Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
740 PM EST Thu Feb 13 2025
Day 1
Valid 16Z Thu Feb 13 2025 - 12Z Fri Feb 14 2025
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTHERN CALIFORNIA...
0100 UTC update...
Made some changes to the previous outlook given latest radar and
model trends. The slight risk area was removed along the northern
and central California coastal regions, where HREF hourly
probabilities for .50"+ amount decrease significantly after 0000
UTC Fri. No changes made inland in the upslope of the Sierra where
hourly .50"+ HREF probabilities remain fairly high through the
remainder of the outlook period.
Across Southern California...changes here based on hi res model
consensus with respect to the timing of the primary axis of max
850-700 mb moisture flux anomalies along and ahead of the cold
front forecast to push across Southern California after 0000 UTC.
The western end of the slight risk was trimmed significantly
southeastward to near Santa Barbara. The western end of the
moderate risk area was trimmed about 70 miles to the east based on
latest timing of the axis of heavy rains to push east across this
region. No changes to the overall thinking with respect to heavy
rainfall potential here. HREF hourly probabilities are
high...90%+...for .50"+/hr rainfall amounts along and ahead of the
front. The overall excessive rainfall threat will be diminishing
with time after 0000 UTC from west to east across Southern
California. However, there are no changes in the overall forecast
rational with a continuation of damaging and potentially life-
threatening flash flooding and debris flows, especially across
recent burn scar regions.
Oravec
16Z Update...
Changes to the previous outlooks were rather minor overall which
includes removal of the Marginal Risk area across the Southeast
given the downward trends in organized convection and a slackening
of the rainfall rates. While some locally heavy showers and
thunderstorms will still be likely this afternoon ahead of a cold
front, the totals should be rather modest.
Over California, a well-defined/organized deep layer cyclone and
associated atmospheric river continues to impact the region. While
the overall deeper layer axis/fetch of strong moisture transport
has settled south of northern California and the Bay Area, there
will be post-frontal convective showers arriving through the
afternoon which will be capable of producing heavy rainfall rates
of 0.50"+/hour. Therefore a Slight Risk is maintained along the
coastal range of northern California coastal range. We have opted
to remove it however for portions of the Sacramento Valley where
the additional totals for today should be rather modest.
No changes farther south down the coast into southern California
where there will be significant sensitivities around the recent
burn scar activity closer into Los Angeles Basin. The latest HREF
guidance supports 0.50" to 1.0"/hour rainfall rates this afternoon
into the evening hours as strong moisture flux and a corridor of
modest instability arrives which coupled with upslope flow into the
Transverse and Peninsular Ranges will support elevated rates
including pockets of convection. Therefore the Moderate Risk
remains in place. Please consult WPC's latest MPDs for more
details on the short-term trends/impacts over the next 6 to 12
hours.
Orrison
Previous discussion...
...California...
Overall, not much change to the previous forecast, with the
unfolding atmospheric river expected to peak near the start of the
period. A long warm front currently extending along the coast is
expected to press inland this morning, with IVTs climbing to around
700-800 kg/m/s near the upstream cold front as it approaches the
coast later this morning. The strongest moisture flux is expected
to center near SF Bay at the start of the period, before steadily
shifting south along the coast today. Models continue to show IVTs
decreasing as the axis moves south, dropping to around 400-500
kg/m/s as the AR reaches Southern California later this afternoon
into the evening. Guidance has remained fairly consistent,
indicating precipitation amounts of 4-6 inches along the central to
southern coastal ranges and the Sierra Nevada, falling mostly as
rain below 6000 ft.
A Moderate Risk (level 3/4) remains in place across portions
Southern California, from Santa Barbara County southeastward into
the San Gabriel and San Bernadino mountains, where damaging and
potentially life-threatening burn scar flash flooding and debris
flows remain a primary concern. Even outside of the burn scar areas,
intense rain rates over complex terrain and urban areas may produce
scattered to numerous instances of flash flooding. Elsewhere, a
broad Slight Risk (Level 2/4) exists for much of the remaining
coastal ranges as well as parts of the Central Valley and the lower
elevations of the Sierra Nevada.
...Florida Panhandle and Southern Georgia...
