SPC 0600Z Day 2 Outlook

Day 2 Outlook Image

Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1149 PM CDT Wed Oct 07 2020

Valid 091200Z - 101200Z


A few tornadoes may accompany the landfall of Hurricane Delta,
mainly across parts of southeastern Louisiana Friday afternoon and

Models continue to indicate that the main belt of westerlies across
the mid-latitude Pacific into North America may begin to undergo
amplification during this period.  It appears that this will include
a digging mid-level trough to the west of the Pacific Northwest
coast. and building large-scale ridging across the Canadian/U.S.
Rockies through the Canadian Prairies and the northern U.S. Great
Plains.  Downstream of the ridging, a short wave trough is forecast
to dig across northwestern Ontario.

Within a lingering weaker branch, split off to the south of the
stronger flow, a short wave trough, with an embedded mid-level low,
may gradually begin to progress into southern California and
adjacent areas of the Southwest.  As this occurs, some eastward
progression of weak downstream troughing east of the southern Great
Plains is expected, though mid-level ridging across the subtropical
western Atlantic into parts of the Southeast may hold firm.

In response to these developments, Hurricane Delta is forecast to
continue accelerating northward, then northeastward, around the
periphery of the ridging, before the mid-level circulation center
becomes increasingly absorbed within the mid-level trough, as it
progresses inland of the Louisiana Gulf coast.  Delta is expected to
be in the process of weakening as it makes landfall by early Friday
evening, and more rapid weakening probably will ensue as it
continues inland across Louisiana, late Friday evening through
daybreak Saturday.

In advance of Delta, and ahead of the weak mid/upper troughing
emerging from the southern Great Plains, seasonably high
precipitable water content likely will advect north-northeastward in
a plume off the northwestern Gulf of Mexico.  However, relatively
dry/potential cool boundary-layer air as far south as portions of
the Gulf coastal plain may be initially slow to modify.

...Gulf Coast...
At the outset of the period, it still appears that surface dew
points near or just above 70 F may be confined to immediate coastal
areas, though perhaps a bit farther inland across southeastern
Louisiana than other areas, to the north of Hurricane Delta.  Across
the remainder of the Gulf coastal plain, surface dew points may
generally be in the mid/upper 60s F, with mid 70s F dew points
mostly well offshore.

Given the warm mid-level environment initially present, and
advecting inland with Delta, mid 70s surface dew points probably
will need to advect inland to contribute to sufficient
boundary-layer destabilization to support an appreciable risk for
tornadoes. Forecast soundings in recent model runs are increasingly
suggestive that this is possible, at least in a corridor inland of
southeastern Louisiana coastal areas, just in advance of and with
the landfall of Delta.  East-northeast and east of the low-level
circulation center, enlarging, clockwise-curved low-level hodographs
in forecast soundings also appear increasingly conducive to tornado
potential, in the presence of weak boundary-layer layer instability,
perhaps by mid to late Friday afternoon.

The tornado threat may persist into Friday evening, before probably
diminishing overnight.

..Kerr.. 10/08/2020

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