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Day 4-8 Severe Weather Outlook Issued on Dec 13, 2017
Updated: Wed Dec 13 10:08:02 UTC 2017
Day 4-8 Severe Weather Outlook Graphics Issued on Dec 13, 2017
Day 4 Severe Weather Outlook Graphics Issued on Dec 13, 2017
Day 4 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
No Risk Areas Forecast
Day 5 Severe Weather Outlook Graphics Issued on Dec 13, 2017
Day 5 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
No Risk Areas Forecast
Day 6 Severe Weather Outlook Graphics Issued on Dec 13, 2017
Day 6 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
No Risk Areas Forecast
Day 7 Severe Weather Outlook Graphics Issued on Dec 13, 2017
Day 7 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
No Risk Areas Forecast
Day 8 Severe Weather Outlook Graphics Issued on Dec 13, 2017
Day 8 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
No Risk Areas Forecast
D4Sat, Dec 16, 2017 - Sun, Dec 17, 2017 D7Tue, Dec 19, 2017 - Wed, Dec 20, 2017
D5Sun, Dec 17, 2017 - Mon, Dec 18, 2017 D8Wed, Dec 20, 2017 - Thu, Dec 21, 2017
D6Mon, Dec 18, 2017 - Tue, Dec 19, 2017 (All days are valid from 12 UTC - 12 UTC the following day)
Note: A severe weather area depicted in the Day 4-8 period indicates 15%, 30% or higher probability for severe thunderstorms within 25 miles of any point.
PREDICTABILITY TOO LOW is used to indicate severe storms may be possible based on some model scenarios. However, the location or occurrence of severe storms are in doubt due to: 1) large differences in the deterministic model solutions, 2) large spread in the ensemble guidance, and/or 3) minimal run-to-run continuity.
POTENTIAL TOO LOW means the threat for a regional area of organized severe storms appears unlikely (i.e., less than 15%) for the forecast day.
 Forecast Discussion

   ZCZC SPCSWOD48 ALL
   ACUS48 KWNS 131006
   SPC AC 131006

   Day 4-8 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0406 AM CST Wed Dec 13 2017

   Valid 161200Z - 211200Z

   ...DISCUSSION...
   A Pacific trough forecast to move inland across British Columbia and
   the western U.S. on Day 3 (Friday) should amplify on Day 4
   (Saturday) across the Rockies, Four Corners region, and the adjacent
   Plains.  As a result of this evolution, the Day 3 northern Mexico
   trough will eject northeast across TX toward AR/LA.  Further
   amplification of the Rockies trough is expected on Day 5 (Sunday)
   across the Plains and upper MS Valley, with the TX shortwave trough
   weakening as it progresses downstream through the OH/TN Valleys to
   the off the middle and northern Atlantic coast.

   ...Day 4 (Saturday)...
   Strengthening southerly low-level winds into East TX to the lower MS
   Valley Saturday, and especially Saturday night will result in
   moisture return ahead of the ejecting northern Mexico shortwave
   trough.  Although strong vertically veering winds would be
   supportive of organized storms, forecast soundings suggest stronger
   instability should remain offshore the northwestern Gulf coast. 
   There is some potential for surface-based storms near the TX coast
   to the LA coast Saturday night, though overall weak instability
   precludes the issuance of a 15% severe probability at this time.

   ...Day 5 (Sunday)...
   The Day 4 TX shortwave trough will continue ejecting northeast and
   away from the Gulf coast region.  There will be the potential for
   showers and thunderstorms to spread across the central Gulf coast
   region and southeast states, though weak instability should limit
   the severe-weather threat.

   ...Day 6 (Monday)...
   Models diverge, especially after Day 5, with the eastward
   progression of the central U.S. large-scale trough.  The ECMWF is
   faster and more amplified with this trough, shifting it from the
   southern Plains to off the eastern seaboard by 12Z Tuesday.  The
   ECMWF suggests the potential for organized severe storms across part
   of northern FL on Monday, though weak instability and lack of model
   agreement preclude the introduction of a 15% severe probability area
   at this time.

   ..Peters.. 12/13/2017

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Page last modified: December 13, 2017
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