Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
119 PM EDT Sat May 18 2024

Valid 00Z Sun May 19 2024 - 00Z Tue May 21 2024

...Increasing severe weather and excessive rainfall threats over the
Central U.S. through early next week...

...Sweltering heat continues across South Florida and southern Texas,
building into the southern High Plains this weekend...

Stormy conditions should wind down across the central Gulf coast while
persisting along a quasi-stationary boundary over Florida through Sunday.
Attention shifts toward the Central U.S. on Sunday and into early next
week as a deep mid-level low digs into the West. A surface low pressure
system will bring heavy rain and thunderstorms to parts of the Great
Plains and Upper/Middle Mississippi Valley on Sunday. Despite the
Excessive Rainfall threat being Marginal (at least 5%), the Storm
Prediction Center issued an Enhanced Risk (level 3/5) of Severe
Thunderstorms across portions of western and central Kansas on Sunday.
Significant damaging gusts, large to very large hail, and a few tornadoes
are all possible according to the SPC. A Slight Risk (level 2/5) of Severe
Thunderstorms is in effect for portions of Nebraska, Kansas, southwest
Iowa and northwest Missouri. Hail and strong wind gusts will be the
primary threats from these storms. Additionally, some light snow to
moderate snowfall is possible on the backside of this system in the
Northern Rockies over the next couple of days.

Intense Summer-like heat will continue over portions of South Florida and
southern Texas this weekend, and expand in coverage into portions of the
southern High Plains. Forecast highs will be in the 90s for Florida with
mid-90s to mid-100s in Texas, potentially record-tying/breaking levels.
When combined with the humidity, heat indices will soar to near 110 in
South Florida. While not quite as hot, temperatures will still be well
above average more broadly across much of the country this weekend,
particularly from the Central Plains into the Midwest where highs in the
80s to near 90 will be common. Highs will also be above average for
portions of the West, with 70s and 80s in the Great Basin/interior
California and 90s to low 100s in the Desert Southeast. More temperate,
below average conditions are expected along much of the East Coast, with
50s and 60s in New England and 60s and 70s into the
Mid-Atlantic/Carolinas. The Pacific Northwest/Northern Rockies will also
be cooler, with highs in the 50s and 60s expected here as well. Variable
temperatures are forecast for the Southeast due to ongoing storms, with
mainly 80s expected.

Kebede/Putnam

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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
314 PM EDT Sat May 18 2024

Valid 12Z Tue May 21 2024 - 12Z Sat May 25 2024


...Overview...

Multiple progressive upper shortwaves/possible closed lows and
surface low pressure/frontal systems will traverse the West to
northern/central Plains and Midwest during the medium range period.
For Tuesday- Wednesday, a potent shortwave will send a modestly
deep surface low into the Upper Midwest and potential for at least
locally heavy rain and thunderstorms across the region. Its
attendant cold front will push across the East and partway into the
South midweek and beyond, with rain and storms along and ahead of
it and cooler temperatures behind. Heat will continue to be
hazardous for at least southern Texas though. Another couple rounds
of energy may push through the Northwest eastward late week, but
with much more uncertainty.


...Guidance/Predictability Assessment...

Models show reasonable agreement with the first shortwave that
kicks out into the Upper Midwest Tuesday-Wednesday, though with
some uncertainty in the timing and depth of the surface low with
the potential merging of upper-level energies. The next upper low
tracking through the Northwest Wednesday and eastward late week
starts with slightly timing better agreement compared to runs from
a day ago, as 06Z and new 12Z GFS runs are more agreeable in
showing this feature separate from upstream energy, similar to the
non- NCEP guidance. There is still some spread in the deterministic
and AI models though. By Saturday, the 00Z deterministic ECMWF
seemed to become an outlier in producing ridging over the north-
central U.S. as it is faster to shear out the shortwave energy.
Other models including EC-based AI models prefer troughing if not
the upper low remaining closed there, and the 12Z ECMWF trended
toward this, though north with its center into south-central
Canada. Then upstream, models generally show yet another closed low
dropping south into the Northwest for the end of the week. The 00Z
ECMWF was west of other operational and AI models with its
position by Saturday. Another feature of possible concern but on
the smaller scale is a southern stream shortwave moving through the
southern Plains to Mid-South to Mid-Atlantic Thursday-Saturday,
which has implications for QPF among other sensible weather, so
this will continue to be monitored.

