August 20-24, 2023 | Tropical Storm Hilary Flooding Event

[ClicheVortex2014]

Aside from the first-ever tropical storm watch being issued by NWS San Diego (or any west coast office), an extremely rare Day 3 high risk for excessive rainfall has been issued for an area that's very prone to flooding/flash flooding/landslides. This is what happens when you have the right entrance region of an unseasonably strong jet streak interact with a tropical storm... but usually it's not in the west. This has the potential to be a very bad event.

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
434 PM EDT Fri Aug 18 2023

Day 3
Valid 12Z Sun Aug 20 2023 - 12Z Mon Aug 21 2023

...A HIGH RISK OF EXCESSIVE RAINFALL EXISTS FOR THE PENINSULAR
RANGES OF SOUTHERN CALIFORNIA...

...2030 UTC Update...
Made a slight westward expansion of the lower-end threshold mainly
across southern California in response to a couple of the global
models which started to spread rainfall that way. However...the
track forecast from NHC has been consistent and the WPC outlook
areas are right where they should be...conceptually. The biggest
uncertainty becomes how much of an effect that terrain has on the
storm...whether Hilary interacts with land before it ever arrives
in the U.S. or exactly what happens to the storm as it crosses the
complex terrain in the U.S.. Given an uptick in guidance...both
model and WPC QPF...a second QPF maxima appeared with a magnitude
as large as amounts which prompted a High Risk farther south. As
a result...introduced a second but targeted High Risk there. The
WPC storm total QPF rivals 200 percent of annual average rainfall
in some places. Farther north...expanded the Slight Risk over
portions of Idaho where the magnitude of anomalies has shown
consistent model run to run increases. Few other changes were
needed.

Bann



...Previous Discussion...

Portions of the West...
The guidance is unanimous in merging Hilary with an upper low
stuck near central CA. Normally for a tropical cyclone this would
be a problem as convective lows would circle the periphery
parallel to 1000-500 hPa thickness lines, which would otherwise
turn it more north or north-northeast, but since Hilary should be
strongly shearing while moving over cold waters and transitioning
to a post-tropical or remnant low in the process, it should have
diminishing convection in its vicinity, so this isn't the typical
model bias (this time). A large area of precipitable water values
of 1.75-2.5" will approach if not exceed all-time time records, so
there will be moisture to spare. In the Southwest in particular,
flow at 750 hPa is expected to reach or exceed 65 kts, so heavy
upslope rains on the atypical sides of the southern Sierra Nevada
and Peninsula Ranges of CA are anticipated. If the flow is more
southerly than expected due to a slightly more westerly track of
Hilary, there's a chance that both sides of the Peninsular Range
get heavy rainfall. The NAEFS is indicating IVT values 14.8
sigmas above the mean. Even assuming a non-standard distribution,
this is extreme. There is a very real potential for 3" amounts in
an hour in this environment. Some of the guidance shows local
amounts of 7"+, which would be exceeding rare for the region from
a tropical cyclone, potentially unique for Nevada. The 100 year
ARI is forecast to be exceeded. If a 7"+ maximum materialized
over Mount Charleston Sunday into early Monday, it would challenge
Nevada's 24 hour rainfall record, set in 2004. Given the overall
uniqueness of this event, chose to upgrade to a High Risk for
areas near and east of the Peninsular Ranges of Southern CA.

Widespread 3"+ is expected

Edited August 18, 2023 by ClicheVortex2014

[1816]

3 minutes ago, ClicheVortex2014 said:

Aside from the first-ever tropical storm watch being issued by NWS San Diego (or any west coast office), an extremely rare Day 3 high risk for excessive rainfall has been issued for an area that's very prone to flooding/flash flooding/landslides. This is what happens when you have the right entrance region of an unseasonably strong jet streak interact with a tropical storm... but usually it's not in the west. This has the potential to be a very bad event.

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
434 PM EDT Fri Aug 18 2023

Day 3
Valid 12Z Sun Aug 20 2023 - 12Z Mon Aug 21 2023

...A HIGH RISK OF EXCESSIVE RAINFALL EXISTS FOR THE PENINSULAR
RANGES OF SOUTHERN CALIFORNIA...

