Area Forecast Discussion
Issued by NWS Bismarck, ND

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000
FXUS63 KBIS 220948
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
448 AM CDT FRI JUL 22 2016

.SHORT TERM...(Today and tonight)
Issued at 448 AM CDT Fri Jul 22 2016

Multiple rounds of strong to severe thunderstorms are expected
across western and central ND today and tonight. Very heavy rain
is also possible, prompting the issuance of a flash flood watch
for central ND.

Synoptically-speaking, a warm frontal zone is expected to set up
along the ND-SD border region today as low pressure deepens over
MT. That`s in advance of a vigorous shortwave trough moving into
the Pacific northwest early this morning, and expected to quickly
move eastward to the central Dakotas by midday Saturday. There is
a reservoir of very rich, evapotranspirationally-aided moisture
across SD early this morning which is characterized by dewpoints
in the middle and upper 70s F. This air mass is forecast to surge
northward today in response to the developing lee-side pressure
falls. Based on an analysis of overnight model trends compared to
observations upstream, and considering recent verification, we
believe the 00 and 06 UTC NAM simulations are the most viable of
the global model guidance with regard to low-level moisture and
thus the degree of destabilization today and tonight.

Forecast soundings from the aforementioned NAM simulations show
strong to extreme instability developing today along and east of
Highway 85 and south of Highway 2, with afternoon MLCAPE ranging
from 3000 to 5000 J/kg in the face of steep mid-level lapse rates
around 8 C/km and boundary layer dewpoints surging well into the
70s F. Coupled with veering wind profiles and 0-6-km bulk wind
differences around 50 kt, the scenario will readily support both
organized and rotating updrafts, including potential supercells.
Weak mid-level height falls will overspread the area today, and
that should favor a large-scale setting supportive of deep moist
convection, especially given relatively weak MLCIN shown on most
model-derived soundings by afternoon. Forcing will be subtle and
could be tied to surges of low- and mid-level Theta-E advection,
differential heating boundaries which develop from early-day low
clouds and/or convective debris from morning elevated storms, or
broad confluence associated with the low- and mid-level warm
frontal zone, to name a few. Given the subtle forcing and weakly
capped air mass, it`s not surprising that run-to-run and model-
to-model continuity has been rather lackluster in the plethora of
convection-allowing model guidance that we have viewed tonight.
However, generally-speaking the primary themes we have observed in
multiple model cores and simulations have been for the following:
1) Early-day elevated convection, including the risk of elevated
supercells, in south central ND, 2) Afternoon development or re-
development of intense, surface-based storms centered along and
north of the warm front in south central ND, and 3) Additional
rounds of thunderstorms, including the possibility of multiple
MCSs, crossing western and central ND tonight after high-based
convection which develops from MT into WY merges and propagates
across the state.

The impressive reservoir of instability that exists upstream of
the area is likely to be continually re-advected into the area
on a 40-50 kt low-level jet based near 850 MB tonight. Thus, we
believe that not only will multiple rounds of storms occur, but
each round will have severe potential. Typically, we expect the
instability to be overturned and the moisture redistributed with
the passage of intense convection, but in this case the air mass
to the south is not expected to have a wide expanse of storms and
thus it will remain ready to replenish the instability across ND
through the night. That scenario is well-supported by a range of
model simulations, including convection-allowing guidance to
include the 00 UTC NSSL WRF-ARW and the UND-run WRF-ARW nest that
is centered on western and central ND.

The CAPE-shear setting and expected mid-level lapse rates favor a
threat of large hail and damaging winds today and tonight. We are
somewhat less-confident in whether or not significant-caliber hail
in excess of 2 inches will occur on anything more than an isolated
basis, largely owing to the high tropospheric moisture content, as
that may tend to favor hailstone melting, as it likely did during
the Wednesday afternoon and evening severe weather episode. More-
over, only one or two members in the 00 UTC NSSL WRF-ARW ensemble
produced hailstones greater than 2 inches in diameter with the
HAILCAST model applied to their simulations. However, based on the
significant hail parameter exceeding 2 on NAM forecast soundings,
and the possibility of initial supercells, we have chosen to make
a mention of large hail up to the size of tennis balls in our
hazardous weather outlook. Given the degree of CAPE, veering wind
profiles, and very rich boundary layer moisture, there`s a risk of
isolated tornadoes with any surface-based supercells during the
afternoon and evening, especially in south central ND. That risk
may be modulated by relatively small low-level hodographs, though.

