Valid 041300Z – 051200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE NORTHERN PLAINS TO NORTHERN MINNESOTA….
…SUMMARY…
Severe hail and wind are possible from afternoon and evening
thunderstorms across parts of the northern Plains to northern
Minnesota.
…Synopsis…
In mid/upper levels, the persistent western-ridge/eastern-trough
longwave will continue across the CONUS this period, with only minor
perturbations. One of those — a weak shortwave trough apparent in
moisture-channel imagery over central/south-central MT then over
northwestern/north-central WY — will be the most meaningful for
today’s severe potential. This feature is moving through the mean
ridge this morning, and will move generally east-southeastward
across much of SD through early evening. Troughing in the eastern
CONUS will be high-amplitude but with broad/generally weak cyclonic
flow aloft, and weak shear. Accordingly, any eastern severe
potential remains too disorganized, low-end and poorly focused to
warrant an unconditional risk area at this time.
The 11Z surface analysis showed a cold front offshore from eastern
New England then across NJ to central VA, becoming a wavy, slow-
moving to stationary boundary over the south-central Appalachians,
southern IL, northern MO, and eastern NE, and a diffuse warm front
across northern NE and southwestern SD. The western part of the
front will move northward across the northern Plains and become more
ill-defined through the day. Meanwhile, an initially separate cold
front – across northern/western ND and southeastern/central MT —
should move slowly eastward/southeastward over ND and northern MN
through the period, while lingering over southeastern MT.
…Northern Plains to northern MN…
A cluster of convection is ongoing across portions of southeastern
MT, moving into northwestern SD, within a plume of elevated low-
level warm advection and moisture transport. Modified RAOBs and
model soundings suggest this activity is rooted aloft in a regime of
steep midlevel lapse rates, but with sufficiently dry air between
the shallow near-surface layer and LFC that isolated damaging gusts
or severe hail cannot be ruled out.
Additional scattered thunderstorms are expected to develop though
this afternoon along and ahead of the surface front, and in the
Black Hills, impinging on a moist and diabatically destabilized
boundary layer, in the presence of favorable deep shear for
organized multicells and a few supercells. Development should occur
sooner in the southwestern part of the outlook area across SD than
over the southern ND to northern MN sector, given the early
influence of the aforementioned shortwave perturbation. Damaging
gusts and hail remain the dominant threats. Only relatively minor
spatial changes were made to the previous outlook to account for:
1. Somewhat faster/more eastern positioning of the frontal zone and
convection-initiation potential on the north part, and
2. Larger area of favorable moisture/instability late this
afternoon into evening, potentially supporting more southeastward
persistence of convection in the Badlands/Sandhills region.
The corridor of favorable low-level moisture ahead of the front
should be characterized mostly by 60s F surface dew points, even
after some mixing, yielding peak preconvective MLCAPE values of
1500-2500 J/kg, amidst effective-shear magnitudes varying from
around 30-35 kt in the northeastern areas to 45-50 kt around
southwestern SD. Activity should weaken considerably through the
evening as the foregoing boundary layer stabilizes, and over SD/NE,
as the supportive shortwave trough outruns remaining the convection
to the east-southeast.
..Edwards/Broyles.. 08/04/2019
$$