Valid 191200Z – 201200Z
…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE
NORTHERN PLAINS AND MIDDLE MO VALLEY….AND OVER THE CENTRAL HIGH
PLAINS…AND ACROSS NEW ENGLAND…
…SUMMARY…
Isolated severe storms are possible over parts of the northern
Plains and Middle Missouri Valley Monday evening and overnight.
Isolated severe storms are possible across the central High Plains
and New England.
…Synopsis…
Overall upper pattern across the northern third of the CONUS will
continue to trend more zonal as the shortwave trough currently
moving through Ontario progresses eastward and upper ridging
centered over the southern Plains builds. An additional shortwave
trough is expected to move across Canadian Prairie Provinces during
the second half of the period, helping to push a cold front through
the much of the northern Plains by early Tuesday morning.
Before this cold front moves through, the surface pattern will be
dominated by high pressure gradually shifting northeastward across
the Upper Midwest and Upper Great Lakes. Southerly flow around this
high will allow the very moist air mass across the central Plains to
advect northward into the northern Plains.
…Northern Plains into the Mid MO Valley…
Moisture advection is anticipated across the region throughout the
day, with dewpoints in the mid to upper 60s (isolated low 70s) by
the late afternoon/early evening. At the same time, strong diurnal
heating will lead to deep boundary-layer mixing and temperatures
into the upper 80s and 90s across the western Dakotas. Large-scale
forcing for ascent will be negligible (with minor height rises
anticipated) but with little convective inhibition and surface
convergence near a weak surface low over western ND, thunderstorm
initiation is possible. If initiation can occur, the air mass across
the western Dakotas (and downstream across the central Dakotas) will
support strong updrafts capable of hail and strong wind gusts.
Southeasterly low-level flow also supports low-probability tornado
threat.
Steep low-level lapse rates and relatively dry mid-levels also
suggest an environment favorable for strong cold pools and a
resulting potential for upscale growth into an organized,
forward-propagating MCS. A strengthening low-level jet would provide
the moisture needed for MCS maintenance. Main uncertainty regarding
this scenario is the initiation of surface-based storms during late
afternoon across the western Dakotas. Guidance has shown high
run-to-run variability regarding initiation. Upgrades may be needed
in subsequent outlooks if initiation appears likely.
Additionally, a strengthening low-level jet and resulting warm-air
advection is expected to promote elevated thunderstorm development
across IA late today/early Tuesday morning. Hail is possible within
the strongest updrafts.
…Central High Plains…
Isolated thunderstorm development is possible along the lee trough
extending across the region. Most probable location for initiation
is across northeast CO where a lee low will likely exist. Given the
deeply mixed boundary-layer, strong wind gusts are possible with a
few of these high-based storms. Some isolated hail is also possible
with the strongest updrafts.
…New England…
Isolated showers and thunderstorms are expected this morning, but
air mass recovery is still anticipated ahead of an approaching
shortwave trough and attendant surface trough. The strongest shear
will be confined to ME, where the potential for a few supercells
exists. Modest flow aloft will exist farther south, but bowing line
segments capable of isolated damaging wind gusts are still expected.
..Mosier/Nauslar.. 08/19/2019
$$