Lingering heavy rainfall will continue this morning across
portions of the Florida Panhandle and southern Georgia ahead of an
approaching cold front. HREF neighborhood probabilities indicate
that additional rainfall amounts of 1-2 inches are likely within
the highlighted area, with some potential for localized rain rates
high enough to cause some urban/localized flooding concerns.
Pereira
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
308 AM EST Fri Feb 14 2025
Day 1
Valid 12Z Fri Feb 14 2025 - 12Z Sat Feb 15 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE MID-SOUTH...
Models continue to show ingredients coming together late in the
period to support a developing heavy rain that will evolve into a
much larger threat on Day 2. Deepening moisture coincident with a
southwesterly 50 kt low level jet, interacting with mid-level
energy, is expected to support showers and storms developing
across the region Saturday morning. There is some signal in the hi-res
models showing storms beginning to train, raising an increasing
threat for heavy accumulations near the end of the period. While
widespread heavy amounts are not expected prior to 12Z, this
continued signal in some of the guidance for training storms with
locally heavy amounts beginning to develop, warranted leaving the
Marginal Risk in place across portions of the region. However, the
areal extent of the risk was reduced some from the previous,
confining the area mostly to where the 00Z HREF showed the highest
neighborhood probabilities for accumulations of an inch or more.
Pereira
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
308 AM EST Fri Feb 14 2025
Day 2
Valid 12Z Sat Feb 15 2025 - 12Z Sun Feb 16 2025
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE TENNESSEE AND OHIO VALLEYS AND THE CENTRAL APPALACHIANS...
...Ohio and Tennessee Valleys...
Confidence remains high in significant heavy rainfall and flash
flood event across the region on Saturday and Saturday Night. Models
continue to depict a swath of 3-6 inches of rainfall in the 24-hour
period, although there continues to be some variability on the exact
placement. Therefore, the maximum risk level was held at a Moderate
Risk for this outlook, and the biggest change was to shift the
overall footprint slightly further to the northwest (by 25-50 mi)
based on trends in the 00 UTC model guidance. The Moderate Risk area
was also trimmed a bit in the Central Appalachians where model QPF
has been trending lower and the region will be removed from the area
of greatest instability (and thus likely to see lower rain rates.)
Based on cluster analysis, a primary source of uncertainty seems to
be related to the position of the surface low. This makes sense as a
more northward track of the low would allow for a more significant
poleward push of the warm front and overall warm sector. Although
the 00 UTC NAM, NAM Nest, and GFS have trended the swath of rainfall
further northwest, this is far from unanimous. In fact, AI-based
versions of the ECMWF and GFS have been fairly consistent in their
depiction of the overall scenario and are among the furthest south
with the position of the surface low. Therefore, although the
overall risk areas were trended slightly to the northwest out of
deference to the overall ensemble envelope, the changes were
incremental at this point and still include a reasonable chance of
some of the southern scenarios unfolding.
An emerging point of consensus among many models, though, appears to
be a focus of the heaviest rainfall generally between 85W and 90W
longitude, or likely somewhere in W/C KY or NW TN. This is an area
that would be favored to receive both the early round of training
and backbuilding convection along the developing warm front in the
morning and early afternoon, as well as more vigorous convection as
the surface low tracks nearby with increasing surface-based
instability. This would potentially maximize both the peak intensity
of rain rates as well as the overall duration of heavy rain. It also
happens to coincide with the most significant above-normal precip
departures over the past 30 days, with precip in that period along
an axis from DYR-LEX generally 1.5 to 2.0 times the normal amount.
Therefore, this is an area that will continue to be monitored for
any potential risk upgrades to a High Risk if it becomes apparent
that the duration of heavier rain rates will last for at least
several hours. More transient bursts of heavy rain would be less
likely to lead to severe impacts, and it is the uncertainty of both
the placement of the heavy rain and the extent to which it will be
concentrated in a sustained period that preclude a risk upgrade at
the present moment.