Thus the WPC forecast utilized a blend of the deterministic
guidance early in the period, but introduced and increased the
proportion of ensemble means to over half by the end of the period
given the model spread. This maintained reasonable continuity with
the previous forecast, though slowed the central U.S. surface low
and front by Saturday with most models trending that way.


...Weather/Hazards Highlights...

An upper shortwave and leading cold front, with deepening surface
low, will push into the Upper Midwest Tuesday/Wednesday, helping to
fuel showers and thunderstorms across this region. Ample moisture
will be in place while the low track and upper level flow provide
significant lift for heavy rain on Tuesday. Thus will embed a
Slight Risk in the Day 4/Tuesday Excessive Rainfall Outlook across
parts of the Upper Midwest. Notable instability will be present on
the warm side of the low, which will also provide support for
likely severe thunderstorms over the Middle Mississippi Valley
according to the Storm Prediction Center. But, a limiting factor
for flash flooding farther north will be the lack of instability on
the backside of the low, despite model guidance showing heavy
amounts across northern Minnesota for example. A broader Marginal
Risk extends around and south just ahead of the cold front. After
Tuesday, expect northeastward progression of the surface low to
push the trailing cold front and accompanying rain/storms farther
east and south with some more potential for heavy rainfall in the
eastern and southern U.S. for Wednesday-Thursday. The front is
likely to slow or stall west-east in a moist and unstable
environment, and with multiple rounds of convection with possible
heavy rain rates, will embed a Slight Risk in the ERO for Day
5/Wednesday from around Texarkana into the Mid-South. Also expanded
the Marginal Risk into west- central Texas with possible dryline
convection with heavy rain rates. Approach/arrival of another
couple of systems as currently advertised into the Northwest by
midweek and next weekend would produce somewhat more organized
precipitation there, and potential for heavy snow in the mountains,
with snow levels dependent on the depth of the upper low(s).

Expect South Texas to see multiple days of hazardous heat during
the period with highs persistently running 10-15F above normal with
max heat index values possibly reaching at least 110F. Highs near
100 degrees could stretch farther north across the southern High
Plains at times as well. Some daily records for highs/warm lows
will be possible. Above normal highs will also track east across
the Midwest and Northeast into Wednesday but should moderate by
Thursday-Friday as the cold front pushes through the region. The
forecast pattern will favor below average highs over the Northwest
to northern Plains for most of next week.


Tate/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
330 AM EDT Sat May 18 2024

Valid 00Z Sun May 19 2024 - 00Z Sun May 26 2024

...Heavy rain threat continues through Saturday for western
islands...

The latest guidance continues to indicate that the deep Kona low
north-northwest of Hawaii and associated surface boundary reaching
the main islands will support heavy rainfall and flash flood
threats mainly for western islands into this weekend. By early
next week, the models show good agreement the low should weaken
and lift north, but another shortwave/upper low will drop in to
the northwest of the islands behind it. This will bring moderate
east-southeasterly winds and both a windward and sea breeze focus
for showers with rainfall generally lighter than the weekend,
though the combination of PWATs staying somewhat above normal and
upper level impulses could still provide some localized
enhancement. By later in the week and into next weekend, upper
ridging and surface high pressure will build back in to the north
of Hawaii with a return to more normal trade wind pattern.

Santorelli

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
405 PM EDT Sat May 18 2024
Day 1
Valid 16Z Sat May 18 2024 - 12Z Sun May 19 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST/CENTRAL GULF COAST...

...16Z Update...

