Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
417 PM EDT Thu May 23 2024

Valid 00Z Fri May 24 2024 - 00Z Sun May 26 2024

...There is an Enhanced Risk of severe thunderstorms over parts of the
Central Plains on Thursday and the Central/Southern Plains on Saturday...

...There is a Slight Risk of severe thunderstorms over parts of the
Western Ohio Valley, Middle/Lower Mississippi Valley, and Southern Plains
on Friday...

...There is a Slight Risk of excessive rainfall over parts of the Southern
Plains/Lower Mississippi Valley and Tennessee/Ohio Valleys on Thursday...

... There are Heat Advisories over southern Texas...

On Thursday evening, a front extending from the Northern/Central Plains to
the Central Rockies will move eastward to the Lower Great Lakes/Ohio
Valley and westward to the Central High Plains by Saturday evening. As the
boundary moves across the Plains, showers and severe thunderstorms will
develop over parts of Nebraska and western Iowa. Therefore, the SPC has
issued an Enhanced Risk (level 3/5) of severe thunderstorms over parts of
the Central Plains/Middle Mississippi Valley through Friday morning. The
hazards associated with these thunderstorms are frequent lightning, severe
thunderstorm wind gusts, hail, and a few tornadoes. In addition, there
will be an added threat of severe thunderstorm wind gusts of 65 knots or
greater over the region.

Furthermore, there will be another region with a slight risk of severe
thunderstorms over parts of the rest of the Plains and parts of the
Middle/Lower Mississippi Valley. Therefore, the SPC has issued a Slight
Risk (level 2/5) of severe thunderstorms over parts of the
Northern/Central/Southern Plains and the Middle/Lower Mississippi Valley
through Friday morning. The hazards associated with these thunderstorms
are frequent lightning, severe thunderstorm wind gusts, hail, and a few
tornadoes. In addition, there is an increased threat of hail, two inches
or greater, over parts of the Northern/Central/Southern Plains.

Moreover, the showers and thunderstorms will cause heavy rain to develop
over parts of the Southern Plains and Lower Mississippi Valley. Therefore,
the WPC has issued a Slight Risk (level 2/4) of excessive rainfall over
the Lower Mississippi Valley and Southern Plains through Friday morning.
The associated heavy rain will create mainly localized areas of flash
flooding, with urban areas, roads, small streams, and low-lying areas the
most vulnerable.

Further, another front extending from Maine across the Northeast Coast to
the Mid-Atlantic, then westward to the Tennessee Valley, will get
reinforced by a third front moving across the Great Lakes by Friday
afternoon. The boundary will create two areas of severe thunderstorms over
Maine and parts of the Ohio/Tennessee Valleys. Therefore, the SPC has
issued a Slight Risk (level 2/5) of severe thunderstorms over two areas:
one over Maine and a second over the Ohio/Tennessee Valleys through Friday
morning. The hazards associated with these thunderstorms are frequent
lightning, severe thunderstorm wind gusts, hail, and a few tornadoes.

Moreover, some showers and thunderstorms will produce heavy rain over
parts of the Ohio/Tennessee Valleys. Therefore, the WPC has issued a
Slight Risk (level 2/4) of excessive rainfall over parts of the
Ohio/Tennessee Valleys through Friday morning. The associated heavy rain
will create mainly localized areas of flash flooding, with urban areas,
roads, small streams, and low-lying areas the most vulnerable.

On Friday, showers and severe thunderstorms will develop as the front over
the Plains moves onto the Mississippi Valley. Therefore, the SPC has
issued a Slight Risk (level 2/5) of severe thunderstorms over parts of the
Western Ohio Valley and Middle/Lower Mississippi. The hazards associated
with these strong to severe thunderstorms are frequent lightning, severe
thunderstorm wind gusts, hail, and a few tornadoes. However, there will be
an added threat of hail, two inches or greater, over parts of the Southern
Plains/Lower Mississippi Valley.

