Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
400 PM EDT Tue Oct 01 2024

Valid 00Z Wed Oct 02 2024 - 00Z Fri Oct 04 2024

...Record breaking heat continues over the Southwest U.S....

...Critical fire weather risk over the Northern/Central High Plains today
and tomorrow...

...Generally dry conditions across much of the Lower 48 through midweek...

Approximately 35 million people in the Southwest U.S. fall within a heat
alert today as a late-season heat wave keeps a firm grip over the region.
Continuing the trend from the last several days, numerous record breaking
high temperatures in the triple digits are expected through at least this
Friday, which will support widespread Moderate to Major heat impacts
across the San Francisco, Las Vegas, and Los Angeles metropolitan areas
per the most recent experimental HeatRisk runs. Locally Extreme heat
impacts are also expected atop the Phoenix metropolitan area today. As a
reminder, the Major to Extreme HeatRisk categories mean that health
impacts from heat become more likely in general, and may occur in anyone
without adequate hydration or cooling.

Outside of the Southwest, well-above average temperatures are expected
through midweek across much of Intermountain West and Plains states within
the warm sector of a developing surface low in the Canadian prairies.
While temperatures will not reach the hazardous levels of the Southwest,
the warmth combined with strengthening surface winds and dry conditions
will support Critical fire weather conditions over the Northern/Central
High Plains today and tomorrow, respectively. Widespread Red Flag Warnings
are in effect across much of the aforementioned area as of this afternoon.

Precipitation-wise, lingering moderate to locally heavy rainfall over the
Mid-Atlantic and central Florida should clear out tonight as approaching
high-pressure kicks a pesky offshore low to the east. A second batch of
showers may arrive to the Eastern U.S. associated with a cold front
tomorrow, but rainfall amounts should be modest at best. Otherwise, a weak
front in the Gulf of Mexico could support locally heavy rainfall along the
Gulf Coast later tomorrow, although much of the heaviest rainfall is
forecast to remain offshore.


Asherman


Graphics available at
https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Tue Oct 1 2024

Valid 12Z Fri Oct 04 2024 - 12Z Tue Oct 08 2024

...Anomalous heat will persist across the Southwest into the
weekend...

...Overview...

Latest models and ensembles continue to show a pattern transition
from progressive northern tier flow late this week toward an
amplifying eastern U.S./Atlantic Coast trough early next week while
a mean ridge covering much of the West extends into the Plains,
producing an extended period of unseasonably warm to hot weather.
Separate fronts affecting the Northeast and Northwest late this
week should generate mainly light rain, but a front and potential
embedded surface system reaching near the East Coast next week
could support more organized rainfall over that region. Farther
south, not much has changed regarding the overall forecast
evolution favoring low pressure and heavy rainfall potential over
the Gulf of Mexico with some pockets of enhanced moisture possibly
extending into some areas along the Gulf Coast, but with poor
agreement/consistency for details. There may be multiple surface
features to focus rainfall, including one lifting from the
northwestern Caribbean into the Gulf and another that may be over
the western Gulf, with some potential for these to combine into
early next week.


...Guidance/Predictability Assessment...

Guidance agreement for northern stream flow late this week into
the weekend has been rather good other than a lingering stray
solution or two. The 00Z CMC seemed to be the biggest outlier as it
showed a closed low embedded within the trough atop the Upper
Midwest to Great Lakes Sunday-Monday unlike other models with their
phased trough. Trough details become more of a question mark
toward the East Coast by next Monday-Tuesday as energy pivots
eastward. GFS runs lately have indicated an upper low closing off
near New England and the nearby western Atlantic by Tuesday, with
the 00/12Z runs showing fairly similar position while the 06Z was
more north. While EC runs were displaying a closed upper low albeit
a bit farther south toward the Mid-Atlantic, the new 12Z EC has
become more like the AI/ML models by showing a phased trough. These
differences will also affect the associated surface low's depth
and position near New England; 12Z model runs are generally weaker
than the 00Z/06Z guidance. Also worth keeping an eye on, by Tuesday
a few ML models show more potential for a shortwave from Hudson
Bay south/southwestward versus what most dynamical guidance shows.
The 12Z CMC also goes all in on this energy, digging it into the
Midwest by Tuesday, unlike other models.

