SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0649 AM CST Mon Nov 09 2020

Valid 091300Z - 101200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
SOUTH FLORIDA AND THE KEYS...AND OVER PORTIONS OF THE
CENTRAL/SOUTHERN PLAINS...

...SUMMARY...
A tornado or two may occur today over parts of south Florida or the
Keys on the back side of Tropical Storm Eta.  Marginally severe hail
or gusts cannot be ruled out tonight over portions of the
south-central Plains.

...Synopsis...
In mid/upper levels, a longwave trough will persist over the western
CONUS, but with some important synoptic to subsynoptic developments.
 A currently upstream trough over the Gulf of AK and southern AK
will dig southeastward to BC and the Pacific Northwest by the end of
the period.  As that occurs, the broad, marginally closed cyclone
now over the Great Basin will eject east-northeastward across the
central Rockies to the adjoining High Plains.  By 12Z tomorrow, the
associated 500-mb low should be located over the northwestern KS/
southwestern NE area, with trough south-southwestward over
southeastern NM.  Elsewhere, a broad mid/upper cyclone over the
southeastern Gulf, southern FL, the northwestern Caribbean, and
central/western Cuba, has enveloped the circulation of T.S. Eta,
which should become the dominant midlevel vorticity max for the
broader cyclone.

At the surface, 11Z analysis showed a cold front across portions of
northeastern/central MN to a weak low in the ONL/YKN area, then
southwestward to another low near LHX.  The Arctic frontal branch
extended from there northwestward over northwestern CO to the
northern Rockies, while a Pacific front extended from the southern
low south-southwestward over south-central NM.  A dryline was drawn
from the southern low southward close to the eastern NM/TX line,
then across the Big Bend area of TX.  By 00Z, the combined cold
front should extend from the western Lake Superior/MN Arrowhead
region to western IA, central KS, through a low over the
southwestern KS/southeastern CO area, then south-southwestward over
far west TX.  The front will overtake the dryline generally from
north-south this evening and overnight.  By 12Z, the front should
reach eastern Lower MI, southwestern WI, to a weak low over
northwestern MO, then across eastern KS to west-central TX and
northern Coahuila.

...South FL/Keys...
T.S. Eta is forecast by NHC to take a broadly cyclonically curving
and generally southwestward track through the period, around the
broader cyclone noted above.  The T.S. should shift toward a
position over the southeastern Gulf that will effectively render it
a meandering, cutoff low amidst weak steering flow on days 2-3.  See
latest NHC advisories for tropical watch/warning info and specific
track/intensity guidance.

Forecast wind radii and existing/progged convective structure
indicate that Eta will remain asymmetric this period with convective
bands well east and northeast of center, across the outlook area. 
Weak lapse rates and marginal instability -- as sampled by available
central/southern FL RAOBs since 06Z -- are expected to continue, but
with typically rich near-surface moisture for a tropical cyclone,
and sufficient SRH (especially for any rightward/deviant motions of
sustained, deeper cells) to support a threat for a brief tornado or
two.  Outer/peripheral convection associated with Eta may continue
to affect parts of this area well into tonight.  However, as the
system pulls away westward, low-level flow, boundary-layer shear,
and hodograph size each will decline gradually with time over land,
likewise ramping down the already marginal supercellular tornado
potential.

...Central/southern Plains...
Isolated to scattered thunderstorms are expected to develop this
evening and especially overnight across western parts of the outlook
area, near the front and dryline, with greater convective coverage
likely over the more-strongly frontally forced KS portion.  Activity
should spread eastward and northeastward across the remainder of the
area overnight, with the most intense cells offering a threat for
marginally severe hail and or gusts.

Strengthening deep-layer shear and lift with time will be the most
favorable factors, while instability remains modest.  Time series of
forecast soundings suggest steep low/middle-level lapse rates above
a nocturnally stabilizing airmass near the surface, with increasing
moisture in and just atop the boundary layer.  This will support
500-800 J/kg MUCAPE, locally/briefly reaching near 1000 J/kg, amidst
40-50 kt effective-shear vectors.  The most favorable juxtaposition
of CAPE-shear parameter space and greatest coverage probabilities
appears to be over parts of central/south-central KS this evening. 
Large-scale ascent should decrease southward across OK into greater
moisture, as will frontal forcing, while forecast soundings show a
warm nose or "CAPE robber" around 500 mb.  This leads to questions
over coverage, but with the potential for isolated strong-severe
convection as far south as west-central OK.

..Edwards/Kerr.. 11/09/2020

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