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LU9DCE > ALERT 12.09.20 18:03z 100 Lines 5835 Bytes #999 (0) @ WW
BID : 25297_LU9DCE
Read: GUEST
Subj: STORM PREDICTION CENTER 12 09
Path: HB9ON<IW8PGT<IZ3LSV<I3XTY<I0OJJ<LU4ECL<VK2RZ<W0ARP<AL0Y<K5DAT<N7HPX<
LU9DCE
Sent: 200912/1800Z 25297@LU9DCE.TOR.BA.ARG.SOAM LinBPQ6.0.20
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ESTACION DE PACKET RADIO - BUENOS AIRES (GF05OM)
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SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0749
AM CDT Sat Sep 12 2020
Valid 121300Z - 131200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
OF THE MIDWEST AND SOUTH FLORIDA...
...SUMMARY... A few strong to marginally severe thunderstorms are
possible across portions of the Midwest and south Florida.
...Synopsis... The mid/upper-level pattern will be dominated on
the synoptic scale by ridging related to anticyclones whose centers
should move slowly eastward across NV and NC. In between, a cyclone
was evident in moisture-channel imagery over the north-central
Plains, centered between PIR-YKN, with trough southwestward across
the central High Plains to northeastern NM. The cyclone is forecast
to devolve gradually to a shortwave trough through the period, as
it crosses portions of the mid/upper Mississippi Valley. By 12Z
tomorrow, the trough should extend from eastern Upper MI down Lake
Michigan to near STL.
At the surface, 11Z analysis showed a wavy/quasistationary
frontal zone across northern NC, central KY and southern IL,
bending southward over south-central MO across the Arklatex region
to near GLS, and just east of BRO. Gradual frontolysis of this
boundary is expected through the period, before parts of it are
overtaken by another cold front developing over the north-central
Plains. The latter boundary should move eastward over western IA,
northeastern MO and eastern KS by this evening. By 12Z, that cold
front should reach Lake Huron, southern IL, and western AR.
...IA/northwestern MO... An arc of relatively low-topped convection
should develop for a few hours late this afternoon, in a zone of
strong frontal and deep- layer forcing related to the approaching
cyclone/trough aloft. This convection may include isolated
thunderstorms and a supercell or two, with attendant potential
for isolated hail/gusts near severe limits. A tornado cannot be
ruled out, but the threat appears marginal at this time. Lack of
stronger instability is the substantial limiting factor for a
better-organized/higher-magnitude, unconditional severe threat.
A narrow plume of marginally favorable destabilization, related to
surface heating and DCVA aloft preceding the ejecting mid/upper
perturbation, will shift east-northeastward close to the surface
cold front. The swath of combined ascent will encounter increasing
boundary-layer moisture with eastward extent and proximity to
the weakening eastern frontal zone. Forecast soundings suggest
surface dew points generally in the upper 50s to low 60s will
support MLCAPE in the 200-700 J/kg range, amidst strong low-level
shear and enlarged hodographs, with 0-3-km SRH 150-300 J/kg and
35-45-kt effective-shear magnitude. Storm motion should outpace
the eastward shift of the narrow supportive thermodynamic regime
across northern MO and southern IA, and weaken substantially this
evening with surface diabatic cooling.
...South Florida... A brief tornado or two cannot be ruled out
through 12Z tomorrow across the outlook area as Tropical Depression
19 pulls away from south FL, but also, intensifies over the Gulf
according to NHC forecasts. Such strengthening will result in an
increasingly favorable system-relative wind field for supercells
in the eastern and southeastern sectors. The outer parts of that
wind field will shift westward across the outlook area, but more
gradually than the TC-translation vector, when accounting for
predicted wind-radii expansion related to cyclone intensification.
The southeastern sector in particular will represent a favorable
downshear portion with respect to the deep-tropospheric northerly
shear component affecting the system, per NHC discussions. Given the
high-theta-e air mass involved, any sustained cells that can persist
in that environment may rotate and offer a brief tornado threat.
See latest NHC advisories for forecast guidance on track and
intensity of this system, and for tropical watches/warnings.
...IL/IN/southeastern MO... A low-confidence, marginal threat for
damaging to severe gusts remains apparent in a narrower (yet slightly
longer) corridor than in the prior outlook. Widely scattered to
scattered thunderstorms should form close to the eastern frontal
zone, in parts of eastern MO and western/central IL, during early/mid
afternoon, then strengthen somewhat and increase in coverage while
crossing the outlook area. This regime will be characterized by
height falls aloft and substantially higher low-level theta-e,
but also, generally weak midlevel lapse rates and low-level
shear, along with less large-scale lift than over the IA/MO area.
Training of echoes should render messy storm modes fairly early in
the convective cycle, given the deep-shear and mean-wind vectors'
orientation being very close to that of the axis of low-level forcing
driving the convective plume. Given these counterbalancing factors,
strong- severe gusts cannot be ruled out, but potential appears
too conditional and poorly organized for greater probabilities.
..Edwards/Broyles.. 09/12/2020
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El pensador sabe considerar las cosas mas sencillas de lo que son.
-- Friedrich Nietzsche. (1844-1900) Filosofo aleman.
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