Also, on Friday, the threat of excessive rainfall will decrease slightly
with two areas of a Marginal Risk (level 1/4) of excessive rainfall over
the Southern Plains and Lower Mississippi, Tennessee Valleys, and Central
Appalachians and the second over parts of the Upper Mississippi Valley and
Upper Great Lakes/Western Ohio Valley. The associated heavy rain will
create localized areas of flash flooding, affecting areas that experience
rapid runoff with heavy rain.

Moreover, showers and severe thunderstorms will develop ahead of a
dryline over parts of the Central/Southern Plains on Saturday. Therefore,
the SPC has issued an Enhanced Risk (level 3/5) of severe thunderstorms
over parts of the Central/Southern Plains on Saturday. The hazards
associated with these thunderstorms are frequent lightning, severe
thunderstorm wind gusts, hail, and a few tornadoes.

The showers and thunderstorms will also produce heavy rain over parts of
the Central Plains/Middle Mississippi Valley on Saturday. Therefore, the
WPC has issued a Slight Risk (level 2/4) of excessive rainfall over parts
of the Central Plains/Middle Mississippi Valley. The associated heavy rain
will create mainly localized areas of flash flooding, with urban areas,
roads, small streams, and low-lying areas the most vulnerable.

Meanwhile, a sub-tropical upper-level high over Mexico will aid in
spawning Heat Advisories over southern Texas through Saturday. The ridging
will create a dangerous early-season heat wave over south Texas, Southern
Florida, and into the Central Gulf Coast. Record or near-record warm
overnight temperatures will provide little to no relief to those without
adequate or reliable cooling. By this weekend, record daily high
temperatures and heat index readings over 115 degrees over the regions
will also be possible. The heat is expected to continue next week,
although some relief may reach the Gulf Coast after Memorial Day with a
cold front. Check local media and government websites for cooling center
locations and hours, especially if you encounter or are dealing with a
loss of power. If you use a portable generator, do so safely to avoid
carbon monoxide poisoning. Never use a generator inside a house, garage,
or other enclosed space!


Ziegenfelder


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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
248 PM EDT Thu May 23 2024

Valid 12Z Sun May 26 2024 - 12Z Thu May 30 2024


...Heavy Rainfall/Flooding possible from the Midwest to the
Northeast Sunday and Monday...

...Hazardous heat will continue through Memorial Day weekend for
South Texas, the Central Gulf Coast, and South Florida...


...Overview...

An upper trough (which may be a combination of different energy
streams) will traverse the CONUS through the medium range period.
This will serve as the main driver for heavy rainfall potential
early in the period from the Midwest/Mid-South into the East. After
Monday, the overall pattern should turn more amplified as an upper
ridge builds over the interior West, causing the downstream trough
to deepen and slow as it moves over the East early next week.
Upstream, a deep upper low anchored off the West Coast should
eventually send energy into the Northwest and northern tier around
the middle of next week. Meanwhile, a strong ridge to the south
will maintain its influence over Mexico and will continue a
hazardous heat threat from South Texas through the Gulf Coast and
for South Florida through Monday. A cold front will drop south
into the northern Gulf mid-next week and bring some much needed
relief from the heat.


...Guidance/Predictability Assessment...

Model guidance shows good agreement on the overall large scale
upper level pattern through the medium range period, but there are
some small/mid scale differences between models and runs that will
affect how upper lows and shortwaves interact. Some of the highest
uncertainty surrounds the timing and strength of a shortwave that
will move over the northern Rockies and Central Plains on Sunday
before interacting with a deep upper low over the Great Lakes
region. The interactions between these two features will influence
the timing and position of several successive frontal systems that
will impact the eastern third of the nation next week.

Shortwave energy should move into the Pacific Northwest and cross
the northern tier of the nation mid-next week, but there is still a
significant amount of uncertainty surrounding the speed and
amplitude of this feature. Recent runs of the GFS and ECMWF have
shown poor run-to-run consistency, so this will be a feature to
keep an eye on in future forecast updates.

The afternoon WPC forecast used a blend of the deterministic
GFS/ECMWF/CMC/UKMET along with the previous WPC forecast blend
through Tuesday, with the heaviest weight placed on the ECMWF and
GFS. This method aims to find a middle ground solution given the
differences in the latest model runs and lack of run-to-run
consistency. After Tuesday, the WPC blend used a combination of the
deterministic GFS/ECMWF/CMC and ensemble means from the GEFS/ECE to
help smooth out model differences. This also maintained good
continuity with the previous WPC forecast for Days 3-6.


