Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
247 AM EDT Sun Jul 27 2025

Valid 12Z Sun Jul 27 2025 - 12Z Tue Jul 29 2025

...Dangerous, long-lasting extreme heat expected across the Southeast this
week...

...Severe weather and flash flooding possible for portions of the Upper
Midwest, Ohio Valley and Northeast/Mid-Atlantic today...


A growing upper ridge in the South will send moist southerly air
northward, where it'll interact with a surface low pressure system and
shortwave energy over the Upper Midwest. Thunderstorms are likely to form
somewhere in northern Minnesota this afternoon and potentially congeal
into a southward-moving MCS tonight. There's a slight risk (at least 15%
chance) of excessive rainfall leading to flash flooding over parts of
northeastern Minnesota where saturated soils are more susceptible to
runoff. There's also a slight risk (level 2/5) of severe thunderstorms
across many of the same areas, due to the threat of wind-damage and
isolated large hail. Some weak impulses embedded within the aforementioned
upper ridge should generate some scattered to isolated thunderstorms
across the Ohio Valley and Northeast/Mid-Atlantic this afternoon, where
relatively high soil moisture should allow for greater chances of flash
flooding. A slight risk excessive rainfall is in effect from eastern
Missouri to southwestern Pennsylvania/West Virginia and from southern New
York to far southern Maryland to reflect these conditions.

The heavy rain and severe weather threats shift a bit to the south on
Monday to encompass portions of eastern South Dakota, southern Minnesota
and northern Iowa where slight risks of excessive rainfall and severe
thunderstorms are in effect. The Storm Prediction Center cited concerns
for hail and a potential tornado in South Dakota and the Upper Mississippi
Valley, while severe wind potential and isolated marginal hail are
possible over portions of Montana and the western Dakotas. Monsoonal
moisture continues to work it's way into the Southwest where marginal risk
(at least 5% chance) of excessive rainfall areas are in effect for the
next several days.

Extreme heat is expected to intensify across much of the Southeast and
Tennessee Valley today, with the most dangerous combination of high
temperatures and humidity occurring from Monday through Wednesday. This
will lead to a prolonged and extremely hazardous heat wave. Heat levels
will become dangerous for anyone without adequate cooling or hydration.
High temperatures will soar into the upper 90s to low 100s, with heat
index values ("feels like" temperatures) surpassing 110-115 degrees.
Several major metropolitan areas--including Raleigh, Charlotte, Nashville
and Orlando--are expected to face Extreme HeatRisk for multiple days, with
over 20 million people impacted at the peak.


Kebede


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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
310 AM EDT Sun Jul 27 2025


...Dangerous heat wave persists in the Southeast through midweek
with gradual relief later week...

...Heavy rain and flash flooding potential in the central
Plains for multiple days...


...Overview...

By the start of the medium range period midweek, the sprawling
upper high atop much of the U.S. early this week will be weakening
somewhat and be pressed south by a northern stream upper trough.
Dangerous heat is possible across the Lower Ohio/Tennessee Valleys
and Mid-Atlantic Wednesday before a cold frontal passage tied to
the upper trough, while the Southeast is forecast to see the heat
continue into late week but gradually shrink in scope. Complexes of
showers and storms are likely near and ahead of the frontal system
stretching from the High Plains eastward across the central
Plains/Mid-Mississippi Valley into the Northeast Wednesday. The
front is likely to stall near the central Plains and lead to
multiple days of storms with heavy rain that could cause flash
flooding. Farther east the cold front should be more progressive,
making its way southward across the Mid-Atlantic and Southeast but
creating an environment for possibly heavy rain each day as it
moves. Meanwhile, southerly flow will bring monsoonal moisture into
the Southwest states for possibly heavy rain.


...Guidance/Predictability Assessment...

