Valid 011300Z – 021200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS CENTRAL AND
SOUTHERN AZ…
…SUMMARY…
Scattered severe wind gusts are possible across parts of central and
southern Arizona during the late afternoon and evening.
…Central/southern Arizona…
A stout mid-level anticyclone will remain centered over the Four
Corners area. Orographic lift will be the primary driver of isolated
to widely scattered thunderstorms by early afternoon
across the higher terrain of western NM into eastern AZ. With a belt
of 15-25 kt easterlies across the southern half of AZ, this
convection will spread west over the lower deserts during the late
afternoon and evening. Surface dew points should mix into the upper
50s to mid 60s at peak heating across the Lower Colorado and Gila
Valleys, yielding at least moderate buoyancy with MLCAPE above 1000
J/kg. One or two clusters may organize off the terrain with a risk
for scattered severe wind gusts until convection weakens overnight
amid increasingly substantial MLCIN in the Mojave Desert.
…OH Valley…
Within broadly cyclonic flow over eastern Canada and the Great
Lakes, a minor shortwave impulse will eject east from IL towards
PA/NY. Modest ascent ahead of this feature has encouraged a couple
thunderstorm clusters from central IL to Lower MI and over the Upper
OH Valley. This activity should remain weak through midday, but will
likely increase in coverage and intensity this afternoon as MLCAPE
reaches 1000-1500 J/kg across parts of IN/OH ahead of the IL
cluster. The primary risks from multicell clusters should be
isolated strong gusts and marginally severe hail. However, guidance
is fairly consistent in strengthening low-level winds this evening
from the Upper OH Valley to northern Appalachians. A small temporal
window should exist for a brief tornado risk near the OH/PA/WV
border area as low-level hodographs enlarge prior to surface-based
instability decreasing with eastern extent in PA.
…MT to NE…
A mid-level ridge will remain prominent across much of the
northern Great Plains through the period, although some flattening
is anticipated towards 12Z/Monday as a vigorous shortwave trough
amplifies over the Canadian Rockies. Considerable uncertainty
remains regarding possible convective initiation during the late
afternoon and early evening. Isolated thunderstorms may develop
along the lee trough from north-central MT to the NE/CO border area.
Buoyancy will be weak to the northwest but may be compensated by
plentiful mid to upper-level speed shear. A couple cells with
mid-level updraft rotation may form but will likely struggle to be
sustained eastward given pronounced MLCIN east of the trough. Have
expanded/combined the MRGL risk areas to account for this scenario.
Otherwise, an arc of 700-mb warm theta-e advection should yield
isolated to perhaps scattered elevated thunderstorms overnight
across eastern MT and the Dakotas into NE. Strongest speed shear
will be across the northern portion of the region, with MUCAPE
increasing as richer 850-mb moisture advects north. It is unclear
whether uncapped parcels can become rooted beneath 700-mb prior to
12Z. For now, will maintain MRGL risk, mainly for the potential of
isolated severe hail.
..Grams/Leitman.. 09/01/2019
$$