Tornado in North Dakota, July 20, 2020

Area Forecast Discussion
National Weather Service Bismarck ND
1249 PM CDT Mon Jul 20 2020

.UPDATE...
Issued at 1244 PM CDT Mon Jul 20 2020

Shallow cumulus field has developed in southwest North Dakota
where surface temperatures are nearing 70 behind this morning's
convection. While surface dew points are less than ideal for
strong convection, steeper lapse rates aloft advected via the
incoming shortwave may provide enough destabilization for at
least a few stronger thunderstorms.

UPDATE Issued at 1009 AM CDT Mon Jul 20 2020

Showers and thunderstorms continue this morning from Bowman/Adams
counties north through Stark county with more stable, stratiform
precipitation ongoing further north into northwest North Dakota.
With this update we attempted to better time precipitation with
recent trends as it progresses east an hour or two quicker than
current convective-allowing models are showing. We also dropped
today's high temperatures in the north/northwest where persistent
cloud cover should dampen the diurnal curve, making highs in the
60s to around 70.

Still great uncertainty lies in the severe threat for this
afternoon/evening, contingent on magnitude of destabilization. No
changes in thinking from earlier forecast at this time.

UPDATE Issued at 643 AM CDT Mon Jul 20 2020

Patchy fog has developed from the Devils Lake Basin to James River
Valley. Expect the fog to dissipate by 9 AM CDT.

Showers and isolated thunderstorms continue to work their way into
western North Dakota ahead of a series of mid level impulses
embedded in shortwave troughing. Recent iterations of the HRRR and
Experimental HRRR are doing a fair job overall, but are now
failing to capture stronger convection near Miles City.

Recent guidance has trended toward a lower probability of
ingredients coming together to support an organized severe
storm threat over southwest and south central North Dakota this
afternoon, but we won't know anything for certain until at least
later this morning or early afternoon, based on observational
trends.

&&

.SHORT TERM...(Today and tonight)
Issued at 403 AM CDT Mon Jul 20 2020

Scattered showers and thunderstorms, some strong, are expected today
into tonight. There is a conditional threat for severe storms south
and west of the Missouri River this afternoon and early evening.

A mid level shortwave currently seen on water vapor imagery moving
into southwest Saskatchewan/northern Montana will continue to
progress east-southeastward through the day. Scattered showers and
isolated thunderstorms ahead of the wave will continue moving into
western North Dakota this morning. Despite mid level lapse rates
around 7 C/km and effective bulk shear near 40 kts, there does not
appear to be enough MUCAPE to support stronger convection early this
morning, although wind gusts near 40 mph have been recorded near
Beach. Later in the morning, atmospheric instability will begin to
increase in response to diurnal heating and weak low to mid level
theta-e advection. This should result in an increase in coverage
of thunderstorms as the precipitation complex moves toward central
North Dakota by the afternoon. There may also be an increase in
intensity, though deep layer shear and mid level lapse rates are
actually forecast to become weaker. The aforementioned late
morning trends are supported by CAM reflectivity output.

Exactly how this morning's convection plays out will have a very
strong influence on the potential for severe storms over southwest
and south central North Dakota this afternoon. There may be as many
as 3 factors that will determine this potential that remain
uncertain at this time: 1) How much cloud cover will be left over
from morning convection, 2) How much morning convection will
interfere with moisture advection, and 3) How well-defined of a
surface trough can establish itself upstream of the moisture return.
The second and third factors are inherently tied together, and the
first two are critical to the amount of destabilization that can
occur in the wake of morning convection. The entire suite of
guidance, including previous cycles within the last 24 hours, show a
wide array of solutions, with forecast SBCAPE over southwest North
Dakota this afternoon ranging anywhere from near zero (the 00Z NSSL-
WRF and NMM) to near 2000 J/kg (a few recent runs of the HRRR and
RAP, as well as the 06Z NAM). There is also uncertainty in the exact
placement of the potential surface trough, which would likely aid in
convective initiation. Another consideration is that the presence of
the surface trough may not actually be necessary for sustained
convection, as mid level forcing may be sufficient. But its lack of
presence would also likely infer lower boundary layer moisture over
southwest North Dakota due to weaker southeasterly return flow.

If cloud debris from morning convection can clear southwest North
Dakota by mid afternoon, and morning convection does not interfere
with moisture return of around 60 F surface dewpoints, sufficient
destabilization will likely occur to support a severe thunderstorm
threat given the presence of 40-50 kts 0-6 km bulk shear. Shear and
anvil-level flow vectors orthogonal to the surface boundary favor a
discrete mode, and CAMs that do produce UH tracks suggest
supercells. Large hail would likely be the main threat, with
isolated damaging winds also possible.

Regardless of whether the severe threat materializes in the
southwest and south central, rounds of scattered thunderstorms are
likely through the afternoon. A few storms could be strong
statewide, but the threat for severe weather outside of what has
been previously discussed seems low, with less than 1000 J/kg
MLCAPE, mid level lapse rates less than 7 C/km, and 0-6 km bulk
shear less than 30 kts across the remainder of western and central
North Dakota, although there is appreciable shear in the 0-3 km
layer. Interestingly, the RAP is simulating relatively high values
of the non-supercell tornado (NST) parameter across much of the
state this afternoon. This would usually require the presence of a
more well-defined surface boundary, so this potential would likely
only be realized due to strong outflow boundary interactions, which
seem rather unlikely but not impossible.

The last issue to address with regards to convection today is heavy
rain. While this does not have the appearance of a heavy rain/flash
flooding scenario, precipitable water near 1.25 inches, deep layer
moisture, and antecedent conditions may lead to localized concerns
if multiple storms move over a similar area. This potential appears
maximized over north central North Dakota where some models hint at
the development of a west-to-east oriented deformation rain band.