Model agreement is excellent for the overall pattern through late
this week. Models have been persistent in showing the expansive
upper high shrinking in scope by Wednesday and becoming centered
over the southern Plains late week, with perhaps another
subtropical high moving across Florida. Meanwhile the ridge axis
will stretch north across the northern Rockies into western Canada
starting Tuesday, coincident with the upper trough edging into the
north-central to northeastern U.S. anchored by a large Hudson Bay
to eastern Canada upper low. Since model differences were mostly
negligible through late week, aside from the 12Z UKMET which showed
some differences in its handling of the ridge, used a 18Z GFS/12Z
ECMWF/12Z CMC blend for the first half of the forecast period.
Regardless of the large scale agreement, shortwaves riding around
the ridge will cause more differences in sensible weather like
frontal placement, and convection will have further uncertainties
due to small boundaries like outflows interacting. These tend not
to become clearer until the short range or near term, so the
details of QPF remain uncertain, but at least the overall pattern
seems set.

By next weekend, model differences start to increase as the
pattern becomes more quasi-zonal and uncertain shortwaves dominate
more. One larger scale model difference was the pattern in the
Northwest especially by next Sunday. This will be influenced by a
large Northeast Pacific upper low with somewhat uncertain
placement. The 12Z GFS was farthest southwest with the upper low
and did not allow for much troughing to come into the Northwest
compared to the non-NCEP guidance. AI/ML models are split but
leaned toward the Northwest troughing. Newer 00Z models have
trended east toward Southeast Alaska with the upper low, but still
have some differences with how much energy may split off this low
and when to create troughing in the West. Used some GEFS and EC
ensemble means by the latter part of the medium range to temper the
individual model differences by that time.


...Weather/Hazards Highlights...

Another hot day is forecast from the south-central U.S. into the
Southeast, Tennessee and Ohio Valleys, and much of the Eastern
Seaboard on Wednesday underneath the (albeit shrinking) upper high.
HeatRisk is mainly in the Major (level 3/4) category with some
embedded Extreme (level 4/4) areas with highs generally in the 90s
to near/exceeding 100 with higher heat indices, while morning lows
well into the 70s and nearing 80 will not provide much relief. A
few more daily records for temperatures could be set into
Wednesday. On Thursday, the cold front will pass through the Ohio
Valley to Mid-Atlantic and bring near to below normal temperatures,
with the heat wave becoming more limited to the
Southeast/Carolinas. The gradual movement of the front southward
should finally lead to heat relief in the Southeast Friday-
Saturday, though Florida could still be hot. Cooler than average
temperatures (especially highs) are likely in the northern/central
Plains on the cool side of the front and with the clouds and rain
chances.

Rain and thunderstorms are forecast to focus on the periphery of
the ridge, enhanced by a frontal boundary across the northern
Rockies into the north-central Plains and Mississippi Valley. Some
shortwave forcing and above normal moisture and instability should
lead to multiple rounds of convection with high rain rates, and
some areas could see repeating rounds of storms with the front
stalling. Thus for the Day 4/Wednesday and Day 5/Thursday ERO, show
Slight Risks over the central Plains for the possibility of flash
flooding. On Day 4/Wednesday, also have the Slight Risk looping
east into the Middle Mississippi Valley where models show MCSs and
antecedent conditions are generally wet. Marginal Risks stretch
north into the northern Rockies/High Plains near the back end of
the front as much higher than average instability (per the Extreme
Forecast Index) is in place. Meanwhile the front extends farther
east across the Ohio Valley and Northeast on Wednesday, promoting
potentially heavier rain in a moisture-laden pre-frontal
environment coincident with areas that have seen well above normal
rainfall the past several weeks, allowing for a Marginal Risk in
the Day 4/Wed ERO. By Thursday, the front will gradually move south
and lead to pooling moisture and instability in the Mid-Atlantic
along with increasing dynamical support in the form of the right
entrance region of the upper jet. Thus will show a Slight Risk in
the Day 5/Thu ERO for this activity. Enhanced rain is possible in
the Carolinas by Friday and Saturday near this front.

