- Day Three
acus01 kwns 201258
Storm Prediction Center ac 201256
Day 1 convective outlook
National Weather Service Storm Prediction Center Norman OK
0756 am CDT sun Oct 20 2019
Valid 201300z - 211200z
..there is an enhanced risk of severe thunderstorms over parts of
eastern Oklahoma...western Arkansas and north-Central/Northeast
The greatest severe-weather threat appears to be this evening into
tonight across parts of eastern Oklahoma, western Arkansas and
north-Central/Northeast Texas. Large hail, damaging gusts and
tornadoes are all possible.
In mid/upper levels, a broad area of cyclonic flow will cover most
of the Continental U.S. Between the Appalachians and the western intermountain
region this period. A broad area of height falls will spread across
the plains and Mississippi Valley, ultimately eastward toward the
Appalachians, once amplifying ridging crosses the Appalachians
tonight. As this occurs, the remnants of former T.S. Nestor, and a
related midlevel shortwave perturbation, will eject across the
remainder of the Carolinas and over the Atlantic.
A mid/upper-level low already appears to be forming over north-
central/northeastern WY, along a strong trough apparent in moisture-
channel imagery from eastern Montana to southern Colorado. This feature is
responsible for record to near-record cold 500-mb temperatures at
proximal radiosonde observation sites this morning, including -28 c at gjt. The
resultant 500-mb cyclone should be centered over south-central South Dakota
by 00z, with troughing south-southwestward across northeastern nm.
By 12z, this low should reach the fsd area, becoming nearly stacked
atop its low-level counterpart.
The related surface low was analyzed at 11z between cdr-rap, with
cold front across western NE and eastern Colorado. By 00z, the low should
reach south-central South Dakota north of vtn, with cold front arching across
east-central NE and central Kansas to a weak low over southwestern OK,
southwestward across southeastern nm. A dryline should develop
today and, by 00z, set up from central/west-central OK to near an
Special Weather Statement-drt line. The front will overtake the dryline from north to
south overnight, reaching a 12z position from central Iowa to
southeastern MO, southwestern AR, and south-central Texas.
..Ozarks to north-central Texas...Arlkatex...mid-south...
scattered to numerous thunderstorms, mainly this evening and
overnight, will offer the risk for large hail, damaging gusts, and
tornadoes over this part of the outlook area.
The most consistently progged and physically reasonable overlap
between returning moisture (supporting surface-based, favorably
buoyant parcels), convective forcing, and strong shear still appears
to be this evening into the overnight hours across the region
encompassing southeastern OK, northeast TX, and western Arkansas. Shear
and lift will diminish southwestward, while instability will
decrease northward. As such, only minor adjustments have been made
to the various probability lines based on newer guidance.
Indirectly, Nestor has contributed to some of the uncertainty here,
through offshore flow across parts of the northern/northeastern
Gulf, leading to a late-returning, somewhat narrowed, but still-
favorable moisture plume.
By this evening, the cold front is expected to impinge on a corridor
of increasingly favorable boundary-layer Theta-E across eastern OK
into north-central Texas. Forecast soundings show some lingering
low-level stability atop the boundary layer, related to a modest
eml. Still, surface-based effective-inflow parcels will be possible
beneath steep mid/upper-level lapse rates, with MLCAPE ranging from
around 2000-2500 j/kg over north-Central/Northeast Texas to 500-1000
j/kg in the Ozarks. The low level jet should strengthen to 55-65 kt between
00-06z, leading to very large hodographs with effective srh commonly
300-500 j/kg, amidst 50-60-kt effective-shear magnitudes.
Conditionally, this can support a significant-tornado risk as well,
but Mode uncertainties still appear too great to draw such an area
at this time.
While that parameter space is quite favorable for all forms of
severe, storm type and duration of any supercell-linear transition
will be the biggest factors influencing change of hail to wind as
the main coverage threat, and of sourcing (supercell vs. Qlcs) for
the tornado threat. A quasi-linear arc will evolve overnight,
shifting a severe-wind threat and at least isolated potential for
tornadoes across parts of the arklatex and western mid-south regions
..lower-mid Missouri Valley...
A narrow, strongly forced band of convection is expected to develop
close to the front late this afternoon and persist into early
evening, in an arc from central South Dakota across eastern NE to northeastern
Kansas. Isolated/conditional potential exists for damaging to severe
gusts and marginal hail, and a tornado cannot be ruled out.
