Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 211721
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
1221 PM CDT THU JUL 21 2016

.UPDATE...
Issued at 1220 PM CDT Thu Jul 21 2016

Drier high pressure continued to push into the state from the west.
Dewpoint temperatures in Montana were in the 50s to lower 60s F and
expect this drier air to push into western and central North Dakota
today. This means that the current dewpoints in the lower to mid 70s
F in the south central and James Valley should drop into the lower
to mid 60s F this afternoon. Overall a much more pleasant day
regarding the oppressive heat/humidity of the past few days. Still
expecting highs in the upper 80s to lower 90s today, but heat index
values should be similar to the actual temperatures instead of well
above 100 F like Wednesday. Current forecast on track for the rest
of today.

UPDATE Issued at 857 AM CDT Thu Jul 21 2016

Quick update for radar trends through 1345 UTC as weak elevated
convection is expected to continue across southern North Dakota
for a few hours in association with a weak upper level impulse.

UPDATE Issued at 641 AM CDT Thu Jul 21 2016

Only minor changes were made with this update to accommodate recent
observational trends and HRRR simulations, which appear to well-
capture the smattering of showers and thunderstorms in southwest
and south central ND early this morning. That model guidance is
still calling for that activity to wane by about 15 UTC.

&&

.SHORT TERM...(Today and tonight)
Issued at 356 AM CDT Thu Jul 21 2016

Today will be hot, but not as hot as Wednesday, and humidity will
also be relatively lower in the wake of a weak overnight frontal
passage.

As of 08 UTC, low-level dry air advection is ongoing behind a weak
surface cool front that extends from roughly Grand Forks southwest
to Aberdeen and nearly to Pierre at this hour. Surface dewpoints
have eased back into the middle and upper 60s F after peaking well
into the middle and upper 70s F Wednesday afternoon. That yielded
an incredible 6100 J/kg of SBCAPE on the 00 UTC Bismarck sounding.
Based on the SPC sounding climatology, that was easily in the top
five values of SBCAPE sampled at Bismarck since 1948. Despite the
early-morning low-level dry air advection, NAM-based soundings in
south central ND suggest MUCAPE will remain in excess of 3000 J/kg
through about 15 UTC owing to the depth of the rich moist layer
and surmounting steep lapse rates. Given that amount of bouyancy
and continued weak ascent manifest as elevated showers and weak
thunderstorms which extend from Bowman toward Regent and New Salem
as 08 UTC, we have some concern that a non-zero chance of isolated
severe thunderstorms will continue through the early morning. That
being said, convection-allowing model guidance including the HRRR
suggests little chance of this convection intensifying. Based on
analysis of radar imagery, it appears the convection is based in a
layer between 12 and 16 thousand feet AGL. Model soundings reveal
a significant dry layer in that zone just above 700 MB, so it may
well be that dry air entrainment and a lack of stronger vertical
motion within and above the mid-level capping inversion has been
and will continue to prevent any stronger updrafts from occurring.

The better part of today will feature mean north or northwest low-
level flow for a continued reduction in surface dewpoints. The 00
UTC multi-model consensus calls for them to range from the mid 50s
to mid 60s F this afternoon, making for a more comfortable day as
far as heat index is concerned (and relatively-speaking). Heights
aloft will remain rather high and the air mass will not cool that
much, so highs will mainly be in the upper 80s to mid 90s F. The
00 UTC guidance suggests that there could be a lingering impulse
or two embedded in westerly flow aloft, but in general heights at
500 MB wil be neutral, and even begin rising again after 00 UTC.
That suggests the overall potential for afternoon thunderstorms is
too low to mention in the forecast. The only convection-allowing
model from 00 UTC which generated storms this afternoon was the
NCEP-run WRF-ARW. The remainder of guidance, including a WRF-ARW
model run at UND and nested over western and central ND, did not
generate any surface-based convection during the diurnal heating
cycle today.

Tonight will mainly be quiet, with lows in the 60s F. However, we
are carrying a low-end chance of thunderstorms in part of western
ND late tonight as moisture advection begins once more.

.LONG TERM...(Friday through Wednesday)
Issued at 356 AM CDT Thu Jul 21 2016

The potential for severe thunderstorms Friday through Saturday is
the highlight of the long term forecast.

The 00 UTC model suite generally agrees that a seasonably vigorous
shortwave trough will track from the Pacific northwest Friday to
central ND by Saturday afternoon. Leading height falls will begin
across the region Friday, and lee-side cyclogenesis will enable a
southeast return-flow of moisture to pick up in earnest then. The
00 UTC global models are at odds over the degree and richness of
boundary layer moisture across western and central ND come Friday
afternoon and evening. We believe the ECMWF`s simulation showing a
wide expanse of surface dewpoints in the middle and upper 60s F is
the most probable outcome among the 00 UTC models. That`s because
the pool of rich moisture upstream of the local area is unlikely
to be overturned before then, and boundary layer moisture will be
aided by the annual peak in regional evapotranspiration, as has
been the case the last several days. Other guidance has displayed
low biases in recent days and thus the lower dewpoints simulated
by the 00 UTC GFS and NAM (or at least the width of its moisture
axis) for Friday and Friday evening are likely incorrect. Based on
the ECMWF output, MLCAPE of 2000 to 3000 J/kg is likely over most
of western and central ND Friday and Friday night, and 0-6-km bulk
wind shear of 40-50 kt will be sufficient for organized, severe
storms. Initially-discrete cells could merge and form into an MCS
by Friday night, as most guidance suggests.

Additional rounds of thunderstorms and potential severe weather
are possible Friday night and Saturday as stronger height falls
occur. Taken at face value, the 00 UTC global models suggest we
could have multiple rounds of convection, finally exiting the
James River valley by late Saturday afternoon as a surface cold
front finally clears the area. There is some potential for this
to be a prolonged or perhaps higher-order event, but we need to
express some reservations as the 21 UTC SREF calibrated severe
thunderstorm probabilities are not particularly high and that
likely reflects some uncertainty in the overall scenario.

Thereafter, the upper-level ridge currently centered across the
Great Plains is forecast to slowly retrograde next week. That in
turn should yield somewhat cooler temperatures across our area,
with forecast highs mainly in the 80s F from Saturday onward.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1220 PM CDT Thu Jul 21 2016

VFR conditions will prevail across western and central ND today and
tonight. A few scattered cumulus clouds may develop this afternoon,
but no showers or thunderstorms are expected through tonight. Early
Saturday, mainly after 12z, the chances of thunderstorms begin to
increase...mainly for KDIK and KISN...but chances of precipitation
are not high enough before 18z to warrant a mention in the TAFS.
Will however carry a mid level cloud deck after 12z at KDIK and
KISN.

&&

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

$$

UPDATE...JV
SHORT TERM...CJS
LONG TERM...CJS
AVIATION...JV



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