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  Monday August 21, 2017

RSS Mesoscale Discussions from Storm Prediction Center

SPC Severe Thunderstorm Watch 460

WW 460 SEVERE TSTM IA MN NE SD 211635Z - 220000Z
      
WW 0460 Image

URGENT - IMMEDIATE BROADCAST REQUESTED
Severe Thunderstorm Watch Number 460
NWS Storm Prediction Center Norman OK
1135 AM CDT Mon Aug 21 2017

The NWS Storm Prediction Center has issued a

* Severe Thunderstorm Watch for portions of 
  Northwest Iowa
  Southwest Minnesota
  Northeast Nebraska
  Southeast South Dakota

* Effective this Monday morning and evening from 1135 AM until
  700 PM CDT.

* Primary threats include...
  Scattered damaging winds and isolated significant gusts to 75
    mph possible
  Scattered large hail and isolated very large hail events to 2
    inches in diameter possible

SUMMARY...Cluster of severe storms over south-central South Dakota
should continue east-southeast with additional storm development
possible to its south and east. A swath of large hail and damaging
winds is expected this afternoon.

The severe thunderstorm watch area is approximately along and 75
statute miles north and south of a line from 45 miles south of
Chamberlain SD to 30 miles north of Storm Lake IA. For a complete
depiction of the watch see the associated watch outline update
(WOUS64 KWNS WOU0).

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 459...

AVIATION...A few severe thunderstorms with hail surface and aloft to
2 inches. Extreme turbulence and surface wind gusts to 65 knots. A
few cumulonimbi with maximum tops to 550. Mean storm motion vector
29030.

...Grams

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SPC Severe Thunderstorm Watch 460 Status Reports

WW 0460 Status Updates
      
WW 0460 Status Image

STATUS REPORT ON WW 460

SEVERE WEATHER THREAT CONTINUES RIGHT OF A LINE FROM 30 WSW OFK
TO 30 ENE SUX TO 10 S OTG TO 15 SW FRM.

..DEAN..08/21/17

ATTN...WFO...FSD...OAX...LBF...


STATUS REPORT FOR WS 460 

SEVERE WEATHER THREAT CONTINUES FOR THE FOLLOWING AREAS 

IAC021-035-041-059-093-133-141-143-149-193-212140-

IA 
.    IOWA COUNTIES INCLUDED ARE

BUENA VISTA          CHEROKEE            CLAY                
DICKINSON            IDA                 MONONA              
O'BRIEN              OSCEOLA             PLYMOUTH            
WOODBURY             


NEC021-039-119-167-173-179-212140-

NE 
.    NEBRASKA COUNTIES INCLUDED ARE

BURT                 CUMING              MADISON             
STANTON              THURSTON            WAYNE               


THE WATCH STATUS MESSAGE IS FOR GUIDANCE PURPOSES ONLY.  PLEASE
REFER TO WATCH COUNTY NOTIFICATION STATEMENTS FOR OFFICIAL
INFORMATION ON COUNTIES...INDEPENDENT CITIES AND MARINE ZONES
CLEARED FROM SEVERE THUNDERSTORM AND TORNADO WATCHES.

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SPC Severe Thunderstorm Watch 459 Status Reports

WW 0459 Status Updates
      
WW 0459 Status Image

STATUS REPORT ON WW 459

SEVERE WEATHER THREAT CONTINUES RIGHT OF A LINE FROM 20 NE ANW TO
25 SSE 9V9 TO 15 NNE MHE.

..DEAN..08/21/17

ATTN...WFO...FSD...ABR...UNR...


STATUS REPORT FOR WS 459 

SEVERE WEATHER THREAT CONTINUES FOR THE FOLLOWING AREAS 

SDC003-023-035-043-053-211840-

SD 
.    SOUTH DAKOTA COUNTIES INCLUDED ARE

AURORA               CHARLES MIX         DAVISON             
DOUGLAS              GREGORY             


THE WATCH STATUS MESSAGE IS FOR GUIDANCE PURPOSES ONLY.  PLEASE
REFER TO WATCH COUNTY NOTIFICATION STATEMENTS FOR OFFICIAL
INFORMATION ON COUNTIES...INDEPENDENT CITIES AND MARINE ZONES
CLEARED FROM SEVERE THUNDERSTORM AND TORNADO WATCHES.

