Severe Storm Outlook Narrative (AC)
Issued by NWS
ACUS01 KWNS 251255
SPC AC 251253
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0753 AM CDT Tue Apr 25 2017
Valid 251300Z - 261200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER NORTHEASTERN
OKLAHOMA...PARTS OF EASTERN KANSAS AND WESTERN MISSOURI...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM
CENTRAL/EASTERN OKLAHOMA TO NORTHERN MISSOURI...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE TEXAS AND OKLAHOMA PANHANDLES INTO NORTHWESTERN OKLAHOMA...
Thunderstorms capable of large hail and damaging wind are expected
to develop across parts of northeast Oklahoma, southeast Kansas and
western Missouri this evening into overnight hours. Isolated hail
and/or damaging wind may occur over parts of the Texas and Oklahoma
Panhandles this afternoon and evening.
The upper-air pattern over the contiguous. U.S. will become
increasingly dominated by cyclonic flow associated with a western
synoptic-scale trough. Several shortwaves will traverse that flow
field, including a zonally elongated vorticity banner now apparent
in moisture-channel imagery over parts of AZ and NM. This feature
is forecast to pivot northeastward across the southern High Plains
and KS today, reaching the upper Mississippi Valley by the end of
the period. Meanwhile, a stronger/upstream shortwave trough -- now
located over portions of UT and NV -- will dig southeastward across
the southern Rockies through tonight, reaching the TX Panhandle and
southeastern NM by 12Z. Ahead of these features, shortwave ridging
will build across the Tennessee and Ohio Valleys. The persistent
deep-layer low over the Carolinas will eject northeastward up the
Atlantic Coast to just offshore DE/NJ by 12Z, while gradually
At the surface, an 11Z chart showed a weak frontal-wave low over
southwestern KS near P28, with a warm front arching northeastward
through parts of central/northeastern KS and northwestern MO, and a
cold front to southeastern CO. The frontal low is forecast to
move/re-develop northeastward to southern IA by around 00Z, then
deepen and move northeastward across the LSE area, just ahead of the
ejecting/initial mid/upper perturbation. Meanwhile, the cold front
will progress to near an MKC-CNU-FSI-TCC line by 00Z, then by 12Z,
to northern MO, eastern OK, west-central TX, and southeastern NM. A
developing dryline will intersect the cold front over northern OK by
00Z, extending southward across central and deep south TX. The
dryline will retreat slowly westward overnight, where not overtaken
by the cold front.
...OK to northern MO...
Isolated to widely scattered thunderstorms may develop along the
KS/OK portion of the front from late afternoon into early evening,
increasing in coverage after dark from northeastern and perhaps
central OK into eastern KS and western MO. Large hail and damaging
gusts will be possible. The warm sector, front and dryline largely
will remain too capped for deep/sustained convective development
through daylight hours. One exception may be after 21Z along a
segment of the front from the dryline intersection northeastward
over eastern KS, where a combination of frontal forcing, lingering
warmth from strong diurnal heating, and moisture advection may
combine to support a few thunderstorms.
Forecast soundings suggest effective-shear magnitudes locally
exceeding 60 kt in the outlook area. MLCAPE near 3000 J/kg may
develop by 00Z over central OK, but remaining capped, while buoyancy
decreases northeastward in keeping with progressively weaker
boundary-layer mixing ratios and PW. Looking upstream this morning:
surface obs, GPS data and available RAOBs show the richest Gulf
moist layer still over south TX near and south of I-10, where
surface dew points exceed 60 F and PW is above an inch, while total
PW in the .75-1.0 inch range is approaching the Red River at this
moment. The moist layer in the 12Z FWD sounding was very shallow
and intensely capped, with a mean mixing ratio of only 9 g/kg.
Strengthening low-level flow will advect this limited moisture
northeastward throughout the day, then richer moisture from south
TX, offsetting strong heating and boundary-layer mixing enough to
get surface dew points 60s F into OK by this evening.
Given aforementioned upstream conditions, most progs still appear to
be rather aggressive with moisture return at the surface, though
pockets of upper 50s to near 60 F dew points may develop across
eastern KS and northeast OK as downward mixing occurs from a layer
of vigorous moisture transport. Deep shear and mid/upper-level
large-scale lift each will strengthen amidst:
1. General height falls and tightening gradients aloft associated
with the progressive synoptic trough, and
2. DCVA preceding the ejecting/leading vorticity max.
Any relatively discrete storms could become supercellular, and an
isolated hail event of 2+ inches cannot be ruled out. At this time,
however, confidence in coverage of the threat is insufficient to
draw a 10%/hatched significant-hail area.
Coverage is forecast to increase in quasi-linear form overnight,
near the front and northeast of the dryline. Backbuilding is
probable from eastern KS into northeastern and perhaps
eastern/central OK, as the frontal zone impinges upon progressively
more moist low-level flow in the surface-850-mb layer.
...Portions of TX/OK Panhandles...
Confidence has increased in the development of widely scattered,
high-based thunderstorms this afternoon and evening, along or even
slightly behind the surface cold front, related to a combination of
destabilization aloft, frontal forcing, and marginal low-level
moisture. Activity should move rather quickly eastward to
east-northeastward over this region, atop a dry and well-mixed sub
cloud layer suitable for supporting strong-severe gusts to the
surface, as well as hail. Uncertainties remain regarding peak
convective coverage, timing and duration, but this area appears to
be the most probable for any such activity. The threat should
diminish quickly tonight as the air mass stabilizes from both
diabatic cooling and post-frontal cold advection.