Intensifying deep-layer lift at all scales -- including large-scale
DCVA/cooling aloft, lift related to the left-exit region of a
cyclonically curved upper jet, and low-level frontogenetic forcing
-- is forecast as the cyclone moves eastward and deepens. This will
yield substantial strengthening of both low-level convergence and
midlevel lapse rates, building a vertical thermodynamic profile
supporting thunderstorms and enough low-level lift to maintain them.
Forecast soundings suggest steep low/middle-level lapse rates with
500-mb temperatures above the surface front ranging from around -18
c over northeast Kansas to the -22 to -25 c range in northeastern NE and
central/southeastern South Dakota. This would yield a narrow corridor of
surface-based effective-inflow parcels and MLCAPE in the 200-700
j/kg range, despite cool surface temperatures in the mid 50s to mid
60s f. Favorable deep shear will spread across the region, with
isallobaric processes keeping near-surface flow backed and
hodographs enlarged. While the lack of greater buoyancy will be the
main limiting factor precluding a better-organized severe threat,
all forms of severe are at least marginally supported for a few
hours near the front.
A marginal tornado threat lingers this morning in the outer
eastern/northeastern fringes of the remnants of Nestor. By
afternoon, boundary-layer destabilization is expected behind the
early convective band and ahead of the surface low. Though
low-level winds will be veered relative to the environment near the
morning convective band, another low-topped arc of convection may
develop in a weakly capped, moist, marginally convergent environment
this afternoon. However, modest low-middle-level winds will keep
hodographs small, while warm midlevel temperatures (related to
trajectories from warm-core tropical-cyclone air) will restrict
buoyancy substantially, with MLCAPE less than 500 j/kg in most
areas. As such, the main severe concern is the marginal tornado
threat near the coast and southern Outer Banks for a few more hours.
See Storm Prediction Center mesoscale discussion 2083 for more details.
..lower Mississippi Valley region...
Widely scattered to scattered thunderstorms may develop this
afternoon in a somewhat strongly heated, weakly capped air mass
signifying the northeastern rim of the Richer return-flow moisture,
from southeast Texas across la, shifting into central MS. Forcing for
convective-scale ascent away from the southeast Texas/southwestern la
sea breeze appears rather nebulous, aside from diurnal heating, and
perhaps subtle boundary-layer confluence/convergence lines. This
still may be sufficient to initiate convection off the southeast
edge of the eml, with around 1500-2000 j/kg MLCAPE developing in the
preconvective warm sector. This regime will be very peripheral to
effects of the western/central Continental U.S. Troughing, though 35-45 kt
effective-shear magnitudes are possible, based on forecast
soundings. Low-level flow will be modest, keeping vector shear and
hodograph sizes small in the boundary layer despite veering with
height. At this time, severe potential appears too conditional for
an outlook, though strong gusts will be possible from the most
acus11 kwns 201209
sels mesoscale discussion
Storm Prediction Center mesoscale discussion 201209
Mesoscale discussion 2083
National Weather Service Storm Prediction Center Norman OK
0709 am CDT sun Oct 20 2019
Areas affected...coastal NC
Concerning...severe potential...watch unlikely
Valid 201209z - 201415z
Probability of watch issuance...5 percent
Summary...a tornado or two will be possible this morning, but threat
appears too limited for watch issuance at this time.
Discussion...at 12z, the remnant circulation of Nestor is centered
over northeast SC, with a warm front extending northeastward into
southeast NC, and east-southeastward from there towards the Outer
Banks. Low-level hodographs remain enlarged near and north of the
warm front, as noted in the recent vwp and 12z sounding from mhx.
Instability remains a limiting factor for the tornado threat, but a
narrow zone of overlap between weak but nonzero surface-based
buoyancy and enhanced low-level shear/helicity will spread northward
in conjunction with the warm front across coastal NC and the Outer
Banks this morning. Any semi-discrete convective elements within the
larger precipitation shield may pose some (likely brief) tornado
threat as they traverse this zone.
The tornado threat is expected to remain too limited in magnitude
and spatial extent for watch issuance. Any remaining tornado threat
will end from west-to-east later this morning, as the midlevel dry
slot overtakes the region.
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 34477736 35137675 36077568 35797546 35487541 35057558
34687597 34227649 34047701 34097723 34477736