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ACUS01 KWNS 032324
SWODY1
SPC AC 032323
Day 1 Convective Outlook AMEND 1
NWS Storm Prediction Center Norman OK
0623 PM CDT Mon Sep 03 2018
Valid 032320Z - 041200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS EVENING
ACROSS PARTS OF WESTERN NORTH DAKOTA...
AMENDED FOR ADDITION OF TORNADO PROBABILITIES IN IOWA
...SUMMARY...
Severe storms capable of large hail and damaging wind are expected
this afternoon and evening over parts of western/central North
Dakota, with other strong to locally severe thunderstorms possible
across the Midwest, south Florida, and northern New England.
...Amendment Update...
Tornado probabilities have been added in Iowa to account for a local
threat of a couple tornadoes associated with a quasi-warm front /
baroclinic zone extending from central Iowa eastward. Higher
effective helicity near the boundary, combined with a moist
low-level environment, may support another tornado or two this
evening. See MCD 1420 for more details.
...20Z Outlook Update...
Some changes to categorical and severe probabilistic lines have been
made, mostly to account for the latest trends concerning
destabilization and ongoing convective development.
...East of the lower Great Lakes...
In most areas still not impacted by thunderstorm activity, surface
temperatures have warmed to near 90F, in the presence of dew points
near or above 70F. This appears to be contributing to moderately
large CAPE in excess of 1500 J/kg, which may support a continued
gradual increase in thunderstorm intensity through late afternoon.
Aided by 25-30+ kt westerly flow in the 700-500 mb layer, as far
south as the central tier counties of New York state (near the
northern periphery of the subtropical ridge), some of this activity
may be accompanied by localized potentially damaging wind gusts
before diminishing this evening.
...Central High Plains...
There are no changes to severe probabilities (less than 5 percent)
across this area. Modest deep layer shear associated with a 30-35
kt mid-level speed maximum may be supporting the sustained ongoing
cluster of storms across eastern Colorado. Activity is occurring in
the presence of a relatively moist environment, which appears to be contributing to CAPE in excess of 1000 J/kg. However, lower/mid
tropospheric lapse rates across the region are generally modest to
weak, and the boundary layer is not expected to become particularly
warm or deeply mixed, which still seems likely to minimize any risk
for severe wind gusts and hail.
..Picca/Kerr.. 09/03/2018
.PREV DISCUSSION... /ISSUED 1131 AM CDT Mon Sep 03 2018/
...Northern High Plains...
In response to CINH-reducing heating and strengthening
convergence/lift along a prefrontal trough, widely scattered to
scattered thunderstorms should form late this afternoon over western
ND/perhaps extreme eastern MT in a south-southwest/north-northeast
band. Coverage should diminish with southward extent into
northwestern South Dakota due to stronger capping and weaker lift.
Severe gusts and large hail will be possible from a blend of
multicellular and supercellular storm modes.
As the warm front moves northward, associated warm and moist
advection will act in tandem with diabatic heating to destabilize
the boundary layer, while boosting surface dew points into the upper
50s to low 60s F range. Resultant MLCAPE of 1500-2500 J/kg, along
with 35-45-kt effective-shear magnitude and substantial veering of
wind with height, will support several hours of severe risk as mode
evolves from discrete to quasi-linear. Late this evening, as the
boundary layer diabatically stabilizes and the mid/upper-level
shortwave trough deamplifies, convection should diminish in
intensity and coverage with time.
...Florida...
Latest NHC guidance suggests that Tropical Storm Gordon will
continue to gradually strengthen and organize as it continues slowly west-northwestward over South Florida. While the overall initial
modest strength/organization of the system and its weak mid-level
winds will tend to limit the overall tornado threat, strong
east/southeasterly winds within the lowest few km AGL will support
some transient supercells. A brief tornado or two could occur aside
from a nearshore waterspout risk.
...Eastern IA/southern WI/northern IL...
Have introduced severe probabilities for expectations of at least
some severe-weather potential across the region this afternoon
through early evening. Similar to prior days, it appears that
thunderstorm development will be focused along a zone of decayed
outflow and differential heating. Zone of moderate instability will
support stout updrafts, with moderately strong mid-level winds
supportive of as much as 35 kt effective shear, which could yield
some transient supercells aside from more common
east/northeastward-moving clusters. Localized wind damage would be
the most probable severe hazard.
...Northern New England...
Scattered thunderstorms are expected to develop over portions of
southern Quebec, and perhaps portions of Vermont/New
Hampshire/northwestern Maine close to the Canadian border. This
activity should move eastward across northern New England, offering
the potential for isolated severe hail and gusts.
A plume of large-scale lift ahead of an Ontario shortwave trough
will coincide with a diabatically heated boundary layer and an eastward-shifting moist axis, characterized by surface dew points
generally in the mid/upper 60s F. Forecast soundings feature MLCAPE
in the 500-1500 J/kg range, locally higher -- but weakening with
northward extent over Maine where antecedent clouds will persist
longer. Deep-layer shear will strengthen as the shortwave trough
approaches, with 30-40-kt effective-shear vectors developing in
support of some convective organization. Nearly unidirectional flow
with substantial westerly component will limit convergence and
storm-relative flow in low levels. Convection should weaken with
eastward extent and time over southern/eastern Maine.
$$
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