Severe Storm Outlook Narrative (AC)
Issued by NWS

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ACUS01 KWNS 210606
SWODY1
SPC AC 210605

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0105 AM CDT Mon Aug 21 2017

Valid 211200Z - 221200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PART OF THE
MID MISSOURI VALLEY TO THE SOUTHWEST GREAT LAKES REGION...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK FROM THE CENTRAL PLAINS TO THE MIDWEST AND WESTERN GREAT
LAKES...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS NORTHERN
VIRGINIA AND MARYLAND INTO EASTERN PENNSYLVANIA...

...SUMMARY...
Severe thunderstorms are forecast to impact parts of the mid
Missouri Valley east-northeastward into the mid to upper Mississippi
Valley and western Great Lakes mainly this afternoon into tonight.
All severe hazards will be possible with the storms this afternoon
within the western part of the Slight risk area, including very
large hail and an isolated tornado threat.  Damaging winds should
become the primary threat through the evening and tonight.

...Synopsis...
Strong westerly flow regime centered over the northern tier of the
U.S. states into southern Canada is expected to become more
amplified this forecast period.  An upper trough, currently moving
east of the Canadian/northern U.S. Rockies per satellite imagery,
will amplify/dig across the northern Plains into the upper MS Valley
and Great Lakes region today through tonight.  Upstream height rises
are expected across the Pacific Northwest and western Canada, as an
upper ridge builds into this region north of a large, weak closed
low centered off the CA coast.  Meanwhile, a mid-level shortwave
trough emerging from the southwest monsoonal regime, currently over
NM per satellite imagery, is expected to track northeast through
western KS this morning, reaching northern KS and
south-central/southeast NE this afternoon.  This feature should then
shift east into the base of the amplifying northern Plains/upper MS
Valley large-scale trough, reaching IL by 12Z Tuesday.  Preceding
this large-scale trough, a weak midlevel impulse or MCV will shift
east into IL/IN this morning and afternoon, while another impulse
farther downstream tracks through PA.

At the surface, a stalled boundary extending from the Upper Great
Lakes through southern MN to NE today will be overtaken by this
evening with a south/southeastward-moving cold front attendant to
the amplifying northern Plains/upper MS Valley trough.  A
sub-synoptic low should develop along the stalled boundary in
northeast NE later this afternoon and track into northern IA this
evening, while a primary synoptic low reaches the Upper Great Lakes
region by 12Z Tuesday.

...Central Plains to mid/upper MS Valley to southwest Great Lakes...

Deep-layer west-southwesterly winds are expected to strengthen from
IA and the upper MS Valley region to the Great Lakes later this
afternoon through tonight to early Tuesday morning.  Seasonably high
moisture content air mass (precipitable water value exceeding 1.5
inches) will spread north across the Slight risk area.  Surface
heating in the wake of early morning storms in IA and steep
mid-level lapse rates will result in very strong instability, with
MLCAPE exceeding 2500-3000 J/kg across parts of eastern NE/western
IA and northern MO.

An increase in forcing for ascent is expected during the mid-late
afternoon across eastern NE/western IA, as height falls develop
ahead of the amplifying northern Plains/upper MS Valley mid-level
trough and low-level warm advection increases within the nose of a
strengthening southwesterly low-level jet into this region.  These
processes will support new thunderstorm development along the
stalled front and convective outflow boundaries in IA and near a
boundary extending from northern MO into eastern NE.  Effective bulk
shear of 30-40 kt will prove favorable for organized storms,
including supercells.  Backed low-level winds near surface
boundaries will enhance low-level shear for a tornado threat, while
steep mid-level lapse rates support a threat for very large hail in
the western extent of the Slight risk area.

As deep-layer westerlies strengthen through tonight, a risk for
damaging winds will become the primary threat this evening through
tonight as storms spread from the upper MS Valley to the southern
Great Lakes.

...IL/IN...
A cluster of storms/MCS should be ongoing at 12Z today across IA,
with downstream destabilization expected ahead of these storms
across IL/IN.  Weak forcing for ascent attendant to a mid-level
impulse or MCV in IA and an increase in deep-layer shear suggest
storms could re-intensify as early morning convection spreads east
into IL.  A marginal severe risk is forecast for this early day into
the afternoon threat for isolated hail and/or locally damaging
winds.

...Northern VA/MD to eastern PA...
Moderate instability is expected to develop across this region
during the afternoon, with forcing for ascent attendant to a
transient weak mid-level impulse moving into this region during peak
heating supporting storm development.  Effective bulk shear around
35 kt will support storm organization, with a threat for isolated
hail and damaging winds.  Overall weak forcing for ascent could
limit the coverage of the severe storms, which warrants only a
marginal severe risk with this outlook issuance.

..Peters/Gleason.. 08/21/2017

$$



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