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  Tuesday August 20, 2019

RSS Mesoscale Discussions from Storm Prediction Center

SPC Severe Thunderstorm Watch 602

WW 602 SEVERE TSTM IA IL MO 201115Z - 202000Z
      
WW 0602 Image

URGENT - IMMEDIATE BROADCAST REQUESTED
Severe Thunderstorm Watch Number 602
NWS Storm Prediction Center Norman OK
615 AM CDT Tue Aug 20 2019

The NWS Storm Prediction Center has issued a

* Severe Thunderstorm Watch for portions of 
  Southeastern Iowa
  Central and northern Illinois
  Northern and eastern Missouri

* Effective this Tuesday morning and afternoon from 615 AM until
  300 PM CDT.

* Primary threats include...
  Widespread damaging winds and isolated significant gusts to 80
    mph likely
  Isolated large hail events to 1.5 inches in diameter possible
  A tornado or two possible

SUMMARY...An expanding, severe complex of thunderstorms will sweep
southeastward across the watch area into early afternoon, the main
severe-weather hazard being wind.  Isolated hail and an embedded
tornado also cannot be ruled out.

The severe thunderstorm watch area is approximately along and 120
statute miles either side of a line from 10 miles west northwest of
Ottumwa IA to 20 miles east northeast of Mattoon IL. For a complete
depiction of the watch see the associated watch outline update
(WOUS64 KWNS WOU2).

PRECAUTIONARY/PREPAREDNESS ACTIONS...

REMEMBER...A Severe Thunderstorm Watch means conditions are
favorable for severe thunderstorms in and close to the watch area.
Persons in these areas should be on the lookout for threatening
weather conditions and listen for later statements and possible
warnings. Severe thunderstorms can and occasionally do produce
tornadoes.

&&

OTHER WATCH INFORMATION...CONTINUE...WW 601...

AVIATION...A few severe thunderstorms with hail surface and aloft to
1.5 inches. Extreme turbulence and surface wind gusts to 70 knots. A
few cumulonimbi with maximum tops to 650. Mean storm motion vector
32045.

...Edwards

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SPC Severe Thunderstorm Watch 602 Status Reports

WW 0602 Status Updates
      
WW 0602 Status Image

STATUS REPORT ON WW 602

SEVERE WEATHER THREAT CONTINUES RIGHT OF A LINE FROM 25 ENE CDJ
TO 10 SSW UIN TO 15 ENE ALN TO 10 ESE CMI TO 35 ENE MMO.

..SMITH..08/20/19

ATTN...WFO...DVN...LSX...ILX...LOT...EAX...


STATUS REPORT FOR WS 602 

SEVERE WEATHER THREAT CONTINUES FOR THE FOLLOWING AREAS 

ILC005-013-019-023-025-027-029-033-035-041-045-049-051-061-075-
079-083-091-101-117-119-121-133-135-139-149-159-163-173-183-189-
197-201720-

IL 
.    ILLINOIS COUNTIES INCLUDED ARE

BOND                 CALHOUN             CHAMPAIGN           
CLARK                CLAY                CLINTON             
COLES                CRAWFORD            CUMBERLAND          
DOUGLAS              EDGAR               EFFINGHAM           
FAYETTE              GREENE              IROQUOIS            
JASPER               JERSEY              KANKAKEE            
LAWRENCE             MACOUPIN            MADISON             
MARION               MONROE              MONTGOMERY          
MOULTRIE             PIKE                RICHLAND            
ST. CLAIR            SHELBY              VERMILION           
WASHINGTON           WILL                


INC007-073-111-201720-

IN 
.    INDIANA COUNTIES INCLUDED ARE

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SPC MD 1810

MD 1810 CONCERNING SEVERE POTENTIAL...WATCH POSSIBLE FOR MOST OF INDIANA AND PORTIONS OF SOUTHERN ILLINOIS
MD 1810 Image


Mesoscale Discussion 1810
NWS Storm Prediction Center Norman OK
1212 PM CDT Tue Aug 20 2019

Areas affected...Most of Indiana and portions of southern Illinois

Concerning...Severe potential...Watch possible 

Valid 201712Z - 201915Z

Probability of Watch Issuance...40 percent

SUMMARY...Locally damaging winds remain possible with the
progression of an MCS southeast through the area.  A Severe
Thunderstorm Watch is possible.