...Weather/Hazards Highlights...

A surface low and associated fronts will be the focus for strong
to severe thunderstorms and heavy rainfall/flooding potential
Sunday and Monday from parts of the Midwest to the East. A Marginal
Risk of Excessive Rainfall (level 1/4) remains in place across
much of the Ohio Valley and into the Tennessee Valley on Sunday
(Day 4), shifting into the Northeast and Mid-Atlantic on Monday
(Day 5). The guidance continues to show elevated QPF totals, but
still with a lot of spread in exactly where. It is possible that
the guidance signal will become more defined and an embedded Slight
Risk (level 2/4) may be needed in the Midwest on Sunday.

The heaviest rainfall should exit the East Coast after Monday with
the cold front, but renewed troughing across the region will allow
for shower chances across the Great Lakes into the
Appalachians/Northeast through mid-next week. Elsewhere,
precipitation chances should increase across the Northwest and
northern/central Plains Tuesday through Thursday while additional
showers and thunderstorms expand across the southern Plains.

Expect hazardous heat to continue through Memorial Day weekend
from South Texas through the Central Gulf Coast and South Florida.
Daytime highs are forecast to be as much as 10-20 degrees above
average with widespread heat index values above 100 degrees, and
max heat index values may exceed 115 degrees in parts of South
Texas. Highs near 100 degrees could stretch farther north across
the southern High Plains on Sunday as well. The combination of
dangerous heat indices and lack of overnight recovery/cooling will
allow for a major to extreme heat risk across South Texas, with
widespread major heat risk also extending into the Central Gulf
Coast region and South Florida. Temperatures should trend closer to
normal on Tuesday across much of the South following a cold front
passage, but South Florida will likely remain warm through mid-
next week.

The forecast pattern will favor below average highs over the
Northwest and northern Plains on Sunday, but above normal
temperatures will build again across the West early next week
underneath of an amplified upper ridge. Above average temperatures
along the eastern seaboard will moderate back to near normal next
week with some chance for below normal highs around mid-next week.


Dolan/Santorelli


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
232 AM EDT Thu May 23 2024

Valid 00Z Fri May 24 2024 - 00Z Fri May 31 2024

An upper low northwest of Hawaii should weaken by later this week,
but combination of energy with another low to its west should keep
a lingering trough west/northwest of Hawaii into next week. Models
do continue to show good agreement that the better moisture and
heaviest rainfall should stay west of the main islands though,
some some enhanced showers could make it to Kauai depending on
exactly how close the trough sets up to the west. Surface high
pressure building north of Hawaii this weekend and next week will
maintain a more typical easterly trade wind pattern, supporting a
mainly windward and sea breeze focus for showers with some locally
breezy trades at times. A cold front approaching the western
islands later next week may allow for trades to shift more
southeasterly.

Santorelli

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
842 PM EDT Thu May 23 2024
Day 1
Valid 01Z Fri May 24 2024 - 12Z Fri May 24 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE ARKLATEX
TO ARKLAMISS AS WELL AS FOR PORTIONS OF THE TENNESSEE VALLEY AND
SOUTHERN APPALACHIANS...

...22Z Update...
Updated the Slight and Moderate Risk areas across the Southern
Plains-ArkLaTex areas to Lower MS Valley and TN-Lower Ohio Valleys,
as well as with the Marginal Risk over the Lower MO Valley, based
on the latest observational and mesoanalysis trends along with the
latest HRRR and HREF guidance. There are active Mesoscale
Precipitation Discussions or MPDs in these outlook areas; for
further information, please go to
www.wpc.ncep.noaa.gov/metwatch/metwatch_mpd.php.

