Valid 301200Z – 011200Z
…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PORTIONS OF
THE UPPER GREAT LAKES TO THE SOUTHERN HIGH PLAINS…
…SUMMARY…
Isolated strong to severe storms will be possible from the upper
Great Lakes to the central Plains as well as over the southern High
Plains.
…Upper Great Lakes to Central Plains…
Strong mid-level speed max is forecast to translate across
WY/Dakotas during the day, then into northwest MN during the evening
hours. In response, a surface cyclone will track across northeastern
SD to near the MN/ON border by 01/00z with a trailing front arcing
through northwestern IA into northeast CO. Large-scale height falls
are forecast to spread across the northern Plains into MB/western ON
with the southern influence of the short-wave trough expected to
glance the international border region. Early in the period,
low-level warm advection will likely be responsible for one or more
clusters of convection across the upper Great Lakes region. While
some of this activity could be strong, possibly generating hail at
times, the primary concern early in the period will be immediately
ahead of the mid-level speed max over eastern ND into northwest MN.
This activity will be an extension of ongoing thunderstorms (at
0530z) that are growing upscale just north of a warm front along the
SD/ND border. Strong/severe elevated thunderstorms should continue
through daybreak north of the surface low where relatively steep
mid-level lapse rates and MUCAPE in excess of 2500 J/kg are
expected. Strong shear suggests storm rotation is possible and a few
supercells can be expected. Large hail is the primary risk before
this activity advances into ON.
Across the warm sector, considerable uncertainty exists regarding
the prospect for surface-based convection. Forecast soundings
suggest parcels will struggle to reach their convective temperatures
except for southwest NE/northeast CO. Even across the central High
Plains it appears any storms that develop along the front will lift
north of the boundary and become elevated. Convection should
increase north of the front after sunset across NE, especially as
LLJ increases into this region. Will maintain 5% severe for this
scenario but a considerable number of storms may ultimately warrant
higher probs for possible hail.
…Southern High Plains…
Strong upper high will hold firm across the Mid-South region
ensuring a very moist southerly trajectory around the western
periphery of the anticyclone into the southern Rockies/High Plains.
While lapse rates will necessarily be poor within this moist plume,
thunderstorms should readily develop by afternoon from northern
Mexico, across far west TX into the TX Panhandle. Modest mid-level
flow should allow for some organization along with a low risk for
damaging winds, and perhaps some hail.
..Darrow/Cook.. 09/30/2019
$$