DISCUSSION...A well-organized MCS with attendant MCV continues
moving southeast at 35-40 kt over Illinois with a slowly broadening
lead convective line arcing from Chicago to near St. Louis.  The
main limiting factor for a more widespread severe wind threat is
weak low-to-mid level wind shear present ahead of the MCS, which
should support a continuation of a significantly upshear-tilted
system and slowly weakening mesoscale rear inflow.  However, ample
instability owing to rich boundary-layer moisture and some daytime
heating (prior to the anvil spreading over the area), and minimal
convective inhibition, should support the continued generation of
some convection along the outflow through the afternoon.  In
addition, widely-scattered storms should continue to develop ahead
of the MCS, particularly in the vicinity of a mesoscale band of
confluence stretching northwest to southeast across southern
Indiana.  Although the MCS should continue south and eastward in a
somewhat disorganized state (compared to this morning), low-level
lapse rates will be sufficient to support locally damaging winds
with the stronger convective pulses.  The main uncertainty is
whether or not the coverage of the threat will be large enough to
warrant a Severe Thunderstorm Watch.

..Coniglio/Thompson.. 08/20/2019

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...LMK...IWX...IND...PAH...LOT...ILX...LSX...

LAT...LON   41198745 41458736 41658678 41638621 41318549 40588525
            39628538 38988573 38598633 38168717 37888793 37808833
            37678913 37828952 38098990 38378995 38578990 38938887
            39538786 40078755 40538748 41198745 

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SPC Aug 20, 2019 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook
      
Day 1 Outlook Image
Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0755 AM CDT Tue Aug 20 2019

Valid 201300Z - 211200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM EXTREME
SOUTHEASTERN IOWA AND NORTHEASTERN MISSOURI TO WESTERN INDIANA AND
SOUTHERN ILLINOIS...

...SUMMARY...
The greatest severe-storm threat will be destructive winds from an
ongoing complex of thunderstorms, which is forecast to continue
southeastward at least across portions of Missouri, Illinois, and
Indiana, as well as remaining parts of Iowa.

...Synopsis...
In mid/upper levels, a belt of seasonally active northern-stream
westerlies will continue across the northern 1/2-2/3 of the CONUS,
while a high shifts/erodes westward from the southern Plains toward
the southern Rockies.  An elongated cyclone across northern portions
of QC/ON/MB and adjoining parts of Hudson Bay will split, with the
western portion digging southeastward across northern ON.  The
trailing strong shortwave trough will approach northern MN and Lake
Superior late in the period.  Meanwhile, a pre-existing shortwave
perturbation over the western IA/southern MN region -- with
reinforcing/convectively generated vorticity to its southeast --
will move southeastward to Indiana by 00Z then across the Ohio
Valley into parts of the central Appalachians by 12Z.

At the surface, 11Z analysis depicted a cold front offshore from
most of New England, transitioning to a wavy/quasistationary front
from northern MD across northern WV, southern OH, southern IN,
becoming a warm front over central IL and a small part of
southeastern IA prior to being overtaken by an ongoing MCS.  The
Ohio Valley States part of this front should move little before
being overtaken by the MCS.  Meanwhile, a separate cold front --
initially analyzed from northwestern MN and southeastern ND across
southwestern SD and northeastern WY -- will move southward across
much of the central Plains and Upper Midwest this period.  By 12Z,
the front should be positioned from Lake Michigan across northern
MO, northern KS and east-central CO.

...Corn Belt...mid Mississippi Valley...
An expansive, severe wind-producing MCS will continue to organize
and offer damaging, possibly significant-severe wind across the mid
Mississippi Valley and toward the lower Ohio Valley through the
early-mid afternoon.  Please refer to SPC watch 602 and related
mesoscale discussions for the latest near-term guidance.

With a favorably moist and diurnally destabilizing air mass
projected downshear, within and south of the warm-frontal zone, the
distance this MCS will travel before weakening is uncertain.  It
will depend to a great extent on internal cold-pool characteristics
such as rear-inflow jet organization and lifting related to
eventually decreasing depth of the density current along and shortly
behind the gust front.  In the meantime, continued lateral expansion
of the cold pool and related forced ascent will encourage additional
convective development along the margins of the MCS, especially in
any bookend vortices, while the greatest severe-wind potential
should continue to be in the bowing portion either side of its apex.
Diabatic heating and rich low-level moisture, beneath favorable
midlevel lapse rates, will help to boost preconvective MLCAPE to the
3000-4500 J/kg range ahead of the complex.

...Central Plains...
Widely scattered thunderstorms are expected to develop late this
afternoon onto evening, initially over higher terrain in the
post-frontal moist/upslope-flow regime.  This activity should
proceed southeastward across parts of western/central NE this
evening and tonight, offering large hail and severe gusts.  Some of
the hail may become especially destructive, exceeding two inches in
diameter.  Additional convection may form tonight ahead of the
downshear progression of early storms, as well.  Uncertainties exist
regarding convective coverage, especially after about 03Z and over
eastern parts of the outlook area; however, any sustained convection
in this environment will be capable of an organized severe threat.