...16Z Update...
The inherited Slight risk area was divided into 2 separate smaller
ones with this update based on the latest guidance. The theme from
the ensemble CAMs was to highlight the area around southern
Arkansas with a separate area on the west side of the southern
Appalachians into Kentucky, eastern Tennessee and extreme
southwestern Virginia. Areas east of the mountains have somewhat
higher FFGs and more tolerance for the possibility of localized
training convection, so the Slight was cut off at the Great
Smokies, which fits well with where current convection is adding
moisture to the environment across southeast Kentucky and northeast
Tennessee. Additional convection expected this afternoon with
daytime heating and a period of a few hours of clearing between
current areas of convection should allow for this afternoon's
storms to have the potential to produce locally heavy rainfall and
resultant flash flooding.

The squall line crossing southeast Arkansas and into northwest
Mississippi is also adding moisture to the environment and locally
lowering FFGs as it crosses the area. The line alone is unlikely to
produce flash flooding given its fast movement perpendicular to the
axis of the line...however it will set the stage for additional
convection likely to develop this afternoon, which in areas with
training could cause widely scattered instances of flash flooding.

Elsewhere only minor changes to the Marginals, including a small
southwestward expansion in central TX for the currently-weakening
convection, trimming out the Sand Hills and dry slot of the
northern Plains low, and including the northernmost counties in
Georgia into the Marginal for possible southward development of
this afternoon's convection.

Wegman

...Previous Discussion...

...ArkLaTex into the Mid-South...
A stationary front is expected to stretched from northeast Texas/
southeast Oklahoma into Arkansas, while slowly weakening. Precipitable
water values expected to remain 1.5 to 2 standard deviations above the
mean along the boundary. Additional shortwave energy moving in from
the west across the Southern Plains will support additional
convection in this high moisture axis and there is potential for
additional heavy rains from northeast Texas/southeast Oklahoma,
east across the Lower Arkansas Valley into the Lower Mississippi
Valley. The stationary front weakens and southerly low level flow
pushes the high PW values farther to the north. Hourly rain totals
to 2.5" and local amounts to 5" are possible. While some model
spread continues, convergence seen in the 00z Canadian Regional,
00z UKMET, and 00z ECMWF in the Slight Risk area led to some
increased confidence, despite the weaker look in the 00z HREF
probabilities and more bifurcated 00z GFS solution in this region.
The Slight Risk remains and has shown some expansion equally due
to the heavy rainfall which occurred Wednesday and because of
future heavy rainfall concerns Thursday and Thursday night.


...Northern Plains/Upper Midwest...
The marginal risk area across eastern portions of the Northern to
Central Plains into the Upper Mississippi Valley remains.
Additional convection is likely ahead of the next strong area of
height falls/cooling aloft rotating from the Northern Rockies into
the region. This area has seen heavy rainfall amounts as of late,
keeping stream flows high. The strong upper trof, and associated
strengthening low level southerly flow and increasing PW values,
there will be potential for localized heavy rainfall amounts that
may fall across areas that received heavy totals over the past 24
hours.

Roth

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
842 PM EDT Thu May 23 2024
Day 2
Valid 12Z Fri May 24 2024 - 12Z Sat May 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM CENTRAL
TEXAS TO THE MID-ATLANTIC AS WELL AS PORTIONS OF THE UPPER MIDWEST...

...2030Z Update...

No significant changes were made to the Marginal risk areas across
portions of the south and the upper Midwest. From Texas to
Virginia, storms are once again expected to develop along the
stalled out front. However, with lesser upper level support and
moisture flow parallel to the front out of the southwest rather
than perpendicular, supporting flow into the storms will be reduced
Friday as compared with today. Thus, the threat level remains
Marginal across this area. It's possible that portions of the front
may need a local Slight by Friday, with the most likely area in the
same portions of the current Slight in and around southern
Arkansas. However, for now the signal for heavy rain in the
guidance is diffuse enough that confidence is not there for a
Slight. This could change if afternoon convective development
across southern Arkansas overperforms tonight.

The Marginal in the Upper Midwest remains largely unchanged with
the cold frontal rainfall expected across Wisconsin and Illinois.
Wisconsin up to Minnesota has been very wet in recent days, while
urbanization in and around Chicago, the Quad Cities, and points in
between locally lower FFGs in this area. Some training storms may
pose an isolated flash flooding threat into Friday evening.

