Scientific Forecaster Discussion
fxus66 kmtr 291609
Area forecast discussion
National Weather Service San Francisco Bay area
909 am PDT sun Mar 29 2020
Synopsis...scattered rain showers will spread inland through
midday as a frontal boundary moves across the region. Dry conditions
will develop tonight and persist through much of next week, aside
from light rain potentially over the North Bay on Monday. Below
average temperatures will continue and begin a slow warming trend by
Discussion...as of 9:09 am PDT Sunday...forecast remains on
track this morning. Visible satellite shows spin of clouds just
offshore as upper low and surface boundary get ready to pass
through the Bay area. Radar showing scattered showers over land
with heavier showers offshore getting ready to move inland from
late morning through early this afternoon. Freezing levels around
5000 feet and small hail possible with heavier showers. Precip
will end by this evening with dry weather overnight though valley
fog formation is likely given recent rains, high boundary layer
relative humidity and light winds expected.
Another weak front approaches norcal by early Monday and 12z NAM
is holding onto light rain chances for coastal and northern Sonoma
County, even into northern Napa on Monday. No significant
rainfall expected up there but forecast will account for continued
precip chances for that part of the district while everyone else
Long range models now show a more zonal/progressive/NW type flow
for the upcoming work week. That results in dry weather and temps
near normal with perhaps some slight warming later in the week.
Generally looking for daytime highs mid 60s to mid 70s. Some upper
30s still possible for the coldest interior valleys but not
expecting any frost potential for crop impacts this week.
Longer range trends suggest renewed rain chances by later next
weekend into the week of April 6th with the long range cfs model
implying above normal precip from mid to late April.
Previous discussion...as of 03:04 am PDT Sunday...a mid/upper level
trough approaching the region will result in an increase in rain
showers through midday as a frontal boundary sweeps inland. These
showers will likely produce anywhere from 0.10" to 0.25" with
locally greater totals of around 0.50" within heavier rain
showers. Precipitation will tapper off from northwest to southeast
this afternoon as the boundary pushes inland and will likely end
region-wide by late this evening. Meanwhile, temperatures will
remain below seasonal averages with daytime highs topping out in
the upper 50s to lower 60s.
Another weather system pushing inland to the north will allow for
light precipitation over northern California and as far south as the
North Bay on Monday. However, most locations south of the Golden
Gate will remain dry and see the start of a gradual warming trend.
The warming trend will continue region-wide through at least
midweek as high pressure builds in from the Pacific and the storm
track remains well to the north.
The latest operational forecast guidance and ensemble members are
keeping temperatures near to slightly below seasonal averages
through the remainder of the week. This is as the ridge of high
pressure does not appear to strengthen as much as previously thought
with broad troughing over the west. However, dry conditions are
likely to continue across our region through at least late week.
Beyond the current 7 day forecast period, the ensemble mean from
both the European model (ecmwf) and GFS suggest wet and unsettled weather conditions
will return the week of April 6th. This will likely be accompanied
by cooler than average temperatures as well as weather systems drop
southward down the West Coast. This is also reflected in the climate
prediction center's 8-14 day outlook with higher probability of above
normal precipitation across much of California.