Although some weaknesses will exist in the midlevel wind profiles, 
strong veering with height will contribute to favorable deep shear,
with effective-shear magnitudes in the 35-45-kt range.  This will
favor supercell characteristics.  An axis of strong buoyancy will
set up across western through central/southeaster NE, with the width
of the favorably unstable air mass increasing eastward.  Rich
low-level moisture -- with surface dew points commonly in the mid
60s to mid 70s F -- will underlie steep lapse rates in support of
MLCAPE in the 3000-4500 J/kg range.

..Edwards/Marsh.. 08/20/2019

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SPC Aug 20, 2019 1630 UTC Day 1 Convective Outlook

SPC 1630Z Day 1 Outlook
      
Day 1 Outlook Image
Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1123 AM CDT Tue Aug 20 2019

Valid 201630Z - 211200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
FOR PARTS OF IL/IN...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
ENHANCED RISK ACROSS THE MIDDLE MS AND OH VALLEYS THIS
AFTERNOON...NORTHERN VA AND VICINITY THIS AFTERNOON...AND PARTS OF
THE CENTRAL PLAINS THIS EVENING INTO TONIGHT...

...SUMMARY...
An ongoing thunderstorm cluster across Illinois will continue to
pose a threat for damaging winds into this afternoon into Indiana. 
Damaging winds will be possible with a few storms this afternoon in
the vicinity of northern Virginia.  Large hail and damaging winds
will be possible this evening into tonight across parts of the
central Plains.

...Mid MS/OH Valley region through this afternoon...
A well-developed MCS and associated MCV will continue to move
east-southeastward across IL/IN through the afternoon.  Convection
will likely be maintained into the afternoon along the gust front as
the cold pool encounters a warming boundary layer with dewpoints in
the mid-upper 70s, resulting in MLCAPE in excess of 3000 J/kg with
weakening convective inhibition.  Vertical shear will weaken with
southeastward extent aside from the MCV itself, which suggests that
convective organization will be driven more by lift along the gust
front by mid-late afternoon.  Still, the established cold pool and
potential for strong downdrafts will maintain a threat for damaging
winds through the afternoon at least into IN/KY, and perhaps into OH
late this afternoon.

...Northern VA and vicinity this afternoon...
A small thunderstorm cluster and diffuse MCV will move eastward from
the eastern WV Panhandle across northern VA/MD this afternoon. 
Surface heating in advance of this minor wave will contribute to
moderate buoyancy (MLCAPE near 2000 J/kg), and some loose clustering
of storms is expected this afternoon.  Steep low-level lapse rates
and DCAPE near 1000 J/kg will favor occasional downbursts with some
potential for wind damage.  

...Central Plains/High Plains this evening into tonight...
The western extent of the outflow is effectively reinforcing a front
across NE.  Dry abiabatic midlevel lapse rates atop boundary-layer
dewpoints in the 70s, with surface heating, will contribute to
MLCAPE of 3000-4000 J/kg.  Regional 12z soundings suggest that
surface temperatures will need to warm to near 100-105 F to
eliminate convective inhibition, thus storm formation this afternoon
along the effective front is in question.  The more probable
scenarios are for storms to form this evening in the upslope regime
near the WY/CO/NE border, and overnight across NE in the
strengthening warm advection immediately north of the front.  The
evening upslope storms could be supercells initially as deep-layer
vertical shear is augmented within the easterly low-level flow along
and north of the front, and large hail would be the main concern. 
Some upscale growth into a cluster with damaging winds will be
possible early tonight as a southerly low-level jet and warm
advection strengthen.  The warm advection could also support
additional elevated storm development farther east across NE
overnight, with an attendant threat for isolated large hail/damaging
winds.

..Thompson/Coniglio.. 08/20/2019

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SPC Aug 20, 2019 1730 UTC Day 2 Convective Outlook

SPC 1730Z Day 2 Outlook
      
Day 2 Outlook Image
Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1230 PM CDT Tue Aug 20 2019

Valid 211200Z - 221200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS WEDNESDAY ACROSS
PARTS OF THE NORTHERN MID ATLANTIC STATES AND NEW ENGLAND...

...SUMMARY...
Severe thunderstorms are possible across parts of the northern Mid
Atlantic Coast states into New England Wednesday.  Additional strong
to severe storms are possible in a corridor from the central Plains
into the lower Ohio Valley, and near the Rockies.

...Synopsis...
The stronger westerlies appear to be undergoing amplification, but
may remain largely confined to Canada and the northern tier of the
U.S. through this period.  Within this regime, larger-scale
mid/upper troughing is forecast to progress inland of the British
Columbia and Pacific Northwest coast, through the Canadian Rockies
and northern U.S. intermountain region by late Wednesday night.  As
this occurs, sharp downstream ridging should shift eastward across
interior Canada, while large-scale troughing to the east develops
across Ontario/Quebec and the Great Lakes region, toward the
northern Atlantic coast.

