U.S. Severe Weather Forecast (Convective Outlook)

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 
by 12z. 

..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. 

..southeastern NC... 
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 
intense cells. 

.Edwards/Dean.. 10/20/2019 


Mesoscale Discussion

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. 

.Dean/Edwards.. 10/20/2019 

..please see www.Spc.NOAA.Gov for graphic product... 


Latitude...Lon 34477736 35137675 36077568 35797546 35487541 35057558 
34687597 34227649 34047701 34097723 34477736