Valid 021300Z – 031200Z
…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS SOUTHEAST ND TO
WEST-CENTRAL WI…
…SUMMARY…
Severe thunderstorms are possible this afternoon through early
morning Tuesday across parts of the Upper Midwest, with the most
likely corridor from southeast North Dakota to west-central
Wisconsin. Large hail and damaging winds should be the primary
hazards, with the risk for tornadoes much more uncertain.
…Upper Midwest…
Details regarding the low-level thermodynamic profile and surface
pattern suggest the forecast is complex and low confidence. While
previous forecast may have been too aggressive for the time frame,
have opted to refrain from a categorical downgrade and instead drive
the Enhanced Slight with hail northwest and wind southeast.
An amplifying shortwave trough over Alberta should dig into ND and
northern MN overnight, yielding increasing mid-level height falls
late in the period. A band of 700-mb warm advection will support
elevated convection spreading from the northeast MT/northwest ND
border area across ND through this afternoon. Thin elevated buoyancy
but pronounced mid to upper-level speed shear will support potential
for a long-track cell or two to produce severe hail towards the Red
River Valley.
By late afternoon/early evening, substantial differences exist in
guidance with the degree of diurnal destabilization. The 06Z NAM has
effectively negligible surface-based buoyancy across most of the
severe outlook area through 00Z with its warm front holding near the
MT/ND border arcing across southeast SD into the Lower MO Valley. It
suggests stratus will remain pervasive to the north/east beneath an
increasingly stout elevated mixed layer. Given that an anticyclone
over southeast MB is expected to only slowly decay as it slides
toward Lake Superior, this scenario is at least plausible. While
guidance such as the RAP indicates much more aggressive
surface-based destabilization northward into ND, with a more
volatile severe setup along/ahead of the impinging cold front during
the evening.
The most probable convective scenario is for elevated convection to
spread southeast and increase in coverage to a broader MCS from the
Red River to Upper MS Valleys tonight. The low-level thermodynamic
profile will be critical to the potential severe wind and tornado
hazards. Consensus of guidance suggests lapse rates from the surface
to 850 mb may remain poor until about 09Z. Still, ample MUCAPE and
approach of fast 700-500 mb westerlies suggest an organized but
largely elevated MCS should develop across parts of MN to WI. This
would likely offer a risk for damaging winds overnight, with a
conditional threat for QLCS tornadoes if near 70 surface dew points
can be realized ahead of the line.
…Southern New England through northern VA…
A convective band is ongoing from eastern PA to western New England
in association with a low-amplitude shortwave impulse. This impulse
will quickly eject into the Canadian Maritimes by evening, resulting
in stronger 850-700 mb winds becoming confined across New England
this afternoon. A pronounced gradient in surface dew points is
currently prevalent from the mid to upper 50s in southern New
England to the low 70s in the Chesapeake/Potomac region. This
suggests that surface-based buoyancy should be weak where low-level
hodographs can remain modestly enlarged, yielding an isolated
damaging wind/brief tornado threat. Farther southwest, isolated to
scattered thunderstorms may develop as a weaker upstream impulse
shifts east from Lower MI and interacts with a lee surface trough.
Adequate deep-layer shear will exist for isolated strong gusts and
marginally severe hail.
…East-central FL Coast…
Hurricane Dorian is forecast to drift northwest through the period,
remaining far enough offshore to limit the overall tornado threat.
However, some veering of the wind profile with height is expected to
persist where north-northwest surface winds are sustained along a
portion of the FL Atlantic Coast. This will support modestly
enlarged low-level hodographs. Any cells that can develop in outer
rainbands this afternoon into tonight might pose a risk for a brief
tornado.
..Grams/Leitman.. 09/02/2019
$$