The forecast over and near the Gulf of Mexico continues to have
low predictability due to a combination of smaller-scale features.
One wave may track into the central/east-central Gulf from the
northwestern Caribbean while another feature may evolve over the
western Gulf, with the surface/QPF details also potentially
affected by energy aloft over Texas and nearby areas. Per
coordination with NHC, these features may phase at some point next
week as they approach and interact. Guidance shows the system(s)
could linger and meander over the Gulf even into the middle of next
week. This appears to be a messy evolution as fronts and moisture
gradients are also evident nearby. Continue to monitor outlooks
from the National Hurricane Center for the latest information on
features of interest.

For the most part, an operational model blend favoring the GFS and
ECMWF sufficed for the early part of the forecast. The CMC was
used minimally and only early on as it diverged with the north-
central U.S. shortwave. By mid-period, included some of the GEFS
and EC ensemble means, and by Day 7 increased the proportion of the
means to over half considering the individual model differences
especially with the eastern troughing/low pressure.


...Weather/Hazards Highlights...

Very little has changed over the past day in terms of the low
confidence for where areas of heavy rain could extend into some
coastal/inland areas near the Gulf Coast during the period. For the
Friday-Saturday time frame covered by the Days 4-5 Excessive
Rainfall Outlooks, there are minority model solutions
showing pockets of enhanced rainfall along parts of the
central/east-central Gulf Coast and/or the Texas coast. However
while separate ensemble systems differ over where the best heavy
rain potential will be over the Gulf, they do not extend their
better probabilities to land. The combination of poor guidance
clustering/continuity and high flash flood guidance values along
the Gulf Coast region continue to favor not depicting any risk
areas on the Days 4-5 EROs. Recent guidance has been suggesting
that some heavy rain may eventually spread across Florida by early
next week, perhaps aided by a front approaching from the north.

From Friday through the weekend, expect lighter rainfall along
portions of the fronts supported by northern stream dynamics. One
front may produce areas of light rain over the Northeast late this
week while a front reaching the Pacific Northwest around Friday
may produce some light rain as well. A frontal system and possible
developing embedded low may produce more organized rainfall from
the Great Lakes region to the East Coast by next Monday-Tuesday.
Coverage and magnitude will depend on system specifics that have
lower confidence at this time. An approaching Pacific front may
bring light rain to the northern Pacific Northwest early next week.

The strong and persistent upper ridge covering the West and
extending into the Plains will support a lengthy period of well
above normal temperatures over many parts of the western/central
U.S. Highs 10-20F or so above normal will be common and numerous
daily records appear likely from the southern half of the West
into parts of the High Plains, especially Friday-Sunday. Eastern
U.S. temperatures will vary in the near to above normal range until
the East Coast trough drops some highs moderately below normal next
Tuesday.


Tate/Rausch


Additional 3-7 Day Hazard information can be found on the WPC
medium
range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw

Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
356 AM EDT Tue Oct 01 2024

Valid 00Z Wed Oct 02 2024 - 00Z Wed Oct 09 2024

Today's guidance maintains fairly good continuity, showing mostly
light showers through the period with precipitable water values
tending to vary within the near to somewhat below normal range.
Moderate trades with primarily mountain/windward shower focus
should continue through Thursday. Then trades will likely trend
lighter into the weekend as a front to the north sufficiently
weakens surface ridging to the north. Sea/land breezes may affect
the distribution of rainfall late week through the weekend.
Consensus depicts a rebound of trades to moderate speeds by early
next week as high pressure bridges across the mid-latitude Pacific
once again, returning the shower focus to windward locales.

Upper level forecasts continue to show a ridge overhead into
Wednesday, followed by a transition to generally cyclonic flow for
a time but still with spread and run-to-run variability among the
guidance, increasing with time. The latest majority cluster
suggests the axis of the mean trough will be east of the main
islands to some degree, with even the potential for ridging just
to the west to move in by around Monday-Tuesday. While the 00Z
CMC/CMCens mean are on their own with their height falls moving in
from the west, latest machine learning models vary widely enough
to include the full range of dynamical solutions. Preferences
remains with a conservative blended/mean approach while the spread
persists.