To the west of the ridge, monsoonal moisture (with precipitable
water values generally in the 75th-90th percentile) will be present
over the Southwest with some instability. Marginal Risks are in
place in the Days 4 and 5 EROs for the Southwest and reaching into
the central Rockies/High Plains as the monsoonal moisture meets the
frontal system to the north.

Elsewhere, moisture and instability could lead to scattered
thunderstorms in the southeastern U.S. from day to day. There may
be some focus for storms near the central Gulf Coast in particular
with weak low level troughs/lows in the vicinity. Marginal Risks of
flash flooding are in place for the central Gulf Coast and parts
of the Southeast due to high rain rates that may overcome the high
Flash Flood Guidance and/or fall atop urban areas.


Tate


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
354 AM EDT Sun Jul 27 2025

Valid 00Z Mon 28 Jul 2025 - 00Z Mon 04 Aug 2025

Increased moisture is likely over Hawaii for the next couple of
days near a surface trough and under a weak trough aloft, leading
to muggy conditions and possibly increasing shower coverage.
Meanwhile trades should be light to moderate. But trades are
forecast to increase Tuesday-Wednesday to breezy levels and remain
there through the rest of the week. This is coincident with a
tightening pressure gradient as a tropical disturbance (that has a
high chance of development into a tropical cyclone per NHC) will
pass south of the state around midweek. Drier air is also likely
as this tropical low's moisture stays south of the state. Another
tropical low just behind the first one may have some increased
moisture come into the Big Island late week (though the low should
remain south), but the better consensus seems to be for continued
dry conditions with the breezy trades.

Tate

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
501 AM EDT Sun Jul 27 2025
Day 1
Valid 12Z Sun Jul 27 2025 - 12Z Mon Jul 28 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE UPPER MIDWEST, NORTHERN APPALACHIANS AND PARTS OF THE NORTHEAST...

...The eastern Dakotas and Upper Midwest...
Continue to make minor adjustments to the placement of a Slight
Risk area mainly confined to Minnesota as a shortwave trough
continues to propagate eastward along the northern tier of states.
There has been an upward trend in the guidance...both in the
coarser/global guidance and with the CAMs and related ensemble
forecast systems. With guidance now showing 2000 to 3000 J per kg
being possible and precipitable water values approaching 1.5
inches,,,another expansion southward of the Slight and Marginal
risk areas was made compared with the previous outlook.

The Marginal Risk area on the northern/western periphery of the
Slight risk area was expanded into North Dakota as a well developed
line of convection was moving across the state and not likely to
diminish prior to the start of the Day 1 period at 12Z.

...The Ohio Valley and Mid Atlantic to southern New England...

The Slight Risks which were raised over the Ohio Valley was
expanded to the west as a complex from overnight moves eastward.
The expectation is that the convection should weaken for a period
before reigniting later. Made a few changes to the outlook area
from the DelMarVa northward based on latest guidance. Locally
enhanced rainfall will persist in the very moist environment near
the west- east frontal boundary.

...Southwest US...

Deeper moisture that made its way across parts of the Gulf and
inland across Mexico should start to be drawn northwest into parts
of the Southwest United States...resulting in scattered late day
and evening convection across parts of New Mexico and a small
portion of Arizona.

Bann

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
501 AM EDT Sun Jul 27 2025
Day 2
Valid 12Z Mon Jul 28 2025 - 12Z Tue Jul 29 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE NORTHERN PLAINS...

