Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 240542
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
1242 AM CDT TUE MAY 24 2016

.UPDATE...
Issued at 1234 AM CDT Tue May 24 2016

Increased PoPS to likely across south central North Dakota per
observed radar trends through 0515 UTC. Overall, convection has
shown a slight increase in coverage and intensity in the last hour
in the vicinity of a weak surface boundary from the Upper James
River Valley into far south central North Dakota, aided by the
interaction of two impulses evident on water vapor imagery. MU
CAPE of around 500 j/Kg is expected to remain in place across the
aforementioned areas through the night with effective deep layer
shear around 25 kts. Thus, storms are expected to remain below
severe limits.

UPDATE Issued at 926 PM CDT Mon May 23 2016

The focus of this update was on refining the chance of showers and
weak thunderstorms through the rest of the night, which is mainly
focused from Bismarck and Fort Yates east to Jamestown. Convection
this evening was most concentrated both 1) along a weak surface
wind shift/pressure trough that extended from near Linton to just
north of Jamestown, and 2) just ahead of a weak shortwave trough
that`s approaching Bismarck as of 0215 UTC. Going forward, expect
the chance of showers and weak storm activity to be focused along
and ahead of the weak shortwave. Most rapid-refresh guidance has
been a bit too slow with eastward propagation of the convection
tied to that wave, but the ESRL-run experimental HRRR from 23 UTC
(the most recent run available) does appear to be doing a decent
job capturing current trends, so we chose to weight the forecast
toward its simulation. Nonetheless, changes with this update were
relatively minor in nature.

UPDATE Issued at 643 PM CDT Mon May 23 2016

We expanded a low-end chance of showers and isolated thunderstorms
to the remainder of central ND with this update (and maintained the
mention in western ND) through mid-evening. As of 2330 UTC, steep
low- and mid-level lapse rates have allowed weak convection to
form as far east as the Rolla and Jamestown areas. The activity is
likely being aided by a weak shortwave trough entering southwest
ND per water vapor imagery. Model guidance, including recent RAP
and HRRR simulations, have been keying on a bit more well-defined
batch of convection ahead of that impulse, but those models could
be a bit too slow with its passage based on radar trends. Finally,
note that the dry boundary layer characterized by temperature-
dewpoint spreads of 40-50 F is contributing to gusty winds with
some of the showers and weak storms, and will continue to do so
until the boundary layer begins to decouple with the approach of
sunset.

UPDATE Issued at 420 PM CDT Mon May 23 2016

A quick update to add in an isolated rain shower central ND with
the shallow cumulus field and weak returns on radar. These should
begin to diminish this evening except in the southwest and south
central, where the next shortwave mentioned in the previous
discussion will maintain development through the evening and
overnight hours. Updates to the text products being sent out now.

&&

.SHORT TERM...(This evening through Tuesday)
Issued at 240 PM CDT Mon May 23 2016

Overall theme will be multiple shortwaves rotating around a
nearly stationary closed upper low in southern Saskatchewan,
resulting in periodic showers and thunderstorms. The best
probability will be Wednesday as the upper low melds into an open
wave as it merges with another stronger shortwave rotating in
from eastern South Dakota. A moist southwest flow remains in tact
through the long term period keeping an unsettled weather pattern
for western and central North Dakota.

Tonight through Tuesday, a weak shortwave in eastern Montana will
shift toward the ND/MT border near Williston this evening. Current
regional radar shows isolated showers in eastern Montana with weak
surface based convective available potential energy or CAPE.
Expect isolated to scattered showers and isolated thunderstorms
with this feature into western ND late this afternoon into early
evening. No severe weather anticipated with cape of 200-400 j/kg
along with low to mid level lapse rates between 7.5 and 8.5 c/km
and 0-6km shear around 15kt.

Another shortwave seen in northern Wyoming will shift into
northern South Dakota this evening, and then into the James River
Valley later tonight. A surface reflection with low pressure in
northern SD will move east along the border. Model data shows an
area of showers and perhaps an isolated thunderstorm developing in
southwest ND early this evening then moving into south central ND
between 10pm Monday and 2am Tuesday before reaching the James
River Valley by 12z Tuesday.

On Tuesday, once the aforementioned shortwave exits the James
River Valley during the morning, expect only a slight chance of
afternoon showers or thunderstorms associated with cyclonic flow
and daytime heating. Areal coverage will remain limited.

.LONG TERM...(Tuesday night through Monday)
Issued at 240 PM CDT Mon May 23 2016

The next shortwave trough mentioned in the short term period
rotating up from eastern South Dakota Tuesday night through
Wednesday will dumbbell and meld with the Saskatchewan upper
low. This will result in likely showers and thunderstorms during
the day Wednesday, then decreasing a bit Thursday.

For Friday through Monday, periodic shortwaves embedded within a
continued southwest flow will produce showers and thunderstorms
through the long term period. Timing of each wave difficult to
discern at this time. High temperatures will mostly be in the 70s.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1234 AM CDT Tue May 24 2016

Thunderstorms are expected to impact KJMS through 08 UTC tonight.
Variable wind gusts of 25-30kts are possible with MVFR conditions
in thunderstorms. Otherwise, VFR conditions are forecast for the
06 UTC TAF cycle across western and central North Dakota.

&&

.BIS Watches/Warnings/Advisories...
NONE.
&&

$$

UPDATE...AYD
SHORT TERM...KS
LONG TERM...KS
AVIATION...AYD


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