Rausch

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
859 PM EDT Tue Oct 1 2024
Day 1
Valid 01Z Wed Oct 02 2024 - 12Z Wed Oct 02 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE FLORIDA Peninsula...

01Z Update...

As low pressure off of the Mid-Atlantic Coast continues to drift
further offshore, the threat for additional widespread heavy
rainfall amounts has ended. Therefore, the Marginal Risk was
removed from the Mid-Atlantic.

Across Florida, the Marginal Risk was maintained. Deep moisture
(PWs at or above 2 inches) ahead of a frontal boundary that is
settling across North Florida, along with weak forcing aloft, will
help continue to support shower and thunderstorm activity across
the region for at least a few more hours before waning overnight.
HREF neighborhood probabilities indicate that some areas,
especially near Lake Okeechobee, could see additional amounts of 2
inches before these storms dissipate.

Pereira

Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
859 PM EDT Tue Oct 1 2024
Day 2
Valid 12Z Wed Oct 02 2024 - 12Z Thu Oct 03 2024

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

There will be a lingering potential for convection near the
eastern coastline that could result in bursts of modest or very
local heavy rainfall intensities. The threat for excessive rainfall
and flooding concerns will remain below the 5 percent threshold for
this period.

Hamrick/Campbell

Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
859 PM EDT Tue Oct 1 2024
Day 3
Valid 12Z Thu Oct 03 2024 - 12Z Fri Oct 04 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR SOUTHERN
LOUISIANA...

There is the potential for moderate to locally heavy rainfall to
move northward from the Gulf of Mexico and affect areas near the
Mississippi River delta, and generally along and south of the
Interstate 10 corridor in Louisiana. This is associated with
moisture convergence along a quasi-stationary frontal boundary over
the northern Gulf of Mexico. The latest 12Z model guidance
suggests the heaviest rainfall should remain just offshore, but any
northward trend could lead to some nuisance level instances of
flooding, thus prompting a Marginal Risk area. Max model QPF for
the 24-hour period is on the order of 3-4 inches over the coastal
waters.

Hamrick
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Tue Oct 1 2024


Very little has changed over the past day in terms of the low
confidence for where areas of heavy rain could extend into some
coastal/inland areas near the Gulf Coast during the period. For the
Friday-Saturday time frame covered by the Days 4-5 Excessive
Rainfall Outlooks, there are minority model solutions
showing pockets of enhanced rainfall along parts of the
central/east-central Gulf Coast and/or the Texas coast. However
while separate ensemble systems differ over where the best heavy
rain potential will be over the Gulf, they do not extend their
better probabilities to land. The combination of poor guidance
clustering/continuity and high flash flood guidance values along
the Gulf Coast region continue to favor not depicting any risk
areas on the Days 4-5 EROs. Recent guidance has been suggesting
that some heavy rain may eventually spread across Florida by early
next week, perhaps aided by a front approaching from the north.

From Friday through the weekend, expect lighter rainfall along
portions of the fronts supported by northern stream dynamics. One
front may produce areas of light rain over the Northeast late this
week while a front reaching the Pacific Northwest around Friday
may produce some light rain as well. A frontal system and possible
developing embedded low may produce more organized rainfall from
the Great Lakes region to the East Coast by next Monday-Tuesday.
Coverage and magnitude will depend on system specifics that have
lower confidence at this time. An approaching Pacific front may
bring light rain to the northern Pacific Northwest early next week.

The strong and persistent upper ridge covering the West and
extending into the Plains will support a lengthy period of well
above normal temperatures over many parts of the western/central
U.S. Highs 10-20F or so above normal will be common and numerous
daily records appear likely from the southern half of the West
into parts of the High Plains, especially Friday-Sunday. Eastern
U.S. temperatures will vary in the near to above normal range until
the East Coast trough drops some highs moderately below normal next
Tuesday.


