Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
149 PM EDT Mon Jul 29 2024

Valid 00Z Tue Jul 30 2024 - 00Z Thu Aug 01 2024

...Flash flooding possible in the Ohio/Tennessee Valleys and
central/southern Appalachians early this week...

...Scattered severe thunderstorms and heavy rain forecast across portions
of the Northern Plains and Midwest...

...Dangerous mid-summer heat wave to expand across the Central U.S. and
Southeast...

Bouts of thunderstorms are expected to continue over portions of the
Ohio/Tennessee Valleys and spread further into the central/southern
Appalachians to start off the work week. An active upper-level pattern
featuring at least a couple shortwaves and an approaching surface frontal
system from the west will help to focus storm development over the
Ohio/Tennessee Valleys and eventually into the southern Appalachians
through tonight. Plentiful moisture in place will also continue to lead to
the threat of heavier rain rates, with increasing storm coverage into the
evening and potential back building/repeated rounds of storms raising the
chance for locally heavy rainfall totals. As such, a Slight Risk of
Excessive Rainfall (level 2/4) is in place for the threat of some
scattered flash flooding. In addition, sufficient instability/shear will
be in place over the Ohio Valley for a couple more intense storms, as well
as the threat for a more organized storm complex later this evening. The
Storm Prediction Center has included a Slight Risk (level 2/5) for severe
weather as well mainly for the threat of damaging winds. A similar pattern
will be in place on Tuesday, with the focus shifting further into the
southern and central Appalachians as the upper-level energy and surface
frontal system move eastward. Another Slight Risk of Excessive Rainfall is
in effect here for additional instances of flash flooding. Outside of the
flash flooding threat, scattered thunderstorms with moderate to locally
heavy rainfall are expected more broadly over the Midwest/Southeast
Monday. Rain chances will expand over the Mid-Atlantic/Northeast as the
system approaches from the west on Tuesday. Forecast high temperatures
across the East will vary depending on cloud/storm coverage, with mostly
mid-80s to low 90s expected.

Some additional storms will be possible further west along the frontal
boundary into the Upper Midwest and Northern Plains. Embedded upper-level
energy will help to trigger one round of storms over the Northern Plains
today. Stronger upper-level flow will lead to more deep-layer shear here
than further east, with another Slight Risk of severe weather for the
threat of some very large hail as well as significant damaging winds if
storms consolidate/grow upscale into an organized system later in the
evening. Another round of severe weather is possible downstream over the
Upper Midwest Tuesday as yet another upper-level impulse helps to trigger
storms along the frontal boundary. Very high moisture will lead to strong
to extreme instability, with a Slight Risk in place for the chance of more
damaging winds.

Outside of the severe weather threat, heat will become the big story more
broadly over the central U.S. over the next few days as an upper-level
high strengthens/expands over the region. Forecast high temperatures
Monday and Tuesday are expected to soar into the triple digits over the
Central Plains, with upper-90s to low 100s to the west over much of the
High Plains, and mid- to upper 90s for the Middle and Lower Mississippi
Valley. High humidity values over the Mississippi Valley and eastern
portions of the plains will lead to heat indices in the 105-110 degree
range, potentially as high as 115 for some locations, with widespread
heat-related warnings/advisories in place. Warm morning lows only dropping
into the mid- to upper 70s will provide little relief from the heat
overnight. This combination of hotter temperatures to the west, higher
heat indices to the east, and the multi-day duration of this heat wave
will increase the danger not only to more sensitive groups, but also the
general public, particularly those without adequate air conditioning.

A stagnant troughing pattern over the West will keep temperatures mostly
below average across the region, especially over portions of the Pacific
Northwest and northern Great Basin/Rockies. Forecast highs Monday-Tuesday
range between the 60s and 70s along the Pacific Coast, the 70s and 80s in
the Pacific Northwest and northern Great Basin/Rockies, the 80s and 90s
for interior California and the central Great Basin/Four Corners Region,
and 100s in the Desert Southwest. A cold front passing through the Pacific
Northwest will bring some rain chances on Monday, spreading into the
northern Rockies Tuesday. Smoke from area wildfires will also continue to
plague parts of the West, particularly over portions of the northern Great
Basin, resulting in poor air quality and areas of reduced visibility.

