FOUS30 KWBC 302026
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
326 PM EST Fri Dec 30 2022
Day 1
Valid 16Z Fri Dec 30 2022 - 12Z Sat Dec 31 2022
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA AND THE CENTRAL GULF COAST...
...California...
An atmospheric river driving eastward along a 170 kt upper level
jet is forcing light to moderate rain across portions of northern
CA this morning. For the most part, this forcing will remain
stationary, at least for the first half of the day. After that, a
strengthening shortwave trough digging into the jet will both
begin to weaken and shift it south overnight tonight. Since for
most of the day the plume of moisture will be stationary, expect
prodigious rainfall into the coastal ranges and the Sierras of
northern CA. Rainfall totals of 4-7 inches are expected by
Saturday morning in both mountain ranges, particularly between
Eureka and the Bay Area and from Fresno north along the Sierras.
Snow levels are not expected to move much today, hovering around
9,000 feet, meaning most areas will see rain except the highest
peaks of the Sierras, where uplift may help locally lower snow
levels. PWATs between 1.0 and 1.25 inches remain in the forecast
with anomalies around +3 standard deviations above normal. As
usual, the valley locations in between the aforementioned ranges
will get significantly less rainfall due to downsloping off the
coastal ranges. Finally, the most susceptible areas continue to be
those areas high in soil moisture, burn scars, canyons, and urban
areas. -Wegman
No changes were made to the inherited outlook for the 16Z ERO.
Bann
...Central Gulf Coast & Mississippi River Valley...
The previously issued Slight Risk area from eastern Louisiana
extending across the southern portion of Mississippi and Alabama
into the western Florida panhandle looks to be in good shape as
convection spreads from west to east. Radar has been showing the
core of heaviest rainfall rates/amounts being increasingly
confined to areas near the coast while 1-hr rainfall rates have
been decreasing to just over 0.5 inches per hour farther north in
Mississippi. In addition, the area of heaviest rainfall is
expected to remain progressive through the remainder of the
morning...which should mitigate some of the risk for excessive
rainfall. Farther north...removed the Marginal Risk areas along
and near the Mississippi and Ohio Rivers where the morning runs of
the CAMs had rainfall amounts generally under 3/4 of an inch.=20
Considering the lack of instability that far north and the
on-going heavier convective to the south...rainfall rates should
remain sufficiently low to not challenge flash flood guidance.
Bann
Day 2
Valid 12Z Sat Dec 31 2022 - 12Z Sun Jan 01 2023
...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR THE COASTAL
RANGES OF CENTRAL CALIFORNIA, THE TRANSVERSE RANGES OF SOUTHERN
CALIFORNIA, AND THE WESTERN SLOPES OF THE SIERRA NEVADA...
...2030Z Excessive Rainfall Update...
An on-going atmospheric river will continue to make its way south
and east on Saturday into the early hours of New Years Day. Model
guidance has been consistent with that feature and associated
precipitation forecast. No changes were needed.
...California...
The atmospheric river impacting much of northern California on
Friday continues into New Year's Eve Saturday from the Bay Area
east into the Sierra Nevadas. Unlike on Friday, by Saturday much
of the heaviest rainfall totals are expected in the Sierra Nevada,
and less-so into the coastal mountains. This is likely because the
digging trough Saturday morning will place the Sierras and
interior CA and far western NV more in the left exit region of the
jet as it becomes much more amplified with time. Expect another 2
to 4 inches of rain across the northern Sierras Saturday morning
before the rain associated with the AR finally begins shifting
southward. Behind the atmospheric river, expect snow levels to
rapidly fall to between 4,000 and 5,000 feet into New Year's
night. This will change over any lingering upslope precipitation
over to snow as it ends. The coastal ranges will see rainfall
associated with the locally enhanced A.R. being helped by the
digging trough being somewhat cancelled out by the increasingly
fast southward movement of the moisture plume. The amplified
nature of the moisture will direct the rainfall more south to
north once it reaches SoCal on New Year's night. Therefore the
Transverse Ranges will be favored by a short period of heavy rain
during which 2-3 inches of rain are expected.
The inherited Slight Risk along the coast was trimmed on the
northern end as little lingering rain is expected on the northern
CA coast, and also trimmed on the southern end to exclude the
Peninsular Ranges. With the atmospheric river more directed south
to north, the Peninsular Ranges will not effectively upslope the moisture...unlike the Transverse Ranges. The interior Slight Risk
for the western slopes of the Sierras was left unchanged with this
update.=20
Wegman
Day 3
Valid 12Z Sun Jan 01 2023 - 12Z Mon Jan 02 2023
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL AND SOUTHERN ARIZONA...
The digging trough from New Years Night will press eastward into
Arizona on New Year's Day Sunday. Expect rainfall to begin early
New Year's morning and continue through the day across the
Marginal Risk area. Generally expect rainfall totals of 1-2
inches, but with locally higher amounts in the mountains east of
Phoenix. Meanwhile further north along the Mogollon Rim and points
north, expect snow levels to be low enough that any rain that
occurs initially will quickly change over to snow. PWATs of 0.5 to
0.75 inches, or +3 standard deviations above normal, will start
out the day in the Marginal Risk area. Thus, moisture amounts will
not be a problem on Sunday. Snow levels are expected to begin the
day around 8,000 feet...but drop to around 5,000 feet by sunset on
Sunday. This should allow many lower elevation areas to see all
rain.
Wegman
The 12Z guidance was larely consistent with the previous runs and
on-going outlook needed few...if any...changes.
Bann
Day 1 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7j2q3A58IarORg1e88HFNcNiZXRFPnehCugJKWcttPSz= EKwkEvEp5Wkxl0Knv_qNU6woc_K_Bqf69IuvBWzzrs8-He8$=20
Day 2 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7j2q3A58IarORg1e88HFNcNiZXRFPnehCugJKWcttPSz= EKwkEvEp5Wkxl0Knv_qNU6woc_K_Bqf69IuvBWzz5cCBPdU$=20
Day 3 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7j2q3A58IarORg1e88HFNcNiZXRFPnehCugJKWcttPSz= EKwkEvEp5Wkxl0Knv_qNU6woc_K_Bqf69IuvBWzzQEXGaZM$=20
$$
=3D =3D =3D
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