...Plains of Eastern Montana to parts of the Upper Midwest...
A surface cold front and mid-level shortwave trough will be
approaching an area rich in moisture and instability over parts of
South Dakota and Minnesota/Iowa on Monday afternoon. With low
level flow likely to increase the amount of moisture transport into
the region during the evening with a corresponding increase in
moisture flux convergence along the front...the potential exists
for excessive rainfall especially in the event of cell training or
repeat convection. Uncertainty remained with respect to placement
in the numerical guidance...but the RRFS seemed too far north. The
WPC deterministic QPF was closer to a GEFS/ECMWF ensemble mean
position...which still left some room for excessive rainfall to
build south. Consequently...allowed for a somewhat broader apron of
Slight Risk than shown by guidance.

Surrounding the Slight risk area...only modest changes were made
to the Marginal risk area extending northwest into the plains of
eastern Montana where a secondary QPF maximum is expected. The
expectation is that there will be two discrete QPF area with a risk
of excessive rainfall associated with each...but maintained the
one area given the range of possible solutions.

...Southwest US...

A continuation of flow with deeper moisture into the Southwest US
will linger on Monday and Monday night...with a continued chance
for scattered late day and evening convection across parts of New
Mexico and a small portion of Arizona that extends northward along
the southern and central Colorado front range.

Bann


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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
501 AM EDT Sun Jul 27 2025
Day 3
Valid 12Z Tue Jul 29 2025 - 12Z Wed Jul 30 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE WESTERN HIGH PLAINS...

Northern Rockies eastward to Great Lakes and southward to the
Southwest US...

Building mid-level ridge over British Columbia and Albert will
result in lowering heights over parts of the northwestern US and
increasing mid- and upper flow. This results in an increasingly
well defined surface boundary that serves to focus showers and
thunderstorm on Tuesday. Ingredients for thunderstorms to produce
heavy to potentially excessive rainfall appear to be aligning from
southeast Montana into northern Nebraska along a corridor of 1000
to 2000 J per kg instability and 1 to 1.25 inch precipitable
water values...and even some 1.5 inch PW values in northern
Nebraska. Showers and thunderstorms are also likely to spread
eastward toward the Great Lakes where the instability and synoptic
scale forcing looks to be weaker. In addition...another round of
late day convection is possible from the Southwest US into the
Rockies and some of this moisture may approach...if not come into
play,,,across the western High Plains late in the period.

...Southeast US...
Precipitable water values in excess of 2 inches will be in place
with 1000-1500 J per kg of CAPE in place and weakening of the broad
upper level ridge should lead to a somewhat better chance for
showers or thunderstorms on Tuesday. The GFS and ECMWF both show
hints at weak vorticity centers in the vicinity to help
focus/support some activity. The expectation is that isolated but
very intense rainfall rates could lead to isolated instances of
excessive rainfall.


Bann

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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
310 AM EDT Sun Jul 27 2025


Another hot day is forecast from the south-central U.S. into the
Southeast, Tennessee and Ohio Valleys, and much of the Eastern
Seaboard on Wednesday underneath the (albeit shrinking) upper high.
HeatRisk is mainly in the Major (level 3/4) category with some
embedded Extreme (level 4/4) areas with highs generally in the 90s
to near/exceeding 100 with higher heat indices, while morning lows
well into the 70s and nearing 80 will not provide much relief. A
few more daily records for temperatures could be set into
Wednesday. On Thursday, the cold front will pass through the Ohio
Valley to Mid-Atlantic and bring near to below normal temperatures,
with the heat wave becoming more limited to the
Southeast/Carolinas. The gradual movement of the front southward
should finally lead to heat relief in the Southeast Friday-
Saturday, though Florida could still be hot. Cooler than average
temperatures (especially highs) are likely in the northern/central
Plains on the cool side of the front and with the clouds and rain
chances.