Putnam/Snell

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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
303 PM EDT Mon Jul 29 2024

Valid 12Z Thu Aug 01 2024 - 12Z Mon Aug 05 2024

...Hazardous heat from the south-central Plains eastward gradually
becoming more suppressed with time while building over the
Northwest later this week...


...Overview...

A strong upper ridge over the central U.S. into the Southeast as
the period begins Thursday will continue to promote unseasonably
warm temperatures over central parts of the Plains and into the
Southeast. Then guidance shows upper ridging building over the
West while a Midwest shortwave drifts into a developing mean trough
over the East. Trailing northern stream flow should maintain a more
shallow eastern trough by early next week. This sets up what may
become a stagnant upper pattern supporting increasing heat across
the West and unsettled weather over the East. The National
Hurricane Center is also monitoring the potential for tropical
development that could at least come close to affecting locations
near the southeastern coast.


...Guidance/Predictability Assessment...

Most guidance agrees fairly well for the large scale pattern but
still displays important differences for some details. The most
significant issues involve the upper trough crossing the
northeastern quadrant of the lower 48 (plus amplitude of lingering
mean troughing by next Monday) and potential tropical development
that could track near the southeastern coast.

Regarding the initial Midwest upper trough, incoming 12Z guidance
generally maintains the spread from prior runs with the ECMWF
leaning on the slower side and the GFS leaning faster. The 06Z/12Z
GFS runs have adjusted somewhat slower to eject the feature than
the 00Z run though. Machine learning (ML) models show some spread
as well but on average lean somewhat faster than at least the
operational ECMWF. By next Monday there is a significant majority
of dynamical and ML models recommending more of a lingering trough
across the Great Lakes/Northeast versus what the 06Z/12Z GFS runs
advertise. The 00Z GFS ended up closer to consensus by next Monday.

Most guidance continues to support the idea of possible tropical
development in the Atlantic, as highlighted in the National
Hurricane Center outlook. The operational ECMWF and its ensembles
maintain a more eastern track, bringing a low northward off the
coast of Florida/the Southeast by the weekend into early next week.
The new 12Z CMC has adjusted eastward from its 00Z run, getting
closer to the ECMWF idea. ML models (including the 18Z/00Z
GraphCast GFS) generally support some degree of development with a
track closer to the ECMWF cluster. Latest operational GFS runs have
been very ill- defined with any possible system, with this
scenario very much in the minority at the moment.

The updated forecast based on 00Z/06Z solutions started with an
operational model composite, with a later-period trend toward a
model/ensemble mean mix that tilted a little more than average
toward the ECMWF/ECens mean given WPC-NHC preferences toward that
cluster of solutions for possible tropical development. Meanwhile
this approach provided a reasonable intermediate solution for the
Midwest through Northeast trough and reflected consensus for
lingering eastern troughing by next Monday--with fairly good
continuity relative to previous forecast.


...Weather/Hazards Highlights...

A wet pattern is likely from the Midwest to the Eastern Seaboard
late this week with ample moisture and instability in place while a
slow moving upper trough and surface/frontal systems all
support areas of showers and thunderstorms. The Days 4-5 Excessive
Rainfall Outlooks covering Thursday-Friday night show fairly broad
Marginal Risks across parts of the Midwest to Ohio Valley and the
Northeast/Mid-Atlantic. Latest guidance continues to show potential
for embedded mesoscale systems to produce locally heavy rainfall
across parts of the above areas. However there is still a lot of
spread for location of heaviest activity, plus dependence on still-
uncertain shorter term rainfall and resulting soil sensitivity.
Therefore preference remains to wait before introducing any Slight
Risk areas. Also, ahead of the late week system, a departing
shortwave and sufficient moisture/instability may produce locally
heavy rainfall over northern New England where the Day 4 outlook
depicts a Marginal Risk area. Much of the north-central U.S. to the
eastern third of the country should remain generally unsettled
through the weekend and into early next week with the
aforementioned upper trough continuing northeastward and general
mean troughing likely to persist in its wake. Locations near the
southeastern coast should monitor latest forecasts regarding
possible tropical development during the weekend/early next week
time frame.