Maintained continuity through a majority of the previously covered
areas within the SLGT and MRGL risk areas. The prospects for flash
flooding remain highest over the Southeast U.S with convergence
along the cold front slowly progressing eastward through the
period. The primary period will be through the 00z hour before the
threat wanes as the best ascent moves away from the region and we
see more isolated to scattered rainfall coverage with less
prominence in the hourly rates. Further north over AL, a remnant
MCV will move east-southeast into northern MS/AL by this afternoon
providing a focal point for convective initiation during peak
instability. The PWAT anomalies are fairly tame for a higher end
threat, but some isolated spots could see 2-3" over the course of
the afternoon with a risk of flash flooding while cells move
overhead. Given the probabilities for 1"/hr running between 40-60%
and 2"/hr around 10-15% at peak, the threat is certainly capped for
any prospects for widespread flooding, so the MRGl risk in place
will suffice for the setup. Across NC, antecedent conditions favor
a better opportunity for flash flooding concerns with streamflow
anomalies relatively high after the recent active pattern across
the region. A backdoor front will progress to the south during the
afternoon and evening with a convergent low-level ascent pattern
developing from western NC through east-central portions of the
state. Instability and PWAT anomalies are generally modest with
scattered convection likely to form within the next few hours
across the terrain west of the Triad. Some isolated totals will
likely hit the 2-3" marker with perhaps a max around 4" based on
the latest HREFpmm and higher-end deterministic, so the threat is
certainly within reason, but the isolated nature of the potential
leans this more on the cusp of a true SLGT risk and a MRGL.

Considering the downward trend of some of the probability fields
from the previous iteration of the ensemble, as well as
collaboration with the affected WFO's, decided to maintain
continuity to allow for consistent messaging and the threat being
truly isolated. There is a potential for a short term upgrade, but
not at this time.

Kleebauer


...Previous Discussion...

...Southern Alabama/Georgia into the Florida Panhandle...

Mid-level trough/weak closed low over the Lower MS Valley/Mid-
South this morning will continue eastward across southern TN today
and into the southern Appalachians by early Sunday. On its
southeast side, a stream of mid- level impulses will move across
the FL Panhandle and southern AL/GA today ahead of a cold front
that will finally clear through the area by tomorrow. SW flow out
of the Gulf will maintain relatively high precipitable water values
(~1.75" or +1.5-2 sigma) into the region ahead of the trough and
surface front, with more impressive 850-700mb moisture flux
anomalies into the region (+3 to +4 sigma). With ongoing convection
over southern LA moving eastward, expect this to continue past 12Z
along the I-10 corridor from southern MS eastward through early
afternoon. Thereafter, models suggest a loss of mid-level vorticity
dynamics to sustain heavier rainfall but this may be underdone
given the approach of the cold front with still sufficient moisture
available. Slight Risk outline was generally constrained to the
higher probabilities of >2" QPF in the 12-18Z period this morning.
Broader Marginal Risk covers much of the rest of the Southeast into
the Carolinas where isolated heavier rain rates are possible.
Trimmed away the northern/northeastern side of the Marginal Risk
where instability is low (or non-existent) on easterly flow.

Fracasso

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
405 PM EDT Sat May 18 2024
Day 2
Valid 12Z Sun May 19 2024 - 12Z Mon May 20 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL PLAINS, UPPER MIDWEST, AND COASTAL SOUTH
CAROLINA/GEORGIA...

...20Z Update...

The previous forecast necessitated very little change across the
central plains and corn belt with the synoptic pattern very much on
track from prior NWP iterations. The only change was the expansion
of the MRGL risk further north into MN and now the western portion
WI, mainly the region north of LaCrosse. Recent trends in the hi-
res convective evolution downstream are the reason for the addition
with much of the CAMs showing fairly robust coverage within an
upscale growth regime that originates over SD into western MN. A
tongue of elevated theta-E's is forecast to be advected into
southern MN with the multi-cell cluster out of the Dakotas to pivot
eastward along the leading edge of the instability gradient. 12z
HREF signal for rates >1"/hr have risen substantially from previous
output with a swath of 40-60% probabilities and some bullseye's
eclipsing the 70% marker, a signal indicative of greater potential
overall. The rates are likely to be capped, however as the best
instability will lie south of the MN border by the end of the
period when the convection makes its way to the MN/WI border.
Regardless, shortwave propagation within the broad southwest flow
regime will be enough to maintain the organized nature of the
convection and provide some locally heavy precip across the
northern Midwest. The ECMWF is still holding on to the primary
threat residing within IA, but there seems to be a split with one
batch in KS and another in MN. Considering the bit of uncertainty
and with the environment favorable for heavy rainfall from KS and
points northeast, kept the MRGL risk with an opportunity for a SLGT
risk upgrade if CAMs latch on to a particular area with greater
impact (ie: Kansas or MN/WI).

An additional MRGL risk was added portions of coastal SC/GA. More
on that in the sub-section below...

