Valid 011630Z – 021200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS CENTRAL AND
SOUTHERN ARIZONA…
…SUMMARY…
Scattered severe wind gusts are possible across parts of central and
southern Arizona during the late afternoon and evening, with other
severe storms possible across the northern High Plains and Ohio
Valley.
…Central/southern Arizona…
The mid/upper-level ridge will remain centered over the Four Corners
area through tonight, with a belt of moderately strong easterly
mid-level winds on its immediate southern periphery as sampled by
12Z observed soundings from Phoenix/Tucson. Orographic lift will be
the primary driver of isolated to widely scattered thunderstorms by
early afternoon across the higher terrain of western New Mexico into
central/eastern Arizona. With a belt of 15-25 kt easterlies across
the southern half of Arizona, this convection will spread westward
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-caliber wind gusts, with at least a couple of these storms
likely to reach the Lower Colorado River Valley.
…Ohio Valley…
A convectively influenced shortwave trough centered over central
Indiana at late morning will continue east-northeastward toward the
Upper Ohio Valley and western portions of Pennsylvania/New York.
Modest ascent ahead of this feature will continue to influence
thunderstorm development across the region, with some increase in
coverage/intensity this afternoon as MLCAPE reaches 1000-1500 J/kg.
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 early this evening from
the Upper Ohio Valley to northern Appalachians. A small temporal
window may exist for a supercell-related brief tornado risk near the
Ohio/Pennsylvania/West Virginia border area as low-level hodographs
enlarge prior to surface-based instability decreasing with eastern
extent in Pennsylvania.
…Northern High Plains including Montana to Nebraska…
No changes warranted for the region. 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.
Isolated thunderstorms may develop along the lee trough from
north-central Montana to the Nebraska/Colorado 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 Montana and the Dakotas into Nebraska. 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.
..Guyer/Nauslar.. 09/01/2019
$$