Elsewhere, monsoonal convection over the Southwest continues to
support Marginal Risk areas on the Days 4 and 5 EROs. Chances for
convection may expand northeastward late this week depending on the
shape of the southern Rockies/Plains ridge and upper level energy
rotating around its western periphery. Thus the Day 4/Thursday
Marginal Risk expands a bit northward into southern Colorado, and
rain chances increase into the central High Plains by Friday. From
the weekend into early next week, flow between the upper ridge and
a weak trough setting up off the West Coast may support a
northwestward/northward expansion of scattered convection.

The heat wave over the Central Plains early this week should
continue through Thursday, with daytime highs around 10F above
normal and widespread major to locally extreme HeatRisk threats.
An incoming front will bring relief to the Central Plains by
Friday, but heat will continue across the Southeast through Friday
or Saturday with warm overnight temperatures only exacerbating heat
stress in that region. Meanwhile, much of the West will trend
warmer, with lower elevations seeing 100F+ high temperatures.
Scattered daily record highs are possible and appear most likely
over the Interior Northwest on Friday-Saturday. Shorter bursts of
above normal temperatures are also likely for the Midwest to Ohio
Valley to the Mid- Atlantic/Northeast late this week, with perhaps
some moderation next weekend after cold fronts pass through.


Rausch/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
256 AM EDT Mon Jul 29 2024

Valid 00Z Tue Jul 30 2024 - 00Z Tue Aug 06 2024

High pressure north of Hawaii will continue to support some
moderate to locally gusty winds Monday, with speeds generally
decreasing thereafter as the high pushes westward and upper
troughing builds to the north/northeast of the state. Overall
stable conditions will focus showers mainly along
windward/mountain areas, but below-climatology precipitable water
values for much of the week should keep rainfall totals on the
light side. Trades may increase again later next week as an upper
ridge builds east/over the islands.

Santorelli

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
758 PM EDT Mon Jul 29 2024
Day 1
Valid 01Z Tue Jul 30 2024 - 12Z Tue Jul 30 2024

....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO VALLEY, UPPER TENNESSEE VALLEY INTO THE SOUTHERN
APPALACHIANS...


...Ohio/Tennessee Valleys & Southern Appalachians...
West-northwest mid to upper level flow will persist from the
Northern Plains through much of the Mississippi Valley, Ohio
Valley, Southern to Central Appalachians, Mid- Atlantic and
Southeast. Embedded shortwaves in this flow will support another
day of active convection across much of these areas where PW values
will be mostly above seasonal averages. A Slight risk area remains
where the latest HREF neighborhood probabilities were the highest
for 1, 2 and 3"+ totals and overlapped the lower FFG values and
observed precip axis from recent rains. This results in a slight
risk from southern Illinois, east-southeast through the Ohio
Valley, Upper Tennessee Valley into the Southern Appalachians.
Changes from the previous issuance were minimal.


...Southeast Arizona into southwest New Mexico...
The Marginal Risk area across the Southwest remains in place.
Existing scattered convection should persist past sunset, when CIN
will set in and begin to diminish coverage and the heavy rainfall
potential. In the meantime, there is the potential for isolated
heavy totals and localized runoff issues.

Roth/Oravec


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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
758 PM EDT Mon Jul 29 2024
Day 2
Valid 12Z Tue Jul 30 2024 - 12Z Wed Jul 31 2024

...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE NORTHERN MID-ATLANTIC INTO THE CENTRAL APPALACHIANS...

...2030Z Update...

...Northern PA & Southern NY...