Tate/Rausch

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Tue Oct 1 2024


Very little has changed over the past day in terms of the low
confidence for where areas of heavy rain could extend into some
coastal/inland areas near the Gulf Coast during the period. For the
Friday-Saturday time frame covered by the Days 4-5 Excessive
Rainfall Outlooks, there are minority model solutions
showing pockets of enhanced rainfall along parts of the
central/east-central Gulf Coast and/or the Texas coast. However
while separate ensemble systems differ over where the best heavy
rain potential will be over the Gulf, they do not extend their
better probabilities to land. The combination of poor guidance
clustering/continuity and high flash flood guidance values along
the Gulf Coast region continue to favor not depicting any risk
areas on the Days 4-5 EROs. Recent guidance has been suggesting
that some heavy rain may eventually spread across Florida by early
next week, perhaps aided by a front approaching from the north.

From Friday through the weekend, expect lighter rainfall along
portions of the fronts supported by northern stream dynamics. One
front may produce areas of light rain over the Northeast late this
week while a front reaching the Pacific Northwest around Friday
may produce some light rain as well. A frontal system and possible
developing embedded low may produce more organized rainfall from
the Great Lakes region to the East Coast by next Monday-Tuesday.
Coverage and magnitude will depend on system specifics that have
lower confidence at this time. An approaching Pacific front may
bring light rain to the northern Pacific Northwest early next week.

The strong and persistent upper ridge covering the West and
extending into the Plains will support a lengthy period of well
above normal temperatures over many parts of the western/central
U.S. Highs 10-20F or so above normal will be common and numerous
daily records appear likely from the southern half of the West
into parts of the High Plains, especially Friday-Sunday. Eastern
U.S. temperatures will vary in the near to above normal range until
the East Coast trough drops some highs moderately below normal next
Tuesday.


Tate/Rausch

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
325 AM EDT Tue May 7 2024

Valid 12Z Tue May 07 2024 - 12Z Fri May 10 2024

...Northern Rockies...
Days 1-2...

An anomalously deep, negatively-tilted, upper low will drift north
and stall over the northern High Plains today as it interacts with
a blocking high to its north. Moderate to heavy precipitation
supported by strong upper forcing and low-to-mid level
frontogenesis will shift west into central and western Montana,
bringing high-elevation heavy snow into the region. Guidance
continues to show that the Glacier National Park region and the
central Montana mountains, including the Little Belt Mountains,
will see some of the greatest impacts today. By early Wednesday,
WPC PWPF indicates that accumulations of 8 inches or more will be
common in these areas, with amounts quickly exceeding a foot in
some locations, especially over the Little Belt Mountains. In
addition to heavy snow, strong winds will be a threat. Even in
areas where lighter snow accumulations are expected to occur, gusty
winds and blowing snow will contribute to disruptive impacts,
including hazardous driving conditions.

The low is forecast to gradually weaken and settle south on
Wednesday. However the threat for areas of moderate to heavy snow
will continue, especially from the Little Belt Mountains south into
the Absaroka and the Big Horn mountains. The Little Belt Mountains
could see several more inches before the snow ends Wednesday
night, pushing storm totals well over 2 feet across the higher
peaks.


The probability of significant icing across the CONUS is less than
10 percent.

Pereira

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Tue Oct 1 2024

Valid 12Z Fri Oct 04 2024 - 12Z Tue Oct 08 2024

...Anomalous heat will persist across the Southwest into the
weekend...

...Overview...

Latest models and ensembles continue to show a pattern transition
from progressive northern tier flow late this week toward an
amplifying eastern U.S./Atlantic Coast trough early next week while
a mean ridge covering much of the West extends into the Plains,
producing an extended period of unseasonably warm to hot weather.
Separate fronts affecting the Northeast and Northwest late this
week should generate mainly light rain, but a front and potential
embedded surface system reaching near the East Coast next week
could support more organized rainfall over that region. Farther
south, not much has changed regarding the overall forecast
evolution favoring low pressure and heavy rainfall potential over
the Gulf of Mexico with some pockets of enhanced moisture possibly
extending into some areas along the Gulf Coast, but with poor
agreement/consistency for details. There may be multiple surface
features to focus rainfall, including one lifting from the
northwestern Caribbean into the Gulf and another that may be over
the western Gulf, with some potential for these to combine into
early next week.