...2030 UTC Update...
Made a slight westward expansion of the lower-end threshold mainly
across southern California in response to a couple of the global
models which started to spread rainfall that way. However...the
track forecast from NHC has been consistent and the WPC outlook
areas are right where they should be...conceptually. The biggest
uncertainty becomes how much of an effect that terrain has on the
storm...whether Hilary interacts with land before it ever arrives
in the U.S. or exactly what happens to the storm as it crosses the
complex terrain in the U.S.. Given an uptick in guidance...both
model and WPC QPF...a second QPF maxima appeared with a magnitude
as large as amounts which prompted a High Risk farther south. As
a result...introduced a second but targeted High Risk there. The
WPC storm total QPF rivals 200 percent of annual average rainfall
in some places. Farther north...expanded the Slight Risk over
portions of Idaho where the magnitude of anomalies has shown
consistent model run to run increases. Few other changes were
needed.

Bann



...Previous Discussion...

Portions of the West...
The guidance is unanimous in merging Hilary with an upper low
stuck near central CA. Normally for a tropical cyclone this would
be a problem as convective lows would circle the periphery
parallel to 1000-500 hPa thickness lines, which would otherwise
turn it more north or north-northeast, but since Hilary should be
strongly shearing while moving over cold waters and transitioning
to a post-tropical or remnant low in the process, it should have
diminishing convection in its vicinity, so this isn't the typical
model bias (this time). A large area of precipitable water values
of 1.75-2.5" will approach if not exceed all-time time records, so
there will be moisture to spare. In the Southwest in particular,
flow at 750 hPa is expected to reach or exceed 65 kts, so heavy
upslope rains on the atypical sides of the southern Sierra Nevada
and Peninsula Ranges of CA are anticipated. If the flow is more
southerly than expected due to a slightly more westerly track of
Hilary, there's a chance that both sides of the Peninsular Range
get heavy rainfall. The NAEFS is indicating IVT values 14.8
sigmas above the mean. Even assuming a non-standard distribution,
this is extreme. There is a very real potential for 3" amounts in
an hour in this environment. Some of the guidance shows local
amounts of 7"+, which would be exceeding rare for the region from
a tropical cyclone, potentially unique for Nevada. The 100 year
ARI is forecast to be exceeded. If a 7"+ maximum materialized
over Mount Charleston Sunday into early Monday, it would challenge
Nevada's 24 hour rainfall record, set in 2004. Given the overall
uniqueness of this event, chose to upgrade to a High Risk for
areas near and east of the Peninsular Ranges of Southern CA.

Widespread 3"+ is expected

Oof that's bad. Nelson County VA says hello. That happened after Camille, liquidated mountainsides basically. 

Those precip maps can't capture a training event in one spot so whole most areas might get away with 3 inches, if it plays out just right one spot could get a couple feet. Pray not. 

[ClicheVortex2014]

Don't see this... ever. Flood watches/tropical storm watch in the west, heat products in the south/central US.

[btbucks]

Get ready for video clips on Vegas casinos flooding.

[ClicheVortex2014]

Edit: I posted about the direct tropical storm aspect... this thread is meant for the flooding/tornadoes

Edited August 19, 2023 by ClicheVortex2014

[1816]

This is one of the most populated areas in the whole country. And this is basically unheard of in anyone's lifetime. Hope people heed these warnings but I'm 100 percent sure there will be untold amounts of stupidity associated with this. Hope the emergency responders and rescue people stay safe. 

[MaineJay]

It wouldn't be 2023 if someone wasn't getting catastrophic flooding.

[ClicheVortex2014]

A few bits from the WPC ERO 

>70% chance for rainfall to be at or exceed the 100 year annual recurrence interval 

PWATs between 1.75"-2.25" expected... all-time PWAT records expected to be broken for stations in the Southwest

IVT being shown as 19 sigmas above normal which is likely too high because it excludes the 1970s, but they note even half of that is still extreme. 

3"/hr rain rates possible

Day 2
Valid 12Z Sun Aug 20 2023 - 12Z Mon Aug 21 2023

...A HIGH RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF
SOUTHERN CALIFORNIA...

...2030Z Update...
Made a slight expansion northward of the High Risk area into
portions of Nevada...in part due to an uptick in amounts and
expansion of the areal coverage both by in the WPC deterministic
QPF and model guidance. Based on the latest WPC QPF...40 km
neighborhood probabilities had a swath along the axis of the High
Risk where Annual Recurrence Intervals in excess of 70 percent at
the 100 year ARI. Admittedly...there could be some blossoming of
areal coverage resulting for being a 40 km neighborhood but the
magnitude of signal at that interval is impressive. Latest QPF
placement was good for a typical land falling tropical cyclone
followed by expansion and growing areal coverage during the day
given the moisture stream interacting with the terrain and the
increasing dynamics. Moisture streaming northward should lead to
increasing coverage of convection capable of producing heavy
rainfall amounts/rates which spreads into parts of the Northwest
U.S.. Remainder of the outlook was in good shape and largely
unchanged.

