&

Mtr watches/warnings/advisories...
... Arena to Pigeon pt 10-60 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm from 5 PM
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm from 5 PM
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm from 3 PM

&&

$$

Public forecast: mm
aviation: CW
marine: CW
fire weather: mm

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