Subj : HVYRAIN: Excessive Rainfall Discussion
To   : wx-storm@lists.illinois.edu
From : COD Weather Processor
Date : Mon Jan 29 2024 15:23:10

FOUS30 KWBC 291523
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1022 AM EST Mon Jan 29 2024

Day 1
Valid 16Z Mon Jan 29 2024 - 12Z Tue Jan 30 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Hurley/Kleebauer


Day 2
Valid 12Z Tue Jan 30 2024 - 12Z Wed Jan 31 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Hurley

Day 3
Valid 12Z Wed Jan 31 2024 - 12Z Thu Feb 01 2024

....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
NORTHWEST CALIFORNIA, INCLUDING THE COASTAL RANGES, PARTS OF THE
SACRAMENTO VALLEY, AND THE NORTHERN BAY AREA...

Mid-upper level flow becomes increasingly meridional along the
West Coast by day 3 (Wed-Wed night), as the deep cyclone
approaches British Columbia while a robust NW-SE oriented upper
level jet streak (130-150kt at 250 mb) allows the longwave trough
to dig into southern CA by Thursday. Strong deep-layer synoptic
scale support will ensue within increasing difluence aloft, as
both the upper level divergence and south-to-north transport of
low-mid layer subtropical moisture become enhanced within the
left-exit region of the aforementioned upper jet streak. By late
Wed and Wed night, robust 850 mb SW-SSW flow will peak between 4
to 5 standard deviations above normal for late Jan per the 00Z
GEFS and SREF. Meanwhile, 850-700 mb moisture flux anomalies are
expected to reach +3 to +4 standard deviations for a little while
as the 1.00-1.25" PW plume skirts the CA coast.

Strength and duration of the IVT anomaly would suggest a moderate
AR based on the CW3E AR Scale per both the GEFS and ECMWF ensemble
means, with peak IVT magnitudes of 600-800+ kg/m/s. However the
relatively swift duration of the AR should limit the coverage of
short-term runoff impacts across the outlook area, along with the
weakening magnitude of the AR with time Wed-Wed night as it
becomes more dislodged from the main (deep) low approaching
western BC. The latest CSU GEFS-based first-guess fields,
particularly the UFVS-verified version, support the continuation
of the Slight Risk that was noted in yesterday's Day 4 ERO, with
minor adjustments. Signals for a 'high end' Slight are evident per
the guidance, i.e. one where the neighborhood probability of
rainfall exceeding current FFGs is at least 25% yet still less
than 40% (threshold for Moderate). This especially in WFO EKA's
CWA where the latest guidance ensemble mean QPF on Day 3 is
between 3-5+ inches over most areas, with spotty 6-8+ inches along
some of the more elevated coastal ranges. Cannot rule out the need
for a targeted Moderate Risk in future outlooks, especially if we
begin to see a slower AR progression (and thus uptick in QPF) in
the guidance trends.

Hurley









Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!7hSMLsY0-3rAbTj2tYB26TLx6mmIkMp2xmScTpZANE-3=
NZBVJ9yVQv9Pk0qiKnrz8xYX1Bt3aEBF4OWo0fjJqx8pFsA$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!7hSMLsY0-3rAbTj2tYB26TLx6mmIkMp2xmScTpZANE-3=
NZBVJ9yVQv9Pk0qiKnrz8xYX1Bt3aEBF4OWo0fjJ7a3DG3s$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!7hSMLsY0-3rAbTj2tYB26TLx6mmIkMp2xmScTpZANE-3=
NZBVJ9yVQv9Pk0qiKnrz8xYX1Bt3aEBF4OWo0fjJpgO44Cs$=20


$$



=3D =3D =3D
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