Convective Outlook

Day 4-8 Outlook

Day 4-8 Convective Outlook  
NWS Storm Prediction Center Norman OK
0314 AM CST Sat Dec 04 2021

Valid 071200Z - 121200Z

...DISCUSSION...
Modest amplification of the mid-level westerlies may occur over
portions of the central/eastern CONUS on Day 4/Tuesday into Day
5/Wednesday. Regardless, a prior frontal passage into the northern
Gulf of Mexico should generally inhibit the inland advance of
substantial low-level moisture along most of the Gulf Coast in this
time frame. Parts of FL and southern GA may be an exception, but
weak forecast instability should limit the threat for organized
severe convection.

Latest medium-range guidance is starting to come into better
agreement with the prospect for another upper trough to amplify and
move eastward across the western/central CONUS around Day 6/Thursday
into Day 7/Friday. If this occurs, then the related low-level mass
response would encourage the inland return of rich low-level
moisture from the Gulf of Mexico into parts of the Southeast and
perhaps even the OH Valley. Of course, there are differences in the
placement and evolution of this upper trough and attendant surface
features at this extended time frame. Still, an increase in severe
potential is evident based on some of the latest models for parts of
the lower MS Valley and Southeast on Day 8/Saturday. While it is too
soon to include a 15% severe delineation due to model differences
and inherent uncertainty, this scenario will be closely monitored.

Updated 4 December 2021 | 3:17 am

SPC 0830Z Day 3 Outlook

Day 3 Convective Outlook  
NWS Storm Prediction Center Norman OK
0215 AM CST Sat Dec 04 2021

Valid 061200Z - 071200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
COASTAL/EAST TEXAS INTO THE LOWER MISSISSIPPI VALLEY...

...SUMMARY...
Isolated strong to severe thunderstorms may occur mainly Monday
morning from parts of coastal/east Texas into the lower Mississippi
Valley.

...Synopsis...
A highly amplified, large-scale upper trough should move from the
central to eastern CONUS on Monday. An upper low in the southern
stream should shift eastward across northern Mexico through the day,
eventually reaching south TX late Monday into early Tuesday morning.
At the surface, a low over the Upper Great Lakes is forecast to
develop northeastward into Ontario/Quebec, with a trailing cold
front likely to continue sweeping east-southeastward over the East
Coast, Southeast, and southern Plains through the period.

...Coastal/East Texas into the Lower Mississippi Valley...
Thunderstorms should be ongoing early Monday morning along/ahead of
the front across parts of coastal/east TX into the lower MS Valley.
The primary forcing associated with the upper trough will remain
displaced to the north of this region. Still, the low-level ascent
associated with the surface cold front should be enough to support
continued convection through at least Monday morning. Generally low
to mid 60s surface dewpoints and modest diurnal heating should
foster weak instability across the warm sector. Even though the
low/mid-level wind field will likely veer and gradually weaken
though the day as the upper trough shifts farther east, there should
be enough enhancement to the mid-level westerlies to support
sufficient deep-layer shear for modest updraft organization.
Isolated strong to severe storms posing a threat for mainly damaging
winds and perhaps marginally severe hail appear possible mainly
Monday morning. As convection develops east-southeastward in tandem
with the cold front, the deep-layer shear should slowly diminish
through the day. There should be a tendency for these thunderstorms
to gradually weaken with southward and eastward extent by late
Monday afternoon.

...Northeast...
Very strong low/mid-level flow is forecast to overspread much of the
Northeast on Monday as the upper trough progresses over the eastern
states. Latest guidance suggests that, at best, low 50s surface
dewpoints may attempt to move inland across parts of southern New
England ahead of the cold front. Weak MUCAPE extending through a
sufficiently deep layer could support charge separation and
occasional lightning flashes with elevated convection. At this
point, the potential for surface-based thunderstorms with any
strong/gusty wind threat appears limited due to the meager low-level
moisture and related lack of boundary-layer instability.

..Gleason.. 12/04/2021

Updated 4 December 2021 | 2:18 am

SPC 0700Z Day 2 Outlook

Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1225 AM CST Sat Dec 04 2021

Valid 051200Z - 061200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
COASTAL/EAST TEXAS TO THE MID-SOUTH AND LOWER OHIO VALLEY...

...SUMMARY...
Isolated strong to severe thunderstorms appear possible mainly
Sunday evening and overnight from parts of coastal/east Texas to the
Mid-South and lower Ohio River Valley.

...Synopsis...
An upper trough initially over the north-central CONUS will likely
undergo substantial amplification on Sunday as it digs
east-southeastward across much of the Midwest and Great Lakes. A
weak upper low over the Baja California vicinity is forecast to move
little through the period. Still, low-amplitude perturbations within
the southern branch of the mid-level westerlies downstream from the
upper low may also aid the amplification of the large-scale upper
trough over the lower/mid MS Valley by Sunday night. A surface low
initially over the Dakotas Sunday morning will develop eastward
towards the Upper Great Lakes by late Sunday night. An attendant
cold front is forecast to sweep east-southeastward over much of the
Plains, Midwest, and mid MS Valley through the period.

...Coastal/East Texas to the Mid-South and Lower Ohio River
Valley...
Modest low-level moisture emanating from the western Gulf of Mexico
should return northward through the day across much of the southern
Plains, lower MS Valley, Mid-South, and perhaps a small part of the
lower OH Valley ahead of the advancing cold front. Various model
forecast soundings across this region suggest this moisture will
likely remain fairly shallow away from the Gulf Coast. Residual
low-level capping will probably tend to inhibit robust convective
development through much of the day across the warm sector.

Still, thunderstorm initiation looks increasingly likely by Sunday
evening along or just ahead of the front from parts of eastern OK to
the Mid-South vicinity as ascent associated from the amplifying
upper trough overspreads these areas. Regardless, MLCAPE should
remain fairly weak, generally in the 250-1000 J/kg range, with mid
to upper 50s and lower 60s surface dewpoints prevalent. But, both
low-level and deep-layer shear are forecast to strengthen quickly
Sunday evening and through the overnight hours with the approach of
the upper trough. This should foster some convective organization,
and isolated strong to damaging winds may occur with thunderstorms
that form along the front and quickly become linear while moving
east-southeastward. Given the expected strength of the
boundary-layer flow and favorable low-level shear as a southwesterly
low-level jet strengthens through the evening, a tornado or two also
appears possible. The lack of stronger forecast instability
currently limits confidence in the potential for a more robust
severe threat.

There also appears to be some chance for open warm-sector
development from parts of east/coastal TX and southeastern OK into
AR and western LA Sunday evening/night. Steepened mid-level lapse
rates and slightly greater low-level moisture across these areas
should support MUCAPE up to 1000-2000 J/kg. Isolated severe hail may
occur with any discrete storms that can form given the strong
effective bulk shear, in addition to gusty downdraft winds and
perhaps a tornado. The convection that develops farther north along
the front should outpace the modest low-level moisture return late
Sunday night into early Monday morning, especially with eastward
extent across the Mid-South into the TN and OH Valleys. Accordingly,
these thunderstorms should gradually weaken in this time frame.

..Gleason.. 12/04/2021

Updated 4 December 2021 | 12:29 am