Valid 091200Z – 101200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER CENTRAL SOUTH
DAKOTA AND FAR NORTH-CENTRAL NEBRASKA…AND CENTRAL OREGON…
…SUMMARY…
Severe storms are likely over parts of the Northern Plains and over
parts of Oregon. A few strong to severe storms are also possible
over parts of the Mid Atlantic and central High Plains.
…Synopsis…
An upper high will remain centered over the southern Plains, with
upper troughs over the West Coast and from the Great Lakes into
Northeast. A surface low will develop from SD southward across the
High Plains, with a stationary front extending east from the SD low
across the MO and OH Valleys. A moist air mass will remain over much
of the Plains, and will result in unstable conditions especially
over SD and NE where minor disturbances will round the upper ridge,
enhancing shear and lift. Farther east, the primary synoptic
boundary will move east as a cold front across VA and the Delmarva,
providing a focus for storms.
To the west, cooling aloft will aid destabilization over OR and
western ID, with increasing winds aloft aiding in severe potential
during the afternoon and evening.
…Northern Plains…
A few storms are possible early in the day moving from west to east
across SD in a zone of low-level warm advection with 30 kt southerly
winds at 850 mb. In the wake of this activity, strong heating will
occur across WY and western NE, with the low-level lapse rate plume
extending into southwest SD. Heating combined with a plume of mid
60s F dewpoints will lead to areas of strong instability, beneath
modest westerly midlevel winds. Storms are expected to initiate by
21Z along a line from west-central SD into parts of the NE
Panhandle, and few could be supercells. Lapse rates aloft will not
be particularly steep due to warming aloft but large hail is still
likely given cellular storm mode. A brief tornado or two cannot be
ruled out as effective SRH increases to 200-400 m2/s2 east of where
storms initiate.
…Oregon…
Strong heating beneath cooling profiles aloft will lead to very
steep lapse rates through a deep layer, while winds aloft also
increase with the approaching upper trough. Storms are expected to
initiate over southern OR by afternoon in a zone of increasing
differential divergence, and will spread northward across the state,
and possibly into western ID as well. A few storms may contain small
hail but the main threat is expected to be damaging outflow winds.
…Mid Atlantic…
Relatively strong northwest flow aloft will remain over the region
with the main upper trough over the Northeast. Strong heating will
occur ahead of the cold front, with storms initiating after 18Z over
central VA, spreading southeastward during the afternoon. Although
storm coverage may remain isolated, any storms will have both hail
and wind potential given favorable time of day, and sufficient
deep-layer shear to sustain storms for several hours.
..Jewell/Cook.. 08/09/2019
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