...Coastal South Carolina and Georgia...

Backdoor front will be pressing south through the Carolinas the
beginning of the period with a surface wave riding up along a cold
front present across southern GA. A combination of focused low-
level forcing from the north and onshore development near the SC/GA
border will create a time frame for prime convective development in
the overlap of the onshore regime and frontal convergence over
land. The instability fields depict this setup very well with a
sharp inflection in the MUCAPE fields with little to no CAPE north
and west of the coastal plain, but a skinny area of 1500-2500 J/kg
located from Charleston down to just south of Savannah. This is
where the latest CAMs are pinpointing the best threat of convection
and rates bordering 1-2"/hr with very high neighborhood
probabilities showing up within the latest HREF for at least 1"/hr
(60-80%) and reasonable up to 2"/hr (20-30%). The biggest factor
will be the speed of the front to the north and where does the
intersection point occur. Some are further into GA and others
closer to Charleston, which is a major factor in the flash flood
potential as the urban corridors will undoubtedly be the most
susceptible. After some collaboration with the local WFO, the
threat was great enough to warrant a MRGL, but likely on the lower-
end of the threshold necessary with a focus on the population
centers.

Kleebauer


...Previous Discussion...

...Central Plains and Upper Midwest...

Broad SW flow over the central Plains across the Corn Belt will
bring in increasing moisture to the region on Sunday with a surface
frontal boundary hung up over Iowa. Mid-level vorticity will exit
the central Rockies and moves across the central Plains as
precipitable water values rise to around 1.50" (+1.5-2 sigma) with
some instability in the afternoon/early evening (500-2000J/kg) that
could yield some 1"/hr rates (per 00Z HREF thru 00Z Mon) which are
near FFG values over Iowa and nearby areas. The region has been
fairly dry the past week (most of the region has had about 50% of
the normal rainfall) so much of the rain will be beneficial, but
given 1-hr FFG values near 1"/hr (between MCI- OMA- DSM),
maintained the Marginal Risk outline for the region. Higher
probabilities exist farther southwest into central Kansas (to where
the Marginal was extended), but FFG values are higher as well
(2"/hr). 12Z CAM guidance through 12Z Mon may shed a little more
light on the higher rainfall potential over the entire 24 hours.


Fracasso

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
405 PM EDT Sat May 18 2024
Day 3
Valid 12Z Mon May 20 2024 - 12Z Tue May 21 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL PLAINS/CORN BELT...

...20Z Update...

The MRGL risk was maintained, but the orientation was changed
from the previous forecast issuance considering the latest trends
within the QPF fields in the bias corrected ensemble and ML
guidance. The premise for the heaviest rainfall over the Sandhills
of NE remains the same with that area mostly void of the risk area
due to very high FFGs. Further to the south, models are beginning
to focus an initiation for heavy rainfall across northeast CO into
southwest NE where the FFG indices are more favorable, albeit not
significantly to favor a flash flood potential with the current
precip forecast. We still reside outside of the CAMs window, but
the pattern is ripe for elevated convective concerns and locally
enhanced rates within the diffluent side of the sharpening
shortwave trough entering the high plains. The MRGL was maneuvered
towards the eastern fringes of the Sandhill's down towards the
KS/CO/NE border where the latest ML guidance continues to hint a
QPF maximum where the frontal boundary will be focused. This
extends back into northeast CO where the convective initiation is
being depicted, as well as north of the Sandhill's in southern SD
where the best QPF footprint outside the higher FFG areas is
located.

Kleebauer


...Previous Discussion...

Upper pattern remains fairly similar to D2 across the Plains, with
broad SW flow over the region. One wave will exit through the
Upper Midwest as troughing over the West deepens via northern and
incoming southern stream vorticity. Upper diffluence east of the
Rockies will support widespread showers/storms over the central
Plains Monday into Monday night along the stalled surface boundary,
with sufficient moisture in place and sfc CAPE 500-1500J/kg.
Heaviest rainfall per the ensembles lie over western Nebraska (Sand
Hills) where FFG values are highest, so focused the area to the
southeast and east into Iowa where rainfall on D2 may lower FFG
values a bit (from already somewhat lower numbers).