An approaching 500mb vort max that is also associated with a
remnant MCV moving through the Great Lakes on Monday will make its
way into the northern Mid-Atlantic on Tuesday. The upper trough
will generate ascent atop the atmosphere while the environment
sports MLCAPE up to 1,000 J/kg and PWs maxing between 1.8-2.0".
While storm motions should generally range between 15-20 kts, ECMWF
area averaged soundings in the Slight Risk area do show upshear
Corfidi vectors <10 kts, suggesting the potential for back-building
thunderstorms. This is particularly the case along mountain ranges
that are oriented favorably to the 850-300mb mean flow out of the
SW. 1-hr FFGs are lowest over the southern tier of NY where values
are <1"/hr in some spots. For these reasons, a Slight Risk was
introduced for parts of Northeast PA and the southern tier of NY
given their more sensitive soils.

...Eastern IA & Northern IL...

A 500mb disturbance tracking across the Northern Plains will track
towards the western Corn Belt while a 50kt 500mb jet over the Upper
MS Valley helps to provide favorable synoptic-scale lift over the
region. At the same time, 850mb ridging over the Southeast and
troughing over the northern Plains will cause an acceleration in
the LLJ (925mb winds 2-3 standardized anomalies above normal in
the Central Plains) to deliver a surge of 925-850mb moisture over
the Midwest. This LLJ will also favorably intersect a stationary
front draped WNW to ESE, which along with veering winds as air
parcels rise will support organized thunderstorm activity. PWs up
to 1.8", MUCAPE >2,000 J/kg, and hodographs that provide sufficient
shear to sustain long duration thunderstorm activity could result
in back building and training thunderstorms either along the
stationary front and/or along outflow boundaries that are supported
by deep cold pools. Latest 12Z HREF's 3-hr QPF > 3-hr FFGs product
depict up to moderate chance probabilities (40-50%) for 3-hr FFG
exceedance while low-to-moderate probabilities (20-40%) for 1-hr
QPF > 1-hr FFGs were present over eastern IA. There remains some
notable spread in guidance with regards to the position and
orientation of where the heaviest QPF area will set up. That said,
the region highlighted is also currently sporting 1-hr FFGs that in
most areas are <2"/hr. Given the optimal atmospheric setup for
storms generating >2"/hr rates and sensitive soils, a Slight Risk
was introduced for portions of eastern IA and northern IL.

...Northern Rockies...

A fast moving -1.5 to -2.0 standardized anomalies 500mb trough
tracking through the Northern Rockies will also be accompanied by
PWs that are hovering around 1.0", which are as much as +2
standardized anomalies over the region. In addition, MUCAPE up to
500 J/kg will support rainfall rates as high as 1.5"/hr in the most
intense areas of convection. Storms should be progressive so
that will help keep the flash flood threat to a very localized
scale, but the available atmospheric moisture and instability could
produce intense enough rainfall rates to cause rapid runoff along
complex terrain and burn scars. A Marginal Risk was introduced to
account for the very localized flash flood potential Tuesday
afternoon and evening.

Outside of minor tweaks to the Southwest's Marginal Risk, the
remainder of the previous shift's forecast discussion is on track.

Mullinax

---Previous Discussion---

...Southwest Florida...
The previous marginal risk area was decreased in size and
concentrated over Southwest Florida. This corresponds to where the
HREF neighborhood probabilities for the 12 hour period from 12Z Tue
to 00Z Wed are highest for 2 and 3"+ amounts. Isolated runoff
issues possible, especially over urbanized regions.

...Southwest...
No changes expected to the large scale flow with the upper ridge
remaining centered over the Southern Plains. This will again
support shortwaves rotating northward on the western side of this
ridge into the Southwest, supporting scattered convection,
localized heavy rains and isolated runoff issues.

Oravec

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
758 PM EDT Mon Jul 29 2024
Day 3
Valid 12Z Wed Jul 31 2024 - 12Z Thu Aug 01 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE UPPER MISSISSIPPI VALLEY INTO THE MID WEST...

...2030Z Update...

Minor adjustments were made to the Slight Risk in the Midwest as
there was not enough consensus on guidance to make significant
adjustments at this moment. The only tweaks made were to take out
the Marginal Risk in western NC and southern VA where QPF has
dropped compared to the previous cycle and to expand the Marginal
in the Northeast southward to include more of the southern tier of
NY where soils are more sensitive. Otherwise, the previous forecast
discussion remains on track.