Models indicate the eastern troughing will include an embedded
mid-level low, with at least a couple of significant perturbations
migrating through the broader cyclonic flow.  One of these is
forecast to be in the process of digging into the upper Great Lakes
region early Wednesday, before accelerating eastward and
northeastward into and through Quebec by 12Z Thursday.  It appears
that associated forcing for ascent will support significant surface
cyclogenesis across Quebec, with the most rapid deepening to the
northwest of the St. Lawrence Valley Wednesday night.  

A front trailing from the developing cyclone is expected to advance
through much of the Upper Midwest, mid Missouri Valley and northern
Plains by early Wednesday, before continuing southeastward
into/through New England, the Ohio and middle Mississippi Valleys
and the central Plains by 12Z Thursday.  The front will be preceded
by the remnants of convective outflow from the large ongoing
convective system (now advancing into the lower Ohio Valley), which
may still be evident across the central Appalachians and lower Ohio
Valley early Wednesday.

Seasonably moist air along and ahead of pre-frontal surface
troughing across parts of the Northeast, and the convective
outflow/cold front extending westward across the central
Appalachians into the Rockies, appears likely to contribute to
moderate to strong potential instability with daytime heating
Wednesday.  This is expected to provide support  for areas of strong
thunderstorm development, some of which will probably pose at least
some risk for severe wind and hail.

....Northeast...
The extent to which to which the stronger surface cyclogenesis
impacts convective potential across the region during this period
remains unclear, given the apparent overnight timing of the most
rapid surface deepening.  Additionally, it is possible that the
associated surface cold front may become a focus for thunderstorm
development, but this may be mostly over portions of southeastern
Ontario and southwest Quebec prior to Wednesday evening.  However,
the primary convective potential seems likely in association with
forcing for ascent accompanying a remnant convectively generated or
enhanced perturbation, which is forecast to develop northeastward
ahead of the primary troughing within the westerlies, from the lee
of the lower Great Lakes region through New England by Wednesday
evening.

A belt of enhanced southwesterly lower/mid tropospheric flow (on the
order of 30-40+ kt), coupled with at least modest destabilization
(CAPE of 1000-2000+ J/kg) near/ahead of a developing pre-frontal
surface trough, may provide an environment conducive to organized
severe storm development across parts of eastern New York and
Pennsylvania and much of New Jersey through much of New England. 
Potentially damaging wind gusts appear the primary hazard, but there
may be at least some risk for a tornado or two, particularly across
the Hudson/Champlain Valley vicinity into western New England
Wednesday afternoon.

...Ohio and middle Mississippi Valleys into high Plains...
It appears that the focusing boundaries for moderate to strong
boundary layer destabilization and thunderstorm development will be
south of the stronger shear associated with the westerlies. 
However, thermodynamic profiles characterized by relatively steep
lapse rates may still be favorable for convection capable of
producing hail and strong wind gusts associated with downbursts and
convectively generated cold pools.  Too much uncertainty still
exists concerning sub-synoptic features to attempt to delineate
areas of higher ("slight risk") severe probabilities at this time.

...MAXIMUM RISK BY HAZARD...
Tornado:   5%     - Slight
Wind:     15%     - Slight
Hail:      5%     - Marginal

..Kerr.. 08/20/2019

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SPC Day 1 Fire Weather Outlook

SPC Day 1 Fire Weather Outlook
          
Day 1 Fire Weather Outlook Image
Day 1 Fire Weather Outlook  
NWS Storm Prediction Center Norman OK
1155 AM CDT Tue Aug 20 2019

Valid 201700Z - 211200Z

...NO CRITICAL AREAS...

No changes. Previous discussion follows.

..Bentley.. 08/20/2019

.PREV DISCUSSION... /ISSUED 0136 AM CDT Tue Aug 20 2019/

...Synopsis...
An upper-level ridge will continue to amplify over the western CONUS
into western Canada today as an upper-level shortwave trough
approaches the Pacific Northwest. Subtropical moisture will begin to
move northward into the Southwest and eastward into southwest
Montana from the northern Plains. A surface cold front will push
southward up against the eastern slopes of the northern/central
Rockies with hot/dry/unstable conditions prevailing across much of
the Intermountain West.

Isolated dry thunderstorms may form across southwest Montana and
northwest Wyoming this afternoon/evening. However, given the
uncertainty regarding coverage and marginal fuel conditions where
storms are currently forecast to develop, an isolated dry
thunderstorm area was not included.

...Please see www.spc.noaa.gov/fire for graphic product...

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