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SPC MD 1545

MD 1545 CONCERNING SEVERE POTENTIAL...WATCH UNLIKELY FOR NORTHEAST KS...I-70 CORRIDOR OF MO
MD 1545 Image


Mesoscale Discussion 1545
NWS Storm Prediction Center Norman OK
0341 PM CDT Mon Aug 21 2017

Areas affected...Northeast KS...I-70 Corridor of MO

Concerning...Severe potential...Watch unlikely 

Valid 212041Z - 212215Z

Probability of Watch Issuance...20 percent

SUMMARY...Thunderstorm clusters capable of locally damaging wind and
isolated instances of hail are possible for the next few hours.
Watch issuance is considered unlikely.

DISCUSSION...A thunderstorm cluster has recently intensified near
Kansas City, to the north of an old outflow boundary that is
advancing slowly northward as an effective warm front across
northeast KS and central MO. While midlevel lapse rates are
relatively weak across the region, rich low-level moisture is
supporting MUCAPE of 2000-3000 J/kg north of the warm front. Veering
wind profiles noted in the EAX and TOP VWPs support some storm
organization. The potential exists for one or more upscale-growing
clusters to track eastward just north of the warm front this
afternoon. Some locally damaging wind risk will develop with any
clusters that can acquire a sufficiently strong cold pool. Buoyancy
may be sufficient for at least small hail, though the severe hail
risk should be limited by poor lapse rates and warm temperatures
aloft. 

With a nose of dry midlevel air moving into central MO, it is
unclear whether any long-lived cluster will be sustained north of
the warm front, but a marginal severe risk will continue across at
least northwest MO in the short term. Some backbuilding is possible
further west into northeast KS with time, as low-amplitude midlevel
features continue to track northeastward out of the southern High
Plains.

..Dean/Grams.. 08/21/2017

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

ATTN...WFO...LSX...SGF...EAX...TOP...

LAT...LON   39599601 39649420 39659307 39419148 38589156 38519243
            38529321 38609419 38759596 39169602 39599601 

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SPC MD 1544

MD 1544 CONCERNING SEVERE POTENTIAL...WATCH UNLIKELY FOR PORTIONS OF THE MID MISSISSIPPI VALLEY
MD 1544 Image


Mesoscale Discussion 1544
NWS Storm Prediction Center Norman OK
0329 PM CDT Mon Aug 21 2017

Areas affected...Portions of the mid Mississippi Valley

Concerning...Severe potential...Watch unlikely 

Valid 212029Z - 212200Z

Probability of Watch Issuance...20 percent

SUMMARY...Isolated damaging gusts and large hail may occur with the
strongest storms through this evening. However, watch issuance is
not currently expected.

DISCUSSION...Scattered thunderstorms continue to develop along an
outflow boundary stretching eastward from the St. Louis metro area
this afternoon. Along and south of this boundary, ample heating has
combined with very moist low levels (e.g., surface dew points in the
upper 70s) to promote MLCAPE values upwards of 3500 J/kg, despite
unimpressive mid-level lapse rates. Modest mid-level westerlies are
offering around 30 kt of effective shear, sufficient for
multicell/transient supercell modes capable of isolated damaging
downbursts and severe hail. However, deep-layer flow oriented
parallel to the boundary will likely promote continued cell
mergers/destructive interference, limiting the overall threat some.
Additionally, the continued southward advance of the outflow
boundary may favor persistent undercutting of updrafts. These
factors should preclude a more organized threat, such that watch
issuance is not presently expected.

..Picca/Grams.. 08/21/2017

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

ATTN...WFO...IND...PAH...ILX...LSX...

LAT...LON   39119142 39389116 39478944 39398778 39098718 38348730
            38058797 38098955 38359080 38829128 39119142 

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SPC Aug 21, 2017 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook
      
Day 1 Outlook Image
Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0251 PM CDT Mon Aug 21 2017

Valid 212000Z - 221200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS EASTERN
NEBRASKA AND MUCH OF IOWA...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PORTIONS OF
THE CENTRAL PLAINS EASTWARD TO THE WESTERN/CENTRAL GREAT LAKES AND
PORTIONS OF THE MID-ATLANTIC...