Fracasso

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
314 PM EDT Sat May 18 2024


An upper shortwave and leading cold front, with deepening surface
low, will push into the Upper Midwest Tuesday/Wednesday, helping to
fuel showers and thunderstorms across this region. Ample moisture
will be in place while the low track and upper level flow provide
significant lift for heavy rain on Tuesday. Thus will embed a
Slight Risk in the Day 4/Tuesday Excessive Rainfall Outlook across
parts of the Upper Midwest. Notable instability will be present on
the warm side of the low, which will also provide support for
likely severe thunderstorms over the Middle Mississippi Valley
according to the Storm Prediction Center. But, a limiting factor
for flash flooding farther north will be the lack of instability on
the backside of the low, despite model guidance showing heavy
amounts across northern Minnesota for example. A broader Marginal
Risk extends around and south just ahead of the cold front. After
Tuesday, expect northeastward progression of the surface low to
push the trailing cold front and accompanying rain/storms farther
east and south with some more potential for heavy rainfall in the
eastern and southern U.S. for Wednesday-Thursday. The front is
likely to slow or stall west-east in a moist and unstable
environment, and with multiple rounds of convection with possible
heavy rain rates, will embed a Slight Risk in the ERO for Day
5/Wednesday from around Texarkana into the Mid-South. Also expanded
the Marginal Risk into west- central Texas with possible dryline
convection with heavy rain rates. Approach/arrival of another
couple of systems as currently advertised into the Northwest by
midweek and next weekend would produce somewhat more organized
precipitation there, and potential for heavy snow in the mountains,
with snow levels dependent on the depth of the upper low(s).

Expect South Texas to see multiple days of hazardous heat during
the period with highs persistently running 10-15F above normal with
max heat index values possibly reaching at least 110F. Highs near
100 degrees could stretch farther north across the southern High
Plains at times as well. Some daily records for highs/warm lows
will be possible. Above normal highs will also track east across
the Midwest and Northeast into Wednesday but should moderate by
Thursday-Friday as the cold front pushes through the region. The
forecast pattern will favor below average highs over the Northwest
to northern Plains for most of next week.


Tate/Santorelli

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
314 PM EDT Sat May 18 2024


An upper shortwave and leading cold front, with deepening surface
low, will push into the Upper Midwest Tuesday/Wednesday, helping to
fuel showers and thunderstorms across this region. Ample moisture
will be in place while the low track and upper level flow provide
significant lift for heavy rain on Tuesday. Thus will embed a
Slight Risk in the Day 4/Tuesday Excessive Rainfall Outlook across
parts of the Upper Midwest. Notable instability will be present on
the warm side of the low, which will also provide support for
likely severe thunderstorms over the Middle Mississippi Valley
according to the Storm Prediction Center. But, a limiting factor
for flash flooding farther north will be the lack of instability on
the backside of the low, despite model guidance showing heavy
amounts across northern Minnesota for example. A broader Marginal
Risk extends around and south just ahead of the cold front. After
Tuesday, expect northeastward progression of the surface low to
push the trailing cold front and accompanying rain/storms farther
east and south with some more potential for heavy rainfall in the
eastern and southern U.S. for Wednesday-Thursday. The front is
likely to slow or stall west-east in a moist and unstable
environment, and with multiple rounds of convection with possible
heavy rain rates, will embed a Slight Risk in the ERO for Day
5/Wednesday from around Texarkana into the Mid-South. Also expanded
the Marginal Risk into west- central Texas with possible dryline
convection with heavy rain rates. Approach/arrival of another
couple of systems as currently advertised into the Northwest by
midweek and next weekend would produce somewhat more organized
precipitation there, and potential for heavy snow in the mountains,
with snow levels dependent on the depth of the upper low(s).

Expect South Texas to see multiple days of hazardous heat during
the period with highs persistently running 10-15F above normal with
max heat index values possibly reaching at least 110F. Highs near
100 degrees could stretch farther north across the southern High
Plains at times as well. Some daily records for highs/warm lows
will be possible. Above normal highs will also track east across
the Midwest and Northeast into Wednesday but should moderate by
Thursday-Friday as the cold front pushes through the region. The
forecast pattern will favor below average highs over the Northwest
to northern Plains for most of next week.


Tate/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
314 PM EDT Sat May 18 2024

Valid 12Z Tue May 21 2024 - 12Z Sat May 25 2024


...Overview...