Finally, given the strong likelihood of multiple rounds of storms
and pre-convective precipitable water values around 1.80 inches, a
flash flood watch was deemed prudent across central ND from this
afternoon through early Saturday afternoon. Flash flood guidance
is generally in excess of 2 inches, but precipitation-rates could
exceed 2 inches per hour with the most intense storms, and there
is a risk of training storms, especially north of the warm front
this afternoon and evening.

.LONG TERM...(Saturday through Thursday)
Issued at 448 AM CDT Fri Jul 22 2016

A surface cold front will cross western ND Saturday morning and is
forecast to clear the James River valley by late in the afternoon
Saturday. Based on the CAPE-shear setting shown by most of the 00
UTC guidance, a risk of strong and severe storms will continue in
most areas until the frontal passage occurs. However, we certainly
have to express some uncertainty in the magnitude of the risk on
Saturday since preceding convection and cloud cover could mitigate
the amount of bouyancy available for updrafts.

The risk of heavy rain will also continue over central ND, and in
fact, depending on the timing of the front a later forecast could
need to extend the flash flood watch for the James River valley a
bit longer into Saturday afternoon.

Otherwise, drier westerly flow is expected Saturday night through
Sunday night. The 00 UTC multi-model consensus suggests the flow
next week will favor seasonable temperatures mainly in the 80s F
with daily chances of thunderstorms Monday through Thursday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1258 AM CDT Fri Jul 22 2016

Sct to bkn mvfr cigs forecast at KBIS and KJMS between 12z and 17z
Friday. Vcfg was also briefly added to KJMS between 11z and 15z
Friday. Otherwise expect vfr cigs outside of any thunderstorms,
which should develop and increase in coverage throughout the TAF
period. Tried to time the showers/thunderstorms where confidence
seemed highest Friday, which included the KISN/KDIK/KBIS terminals.
A vcts/vcsh was utilized elsewhere.


&&

.HYDROLOGY...
Issued at 448 AM CDT Fri Jul 22 2016

Collaboration with the WPC QPF desk yielded concerns of flash
flooding across central ND Friday afternoon through Saturday
morning. With a prolonged period of precipitable water values of 1.5
to 2.0 inches, hourly rainfall rates above 2 inches possible where
cells merge or backbuild/train over the same area. Successive rounds
of heavy thunderstorms forecast through Saturday morning has
necessitated a Flash Flood Watch central ND. Total rainfall amounts
across central North Dakota of 2 to 4 inches possible through
Saturday morning.

With that said, we have issued a Flash Flood Watch in two segments.
Segment one covers all of south central ND beginning at 18 UTC/1 PM
CDT Friday today through 18 UTC Saturday/1 PM CDT Saturday. Segment
two covers the north central and begins a bit later, 03 UTC
Saturday/10 PM CDT Friday through 18 UTC Saturday/1 PM CDT
Saturday. Decision to begin segment one earlier goes along with
the Short Term Discussion with multiple opportunities for heavy
thunderstorms along and north of a warm front, which is forecast
to initiate this afternoon. The north central is favored for
heavier precipitation late Friday evening through Saturday
morning.

&&

.BIS Watches/Warnings/Advisories...
Flash Flood Watch from this evening through Saturday afternoon
for NDZ002>005-010>013.

Flash Flood Watch from 1 PM CDT /noon MDT/ this afternoon
through Saturday afternoon for NDZ019>023-025-034>037-042-
045>048-050-051.

&&

$$

SHORT TERM...CJS
LONG TERM...CJS
AVIATION...KS
HYDROLOGY...KS



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