Day 1 Outlook Day 2 Outlook Day 3 Outlook Days 4-8 Outlook

Day 4-8 Outlook
Day 4 - 8 Outlook
Day 4-8 Severe Weather Outlook. A depicted severe weather area indicates a 30% or higher probability for severe thunderstorms within 25 miles of a point.
D4Fri, Sep 21, 2018 - Sat, Sep 22, 2018 D7Mon, Sep 24, 2018 - Tue, Sep 25, 2018
D5Sat, Sep 22, 2018 - Sun, Sep 23, 2018 D8Tue, Sep 25, 2018 - Wed, Sep 26, 2018
D6Sun, Sep 23, 2018 - Mon, Sep 24, 2018 (All days are valid from 12 UTC - 12 UTC)
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. 1) large differences in the deterministic model solutions,
  2. 2) large spread in the ensemble guidance, and/or
  3. 3) minimal run-to-run continuity.
POTENTIAL TOO LOW means the threat for a regional area of organized severe storms appears highly unlikely during the entire period (e.g. less than a 30% probability for a regional severe storm area across the CONUS through the entire Day 4-8 period).

Images courtesy of the NWS Storm Prediction Center
 Forecast Discussion


000
ACUS48 KWNS 180908
SWOD48
SPC AC 180907

Day 4-8 Convective Outlook  
NWS Storm Prediction Center Norman OK
0407 AM CDT Tue Sep 18 2018

Valid 211200Z - 261200Z

...DISCUSSION...
Medium-range models appear to be in fairly close agreement through
Day 5 (Saturday 9-22), after which agreement begins to break down
substantially, as an upper trough moves inland from the eastern
Pacific across the Intermountain West.  Substantial differences with
respect to the progression/speed of this trough result in similarly
substantial differences in surface pattern evolution, resulting in
low confidence with respect to any convective forecast beyond Day 5.

On Day 4 (Friday 9-21) however, an initial upper trough is forecast
to be advancing quickly eastward across the Great Lakes and later
the Northeast U.S., and adjacent eastern Canada.  This trough will
be accompanied by a seasonably strong cold front, progged to shift
across the Northeast and Ohio Valley and shift off the New England
coast by the beginning of Day 5.  While instability along the length
of the front -- particularly across New England -- will likely
remain limited, strong westerly/west-southwesterly flow through a
deep tropospheric layer may support potential for gusty/damaging
winds, with a frontal line of convection.  Thus, a severe risk area
is being included Day 4, from parts of the Midwest/Ohio Valley
northeastward into New England.

By Day 5, with the progressing portion of the front shifting into
the western Atlantic, severe weather threat will diminish, and then
with increasing discrepancies amongst the models the rest of the
period, no additional outlook areas will be included at this time.

..Goss.. 09/18/2018