Wegman

...Previous Discussion...

...Upper Mississippi Valley into the Upper Great Lakes...
The next strong area of height falls moving into the northern High Plains
will continue to push eastward through the Northern Plains and into the
Upper Mississippi Valley. The overall favorable large scale conditions
ahead of these height falls from Thursday will continue into Friday
along the associated front where strong frontal convergence in an axis
of PW values 1 to 1.5 standard deviations above the mean will support
potential for moderate to locally heavy rainfall totals. This front
does look progressive, which may limit very heavy amounts. The marginal
risk was drawn to where there may be overlap of locally heavy rains
Friday through Friday night with heavy rains that have fallen as of
late. Hourly rain totals to 2" and local amounts to 4" are expected
where cells merge or manage to train.


...ArkLaTex, Tennessee Valley, & southern Appalachians...
A broad marginal risk area continues and was expanded in areal
coverage from continuity based on the most recent QPF guidance and
to some degree heavy rainfall that occurred near the ArkLaTex on
Wednesday. Precipitable water values across these regions will
remain above average, with additional shortwaves in from the west
supporting scattered convection and locally heavy rainfall amounts.
Spread remains within the guidance, which keeps the threat level
Marginal. The ingredients in place support hourly rain totals to
2.5" with local amounts to 5". Should the guidance show more
convergence in future runs, a Slight Risk could be prudent as
portions of the area saw heavy rainfall Wednesday and should see
heavy rains again on Thursday.

Roth

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
842 PM EDT Thu May 23 2024
Day 3
Valid 12Z Sat May 25 2024 - 12Z Sun May 26 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CENTRAL PLAINS...

...2030Z Update...

Few overall changes were needed as the area covering much of
eastern Kansas and Missouri remains the area of focus for
convection on Saturday. However, as convection in this region will
be largely dependent on the behavior of convection from previous
days, guidance agreement is poor.

Since there is a large amount of spread in this area, and with
recent heavy rainfall, the Slight was expanded to cover more of the
potential scenarios for flash flooding in this area. Most of the
guidance took the unusual step of trending north, which is the
direction much of the expansion of the Slight went, but since
convective behavior tends to trend south, very little change was
made to the southern end of the Slight. It's likely that with
better agreement closer to the event that the Slight will be able
to be shrunk to follow those trends.

Wegman

...Previous Discussion...

The Marginal Risk in the area continues and was expanded in areal
coverage to account for the most recent QPF guidance, with the
Slight Risk a new addition. A cyclone emerging into the central
Plains will bring precipitable water values of 1.5"+ across the
region on the heels of sufficiently strong low level
inflow/effective bulk shear. The region lies within a general
instability gradient, with CAPE expected to rise to 4000+ J/kg
within an increasingly capped atmosphere across the southern
Plains. Pockets of moderate to heavy rainfall are forecast within
this region. There is increasing overlap in the QPF pattern near
the KS/MO border within the 00z guidance where three hourly flash
flood guidance is modest, so went ahead with an upgrade to a Slight
Risk for this update. However, given the ingredients, hourly rain
totals to 2.5" with local amounts to 5" are possible where
mesocyclones form and/or cells manage to train.

Roth

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
248 PM EDT Thu May 23 2024


A surface low and associated fronts will be the focus for strong
to severe thunderstorms and heavy rainfall/flooding potential
Sunday and Monday from parts of the Midwest to the East. A Marginal
Risk of Excessive Rainfall (level 1/4) remains in place across
much of the Ohio Valley and into the Tennessee Valley on Sunday
(Day 4), shifting into the Northeast and Mid-Atlantic on Monday
(Day 5). The guidance continues to show elevated QPF totals, but
still with a lot of spread in exactly where. It is possible that
the guidance signal will become more defined and an embedded Slight
Risk (level 2/4) may be needed in the Midwest on Sunday.

The heaviest rainfall should exit the East Coast after Monday with
the cold front, but renewed troughing across the region will allow
for shower chances across the Great Lakes into the
Appalachians/Northeast through mid-next week. Elsewhere,
precipitation chances should increase across the Northwest and
northern/central Plains Tuesday through Thursday while additional
showers and thunderstorms expand across the southern Plains.