Rain and thunderstorms are forecast to focus on the periphery of
the ridge, enhanced by a frontal boundary across the northern
Rockies into the north-central Plains and Mississippi Valley. Some
shortwave forcing and above normal moisture and instability should
lead to multiple rounds of convection with high rain rates, and
some areas could see repeating rounds of storms with the front
stalling. Thus for the Day 4/Wednesday and Day 5/Thursday ERO, show
Slight Risks over the central Plains for the possibility of flash
flooding. On Day 4/Wednesday, also have the Slight Risk looping
east into the Middle Mississippi Valley where models show MCSs and
antecedent conditions are generally wet. Marginal Risks stretch
north into the northern Rockies/High Plains near the back end of
the front as much higher than average instability (per the Extreme
Forecast Index) is in place. Meanwhile the front extends farther
east across the Ohio Valley and Northeast on Wednesday, promoting
potentially heavier rain in a moisture-laden pre-frontal
environment coincident with areas that have seen well above normal
rainfall the past several weeks, allowing for a Marginal Risk in
the Day 4/Wed ERO. By Thursday, the front will gradually move south
and lead to pooling moisture and instability in the Mid-Atlantic
along with increasing dynamical support in the form of the right
entrance region of the upper jet. Thus will show a Slight Risk in
the Day 5/Thu ERO for this activity. Enhanced rain is possible in
the Carolinas by Friday and Saturday near this front.

To the west of the ridge, monsoonal moisture (with precipitable
water values generally in the 75th-90th percentile) will be present
over the Southwest with some instability. Marginal Risks are in
place in the Days 4 and 5 EROs for the Southwest and reaching into
the central Rockies/High Plains as the monsoonal moisture meets the
frontal system to the north.

Elsewhere, moisture and instability could lead to scattered
thunderstorms in the southeastern U.S. from day to day. There may
be some focus for storms near the central Gulf Coast in particular
with weak low level troughs/lows in the vicinity. Marginal Risks of
flash flooding are in place for the central Gulf Coast and parts
of the Southeast due to high rain rates that may overcome the high
Flash Flood Guidance and/or fall atop urban areas.


Tate

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
310 AM EDT Sun Jul 27 2025


Another hot day is forecast from the south-central U.S. into the
Southeast, Tennessee and Ohio Valleys, and much of the Eastern
Seaboard on Wednesday underneath the (albeit shrinking) upper high.
HeatRisk is mainly in the Major (level 3/4) category with some
embedded Extreme (level 4/4) areas with highs generally in the 90s
to near/exceeding 100 with higher heat indices, while morning lows
well into the 70s and nearing 80 will not provide much relief. A
few more daily records for temperatures could be set into
Wednesday. On Thursday, the cold front will pass through the Ohio
Valley to Mid-Atlantic and bring near to below normal temperatures,
with the heat wave becoming more limited to the
Southeast/Carolinas. The gradual movement of the front southward
should finally lead to heat relief in the Southeast Friday-
Saturday, though Florida could still be hot. Cooler than average
temperatures (especially highs) are likely in the northern/central
Plains on the cool side of the front and with the clouds and rain
chances.

Rain and thunderstorms are forecast to focus on the periphery of
the ridge, enhanced by a frontal boundary across the northern
Rockies into the north-central Plains and Mississippi Valley. Some
shortwave forcing and above normal moisture and instability should
lead to multiple rounds of convection with high rain rates, and
some areas could see repeating rounds of storms with the front
stalling. Thus for the Day 4/Wednesday and Day 5/Thursday ERO, show
Slight Risks over the central Plains for the possibility of flash
flooding. On Day 4/Wednesday, also have the Slight Risk looping
east into the Middle Mississippi Valley where models show MCSs and
antecedent conditions are generally wet. Marginal Risks stretch
north into the northern Rockies/High Plains near the back end of
the front as much higher than average instability (per the Extreme
Forecast Index) is in place. Meanwhile the front extends farther
east across the Ohio Valley and Northeast on Wednesday, promoting
potentially heavier rain in a moisture-laden pre-frontal
environment coincident with areas that have seen well above normal
rainfall the past several weeks, allowing for a Marginal Risk in
the Day 4/Wed ERO. By Thursday, the front will gradually move south
and lead to pooling moisture and instability in the Mid-Atlantic
along with increasing dynamical support in the form of the right
entrance region of the upper jet. Thus will show a Slight Risk in
the Day 5/Thu ERO for this activity. Enhanced rain is possible in
the Carolinas by Friday and Saturday near this front.

