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ALL SPC Mesoscale Analysis Curently in Effect
U.S. Tornado, Severe Thunderstorm & Fire Wx Alerts

No watches are valid as of Sat Jul 22 14:01:01 UTC 2017.No Mesoscale Discussions are in effect as of Sat Jul 22 14:01:01 UTC 2017.SPC 1300Z Day 1 Outlook
Day 1 Outlook Image
Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0757 AM CDT Sat Jul 22 2017

Valid 221300Z - 231200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE UPPER
OHIO VALLEY TO PARTS OF THE MID-ATLANTIC...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
UPPER MIDWEST/UPPER GREAT LAKES REGION...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISKS AND EXTENDING ACROSS THE MIDDLE MISSISSIPPI/LOWER
MISSOURI VALLEYS TO CENTRAL PLAINS...

...SUMMARY...
Scattered severe storms capable of hail or damaging wind are
possible from parts of the Ohio Valley eastward into the
Mid-Atlantic, and across parts of the upper Mississippi Valley and
Upper Great Lakes today.  More isolated and marginal severe storms
are expected across the central Plains to mid Mississippi Valley.

...Synopsis...
The upper-air pattern continues to feature a large area of high
heights covering most of the southern  2/3 of the conterminous U.S.,
while a belt of enhanced west-northwesterly mean flow extends from
the Canadian Rockies across the Upper Midwest and Great Lakes to New
England.  As ridging builds over the northern Rockies, a strong
shortwave trough -- now evident in moisture-channel imagery over
southern MB -- is forecast to move east-southeastward.  By 00Z, this
perturbation should include a compact 500-mb cyclone centered over
ON, just north of the MN border, with trough south-southwestward to
eastern NE.  By 12Z, the mid/upper low should reach central/southern
Lake Superior, with trough to northern MO.  Downstream, an MCV
related to ongoing convection is evident in composite-reflectivity
and satellite imagery over northern OH.  The related perturbation
should reach southern/southeastern PA and northern VA by 00Z. 

At the surface, 11Z analysis showed a cold front from the ACK/Long
Island area to northern VA, arching northwestward across western PA
and northeastern OH as a warm front.  This boundary should stall
over the Mid-Atlantic then move northward before being overtaken by
convection from the west.  Meanwhile, a surface low related to the
MB shortwave trough was analyzed over southeastern MB, with an
occluded front extending to a weak triple-point low over southern
MN.  From there an ill-defined warm front was drawn
east-southeastward over southern WI, and is forecast to move
northward across WI today through an air mass partially modified by
convective outflows over the last few days.  A cold front, initially
arching southwestward from the triple point across western IA and
northwestern KS, should move to eastern WI, central MO and southern
KS by the end of the period.

...Ohio Valley to Mid-Atlantic...
A complex and somewhat conditional scenario exists for severe
potential in this region today.  As such, we will maintain a broad
brushed corridor of 15%/slight-risk unconditional probabilities away
from areas too stabilized by earlier/ongoing storms -- acknowledging
that denser, localized/mesobeta-scale concentrations  of severe
(especially wind) are possible within. 

Two substantial areas of thunderstorms and associated clouds/precip
are ongoing that will influence severe threats in and near this
corridor:

1.  A complex of strong thunderstorms over eastern OH that has
produced isolated strong/damaging gusts overnight, and the northern
OH MCV.  Isolated severe potential exists over the next couple
hours; see SPC mesoscale discussion 1393 for near-term details.  

This convection primarily is located along and north of an outflow
boundary from earlier convection that extends east-southeastward
into central WV.  As the MCV and trailing trough proceed
east-southeastward, either ongoing activity or newly generated
storms will encounter a destabilizing boundary layer across the
central Appalachians and across the adjoining Piedmont and Fall
Line, to the lowlands of NJ and the Delmarva Peninsula.  Pockets of
strong surface heating are probable through breaks in cloud cover,
contributing to minimal MLCINH and well-mixed subcloud layers
supporting wind-gust potential. Forecast soundings suggest modest
midlevel lapse rates will be offset by sufficient low-level
moisture/destabilization to yield 800-1500 J/kg MLCAPE.  Deep shear
will diminish southward, with buoyancy weakening northward past the
front -- the optimal overlaps between supportive westerlies aloft
and instability being in the outlook area.  Some subset of this
swath may need 30%/enhanced wind probabilities as mesoscale
trends/details become better focused. 

2.  A trailing area of initially non-severe thunderstorms across
portions of northern IL and northeastern MO that mostly resides
along and north of the outflow boundary from the leading MCS.  This
activity is being supported by a sheet of elevated inflow associated
with the 30-40-kt westerly to west-southwesterly remnants of the
prior nocturnal LLJ.  As the LLJ continues to veer and weaken, this
activity should diminish gradually and weaken as it moves across IL,
IN and perhaps OH, substantially limiting/delaying strong insolation
and related destabilization over those areas.  Some guidance
indicates widely scattered to isolated thunderstorms developing near
the associated outflow/differential-heating boundary that should be
retreating northward across IL/IN/OH in its wake.  Any such activity
may offer isolated severe hail and damaging gusts; however, the
threat appears too low-end and conditional for more than marginal
probabilities at this time. 

...Upper Midwest...
Scattered to widely scattered thunderstorms are expected to form
this afternoon and evening, primarily in one or two arcs ahead of
the compact upper wave/low over the northern MN/western Lake
Superior area, with more isolated to widely scattered activity
farther southeastward across WI near and south of the warm front. 
Sporadic damaging gusts and severe hail will be the primary
concerns. 

Air-mass recovery and destabilization from both local diabatic
heating and low-level WAA are expected to occur across this region
today, behind the aforementioned IL activity, and around the remains
of a smaller, ongoing convective area over central MN.  This will
occur in tandem with a marked increase in large-scale forcing for
ascent aloft preceding the strong shortwave trough.  Moisture and
low-level theta-e generally will increase southward, while
cooling/DCVA aloft will be greater in the north, with at least
marginally suitable deep shear over the whole area.  Forecast
soundings suggest MLCAPE peaking in the 1000-1500 J/kg range away
from the shallow/cold marine layer of Lake Superior, amidst 30-40 kt
effective-shear magnitudes.  Multicells and at least transient
supercell structures are possible.  The severe threat should
diminish mid/late evening into the early overnight hours.

..Edwards/Thompson.. 07/22/2017

 




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