Expect hazardous heat to continue through Memorial Day weekend
from South Texas through the Central Gulf Coast and South Florida.
Daytime highs are forecast to be as much as 10-20 degrees above
average with widespread heat index values above 100 degrees, and
max heat index values may exceed 115 degrees in parts of South
Texas. Highs near 100 degrees could stretch farther north across
the southern High Plains on Sunday as well. The combination of
dangerous heat indices and lack of overnight recovery/cooling will
allow for a major to extreme heat risk across South Texas, with
widespread major heat risk also extending into the Central Gulf
Coast region and South Florida. Temperatures should trend closer to
normal on Tuesday across much of the South following a cold front
passage, but South Florida will likely remain warm through mid-
next week.

The forecast pattern will favor below average highs over the
Northwest and northern Plains on Sunday, but above normal
temperatures will build again across the West early next week
underneath of an amplified upper ridge. Above average temperatures
along the eastern seaboard will moderate back to near normal next
week with some chance for below normal highs around mid-next week.


Dolan/Santorelli

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
248 PM EDT Thu May 23 2024


A surface low and associated fronts will be the focus for strong
to severe thunderstorms and heavy rainfall/flooding potential
Sunday and Monday from parts of the Midwest to the East. A Marginal
Risk of Excessive Rainfall (level 1/4) remains in place across
much of the Ohio Valley and into the Tennessee Valley on Sunday
(Day 4), shifting into the Northeast and Mid-Atlantic on Monday
(Day 5). The guidance continues to show elevated QPF totals, but
still with a lot of spread in exactly where. It is possible that
the guidance signal will become more defined and an embedded Slight
Risk (level 2/4) may be needed in the Midwest on Sunday.

The heaviest rainfall should exit the East Coast after Monday with
the cold front, but renewed troughing across the region will allow
for shower chances across the Great Lakes into the
Appalachians/Northeast through mid-next week. Elsewhere,
precipitation chances should increase across the Northwest and
northern/central Plains Tuesday through Thursday while additional
showers and thunderstorms expand across the southern Plains.

Expect hazardous heat to continue through Memorial Day weekend
from South Texas through the Central Gulf Coast and South Florida.
Daytime highs are forecast to be as much as 10-20 degrees above
average with widespread heat index values above 100 degrees, and
max heat index values may exceed 115 degrees in parts of South
Texas. Highs near 100 degrees could stretch farther north across
the southern High Plains on Sunday as well. The combination of
dangerous heat indices and lack of overnight recovery/cooling will
allow for a major to extreme heat risk across South Texas, with
widespread major heat risk also extending into the Central Gulf
Coast region and South Florida. Temperatures should trend closer to
normal on Tuesday across much of the South following a cold front
passage, but South Florida will likely remain warm through mid-
next week.

The forecast pattern will favor below average highs over the
Northwest and northern Plains on Sunday, but above normal
temperatures will build again across the West early next week
underneath of an amplified upper ridge. Above average temperatures
along the eastern seaboard will moderate back to near normal next
week with some chance for below normal highs around mid-next week.


Dolan/Santorelli

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
1031 AM EDT Mon May 13 2024

Valid 00Z Tue May 14 2024 - 00Z Fri May 17 2024

...WPC Winter Weather Desk no longer routinely staffed this
summer...

Pending any significant winter weather, the WPC Winter Weather Desk
will not be staffed this summer. The desk will resume continuous
staffing starting in late September 2024.

Most of the winter products from WPC will continue to be produced,
including the Winter Storm Severity Index (WSSI), Probabilistic
Winter Storm Severity Index (WSSI-P), and the Probabilistic Winter
Precipitation Forecasts (PWPF).

WPC

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
248 PM EDT Thu May 23 2024

Valid 12Z Sun May 26 2024 - 12Z Thu May 30 2024


...Heavy Rainfall/Flooding possible from the Midwest to the
Northeast Sunday and Monday...

...Hazardous heat will continue through Memorial Day weekend for
South Texas, the Central Gulf Coast, and South Florida...


...Overview...