To the west of the ridge, monsoonal moisture (with precipitable
water values generally in the 75th-90th percentile) will be present
over the Southwest with some instability. Marginal Risks are in
place in the Days 4 and 5 EROs for the Southwest and reaching into
the central Rockies/High Plains as the monsoonal moisture meets the
frontal system to the north.

Elsewhere, moisture and instability could lead to scattered
thunderstorms in the southeastern U.S. from day to day. There may
be some focus for storms near the central Gulf Coast in particular
with weak low level troughs/lows in the vicinity. Marginal Risks of
flash flooding are in place for the central Gulf Coast and parts
of the Southeast due to high rain rates that may overcome the high
Flash Flood Guidance and/or fall atop urban areas.


Tate

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
214 PM EDT Wed May 7 2025

Valid 00Z Thu May 08 2025 - 00Z Sun May 11 2025


...Colorado...
Day 1...

Lingering vorticity on the western side of the departing upper low
will still trigger some light snow over the San Juans this evening
before tapering off tonight/early Thursday. WPC probabilities for
at least an additional 4 inches of snow after 00Z are low (10-40%)
above 11,000ft.

For days 2-3, the probability of at least 4 inches of snow is less
than 10%.

The probability of significant freezing rain across the CONUS is
less than 10%.

Note - this is the last regularly scheduled discussion (QPFHSD)
for the 2024-2025 winter season. Unless a significant winter
weather threat emerges, this discussion will next be updated on or
about September 25, 2025.


Fracasso

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
310 AM EDT Sun Jul 27 2025


...Dangerous heat wave persists in the Southeast through midweek
with gradual relief later week...

...Heavy rain and flash flooding potential in the central
Plains for multiple days...


...Overview...

By the start of the medium range period midweek, the sprawling
upper high atop much of the U.S. early this week will be weakening
somewhat and be pressed south by a northern stream upper trough.
Dangerous heat is possible across the Lower Ohio/Tennessee Valleys
and Mid-Atlantic Wednesday before a cold frontal passage tied to
the upper trough, while the Southeast is forecast to see the heat
continue into late week but gradually shrink in scope. Complexes of
showers and storms are likely near and ahead of the frontal system
stretching from the High Plains eastward across the central
Plains/Mid-Mississippi Valley into the Northeast Wednesday. The
front is likely to stall near the central Plains and lead to
multiple days of storms with heavy rain that could cause flash
flooding. Farther east the cold front should be more progressive,
making its way southward across the Mid-Atlantic and Southeast but
creating an environment for possibly heavy rain each day as it
moves. Meanwhile, southerly flow will bring monsoonal moisture into
the Southwest states for possibly heavy rain.


...Guidance/Predictability Assessment...

Model agreement is excellent for the overall pattern through late
this week. Models have been persistent in showing the expansive
upper high shrinking in scope by Wednesday and becoming centered
over the southern Plains late week, with perhaps another
subtropical high moving across Florida. Meanwhile the ridge axis
will stretch north across the northern Rockies into western Canada
starting Tuesday, coincident with the upper trough edging into the
north-central to northeastern U.S. anchored by a large Hudson Bay
to eastern Canada upper low. Since model differences were mostly
negligible through late week, aside from the 12Z UKMET which showed
some differences in its handling of the ridge, used a 18Z GFS/12Z
ECMWF/12Z CMC blend for the first half of the forecast period.
Regardless of the large scale agreement, shortwaves riding around
the ridge will cause more differences in sensible weather like
frontal placement, and convection will have further uncertainties
due to small boundaries like outflows interacting. These tend not
to become clearer until the short range or near term, so the
details of QPF remain uncertain, but at least the overall pattern
seems set.

