SPC - No watches are valid as of Tue Sep 18 17:21:02 UTC 2018

No watches are valid as of Tue Sep 18 17:21:02 UTC 2018.


No Mesoscale Discussions are in effect as of Tue Sep 18 17:21:02 UTC 2018.

SPC Sep 18, 2018 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
0753 AM CDT Tue Sep 18 2018

Valid 181300Z - 191200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE UPPER MIDWEST...

...SUMMARY...
Isolated, sporadic severe hail and gusts are possible today over
parts of the Upper Midwest.

...Synopsis...
In mid/upper levels, a belt of relatively low-amplitude westerlies
will shift eastward from the northern Plains and Upper Midwest
across the Upper Great Lakes region.  An embedded/weak shortwave
trough -- now apparent in moisture channel imagery over central/
eastern WY -- is forecast to move eastward to the eastern Dakotas by
00Z, then across MN overnight, perhaps phasing with convectively
induced vorticity maxima to its southeast.  A synoptic-scale trough
is located from southern BC to the OR coast and offshore central/
northern CA.  This trough is forecast to move slowly inland across
the Pacific Coast States through the period, as an upstream speed
max and shortwave trough now about 700 nm W of the OR coast catches
up and phases with the synoptic trough over OR and northern CA
tonight.  Meanwhile, a weak but lengthy,  positively tilted
mid/upper trough -- initially extending from southeastern NY
southwestward over the central/southern Appalachians -- will move
eastward across the Atlantic States through the period, the northern
portion progressing faster.  As this occurs, low/middle-level
remnants of Florence will accelerate offshore.

At the surface, a frontal-wave low was analyzed at 11Z in the
OFK/SUX corridor, with cold front southwestward across southwestern
NE and northern CO.  A warm front extended eastward across north-
central IA to an outflow boundary that extended from there to
north-central IL.  The IA portion of the outflow boundary should
stall in the next few hours and effectively blend with the warm
front, which will move only slowly further northward, if at all,
given convection growing denser to its north.  The cold front
generally will stall through the rest of the period as flow aloft
becomes more southwesterly and weak waves ripple along the boundary.

...Upper Midwest...
A complex and messy convective scenario is underway, and likely to
remain so through much of the period, over the outlook area.  As a
result, unconditional severe probabilities are maintained at
marginal levels, though more focused mesoscale features and
convective trends may compel a stronger risk in a subset of the area
on a later update. 

An ongoing convective complex that earlier produced isolated severe
hail/gusts in eastern SD is moving eastward north of the surface
front.  Refer to SPC mesoscale discussion 1475.  The associated
outflow boundary and low/middle-level perturbation are expected to
shift eastward and east-southeastward, primarily into relatively
stable air north of the baroclinic zone that is being further
processed by growing clusters of elevated convection over southern
MN and extreme northern IA.  As a result, general weakening of the
MCS is expected through the remainder of this morning as it merges
eastward into the elevated convective regime. 

The trailing part of low/middle-level perturbation related to the
MCS may impinge on a destabilizing boundary layer near and south of
the front, over northern/eastern IA, through the remainder of
morning into afternoon.  That destabilization will be related to
both diabatic heating and low-level warm advection, combining with
boundary-related lift to support either ongoing convection now
behind the MCS' southward extending outflow boundary, or new storms
in the same vertical-velocity plume, either of which may offer
sporadic severe hail/gusts.  The wind hazard should diminish with
northward extent from the surface front/outflow boundary and with
eastward extent this evening, into more-stable air. 

In addition to the added wind probabilities, isolated severe hail
still possible as well from any of these regimes:
1.  The merged area of MCS/warm-advection storms this morning into
afternoon north of the front, as embedded updrafts pulse to
strengths supporting marginally severe hailstone production;
2.  Aforementioned newer development near the boundaries today,
and/or
3.  Conditionally, elevated development possible later this
afternoon and evening over the aggregated convective cold pool, as
warm advection and moisture transport occur, and parcels rise
isentropically to LFC.

..Edwards/Peters.. 09/18/2018

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SPC Sep 18, 2018 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
1100 AM CDT Tue Sep 18 2018

Valid 181630Z - 191200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE UPPER MIDWEST...

...SUMMARY...
Isolated hail and damaging wind gusts are possible today over parts
of the Upper Midwest.

...NE/IA/IL...
A persistent cluster of thunderstorms is affecting much of northern
IA and adjacent regions this morning, with the leading-edge outflow
surging southward across eastern IA.  Much of this region has been
stabilized by the convection, leaving a corridor from eastern NE
into northwest IL where sufficient heating is possible later today. 
This axis may see storm intensification by mid/late afternoon, with
the strongest cells capable of hail or locally damaging wind gusts. 
Present indications are that the severe risk will remain isolated
and marginal in nature.

..Hart/Darrow.. 09/18/2018

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SPC Sep 18, 2018 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
1213 PM CDT Tue Sep 18 2018

Valid 191200Z - 201200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE HIGH
PLAINS OF WYOMING AND COLORADO INTO SOUTHERN MINNESOTA AND WESTERN
WISCONSIN...