Mullinax


--Previous Discussion--

...Upper Mississippi Valley into the Midwest...

The progressive northern stream flow will continue into day 3 with
the next strong area of height falls expected to push eastward
across the Northern Plains and into the Upper Mississippi Valley.
There is potential for additional organized convection/heavy
rainfall to push eastward with these height falls across eastern
portions of the Northern Plains and into the Upper Mississippi
Valley. There are some typical latitude differences with the max
qpf axis. The day 3 slight risk area was drawn to favor the more
southern solutions of the EC, NAM and CMC. The previous slight risk
area over the Upper Mississippi Valley was suppressed more to the
south and southwest to better match the more southern solutions
with the next round of potentially organized convection.

...Eastern New York State into western New England...
The strong height falls moving out of the Central Appalachians
into the Northern Mid-Atlantic day 2, will progress across eastern
NY State into New England day 3. There is a typical amount of qpf
spread for the day 3 period, but general consensus for potential of
moderate to locally heavy rainfall amounts in a region of much
above average PW values ahead of these height falls. A broad
marginal risk area was maintained from the previous issuance to
capture the heavy rainfall spread in the models across this area
day 3.


...Southwest...
Similar to the days 1 and 2 period, the mid to upper level ridge
will remain centered over the Southern Plains. Scattered convection
again likely over portions of the Southwest as shortwave energy
again rotates northward on the western side of this ridge and into
the Southwest. Localized heavy rains and isolated runoff issues
again possible for southeast Arizona into southwest to central New
Mexico.

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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
303 PM EDT Mon Jul 29 2024


A wet pattern is likely from the Midwest to the Eastern Seaboard
late this week with ample moisture and instability in place while a
slow moving upper trough and surface/frontal systems all
support areas of showers and thunderstorms. The Days 4-5 Excessive
Rainfall Outlooks covering Thursday-Friday night show fairly broad
Marginal Risks across parts of the Midwest to Ohio Valley and the
Northeast/Mid-Atlantic. Latest guidance continues to show potential
for embedded mesoscale systems to produce locally heavy rainfall
across parts of the above areas. However there is still a lot of
spread for location of heaviest activity, plus dependence on still-
uncertain shorter term rainfall and resulting soil sensitivity.
Therefore preference remains to wait before introducing any Slight
Risk areas. Also, ahead of the late week system, a departing
shortwave and sufficient moisture/instability may produce locally
heavy rainfall over northern New England where the Day 4 outlook
depicts a Marginal Risk area. Much of the north-central U.S. to the
eastern third of the country should remain generally unsettled
through the weekend and into early next week with the
aforementioned upper trough continuing northeastward and general
mean troughing likely to persist in its wake. Locations near the
southeastern coast should monitor latest forecasts regarding
possible tropical development during the weekend/early next week
time frame.

Elsewhere, monsoonal convection over the Southwest continues to
support Marginal Risk areas on the Days 4 and 5 EROs. Chances for
convection may expand northeastward late this week depending on the
shape of the southern Rockies/Plains ridge and upper level energy
rotating around its western periphery. Thus the Day 4/Thursday
Marginal Risk expands a bit northward into southern Colorado, and
rain chances increase into the central High Plains by Friday. From
the weekend into early next week, flow between the upper ridge and
a weak trough setting up off the West Coast may support a
northwestward/northward expansion of scattered convection.

The heat wave over the Central Plains early this week should
continue through Thursday, with daytime highs around 10F above
normal and widespread major to locally extreme HeatRisk threats.
An incoming front will bring relief to the Central Plains by
Friday, but heat will continue across the Southeast through Friday
or Saturday with warm overnight temperatures only exacerbating heat
stress in that region. Meanwhile, much of the West will trend
warmer, with lower elevations seeing 100F+ high temperatures.
Scattered daily record highs are possible and appear most likely
over the Interior Northwest on Friday-Saturday. Shorter bursts of
above normal temperatures are also likely for the Midwest to Ohio
Valley to the Mid- Atlantic/Northeast late this week, with perhaps
some moderation next weekend after cold fronts pass through.