By next weekend, model differences start to increase as the
pattern becomes more quasi-zonal and uncertain shortwaves dominate
more. One larger scale model difference was the pattern in the
Northwest especially by next Sunday. This will be influenced by a
large Northeast Pacific upper low with somewhat uncertain
placement. The 12Z GFS was farthest southwest with the upper low
and did not allow for much troughing to come into the Northwest
compared to the non-NCEP guidance. AI/ML models are split but
leaned toward the Northwest troughing. Newer 00Z models have
trended east toward Southeast Alaska with the upper low, but still
have some differences with how much energy may split off this low
and when to create troughing in the West. Used some GEFS and EC
ensemble means by the latter part of the medium range to temper the
individual model differences by that time.


...Weather/Hazards Highlights...

Another hot day is forecast from the south-central U.S. into the
Southeast, Tennessee and Ohio Valleys, and much of the Eastern
Seaboard on Wednesday underneath the (albeit shrinking) upper high.
HeatRisk is mainly in the Major (level 3/4) category with some
embedded Extreme (level 4/4) areas with highs generally in the 90s
to near/exceeding 100 with higher heat indices, while morning lows
well into the 70s and nearing 80 will not provide much relief. A
few more daily records for temperatures could be set into
Wednesday. On Thursday, the cold front will pass through the Ohio
Valley to Mid-Atlantic and bring near to below normal temperatures,
with the heat wave becoming more limited to the
Southeast/Carolinas. The gradual movement of the front southward
should finally lead to heat relief in the Southeast Friday-
Saturday, though Florida could still be hot. Cooler than average
temperatures (especially highs) are likely in the northern/central
Plains on the cool side of the front and with the clouds and rain
chances.

Rain and thunderstorms are forecast to focus on the periphery of
the ridge, enhanced by a frontal boundary across the northern
Rockies into the north-central Plains and Mississippi Valley. Some
shortwave forcing and above normal moisture and instability should
lead to multiple rounds of convection with high rain rates, and
some areas could see repeating rounds of storms with the front
stalling. Thus for the Day 4/Wednesday and Day 5/Thursday ERO, show
Slight Risks over the central Plains for the possibility of flash
flooding. On Day 4/Wednesday, also have the Slight Risk looping
east into the Middle Mississippi Valley where models show MCSs and
antecedent conditions are generally wet. Marginal Risks stretch
north into the northern Rockies/High Plains near the back end of
the front as much higher than average instability (per the Extreme
Forecast Index) is in place. Meanwhile the front extends farther
east across the Ohio Valley and Northeast on Wednesday, promoting
potentially heavier rain in a moisture-laden pre-frontal
environment coincident with areas that have seen well above normal
rainfall the past several weeks, allowing for a Marginal Risk in
the Day 4/Wed ERO. By Thursday, the front will gradually move south
and lead to pooling moisture and instability in the Mid-Atlantic
along with increasing dynamical support in the form of the right
entrance region of the upper jet. Thus will show a Slight Risk in
the Day 5/Thu ERO for this activity. Enhanced rain is possible in
the Carolinas by Friday and Saturday near this front.

To the west of the ridge, monsoonal moisture (with precipitable
water values generally in the 75th-90th percentile) will be present
over the Southwest with some instability. Marginal Risks are in
place in the Days 4 and 5 EROs for the Southwest and reaching into
the central Rockies/High Plains as the monsoonal moisture meets the
frontal system to the north.

Elsewhere, moisture and instability could lead to scattered
thunderstorms in the southeastern U.S. from day to day. There may
be some focus for storms near the central Gulf Coast in particular
with weak low level troughs/lows in the vicinity. Marginal Risks of
flash flooding are in place for the central Gulf Coast and parts
of the Southeast due to high rain rates that may overcome the high
Flash Flood Guidance and/or fall atop urban areas.


Tate


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