An upper trough (which may be a combination of different energy
streams) will traverse the CONUS through the medium range period.
This will serve as the main driver for heavy rainfall potential
early in the period from the Midwest/Mid-South into the East. After
Monday, the overall pattern should turn more amplified as an upper
ridge builds over the interior West, causing the downstream trough
to deepen and slow as it moves over the East early next week.
Upstream, a deep upper low anchored off the West Coast should
eventually send energy into the Northwest and northern tier around
the middle of next week. Meanwhile, a strong ridge to the south
will maintain its influence over Mexico and will continue a
hazardous heat threat from South Texas through the Gulf Coast and
for South Florida through Monday. A cold front will drop south
into the northern Gulf mid-next week and bring some much needed
relief from the heat.


...Guidance/Predictability Assessment...

Model guidance shows good agreement on the overall large scale
upper level pattern through the medium range period, but there are
some small/mid scale differences between models and runs that will
affect how upper lows and shortwaves interact. Some of the highest
uncertainty surrounds the timing and strength of a shortwave that
will move over the northern Rockies and Central Plains on Sunday
before interacting with a deep upper low over the Great Lakes
region. The interactions between these two features will influence
the timing and position of several successive frontal systems that
will impact the eastern third of the nation next week.

Shortwave energy should move into the Pacific Northwest and cross
the northern tier of the nation mid-next week, but there is still a
significant amount of uncertainty surrounding the speed and
amplitude of this feature. Recent runs of the GFS and ECMWF have
shown poor run-to-run consistency, so this will be a feature to
keep an eye on in future forecast updates.

The afternoon WPC forecast used a blend of the deterministic
GFS/ECMWF/CMC/UKMET along with the previous WPC forecast blend
through Tuesday, with the heaviest weight placed on the ECMWF and
GFS. This method aims to find a middle ground solution given the
differences in the latest model runs and lack of run-to-run
consistency. After Tuesday, the WPC blend used a combination of the
deterministic GFS/ECMWF/CMC and ensemble means from the GEFS/ECE to
help smooth out model differences. This also maintained good
continuity with the previous WPC forecast for Days 3-6.


...Weather/Hazards Highlights...

A surface low and associated fronts will be the focus for strong
to severe thunderstorms and heavy rainfall/flooding potential
Sunday and Monday from parts of the Midwest to the East. A Marginal
Risk of Excessive Rainfall (level 1/4) remains in place across
much of the Ohio Valley and into the Tennessee Valley on Sunday
(Day 4), shifting into the Northeast and Mid-Atlantic on Monday
(Day 5). The guidance continues to show elevated QPF totals, but
still with a lot of spread in exactly where. It is possible that
the guidance signal will become more defined and an embedded Slight
Risk (level 2/4) may be needed in the Midwest on Sunday.

The heaviest rainfall should exit the East Coast after Monday with
the cold front, but renewed troughing across the region will allow
for shower chances across the Great Lakes into the
Appalachians/Northeast through mid-next week. Elsewhere,
precipitation chances should increase across the Northwest and
northern/central Plains Tuesday through Thursday while additional
showers and thunderstorms expand across the southern Plains.

Expect hazardous heat to continue through Memorial Day weekend
from South Texas through the Central Gulf Coast and South Florida.
Daytime highs are forecast to be as much as 10-20 degrees above
average with widespread heat index values above 100 degrees, and
max heat index values may exceed 115 degrees in parts of South
Texas. Highs near 100 degrees could stretch farther north across
the southern High Plains on Sunday as well. The combination of
dangerous heat indices and lack of overnight recovery/cooling will
allow for a major to extreme heat risk across South Texas, with
widespread major heat risk also extending into the Central Gulf
Coast region and South Florida. Temperatures should trend closer to
normal on Tuesday across much of the South following a cold front
passage, but South Florida will likely remain warm through mid-
next week.

The forecast pattern will favor below average highs over the
Northwest and northern Plains on Sunday, but above normal
temperatures will build again across the West early next week
underneath of an amplified upper ridge. Above average temperatures
along the eastern seaboard will moderate back to near normal next
week with some chance for below normal highs around mid-next week.


Dolan/Santorelli


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