SPC Apr 21, 2017 Day 4-8 Severe Weather Outlook

Day 4-8 Outlook

Day 4-8 Convective Outlook  
NWS Storm Prediction Center Norman OK
0346 AM CDT Fri Apr 21 2017

Valid 241200Z - 291200Z

...DISCUSSION...
...Monday/Day 4...
The medium-range models begin the Day 4 to 8 period with an
upper-level low over northern Georgia and move this feature to the
east coast on Monday/Day 4. Thunderstorm development will be
possible during the day across parts of the eastern Carolinas.

...Tuesday/Day 5...
Both the ECMWF and GFS advect moisture into eastern parts of the
Great Plains on Tuesday/Day 5 and move a relatively low-amplitude
upper-level trough across the region. Although a localized severe
threat would be possible across parts of the southern Plains Tuesday
afternoon and evening, any convective development should remain very
isolated.

...Wednesday/Day 6...
Both the ECMWF and GFS continue moisture advection across the
south-central U.S. as an upper-level trough moves into the Four
Corners region on Wednesday/Day 6. Both models place an axis of
instability by afternoon across east Texas extending north
northeastward into the Ozarks. If this pattern materializes, severe
thunderstorms will be possible along the northern part of the
instability corridor in the Ozarks and mid Mississippi Valley on
Wednesday afternoon and evening.

...Thursday/Day 7...
The ECMWF and GFS differ in their solutions on the shortwave trough
positions for Thursday/Day 7. The ECMWF develops a negatively tilted
shortwave trough over the central Plains while the GFS maintains
southwesterly flow across the central U.S. If the ECMWF were to
verify, a cluster of severe storms would be possible across eastern
parts of the central Plains eastward into the mid Mississippi
Valley. Predictability remains low due to differences in model
solutions concerning smaller-scale features.

...Friday/Day 8...
The ECMWF and GFS are in better agreement in the western U.S.,
developing a vigorous upper-level trough in the Four Corners region.
This coincides with strong moisture advection across the Great
Plains. The synoptic setup which the medium-range models have been
forecasting for a couple days, could result in a significant severe
weather event on Friday into Friday night across parts of the
southern and central Plains with tornadoes, large hail and wind
damage possible. Due to the potential for a higher-end event on
Friday, have added a 15 percent contour where model consensus places
the greatest chance of severe weather.

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