AWUS01 KWNH 202030
FFGMPD
CAZ000-210600-
Mesoscale Precipitation Discussion 1161
NWS Weather Prediction Center College Park MD
329 PM EST Wed Nov 20 2024
Areas affected...Northern California...
Concerning...Heavy rainfall
Valid 202030Z - 210600Z
SUMMARY...Strong, persistent heavy rainfall signal continues into
north-central coastal Range and starting to bleed over to the
northern Sierra Nevada range with times of .5-.75"/hr rates and
additional 3-4" totals by 06z.=20
DISCUSSION...GOES-W WV suite depicts a weak baroclinc leaf with
transverse banding lifting northeast toward and starting to
intersect with the SW Oregon coast, this is indicative of a
broadening right entrance to the polar jet and short-wave ridging
across northern California. As such, the forward progression of
the front has fully stalled with weak rain-cooled support pressing
the pre-frontal convergence/isentropic band across Sonoma/Santa
Rosa counties at this time. The upstream edge of the divergence
pattern aloft well-offshore is helping to spur a weak surface low
along the front which will help lift the best moisture flux
convergence axis northward after 23-00z with some increased
southerly flow at the coastline, shifting the axis back to
Menodcino county later in the valid time period (03-06z).=20
Recent surface observations have seen dew-points rising through
the San Francisco Bay and filtering into the Sacramento Valley,
suggesting the warm front has over-topped or is near over-topping
the coastal range. CIRA LPW hints at this as well with .5-.75" at
the sfc/850mb layer pressing through the north-central CA coast.=20
LPW also denotes better alignment through depth and so moisture
totals are reaching 1.3" and nearing 1.5" just upstream off-shore.
Total IVT ranges are about 750-900 kg/m/s along and just offshore,
given said moisture and continued solid 50-60kts of fairly
orthogonal 850mb flow in the core of the PW plume. As such,
.5"/hr rates in the coastal range are increasingly probable
supported by 50-75% neighborhood probability along the coastal
range wavering north/south with the passing surface inflection
toward 03z. Totals across the coastal range vary from 2-4.5" and
soils are starting to fully saturate. FFG values along the spine
are about 1.5"/3hrs and given the recent saturation, it is
becoming increasingly plausible of exceedance but probably
probability still remain just below threshold.=20
As mentioned, deeper moisture is filtering through the Sacramento
Valley, though cooler and drier air remains coincident with the
front range, this further intensifies the isentropic ascent plane
increasing verticality of of the showers. Core of the plume still
appears directed at the Butte/W Plumas county slopes, and HREF
probability of 1"/hr reach 15-20% suggesting a spot or two for a
few hours are likely to reach .75"/hr of rainfall, peaking in the
23-01z time frame. Even IVT values are in the 600+ kg/m/s range
supporting this potential.Persistence of southwesterly warm-air
advection will press freezing levels steadily up slope as well,
broadening the area of moderate to heavy rainfall in steeper
terrain. Similar but slightly higher 1.5-2"/3hr FFG values exist
across this area and while there is increased potential for higher
intensity rates, the duration of moderate to heavy rainfall
pre-cursory to this surge has been limited relative to the coast.=20
While a spot or two of FFG exceedance is possible, it will need to
be monitored closely...but will continue to maintain a Heavy
Rainfall tag for this MPD valid time.
Further north toward Cape Mendocino into the SW slopes of the
Trinity Range...
The core of the moisture will continue to be focused south, but
given proximity to the stationary front/triple point, convergence
should be maximized to allow for similar occasional upticks to
.5"/hr. Given this area has the climatologically wetter climate
and higher natural FFG values, the risk for flooding will continue
to be better accommodated than further south. Though and
additional 2-3" rainfall expected; this further saturation will
result in greater run-off further setting the stage for expected
activity over the next coming few days.
Gallina
...Please see
https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!9xDAmHd0dfUfszK6USAAuGP2uXiKZFEHFeccEVhVueDyl15euGpcO2pRhLV0bGHSAjo9= jRsvBSK4K2yv0Y6fPc8nx7I$ for graphic product...
ATTN...WFO...EKA...MTR...STO...
ATTN...RFC...CNRFC...NWC...
LAT...LON 41782411 41612388 41312362 40982347 40982270=20
41072204 40152159 39422087 38702061 38422091=20
38292172 38102232 37872287 38332328 38832377=20
39622397 40022422 40352452 40642443 41242428=20
41672428=20
=3D =3D =3D
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