299
FXUS63 KBIS 172311 CCA
AFDBIS

AREA FORECAST DISCUSSION...CORRECTED
National Weather Service Bismarck ND
611 PM CDT Fri May 17 2024

.KEY MESSAGES...

- Scattered severe thunderstorms are possible this evening. The
 expected hazards will be damaging winds up to 70 mph and hail
 up to quarter size.

- Windy and cooler on Saturday with highs mostly in the 60s.

- Daily chances (40%) for showers and thunderstorms continue
  through the weekend and into early next week.

&&

.UPDATE...
Issued at 557 PM CDT Fri May 17 2024

** MESOSCALE UPDATE **

The primary severe-storm risk the next few hours will be in the
form of damaging winds of 60-70 mph from the Hettinger area
toward Fort Yates, Linton, and toward Ashley/Wishek and into
Stutsman, LaMoure, and Dickey Counties. The northern edge of the
more sustained damaging wind risk will likely be limited by the
sagging, though gradually-decelerating outflow boundary/radar
fine-line extending from Carrington to Steele and now to the south
of Bismarck/Mandan. The storms near Hettinger are on the leading
edge of stronger forcing aloft and increasing low- and deep-layer
wind fields, including 0-3-km bulk shear increasing to at least 25
kt. The area ahead of that convection and south of the outflow
boundary will have sufficient bouyancy to present an organized
damaging wind risk, assuming it does not outrun the increasing
winds aloft to the west, or that the outflow boundary does not
progress to the south too quickly. Otherwise, the boundary layer
to the north and west has cooled and the severe-storm risk is
diminishing in west central and north central ND. The storms
immediately north of the outflow boundary, e.g., near the
Bismarck/Mandan area toward Carson and Elgin, are likely elevated
and pose a more marginal severe risk given the cooler boundary
layer in that zone.

UPDATE Issued at 241 PM CDT Fri May 17 2024

Quick update that there is now a Severe Thunderstorms watch for
most of western and central ND until 02z. Hazards are still
70mph winds and quarter sized hail. It may need to be expanded
into the far southeast, but we will see how the storms go.

&&

.DISCUSSION...
Issued at 200 PM CDT Fri May 17 2024

The upper level wave has started to move into the state, as seen
in satellite with agitated mid level clouds at 18z. Near the
Montana/North Dakota border near Glendive, MT, south to Bowman,
ND and over to Glen Ullin, ND cumulus has started. We should be
warmer than our convective temperature now, so more and more
cumulus should start to pop in the next hour, especially with
the diffluence over head from the upper level wave. The latest
SPC update at 1630z expanded the slight risk through the whole
CWA. Some CAMs have isolated storms forming now before the main
cold front moves through and forms a line. The main threat is
still 70mph winds because of a lack in CAPE, and the storm mode.
The storm mode is a line because the shear vector will be
parallel to the cold front, continuously allowing new convection
to form in the line. DCAPE look very favorable with values
around 1000 J/kg, and model soundings have an inverted V with
steep lapse rate. This allows the wind to mix down very easily.
So the isolated storms that form now, will then combine into a
line as the cold front moves in. Timing on the line forming
looks to sometime around 21-23z. The CAMs are all a little
different on the which storm mode they start as. The line will
move west to east, leaving our area around 03z. Most CAMs are
confident on that. Then on the back side of the front, more
showers and possibly thunderstorms will form again in the west,
lasting through the night.

Saturday will be breezy on the backside of this front with cold
air advection and a tight pressure gradient. Westerly winds
will gust to 40mph, with sustained winds at 30mph. We will most
likely need a Wind Advisory from Saturday morning, until around
00z. Since we have active storms today, we will wait until the
overnight period to issue that. Highs on Saturday will be much
cooler in the 60s, this cooler trend will continue through the
weekend. Flow aloft this weekend will remain southwest with
another low and wave moving through Sunday and Monday. Chances
for showers and thunderstorms will continue through the forecast
period with the highest chances ending Monday when the
southwest flow relaxes some. Mid next week the flow is weak in a
quasi-zonal flow. Another big low pressure possibly forms in
the Pacific Northwest Wednesday. This could bring another round
of storms with diffluence flow aloft and southerly flow at the
surface Thursday and Friday. Temperatures look to stay slightly
below average in the low to mid 60s through Friday.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 200 PM CDT Fri May 17 2024

VFR this afternoon until strong thunderstorms move west to east
between 21 and 03z. Overnight light rain may continue until the
early morning. Winds will gust around 20kts today from the
southwest increasing to 35kts from the west by Saturday
morning. Skies will clear out by late morning Saturday except
near KMOT where 3kft ceilings could linger, left it as SCT for
now until we get more confidence.


&&

.BIS WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

MESOSCALE...CJS
DISCUSSION...Smith
AVIATION...Smith