Rausch/Santorelli

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
303 PM EDT Mon Jul 29 2024


A wet pattern is likely from the Midwest to the Eastern Seaboard
late this week with ample moisture and instability in place while a
slow moving upper trough and surface/frontal systems all
support areas of showers and thunderstorms. The Days 4-5 Excessive
Rainfall Outlooks covering Thursday-Friday night show fairly broad
Marginal Risks across parts of the Midwest to Ohio Valley and the
Northeast/Mid-Atlantic. Latest guidance continues to show potential
for embedded mesoscale systems to produce locally heavy rainfall
across parts of the above areas. However there is still a lot of
spread for location of heaviest activity, plus dependence on still-
uncertain shorter term rainfall and resulting soil sensitivity.
Therefore preference remains to wait before introducing any Slight
Risk areas. Also, ahead of the late week system, a departing
shortwave and sufficient moisture/instability may produce locally
heavy rainfall over northern New England where the Day 4 outlook
depicts a Marginal Risk area. Much of the north-central U.S. to the
eastern third of the country should remain generally unsettled
through the weekend and into early next week with the
aforementioned upper trough continuing northeastward and general
mean troughing likely to persist in its wake. Locations near the
southeastern coast should monitor latest forecasts regarding
possible tropical development during the weekend/early next week
time frame.

Elsewhere, monsoonal convection over the Southwest continues to
support Marginal Risk areas on the Days 4 and 5 EROs. Chances for
convection may expand northeastward late this week depending on the
shape of the southern Rockies/Plains ridge and upper level energy
rotating around its western periphery. Thus the Day 4/Thursday
Marginal Risk expands a bit northward into southern Colorado, and
rain chances increase into the central High Plains by Friday. From
the weekend into early next week, flow between the upper ridge and
a weak trough setting up off the West Coast may support a
northwestward/northward expansion of scattered convection.

The heat wave over the Central Plains early this week should
continue through Thursday, with daytime highs around 10F above
normal and widespread major to locally extreme HeatRisk threats.
An incoming front will bring relief to the Central Plains by
Friday, but heat will continue across the Southeast through Friday
or Saturday with warm overnight temperatures only exacerbating heat
stress in that region. Meanwhile, much of the West will trend
warmer, with lower elevations seeing 100F+ high temperatures.
Scattered daily record highs are possible and appear most likely
over the Interior Northwest on Friday-Saturday. Shorter bursts of
above normal temperatures are also likely for the Midwest to Ohio
Valley to the Mid- Atlantic/Northeast late this week, with perhaps
some moderation next weekend after cold fronts pass through.


Rausch/Santorelli

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
1251 PM EDT Mon May 20 2024

Valid 00Z Tue May 21 2024 - 00Z Fri May 24 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
303 PM EDT Mon Jul 29 2024

Valid 12Z Thu Aug 01 2024 - 12Z Mon Aug 05 2024

...Hazardous heat from the south-central Plains eastward gradually
becoming more suppressed with time while building over the
Northwest later this week...


...Overview...

A strong upper ridge over the central U.S. into the Southeast as
the period begins Thursday will continue to promote unseasonably
warm temperatures over central parts of the Plains and into the
Southeast. Then guidance shows upper ridging building over the
West while a Midwest shortwave drifts into a developing mean trough
over the East. Trailing northern stream flow should maintain a more
shallow eastern trough by early next week. This sets up what may
become a stagnant upper pattern supporting increasing heat across
the West and unsettled weather over the East. The National
Hurricane Center is also monitoring the potential for tropical
development that could at least come close to affecting locations
near the southeastern coast.


...Guidance/Predictability Assessment...

Most guidance agrees fairly well for the large scale pattern but
still displays important differences for some details. The most
significant issues involve the upper trough crossing the
northeastern quadrant of the lower 48 (plus amplitude of lingering
mean troughing by next Monday) and potential tropical development
that could track near the southeastern coast.