The environment will be quite unusual for mid-February. The
combination of MUCAPE of 500-1000 j/kg and PWATs reaching around 1.5
inches has only rarely been observed in KY/TN at this time of year,
and would be supportive of 1-2 inch per hour rain rates in the most
organized and intense convection. A powerful southwesterly LLJ over
60 knots, oriented at an acute angle to the developing warm front,
will favor backbuilding into the region of stronger instability. And
the overall mean flow should favor some periods of training
convective clusters or bands near the warm front and surface low
track. All of these ingredients are fairly classic for impactful
flash flooding cases, and significant impacts are possible in this
one as well.
Lamers
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
308 AM EST Fri Feb 14 2025
Day 3
Valid 12Z Sun Feb 16 2025 - 12Z Mon Feb 17 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN PORTIONS OF THE
MID-ATLANTIC...
...Mid-Atlantic...
A Marginal Risk was maintained across portions of the Mid-Atlantic
region on Sunday, primarily due to the potential for briefly heavy
rain rates with convection along an advancing cold front, and fairly
low flash flood guidance values. However, the speed of the cold
front should reduce the duration of any heavy rain, and thus any
flash flood threat should remain rather isolated.
Lamers
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
244 AM EST Fri Feb 14 2025
A closed upper low/trough and deep surface low will work into the
Canadian Maritimes by early next week. Trailing upper energies and
very cold lower atmospheric flow will combine with lingering wrap-
back moisture to fuel Great Lake effect to northern New England
snows. The tight pressure gradient may result in a period of
strong, gusty winds. Behind this system, elongated upper troughing
will linger across the northern tier and an impressive Arctic
airmass takes hold over the central and eastern U.S. next week.
Entrenched cold air will maintain lake effect snows and also offer
a widespread threat of record cold temperatures with focus over
especially the central U.S. in this pattern.
Upstream, several organized shortwaves/frontal systems will dig
southeastard through the West. These systems will bring rounds of
low elevation rain and mountain snow to the Pacific Northwest,
Northern California, and the Northern/Central Intermountain
West/Rockies through the period. A main focus for precipitation
will spread to the Central/Southern Plains early next week as a low
pressure system moves south across the region towards the Gulf
Coast. A WPC Day5/Tuesday Excessive Rainfall Outlook (ERO) Marginal
Threat area has been introduced. Precipitation will expand across
the South and Southeast heading through next midweek as wavy
frontal lows develop and swing along the Gulf Coast to the
Southeast. Impactful wintry weather is expected on the northern
side of this system from the Central/Southern Plains through the
Mid-Mississippi and Tennessee Valleys. The WPC Winter Weather
Outlook outlines the snow/ice threat that may continue into later
next week with possible coastal low/storm development to threaten
up the East Coast from the Mid-Atlantic to eastern New England.
Schichtel
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
244 AM EST Fri Feb 14 2025
A closed upper low/trough and deep surface low will work into the
Canadian Maritimes by early next week. Trailing upper energies and
very cold lower atmospheric flow will combine with lingering wrap-
back moisture to fuel Great Lake effect to northern New England
snows. The tight pressure gradient may result in a period of
strong, gusty winds. Behind this system, elongated upper troughing
will linger across the northern tier and an impressive Arctic
airmass takes hold over the central and eastern U.S. next week.
Entrenched cold air will maintain lake effect snows and also offer
a widespread threat of record cold temperatures with focus over
especially the central U.S. in this pattern.
Upstream, several organized shortwaves/frontal systems will dig
southeastard through the West. These systems will bring rounds of
low elevation rain and mountain snow to the Pacific Northwest,
Northern California, and the Northern/Central Intermountain
West/Rockies through the period. A main focus for precipitation
will spread to the Central/Southern Plains early next week as a low
pressure system moves south across the region towards the Gulf
Coast. A WPC Day5/Tuesday Excessive Rainfall Outlook (ERO) Marginal
Threat area has been introduced. Precipitation will expand across
the South and Southeast heading through next midweek as wavy
frontal lows develop and swing along the Gulf Coast to the
Southeast. Impactful wintry weather is expected on the northern
side of this system from the Central/Southern Plains through the
Mid-Mississippi and Tennessee Valleys. The WPC Winter Weather
Outlook outlines the snow/ice threat that may continue into later
next week with possible coastal low/storm development to threaten
up the East Coast from the Mid-Atlantic to eastern New England.
Schichtel