686
FXUS63 KBIS 061503
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
1003 AM CDT Mon May 6 2024

.KEY MESSAGES...

- Several rounds of showers with low to medium chances for
  thunderstorms across western and central North Dakota today
  through tonight.

- There is a conditional risk of a few strong thunderstorms this
  afternoon and evening, mainly across southwest and south
  central parts of the state.

- Strong southeast winds gusting to 50 mph expected across
  central and eastern North Dakota today.

- Strong westerly winds are possible in southwest North Dakota
  on Tuesday.

- Warmer and drier weather is favored later in the week.

&&

.UPDATE...
Issued at 955 AM CDT Mon May 6 2024

Some showers have developed across northwestern North Dakota,
moving across Jamestown through the past hour or so. Some
showers have had some embedded thunder associated with them as
well. The showers across the southern portion of the state
continue to move north, with some of these showers having some
embedded thunder in them as well. SPC has expanded the Marginal
Risk (level 1 of 5) into our southwestern corner, indicating
increased confidence that a strong storm cannot be ruled out in
that area this evening. The timing for these storms will be
rather narrow, with a current expected timeframe of 3 pm to 7 pm
CDT. The forecast for the rest of the day continues to remain
on track, with only minor adjustments to wind speeds needed with
this update.

.UPDATE...
Issued at 643 AM CDT Mon May 6 2024

An area of showers continues to grow northward over south
central North Dakota early this morning. Occasional lightning
had previously been observed with this activity closer to the
South Dakota border, but this has mostly ceased as the showers
are drifting away from the more unstable air mass aloft.
Overall, the forecast for today remains on track.

&&

.DISCUSSION...
Issued at 444 AM CDT Mon May 6 2024

A strong low level jet resides over the Northern Plains early this
morning, downstream of a potent shortwave/closed low over the
Rockies. Poleward moisture transport over central South Dakota has
recently initiated an area of showers with occasional lightning
along and west of Lake Oahe. This activity will continue to spread
northward into south central North Dakota through mid morning. More
isolated shower activity has also been noted across the rest of the
western half of the state at times during the early morning hours.

The strong upstream mid level low will spin into the Northern High
Plains today, with surface cyclogenesis underway early this morning
across eastern Montana and eastern Wyoming. Expect several areas of
showers with embedded thunderstorms to develop over and track
through western and central North Dakota over the course of the day.
Strong southeast winds will focus more over central and eastern
parts of the state today as the surface low deepens near the Black
Hills. Previously issued Wind Advisories for today remain on track.
Afternoon temperatures are likely to reach the 60s.

We are still monitoring the potential for a few stronger storms this
afternoon and evening, with the primary hazard being a weak tornado
or two. The main area of concern is expected to be along a northward
advancing occluded front with a northwest-southeast orientation from
southwest into far south central North Dakota. It is not certain
that surface-based convection will initiate along this boundary
(although mid to upper level forcing should be substantial), nor that
uninhibited low level buoyancy will build directly ahead of the
occluded front. Having said that, there is now greater ensemble
support for a narrow ribbon of 500-1000 J/kg SBCAPE preceding the
occluded front. Any storms that form along this boundary would
likely experience 20-30 kts of 0-1 km shear, along with 100-200
m2/s2 of 0-500 m SRH. Given those shear parameters, coupled with
very low LCLs and large, looping hodographs showing ample streamwise
vorticity for ingestion by right-moving supercells, it stands to
reason that a couple of brief, weak tornadoes could occur. Once
again, this potential is reliant on the presence of SBCAPE on the
order of 500 J/kg in the storm inflow air mass. Other severe hazards
cannot be completely ruled out, but should be of lesser concern.

A nearly-stacked low will continue to spin over western North Dakota
tonight before it begins to broaden out on Tuesday. There is still
some uncertainty on where the surface low circulation will settle on
Tuesday, but we are still expecting a synoptic dry slot to
significantly limit QPF across all but far western North Dakota
beyond tonight. In fact, there are now only medium odds (less than
70 percent) for an event-total rainfall exceeding one inch along the
Montana border. The far west could still see some periods of heavier
rainfall on Tuesday though, moreso in the southwest where strong
westerly winds could actually be the greater concern, with some
guidance showing BL momentum transfer potential near 50 kts. Tuesday
will be cooler than today, with highs mainly in the 50s, but
possibly as low as the mid 40s in parts of the southwest.

The stacked low pressure is forecast to devolve and shift southward
on Wednesday, though the continued presence of cyclonic vorticity
keeps medium chances for lighter rain in the forecast. A drying out
and warming of temperatures should finally commence on Thursday,
with highs forecast to return to the mid and upper 60s. Global
models are projecting a clipper-like system diving down from Canada
on Friday, but ensemble analysis shows very large spread on the
evolution of the synoptic pattern heading into the weekend.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 643 AM CDT Mon May 6 2024

A strong low pressure system will bring aviation impacts to
western and central North Dakota through the forecast period.
MVFR to IFR ceilings will build in from southwest to northeast
this morning, persisting through the rest of the day and
tonight. Periods of showers and isolated to scattered
thunderstorms can also be expected today through tonight.
Southeast winds will remain strong through this evening,
increasing to sustained speeds around 20-30 kts with gusts to
35-45 kts this afternoon, strongest over central and eastern
North Dakota. Later tonight, winds will temporarily weaken
across western North Dakota while turning more easterly in the
northwest, and westerly in the southwest. Low level wind shear
will continue at KMOT until mid Monday morning.

&&

.BIS WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until midnight CDT /11 PM MDT/ tonight for
NDZ003>005-011>013-019>023-025-034>037-042-045>048-050-051.

&&

$$

UPDATE...Besson
DISCUSSION...Hollan
AVIATION...Hollan