Areas affected…Portions of western/southern ND into northern SD
Concerning…Severe potential…Watch unlikely
Valid 192306Z – 200100Z
Probability of Watch Issuance…20 percent
SUMMARY…An isolated large hail and wind threat should increase
this evening. A watch will probably not be needed unless greater
than expected storm coverage occurs.
DISCUSSION…A surface trough is located across western ND at 23Z
per observations. Weak low-level convergence along this boundary has
fostered a gradually deepening cumulus field across this region, and
convective initiation may occur over the next couple of hours
(through 02Z). The lack of more obvious large-scale ascent suggests
overall storm coverage will probably remain isolated. Low-level
moisture return is occurring across the central into northern Plains
early this evening, with upper 60s to lower 70s surface dewpoints
extending as far north as the ND/SD border region as of 23Z.
Modestly steepened mid-level lapse rates have overspread this region
from the higher terrain of the northern Rockies/High Plains, which
combined with the returning low-level moisture and strong diurnal
heating is supporting weak to moderate instability along and just
ahead of the surface trough. RAP forecast soundings across western
ND show a veering wind profile through about 3 km, with generally
westerly winds strengthening with height through the remainder of
the troposphere. Resultant effective bulk shear of 45-50 kt will
easily support supercells with an associated large hail threat. Some
severe wind gusts may also occur where low-level lapse rates have
become steepened.
Current expectations are for a relatively greater severe threat to
develop across southwestern ND initially, as somewhat greater
low-level moisture and stronger instability should be present. There
is some potential for a supercell or two to move east-southeastward
through the evening along the ND/SD border along an instability
gradient. Overall storm coverage is still somewhat uncertain, and a
watch will probably not be issued unless at least a few supercells
develop.
..Gleason/Grams.. 08/19/2019
…Please see www.spc.noaa.gov for graphic product…
ATTN…WFO…ABR…BIS…UNR…
LAT…LON 45890317 46860339 48290315 49060286 49020145 48030148
47100138 46520064 46010031 45590046 45280099 45310174
45520262 45890317