974 
FXUS63 KBIS 112302
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
602 PM CDT Sun Aug 11 2019

.UPDATE...
Issued at 602 PM CDT Sun Aug 11 2019

Multiple rounds of showers and thunderstorms with heavy rainfall
continues to be the main hazard this evening/tonight. Heaviest
rainfall totals this evening occurred in Bowman and near New 
England with around 2 inches in one hour. Soil moisture has been 
able to handle this well, but will monitor for future rounds and 
location. Movement of these heavier showers and thunderstorms have
been progressive, which is helping to inhibit flash flooding 
potential. Uncertainty on exactly where and how many more rounds 
will actually develop remain high, as the CAMS are all over the 
place in terms of possible scenarios.

&&

.SHORT TERM...(This evening through Monday)
Issued at 259 PM CDT Sun Aug 11 2019

A severe thunderstorm threat exists for far southwest North 
Dakota this afternoon and evening with large hail, damaging wind 
gusts, and a nonzero tornado chance possible. Additionally, heavy
rainfall and several rounds of thunderstorms in the southwest to 
south- central may cause a flash flooding concern overnight.

This afternoon, the synoptic pattern was characterized by a potent 
shortwave rounding the Great Basin region with a 500mb speed max 
pushing into the northern Rockies. Smaller impulses were detected on 
water vapor imagery pushing over central Montana and northern 
Wyoming. At the surface, pressure falls were noted in southern 
Montana and northern Wyoming where cyclogenesis and convergent 
upslope flow was ongoing. In north-central through southeast 
North Dakota a surface high axis was in place. Widespread stratus 
was detected on visible satellite imagery within the western and 
southern peripheries of the surface high, extending from western 
North Dakota through the James River Valley. In the southwest 
through south-central, skies have been clearing with the 
subsequent development of a cumulus field over the last several 
hours. 

A severe thunderstorm threat is expected to develop late this 
afternoon and evening in southeast North Dakota. Surface dew 
points in the low 60s beneath midlevel lapse rates of 7-7.5 C/km 
will develop MLCAPE values in the 1000-2000 J/kg range. Strong 
midlevel flow marked by 30-40 kts of effective bulk layer shear 
will combine with this instability to create a potential severe 
thunderstorm parameter space. With generally unidirectional 
hodographs depicted in southwest ND forecast soundings, splitting 
cells with eventual storm mergers will be likely. Thus the severe 
threat at the moment seems to be contained mostly to our southeast
counties where initial cells may produce large hail and locally 
damaging wind gusts before becoming congealed. Additionally given 
strong deep layer shear, ML LCL's around or below 1000m, and 
potential for discrete supercells...a tornado risk does exist. 0-1
km SRH is rather low, though the other ingredients is enough to 
make it a risk worth highlighting. 

This evening, thunderstorm coverage is expected to increase across 
our southwest as the midlevel speed max exits the Rockies. 
Thunderstorms are forecast to develop in central Montana and move 
into the northwest counties by late this evening as well. By the 
time of their arrival they will likely be clustered and elevated so 
the severe risk further north is low. Heavy rain will be a concern 
with thunderstorms tonight as PWAT values of 1.25" to over 1.5" will 
continue to advect northward this evening. Widespread rainfall 
totals of 1 to over 2 inches will be possible. In the southwest 
where several rounds of convection are likely this evening and 
tonight, the totals may be greater. HREF guidance indicates areas of 
2 to 4 inches will be possible in the southeast to south-central 
counties given multiple thunderstorm passages. That said, the lack 
of a reinforcing mechanism to redevelop thunderstorms in one 
location does tone down the flash flood threat some. Though storm 
propagation should be fast enough to mitigate flash flooding 
concerns, this will still need to be closely monitored especially 
if a couple thunderstorms move over more populated areas. 

Scattered showers and isolated non-severe thunderstorms will 
continue through northern and eastern North Dakota early Monday 
morning before dissipating by mid-day. While the midlevel low is 
expected to briefly close off early Monday, guidance shows it 
becoming more progressive through the day and begin to move 
southeastward from northeast Montana through southeast North Dakota. 
A relatively thin instability corridor ahead of an advancing surface 
trough will pose a marginal severe threat Monday afternoon in the 
central and southeast. This will be highly conditional on daytime 
clearing and destabilization. 

Lingering precipitation and cloud cover Monday will also lead to 
cooler temperatures with highs in the 60s north and east to 70s 
south and west expected.

.LONG TERM...(Monday night through Sunday)
Issued at 259 PM CDT Sun Aug 11 2019

The long term forecast period is highlighted by almost daily chances 
for showers and thunderstorms with the active period continuing 
through the weekend.

Monday evening the previously closed midlevel low rejoins the flow 
and dives southeastward through the Dakotas, dragging an attendant 
cold front through the area overnight. Pressure rises behind the 
front may bring breezy overnight winds however the magnitude is 
fairly uncertain given this occurring through the diurnal minimum
and boundary layer decoupling. Some lingering showers will be 
possible Tuesday though the trend will be towards clearing with a 
surface high developing beneath midlevel height rises. NAM/GFS MOS
guidance shows widespread high temperatures of mid 60s to around 
70s likely Tuesday afternoon though the ECMWF has been a few 
degrees warmer. NBM blend of upper 60s to around 70 captured this 
well.

Wednesday through Friday, the synoptic pattern is characterized by 
generally northwest flow aloft with daily chances for showers and 
thunderstorms. Midlevel flow stays fairly strong through the period 
so chances for severe weather in the region continue through the 
week (though uncertain in coverage and location at the moment). 

This weekend, ensemble and deterministic guidance has been 
consistent in developing a strong northwest trough and tracking it 
eastward through the weekend. While run to run details in wave 
timing, return flow, etc have been inconsistent...ensemble guidance 
in the CIPS analogs and GEFS plumes have shown some sort of signal 
for higher chances of thunderstorms and possibly severe weather.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 602 PM CDT Sun Aug 11 2019

Showers and thunderstorms highlight the taf period through 12z
Monday, with possibly more precipitation across the northern
terminals Monday afternoon. Heavy rainfall will result in MVFR
to IFR cigs/vsbys at times. Expect the precipitation to wane from
west to east 06z-12 Monday. Should see a break through 18z with
MVFR cigs improving to low vfr. More showers/thunderstorms are
forecast after 18z Monday, mainly over the northern and south
central terminals of KISN/KMOT/KBIS.

&&

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

$$

UPDATE...KS
SHORT TERM...AE
LONG TERM...AE
AVIATION...KS