Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 200518

Area Forecast Discussion
National Weather Service Bismarck ND
1218 AM CDT Thu Jul 20 2017

Issued at 1218 AM CDT Thu Jul 20 2017

We added patchy fog until 14 UTC from the Turtle Mountains south
across the James River valley and as far west as Ashley with this
update. The reasoning for that includes the following: 1) Narrow
temperature/dewpoint spreads and light winds in that corridor as
of 05 UTC, 2) NAM and RAP/HRRR forecast soundings which show weak
low-level turbulence, and 3) Crossover temperatures which will be
met or exceeded given expected lows. Recent HRRR simulations also
support this forecast, mainly in the southern James River valley
where their visibility forecasts explicitly depict fog. Otherwise,
the rest of the forecast is on track and only minor changes were
made based on observational trends.

UPDATE Issued at 859 PM CDT Wed Jul 19 2017

Updates forthcoming to include thunderstorms across portions of
northwestern and north central ND. Isolated showers continue to
grow into thunderstorms within an area of low level convergence
and frontogenesis.

UPDATE Issued at 559 PM CDT Wed Jul 19 2017

After collaborating with neighboring offices, decided to decrease
POPS to non-measurable southern James River Valley. Latest
visible imagery shows little in the way of cloud development. Per
SPC mesoanalysis page, surface CIN, as well as ML Cin already in
place. Despite strong shear aloft and departing jet, not expecting
any further development this evening. Did keep a slight chance
mention in the northwest with a shallow cumulus field in place
through 00z/7pm. Rest of forecast on track tonight, with dry
conditions expected.


.SHORT TERM...(This afternoon through tonight)
Issued at 315 PM CDT Wed Jul 19 2017

Current surface analysis places low to the south of the Black
Hills, with secondary trough moving through Manitoba stretching
into North Dakota. Upper level analysis places high over the
southern plains with monsoonal moisture wrapping over the system.
Over our area, a generally western active pattern is in place with
a variety of weak waves moving through.

For the rest of this afternoon into tonight...most convective
activity has pushed to the south and east of the area, though with
aforementioned waves sliding through the flow, an occasional
shower or storm may pop up either over the southeastern portions
of the area where ample instability is noted, or over the
northwest where a bit more notable wave will be approaching.
Confidence has lessened on severe threat over the southeastern
portions of the area as instability is not as strong as previously
noted, but still high enough to keep a close eye on the area.

.LONG TERM...(Thursday through Wednesday)
Issued at 315 PM CDT Wed Jul 19 2017

An active and seasonably warm pattern will continue through much
of the long term on the northern edge of the large, but flattened
subtropical ridge. Two primary opportunities for thunderstorms may
be Thursday afternoon into Thursday night in western ND, and
across central ND from Thursday night into Friday. Initially, it
appears that elevated instability may favor convective sustenance
out of eastern MT Thursday night, fed by a modest low-level jet.
A greater opportunity for more robust, surface-based storms with
an attendant greater probability of severe weather may develop
Friday afternoon and evening across central ND ahead of a cold
front. Latest Nam is still showing the potential for cape values
of 2.5-4 kJ/kg over central North Dakota with 40kts of 0-6 kg bulk
shear (GFS showing similar values but is a tad bit further west
with instability axis), though there are questions about frontal
timing and the potential for capping to limit convective coverage.

Otherwise, a drier and cooler post-frontal air mass may invade the
region for the weekend before a warming trend returns next week.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1218 AM CDT Thu Jul 20 2017

Patchy fog could produce localized MVFR or IFR conditions through
14 UTC from the Turtle Mountains southward through the James River
valley. However, confidence in it impacting KJMS is modest at best
and thus only a VCFG was carried in its 06 UTC TAF. Otherwise, VFR
conditions will prevail Thursday. Thunderstorms may develop across
western ND along a surface pressure trough after about 21 UTC, and
could then move into central ND by late Thursday evening (although
we are not confident they will survive that far eastward).




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