...SUMMARY...
Scattered severe hail and wind is anticipated across parts of the
Mid-Missouri Valley and Upper Midwest this afternoon and evening.
Isolated severe storms are also possible across a broad region from
the central Great Plains to the Mid-Atlantic States.

...20Z Outlook Update...
Minimal spatial changes were made to the ongoing outlook primarily
to decrease probabilities upstream of a mature MCS over far
southeastern South Dakota/northwestern Iowa.  This MCS will continue
to migrate southeastward primarily across the Slight Risk area over
the next several hours and pose a damaging wind/isolated tornado
threat before expected weakening due to lesser instability over
eastern Iowa. Severe probabilities were also decreased in southern
Minnesota, where widespread stratiform precipitation was limiting
surface-based destabilization.  Reference MCD 1543 for more
information.

Elsewhere, scattered severe storms will pose a risk for isolated
instances of hail and damaging wind gusts across the central Plains
eastward into the Midwest and in portions of the Mid-Atlantic.  MCD
1542 has more information for the severe threat in portions of the
Mid-Atlantic.

..Cook.. 08/21/2017

.PREV DISCUSSION... /ISSUED 1119 AM CDT Mon Aug 21 2017/

...Mid-MO Valley and Upper Midwest...
A small forward-propagating MCS over south-central SD is the primary
concern for severe potential this afternoon. Robust insolation is
occurring downstream across eastern NE into western IA. This will
likely result in a strongly buoyant air mass developing by peak
heating amid very steep mid-level lapse rates near 8.5 C/km from
700-500 mb per 12Z Rapid City and North Platte soundings. On the
fringe of faster mid-level westerlies over the Upper Midwest, this
MCS should remain organized with a threat for severe hail/wind and
expand in coverage as it likely evolves east-southeast. The overall
threat with this MCS should diminish during the evening over parts
of IA given a large swath of ongoing non-severe convection across
central IA to northern IL hindering diabatic heating.

...Midwest to Mid-Atlantic States...
Outside of the aforementioned scenario, confidence is low in
highlighting probabilities beyond a Marginal risk. Pockets of
moderate to large buoyancy should develop across parts of southern
IL into IN and northern Lower MI, as well as east of the lee trough
in the Mid-Atlantic States. A broad belt of moderate westerlies
southeast of the amplifying short wave trough over the southern
Canadian Prairies will support multicells and a few transient
supercell structures with risks for isolated damaging winds and
hail.

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SPC Aug 21, 2017 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
1203 PM CDT Mon Aug 21 2017

Valid 221200Z - 231200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE OHIO
VALLEY NORTHEASTWARD INTO WESTERN NEW ENGLAND...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE
SOUTHERN GREAT PLAINS EAST-NORTHEASTWARD TO THE OHIO
VALLEY...MID-ATLANTIC...AND WESTERN NEW ENGLAND...

...SUMMARY...
Severe thunderstorms are possible Tuesday afternoon and evening
across the lower Great Lakes region and Ohio Valley into parts of
the Northeast, accompanied mainly by the risk for potentially
damaging wind gusts.  Scattered strong thunderstorms may also impact
parts of the Ozark Plateau into the south central Plains, with at
least some risk for severe weather.

...Synopsis...
A broad mid/upper low will migrate slowly eastward across Ontario
and into western Quebec Tuesday while spreading enhanced flow aloft
over portions of the Great Lakes, Ohio Valley, and Northeast. 
Meanwhile, another slow-moving low will remain nearly stationary
along the California coast.  A mid-level ridge will remain
stationary from northern Mexico into southwest Texas.

At the surface, substantial cyclogenesis is expected across Quebec
in response to the approaching mid-level low.  A strong cold front
will migrate southeastward across the upper Midwest and central
Plains throughout the day.  Ahead of this front, a warm, humid
airmass will exist and support development of numerous showers and
thunderstorms from midday onward across the Northeast, Ohio Valley,
and westward into the southern Plains.