Multiple progressive upper shortwaves/possible closed lows and
surface low pressure/frontal systems will traverse the West to
northern/central Plains and Midwest during the medium range period.
For Tuesday- Wednesday, a potent shortwave will send a modestly
deep surface low into the Upper Midwest and potential for at least
locally heavy rain and thunderstorms across the region. Its
attendant cold front will push across the East and partway into the
South midweek and beyond, with rain and storms along and ahead of
it and cooler temperatures behind. Heat will continue to be
hazardous for at least southern Texas though. Another couple rounds
of energy may push through the Northwest eastward late week, but
with much more uncertainty.


...Guidance/Predictability Assessment...

Models show reasonable agreement with the first shortwave that
kicks out into the Upper Midwest Tuesday-Wednesday, though with
some uncertainty in the timing and depth of the surface low with
the potential merging of upper-level energies. The next upper low
tracking through the Northwest Wednesday and eastward late week
starts with slightly timing better agreement compared to runs from
a day ago, as 06Z and new 12Z GFS runs are more agreeable in
showing this feature separate from upstream energy, similar to the
non- NCEP guidance. There is still some spread in the deterministic
and AI models though. By Saturday, the 00Z deterministic ECMWF
seemed to become an outlier in producing ridging over the north-
central U.S. as it is faster to shear out the shortwave energy.
Other models including EC-based AI models prefer troughing if not
the upper low remaining closed there, and the 12Z ECMWF trended
toward this, though north with its center into south-central
Canada. Then upstream, models generally show yet another closed low
dropping south into the Northwest for the end of the week. The 00Z
ECMWF was west of other operational and AI models with its
position by Saturday. Another feature of possible concern but on
the smaller scale is a southern stream shortwave moving through the
southern Plains to Mid-South to Mid-Atlantic Thursday-Saturday,
which has implications for QPF among other sensible weather, so
this will continue to be monitored.

Thus the WPC forecast utilized a blend of the deterministic
guidance early in the period, but introduced and increased the
proportion of ensemble means to over half by the end of the period
given the model spread. This maintained reasonable continuity with
the previous forecast, though slowed the central U.S. surface low
and front by Saturday with most models trending that way.


...Weather/Hazards Highlights...

An upper shortwave and leading cold front, with deepening surface
low, will push into the Upper Midwest Tuesday/Wednesday, helping to
fuel showers and thunderstorms across this region. Ample moisture
will be in place while the low track and upper level flow provide
significant lift for heavy rain on Tuesday. Thus will embed a
Slight Risk in the Day 4/Tuesday Excessive Rainfall Outlook across
parts of the Upper Midwest. Notable instability will be present on
the warm side of the low, which will also provide support for
likely severe thunderstorms over the Middle Mississippi Valley
according to the Storm Prediction Center. But, a limiting factor
for flash flooding farther north will be the lack of instability on
the backside of the low, despite model guidance showing heavy
amounts across northern Minnesota for example. A broader Marginal
Risk extends around and south just ahead of the cold front. After
Tuesday, expect northeastward progression of the surface low to
push the trailing cold front and accompanying rain/storms farther
east and south with some more potential for heavy rainfall in the
eastern and southern U.S. for Wednesday-Thursday. The front is
likely to slow or stall west-east in a moist and unstable
environment, and with multiple rounds of convection with possible
heavy rain rates, will embed a Slight Risk in the ERO for Day
5/Wednesday from around Texarkana into the Mid-South. Also expanded
the Marginal Risk into west- central Texas with possible dryline
convection with heavy rain rates. Approach/arrival of another
couple of systems as currently advertised into the Northwest by
midweek and next weekend would produce somewhat more organized
precipitation there, and potential for heavy snow in the mountains,
with snow levels dependent on the depth of the upper low(s).

Expect South Texas to see multiple days of hazardous heat during
the period with highs persistently running 10-15F above normal with
max heat index values possibly reaching at least 110F. Highs near
100 degrees could stretch farther north across the southern High
Plains at times as well. Some daily records for highs/warm lows
will be possible. Above normal highs will also track east across
the Midwest and Northeast into Wednesday but should moderate by
Thursday-Friday as the cold front pushes through the region. The
forecast pattern will favor below average highs over the Northwest
to northern Plains for most of next week.


Tate/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