Valid 011200Z – 021200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS NORTHEAST
CO…FAR NORTHWEST KS…FAR SOUTHWEST NE…
…SUMMARY…
Severe thunderstorms capable of large hail and strong wind gusts are
possible across the central Plains this afternoon and evening.
…Synopsis…
Upper ridging centered over the southern High Plains and extending
into the Canadian Prairie provinces will remain the primary upper
feature today. A compact shortwave trough will move into the Pacific
Northwest/southern British Columbia tonight. A pair of more
subtle/low-amplitude shortwave troughs are expected to move through
the periphery of the upper ridge. The northernmost of this pair is
expected to move out of WY and into SD/NE this afternoon and evening
while the southernmost of the pair moves through the middle MS
Valley into the lower MS Valley. Both of these shortwave trough will
provide the impetus for thunderstorm development.
…Central High Plains into the Northern Plains/Northern High
Plains…
Thunderstorms are expected to develop during the afternoon as
diurnal heating destabilizes the air mass and previously mentioned
shortwave trough provides the forcing for ascent. Highest
thunderstorm coverage is expected across the central High Plains
where the best overlap between favorable low-level moisture,
instability, and forcing for ascent exists. Effective bulk shear
will be around 35 kt, which is sufficient for organized storms. An
initially cellular/discrete mode will likely transition to a more
linear mode as cold pools amalgamate and upscale growth occurs.
Consequently, primary threat will initially be large hail with a
transition to strong wind gusts thereafter.
Farther north (eastern WY/western NE Panhandle/western SD),
shortwave trough may precede diurnal destabilization here, limiting
thunderstorm coverage and intensity. Even so, a few strong to severe
storms are possible.
A somewhat different forcing mechanism is anticipated across eastern
MT, where convergence along a surface trough is expected to provide
the lift needed for convective initiation. Limited low-level
moisture and deep boundary-layer mixing will likely keep storm
coverage isolated, although modest directional shear could support a
few stronger updrafts capable of large hail and/or strong wind
gusts.
…Lower MS Valley…
Shortwave trough enhanced by convection currently ongoing across
eastern KS/western MO will progress southward throughout the day.
Some modest increase in the mid-level southerly flow may accompany
this shortwave as it rounds the upper ridge in LA/east TX. Overlap
between this stronger flow aloft and afternoon thunderstorms may
occur across portions of the lower MS Valley, yielding a few strong
storms capable of damaging wind gusts. Even so, forecast uncertainty
and low coverage result in severe probabilities less than 5%.
..Mosier/Bentley.. 08/01/2019
$$