...SUMMARY...
A few severe storms are possible across parts of the northern Plains
and upper Mississippi Valley on Wednesday, mainly during the
evening.

...Synopsis and Discussion...
An upper high will shift east from the lower Mississippi Valley
toward Kentucky and Tennessee, providing very warm temperatures
aloft, while an upper trough develops across the Rockies. Increasing
southwesterly winds aloft will develop across the northern Plains
into the Great Lakes ahead of the upper trough, with increasing lift
during the evening. At the surface, a stationary front will extend
from a lee trough over eastern Colorado northeastward into central
Wisconsin, with a moist air mass along and south of the boundary.

...Northern Plains into the upper Mississippi Valley...
Scattered thunderstorms may be ongoing across the upper Mississippi
Valley Wednesday morning on the nose of a weak southwesterly
low-level jet. These storms are not likely to be severe, but may
contain small hail and heavy rain. More vigorous and widespread
thunderstorms will occur after 00Z mainly from northeast Colorado
into Wisconsin as warm advection increases during the evening, near
or just after peak destabilization. Hail will be the primary threat
as most of the storms will be north of the surface front.

...MAXIMUM RISK BY HAZARD...
Tornado:  <2%     - None
Wind:      5%     - Marginal
Hail:      5%     - Marginal

..Jewell.. 09/18/2018

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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
1007 AM CDT Tue Sep 18 2018

Valid 181700Z - 191200Z

...CRITICAL FIRE WEATHER AREA FOR EXTREME EAST-CENTRAL
NEVADA...SOUTHWEST INTO NORTHEAST UTAH...AND SOUTHWEST INTO
SOUTH-CENTRAL WYOMING...

Minor adjustments to the elevated and critical area have been made
this morning. These changes are supported by latest hi-res HRRR and
HREF guidance, in addition to surface observation trends this
morning. Overall forecast reasoning remains unchanged. See previous
outlook below for more details.

..Leitman.. 09/18/2018

.PREV DISCUSSION... /ISSUED 0116 AM CDT Tue Sep 18 2018/

...Synopsis...
A mid-level shortwave will impact potions of the Great Basin and
central Rockies today. The strong mid-level flow associated with
this feature will boost surface wind speeds as boundary layers
deepen in the afternoon. At the surface, troughing within the Great
Basin into Central Wyoming as well as high pressure behind a cold
front will build into the northern Rockies will further enhance the
pressure gradient.

...East-central Nevada...west-central into northeastern
Utah...southwestern/south-central Wyoming...
Critical fire weather conditions are expected during the afternoon
as surface winds reach 20-25 mph. Mid-level flow will drive much of
the critical conditions for parts of Nevada and Utah. In Wyoming,
high-resolution guidance appears to be picking up on the
strengthening surface pressure gradient with the approach of a cold
front from the north. Afternoon RH values will range from 5-15%.
Areas surrounding the critical area will see similar RH values but
surface winds should fall short of critical criteria, ranging only
from 15-20 mph.

...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
1217 PM CDT Tue Sep 18 2018

Valid 191200Z - 201200Z

...CRITICAL FIRE WEATHER AREA FOR SOUTHWEST TO NORTHEAST
UT...SOUTHWEST WY AND FAR NORTHWEST CO...

The critical area has been removed from southern ID with this
update. Latest hi-res and deterministic guidance suggests wind
speeds will not meet critical criteria across that area, though
elevated fire weather conditions are still expected. Further south,
the critical area has been expanded to cover southwest to northeast
UT as a band of dry and windy conditions is expected to develop as a
surface trough strengthens. Elsewhere, minor adjustments to the
elevated area have been made based on latest model trends. For more
details, see previous discussion below.

..Leitman.. 09/18/2018

.PREV DISCUSSION... /ISSUED 0153 AM CDT Tue Sep 18 2018/

...Synopsis...
The mid-level trough axis will push through the Great Basin on
D2/Wednesday. Another well-timed mid-level jet will round the base
of the trough and impact portions of Utah and Wyoming during the
afternoon. A deepening surface trough across Utah and southwest
Wyoming will also enhance surface winds.

...Portions of southeast Idaho...northeastern Utah...portions of
west-central Utah...southwestern/south-central Wyoming...northwest
Colorado...
Critical fire weather conditions are expected in several areas.
Within the Sake River Valley and vicinity, high pressure building to
the west in concert with the deepening trough across Wyoming and
Utah will drive surface flow that will be enhanced by the terrain.
Across southwestern/south-central Wyoming and adjacent portions of
Colorado and Utah, afternoon heating will mix strengthening
mid-level flow to the surface with surface troughing helping to
enhance wind speeds further. Farther southwest in west-central Utah,
the core of the mid-level jet will drive surface wind speeds to
critical levels during the afternoon. These areas will all
experience surface winds of 20-25 mph amidst afternoon RH of 5-15%.
A broader area of elevated fire weather concerns will exist
surrounding these critical areas where similar afternoon RH values
will exist with weaker surface winds of 15-20 mph.

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

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