Bann

...Previous Excessive Rainfall Discussion...

Portions of the West...
The guidance is unanimous in merging Hilary with an upper low
stuck near central CA this period, and has picked up the pace on
its acceleration into/across the area. Normally for a tropical
cyclone this would be a problem as convective lows would circle
the periphery parallel to 1000-500 hPa thickness lines, which
would otherwise
turn it more north or north-northeast, but since Hilary should be
strongly shearing while moving over cold waters and interacting
with the Peninsular Ranges of the Baja California Peninsula and
southern CA, it should be transitioning to a post-tropical or
remnant low in the process and have diminishing convection in its
vicinity; the guidance probably isn't displaying its typical model
bias (this time). A large area of precipitable water values of
1.75-2.25" will approach if not exceed all-time records across
portions of the Southwest, so there will be moisture to spare. In
the Southwest in particular, flow at 750 hPa is expected to reach
or exceed 65 kts, so heavy upslope rains on the atypical sides of
the southern Sierra Nevada and Peninsula Ranges of CA are
anticipated. If the flow is more southerly than expected due to a
slightly more westerly track of Hilary, there's a chance that both
sides of the Peninsular Ranges of southern CA could get heavy
rainfall. The 19/12z NAEFS is indicating IVT values 19.1 sigmas
above the mean; it should be noted that it is using a dataset that
does not include the rash of tropical cyclones that impacted the
Southwest in the 1970's, so this value is likely a bit too high.
Even assuming a non-standard distribution and standard deviations
half as large, this is extreme. There is a very real potential
for 3" amounts in an hour in this environment should sufficient
instability be present. Even if instability was completely
eroded, 0.5" an hour totals would be possible; heavy rain appears
inevitable. The 00z Canadian Regional shows local amounts of
towards 10", which would be exceeding rare for the region from a
tropical cyclone, potentially unique for Nevada. The 100 year ARI
is forecast to be exceeded. Some locations within this arid
region are slated to get 1-2 years worth of rain in one day. If a
7"+ maximum materialized over Mount Charleston Sunday into early
Monday, it would challenge Nevada's 24 hour rainfall record, set
in 2004.

The heavy rainfall combined with high winds expected at elevation
could lead to mudslides and landslides, which would be exacerbated
where trees uproot within saturating soils. Debris flows and rock
slides are a given considering the volume of rainfall expected.
The overall combination of effects could block and undermine
roads, particularly sensitive areas such as sections of U.S. 50 in
NV. Towns could get cut off. Given the overall uniqueness of
this event and expected impacts, the High Risk for areas of
southern CA remains justified. The main change was the joining of
the two separate High Risk areas and some slight westward shift of
the risk areas in CA, NV, UT, and AZ and some northward stretching
of the threat areas to account for the slightly accelerated
guidance.

 

 

Worth noting, too, that this isn't the first time a tropical storm has hit southern California. Once in 1858, and again in 1939.

https://en.wikipedia.org/wiki/1858_San_Diego_hurricane

https://en.wikipedia.org/wiki/1939_California_tropical_storm

So, unfortunately we can't really conclude much by looking at those storms. The area has changed significantly since 1939. Yes, there's better drainage/forecasting now, but there's more people in the area and it's far more urbanized (buildings, streets/roads) which is notoriously not ideal for absorbing rainfall.

[Neoncyclone]

What did I just watch lol

 

[Neoncyclone]

So so strange, see above post, anyone smarter than I am know whats going on here?

 

[1816]

20 minutes ago, Neoncyclone said:

So so strange, see above post, anyone smarter than I am know whats going on here?

 

There's some big hills poking up in that general area nw of yuma. They're not super tall but they are pretty prominent relative to the surrounding area.  That's all I got. 

[ClicheVortex2014]

Definitely some tropical soundings in the southwest. Vegas needs some sunshine and there'd be some tornado threat. San Diego has that weird low-level dry layer to work out. Helps explain why they've been seeing some 75+ mph gusts.

Edited August 20, 2023 by ClicheVortex2014

[ClicheVortex2014]

Don't see this often in a tropical storm