...Ohio Valley into the Northeast...
An ongoing, yet decaying, complex of storms may be ongoing along the
northwestern extent of the MRGL/SLGT risk areas at the beginning of
the forecast period.  These clouds/showers should dissipate
throughout the morning hours and may end up mitigating the magnitude
of the severe risk later in the day especially across
Indiana/western Ohio.  Nevertheless, insolation/surface-based
destabilization and lift associated with the approaching mid-level
low and cold front should result in scattered to numerous
thunderstorms developing within the MRGL/SLGT risk areas from
mid-afternoon onward.  Deep shear profiles favor organized
convection, with multiple linear segments/bands of storms favoring a
threat for damaging wind gusts areawide into the evening hours.

Farther northeast (eastern New York into western New England),
low-level shear profiles are expected to be a bit stronger owing to
backed surface winds in response to the deepening Canadian surface
low.  An attendant risk for supercellular structures (and perhaps a
tornado or two) will exist, although the magnitude of this threat
will depend on how unstable the airmass can get during peak heating
hours, which is somewhat uncertain.  A categorical upgrade may be
needed in later outlooks - especially if stronger instability can
develop along the Hudson Valley and vicinity (as suggested by the
12Z Nam).

...Southern Plains eastward to Missouri/Arkansas...
This region will be well displaced from stronger mid/upper flow
associated with the Canadian trough, resulting in meager deep shear
profiles.  Nevertheless, areas of strong instability is expected in
response to strong insolation/surface heating amidst low to mid-70s
F dewpoints.  Scattered thunderstorms will develop with at least an
isolated risk for hail and damaging downburst winds.  These storms
may organize into one or two loosely organized clusters before
weakening after sunset.

..Cook.. 08/21/2017

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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
1151 AM CDT Mon Aug 21 2017

Valid 211700Z - 221200Z

...NO CRITICAL AREAS...

No changes have been made to the ongoing forecast.

..Cohen.. 08/21/2017

.PREV DISCUSSION... /ISSUED 0140 AM CDT Mon Aug 21 2017/

...Synopsis...
Upper ridging will continue to amplify across much of the western
CONUS today as a weak upper low remains off the coast of
central/southern CA. Farther east, a shortwave trough will dig
southeastward from the Canadian Prairie provinces into the northern
Plains and Upper Midwest by tonight. At the surface, some increase
in northerly post-frontal winds will occur across parts of eastern
MT into the western Dakotas and northeastern WY this afternoon.
However, RH values are forecast to become only marginally lowered,
which precludes the introduction of an elevated area at this time.

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

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SPC Day 2 Fire Weather Outlook

SPC Day 2 Fire Weather Outlook
          
Day 2 Fire Weather Outlook Image
Day 2 Fire Weather Outlook  
NWS Storm Prediction Center Norman OK
0203 PM CDT Mon Aug 21 2017

Valid 221200Z - 231200Z

No changes have been made to the ongoing forecast.

..Cohen.. 08/21/2017

.PREV DISCUSSION... /ISSUED 0140 AM CDT Mon Aug 21 2017/

...Synopsis...
Upper ridging will remain prominent over much of the western CONUS
on Day 2/Tuesday, while an upper low located off the coast of
southern/central CA moves little. Mid-level monsoonal-related
moisture will advance northward along the Sierras into the southern
Cascades and vicinity on the northern periphery of the upper low by
Tuesday afternoon.

...Portions of Northern CA into Southern/Central OR and Far
Northwestern NV...
Isolated to widely scattered thunderstorms should develop by Tuesday
afternoon and continue through at least the early evening hours
across portions of northern CA into southern/central OR.
Terrain-driven circulations will likely be the primary impetus for
convective initiation. Forecast precipitable water values around 
0.7 to 1.0 inch and modest (~15-20 kt) mid-level southwesterly flow
along the Sierra/Cascade crest will likely allow a mix of wet/dry
thunderstorms. A dry antecedent low-level airmass coupled with
diurnal heating will act to lower RH values across this region,
which should limit the potential for wetting rainfall outside of the
heaviest thunderstorm cores. Fuels also remain generally dry to very
dry and receptive to large fire starts. Accordingly, an isolated dry
thunderstorm area has been introduced across northern CA into
southern/central OR and far northwestern NV.

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

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