Regarding the initial Midwest upper trough, incoming 12Z guidance
generally maintains the spread from prior runs with the ECMWF
leaning on the slower side and the GFS leaning faster. The 06Z/12Z
GFS runs have adjusted somewhat slower to eject the feature than
the 00Z run though. Machine learning (ML) models show some spread
as well but on average lean somewhat faster than at least the
operational ECMWF. By next Monday there is a significant majority
of dynamical and ML models recommending more of a lingering trough
across the Great Lakes/Northeast versus what the 06Z/12Z GFS runs
advertise. The 00Z GFS ended up closer to consensus by next Monday.

Most guidance continues to support the idea of possible tropical
development in the Atlantic, as highlighted in the National
Hurricane Center outlook. The operational ECMWF and its ensembles
maintain a more eastern track, bringing a low northward off the
coast of Florida/the Southeast by the weekend into early next week.
The new 12Z CMC has adjusted eastward from its 00Z run, getting
closer to the ECMWF idea. ML models (including the 18Z/00Z
GraphCast GFS) generally support some degree of development with a
track closer to the ECMWF cluster. Latest operational GFS runs have
been very ill- defined with any possible system, with this
scenario very much in the minority at the moment.

The updated forecast based on 00Z/06Z solutions started with an
operational model composite, with a later-period trend toward a
model/ensemble mean mix that tilted a little more than average
toward the ECMWF/ECens mean given WPC-NHC preferences toward that
cluster of solutions for possible tropical development. Meanwhile
this approach provided a reasonable intermediate solution for the
Midwest through Northeast trough and reflected consensus for
lingering eastern troughing by next Monday--with fairly good
continuity relative to previous forecast.


...Weather/Hazards Highlights...

A wet pattern is likely from the Midwest to the Eastern Seaboard
late this week with ample moisture and instability in place while a
slow moving upper trough and surface/frontal systems all
support areas of showers and thunderstorms. The Days 4-5 Excessive
Rainfall Outlooks covering Thursday-Friday night show fairly broad
Marginal Risks across parts of the Midwest to Ohio Valley and the
Northeast/Mid-Atlantic. Latest guidance continues to show potential
for embedded mesoscale systems to produce locally heavy rainfall
across parts of the above areas. However there is still a lot of
spread for location of heaviest activity, plus dependence on still-
uncertain shorter term rainfall and resulting soil sensitivity.
Therefore preference remains to wait before introducing any Slight
Risk areas. Also, ahead of the late week system, a departing
shortwave and sufficient moisture/instability may produce locally
heavy rainfall over northern New England where the Day 4 outlook
depicts a Marginal Risk area. Much of the north-central U.S. to the
eastern third of the country should remain generally unsettled
through the weekend and into early next week with the
aforementioned upper trough continuing northeastward and general
mean troughing likely to persist in its wake. Locations near the
southeastern coast should monitor latest forecasts regarding
possible tropical development during the weekend/early next week
time frame.

Elsewhere, monsoonal convection over the Southwest continues to
support Marginal Risk areas on the Days 4 and 5 EROs. Chances for
convection may expand northeastward late this week depending on the
shape of the southern Rockies/Plains ridge and upper level energy
rotating around its western periphery. Thus the Day 4/Thursday
Marginal Risk expands a bit northward into southern Colorado, and
rain chances increase into the central High Plains by Friday. From
the weekend into early next week, flow between the upper ridge and
a weak trough setting up off the West Coast may support a
northwestward/northward expansion of scattered convection.

The heat wave over the Central Plains early this week should
continue through Thursday, with daytime highs around 10F above
normal and widespread major to locally extreme HeatRisk threats.
An incoming front will bring relief to the Central Plains by
Friday, but heat will continue across the Southeast through Friday
or Saturday with warm overnight temperatures only exacerbating heat
stress in that region. Meanwhile, much of the West will trend
warmer, with lower elevations seeing 100F+ high temperatures.
Scattered daily record highs are possible and appear most likely
over the Interior Northwest on Friday-Saturday. Shorter bursts of
above normal temperatures are also likely for the Midwest to Ohio
Valley to the Mid- Atlantic/Northeast late this week, with perhaps
some moderation next weekend after cold fronts pass through.


Rausch/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