Showers and thunderstorms should slowly dissipate and/or move off to
the east later this evening into tonight. With the trough moving
through and the presence of widespread clouds through much of the
day, expect below normal highs ranging from the upper 60s northwest
to near 80 south.

.LONG TERM...(Tuesday through Sunday)
Issued at 403 AM CDT Mon Jul 20 2020

The long term period is highlighted by the return of warm and humid
conditions with several chances for thunderstorms, some possibly
severe.

Northwest flow aloft is forecast for Tuesday, with the downstream
shortwave moving along the Minnesota/Ontario border and upstream
thermal ridge building over the Rockies. A few residual showers in
the cyclonic flow regime cannot be ruled out near the Turtle
Mountains on Tuesday, but low level subsidence will likely prevail.

Models remain in agreement that strong upper level ridging will
build over the central CONUS starting Wednesday and lasting through
the weekend. The last few model cycles have trended toward a
slightly higher amplification of the mid level height field and an
eastward shift of the ridge axis closer to the Mississippi River
Valley, which places North Dakota under southwest flow aloft. This
synoptic pattern favors above normal temperatures, humid conditions,
periods of breezy southeast winds, and multiple chances for
thunderstorms, both due to shortwaves embedded in the southwest flow
and the nocturnal low-level jet. The clearest signal for a stronger
shortwave is Thursday afternoon through Thursday night, but
thunderstorms cannot be ruled out during any 12-hour period
beginning Wednesday night. Instability itself will likely be
sufficient for any round of storms to include severe weather, but
shear appears more marginal until the weekend. There will also
likely be capping concerns with any potential convection.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1244 PM CDT Mon Jul 20 2020

Hazards to aviation include scattered thunderstorms across
west and central North Dakota through 04z.

At noon CDT, a trough of low pressure across western North Dakota
will be the focus for showers and scattered thunderstorms this
afternoon and evening as it moves east.

MVFR conditions are possible in low ceilings associated with rain
showers and thunderstorms. Scattered areas of fog may reduce
visibilities to 3 miles at KDIK, and this area could become more
widespread, although there is much uncertainty. MVFR at KJMS after
06z.

&&

.BIS WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

UPDATE...AE
SHORT TERM...Hollan
LONG TERM...Hollan
AVIATION...WAA

Hazardous Weather Outlook
National Weather Service Bismarck ND
1025 AM CDT Mon Jul 20 2020

NDZ017>020-031>034-040>045-211530-
McKenzie-Dunn-Mercer-Oliver-Golden Valley-Billings-Stark-Morton-
Slope-Hettinger-Grant-Bowman-Adams-Sioux-
1025 AM CDT Mon Jul 20 2020 /925 AM MDT Mon Jul 20 2020/

This Hazardous Weather Outlook is for portions of northwest North
Dakota, south central North Dakota, and southwest North Dakota.

.DAY ONE...Today and Tonight

Scattered thunderstorms are likely today. Some storms may be
strong, with small hail and gusty winds. There may be potential
for a few severe storms later this afternoon and early evening,
depending on early afternoon cloud cover. If this potential
materializes, hail up to half dollar size and damaging winds to 60
mph would be possible.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

The probability for widespread hazardous weather is low.

.SPOTTER INFORMATION STATEMENT...

Spotters are encouraged to report severe weather to NWS Bismarck
by calling 701-223-4582 or messaging NWS Bismarck on social
media.

NDZ001>005-009>013-021>023-025-035>037-046>048-050-051-211530-
Divide-Burke-Renville-Bottineau-Rolette-Williams-Mountrail-Ward-
McHenry-Pierce-McLean-Sheridan-Wells-Foster-Burleigh-Kidder-
Stutsman-Emmons-Logan-La Moure-McIntosh-Dickey-
1025 AM CDT Mon Jul 20 2020

This Hazardous Weather Outlook is for portions of north central
North Dakota, northwest North Dakota, south central North Dakota,
and southeast North Dakota.

.DAY ONE...Today and Tonight

Scattered thunderstorms are likely today. Some storms may be
strong, with small hail and gusty winds.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

The probability for widespread hazardous weather is low.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will not be needed.

Special Weather Statement
National Weather Service Bismarck ND
942 AM CDT Mon Jul 20 2020

NDZ033-034-041>044-201530-
Grant ND-Hettinger ND-Stark ND-Adams ND-Morton ND-Bowman ND-
942 AM CDT Mon Jul 20 2020

...SIGNIFICANT WEATHER ADVISORY FOR SOUTHEASTERN BOWMAN...
HETTINGER...WESTERN MORTON...EASTERN STARK...NORTHWESTERN GRANT AND
ADAMS COUNTIES UNTIL 1030 AM CDT/930 AM MDT/...

At 942 AM CDT/842 AM MDT/, strong thunderstorms were located along a
line extending from near Gladstone to 9 miles west of Regent to 7
miles northeast of Ludlow. Movement was east at 40 mph.

Half inch hail and wind gusts to 40 mph will be possible with these
storms.

Locations impacted include...
Hettinger, Mott, Hebron, Richardton, Gladstone, Reeder, Regent,
Taylor, Burt, Bucyrus, Havelock, Gascoyne, Bowman Haley Dam and
Lefor.

LAT...LON 4595 10331 4641 10278 4692 10265 4693 10200
      4634 10207 4594 10257
TIME...MOT...LOC 1442Z 275DEG 36KT 4685 10265 4641 10276 4593 10332

ABELING