...Guidance/Predictability Assessment...

Guidance agreement for northern stream flow late this week into
the weekend has been rather good other than a lingering stray
solution or two. The 00Z CMC seemed to be the biggest outlier as it
showed a closed low embedded within the trough atop the Upper
Midwest to Great Lakes Sunday-Monday unlike other models with their
phased trough. Trough details become more of a question mark
toward the East Coast by next Monday-Tuesday as energy pivots
eastward. GFS runs lately have indicated an upper low closing off
near New England and the nearby western Atlantic by Tuesday, with
the 00/12Z runs showing fairly similar position while the 06Z was
more north. While EC runs were displaying a closed upper low albeit
a bit farther south toward the Mid-Atlantic, the new 12Z EC has
become more like the AI/ML models by showing a phased trough. These
differences will also affect the associated surface low's depth
and position near New England; 12Z model runs are generally weaker
than the 00Z/06Z guidance. Also worth keeping an eye on, by Tuesday
a few ML models show more potential for a shortwave from Hudson
Bay south/southwestward versus what most dynamical guidance shows.
The 12Z CMC also goes all in on this energy, digging it into the
Midwest by Tuesday, unlike other models.

The forecast over and near the Gulf of Mexico continues to have
low predictability due to a combination of smaller-scale features.
One wave may track into the central/east-central Gulf from the
northwestern Caribbean while another feature may evolve over the
western Gulf, with the surface/QPF details also potentially
affected by energy aloft over Texas and nearby areas. Per
coordination with NHC, these features may phase at some point next
week as they approach and interact. Guidance shows the system(s)
could linger and meander over the Gulf even into the middle of next
week. This appears to be a messy evolution as fronts and moisture
gradients are also evident nearby. Continue to monitor outlooks
from the National Hurricane Center for the latest information on
features of interest.

For the most part, an operational model blend favoring the GFS and
ECMWF sufficed for the early part of the forecast. The CMC was
used minimally and only early on as it diverged with the north-
central U.S. shortwave. By mid-period, included some of the GEFS
and EC ensemble means, and by Day 7 increased the proportion of the
means to over half considering the individual model differences
especially with the eastern troughing/low pressure.


...Weather/Hazards Highlights...

Very little has changed over the past day in terms of the low
confidence for where areas of heavy rain could extend into some
coastal/inland areas near the Gulf Coast during the period. For the
Friday-Saturday time frame covered by the Days 4-5 Excessive
Rainfall Outlooks, there are minority model solutions
showing pockets of enhanced rainfall along parts of the
central/east-central Gulf Coast and/or the Texas coast. However
while separate ensemble systems differ over where the best heavy
rain potential will be over the Gulf, they do not extend their
better probabilities to land. The combination of poor guidance
clustering/continuity and high flash flood guidance values along
the Gulf Coast region continue to favor not depicting any risk
areas on the Days 4-5 EROs. Recent guidance has been suggesting
that some heavy rain may eventually spread across Florida by early
next week, perhaps aided by a front approaching from the north.

From Friday through the weekend, expect lighter rainfall along
portions of the fronts supported by northern stream dynamics. One
front may produce areas of light rain over the Northeast late this
week while a front reaching the Pacific Northwest around Friday
may produce some light rain as well. A frontal system and possible
developing embedded low may produce more organized rainfall from
the Great Lakes region to the East Coast by next Monday-Tuesday.
Coverage and magnitude will depend on system specifics that have
lower confidence at this time. An approaching Pacific front may
bring light rain to the northern Pacific Northwest early next week.

The strong and persistent upper ridge covering the West and
extending into the Plains will support a lengthy period of well
above normal temperatures over many parts of the western/central
U.S. Highs 10-20F or so above normal will be common and numerous
daily records appear likely from the southern half of the West
into parts of the High Plains, especially Friday-Sunday. Eastern
U.S. temperatures will vary in the near to above normal range until
the East Coast trough drops some highs moderately below normal next
Tuesday.


Tate/Rausch


Additional 3-7 Day Hazard information can be found on the WPC
medium
range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw