Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 080920
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
420 AM CDT Sat May 8 2021

.SHORT TERM...(Today and tonight)
Issued at 418 AM CDT Sat May 8 2021

The main story for the short term period is much needed
precipitation across western and south central North Dakota.
Unfortunately, areas from north central North Dakota into the James
River Valley will mostly be left out.

A negatively tilted upper level trough lies from British Columbia
into the Northern Rockies early this morning while a closed upper
low spins just south of Hudson Bay, placing the Northern High Plains
under a deformation zone. Shortwave energy ejecting from the base of
the upstream trough combined with Q-vector convergence and mid level
frontogenesis will bring widespread precipitation to most of western
and south central North Dakota through the short term period. The
two main forecast challenges with this event are the northern and
eastward extent of the precipitation shield and precipitation type.
High pressure over Manitoba continues to direct dry air into the
region, and will likely force a sharp cutoff in precipitation. The
average location of this cutoff across the entire suite of guidance
is estimated to be just north and east of a line from Williston to
Bismarck to Aberdeen, SD. We are fairly confident that the greater
Bismarck/Mandan and Williston areas will receive at least a few
tenths of an inch of QPF, but will likely miss out on higher amounts
approaching one inch just to the west and south. Areas that are
favored to see the highest QPF are generally located along a line
from Sidney, MT to Dickinson to Mobridge, SD. Mesoscale banding and
mid level instability could bring over an inch of liquid to some
areas. In fact, enough MUCAPE is present for a few thunderstorms
across southwest North Dakota through the morning, which have
already been observed.

Precipitation type concerns generally exist across areas that are
expected to lie under the heaviest precipitation and with elevation
above around 2,000 ft. This includes a large area to the south and
west of the Missouri River, excluding the far southwest corner of
the state. The main driving factors for precipitation type are
expected to be surface wet-bulb temperatures and the temperature of
a deep isothermal layer from just above the surface to near 650 mb.
The isothermal layer is forecast to maintain a temperature around
freezing through the morning before dynamic cooling effects take
over this afternoon. Meanwhile, surface wet-bulb temperatures are
more or less forecast to range from around 33 to 35 degrees
Fahrenheit through the day. Therefore, our expectation is that
precipitation will fall as rain through around 8 AM CDT, followed
by a gradual transition to a rain/snow mix for the aforementioned
areas through late morning. However, a one or two degree difference
in either direction for either parameter could result in a very
different outcome. Even if a cooler solution is realized and more
snow falls than expected, very low snow-to-liquid ratios, warm
ground temperatures, and strong solar insolation will mostly prevent
accumulations, with grassy surfaces possibly becoming white. The
main impact from snow is expected to be reduced visibility, with
aggregation likely resulting in monster-sized flakes.

The deformation zone will begin to get stretched this evening into
tonight, resulting in a slow narrowing of the precipitation band and
a decrease in rates. There is some uncertainty on exactly where the
remnant band will set up through the night, but the general
expectation is for it to be somewhere from northwest to south
central North Dakota. Additionally, a stacked low developing over
Montana could bring additional showers back into southwest North
Dakota later tonight. Precipitation that falls after sunset could be
either rain or snow, which will be determined by surface
temperatures and dewpoints that are forecast to hover around to just
above freezing. Grassy surfaces may become white overnight due to
the loss of solar insolation, but snow rates will be much lighter.

It will be a raw and chilly day under the precipitation, with
easterly winds around 15 to 20 mph and daytime highs only in the mid
30s to lower 40s. Areas to the north and east will still experience
the easterly breeze, but should see afternoon temperatures climb
into the lower to mid 50s.

.LONG TERM...(Sunday through Friday)
Issued at 418 AM CDT Sat May 8 2021

A nearly stacked deep layer high pressure building over central
Canada will force the closed upper low over Montana to dissolve on
Sunday. Remnant precipitation will remain possible across western
and south central North Dakota Sunday morning, with chances eroding
from north to south through the afternoon. Sunday should be warmer
than today, but persistent cloud cover could still limit highs to
the 40s over southwest North Dakota. The strengthening deep layer
subsidence regime is forecast to remain over the Northern Plains
through Tuesday, leading to dry weather, lighter winds, and a
warming trend.

Models are trying to bring a shortwave trough through the region
around Wednesday, but are very inconsistent with its evolution. This
could bring a slight chance of rain to parts of the region, but
confidence remains very low at this time. Beyond the middle of next
week, long range guidance appears to favor broad upper ridging
somewhere between the Rockies (ECMWF ensemble) and the central CONUS
(GEFS). Either solution would continue a warming trend, with highs
likely returning to the mid 60s to mid 70s by the end of the week.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 418 AM CDT Sat May 8 2021

Rain is expected at KDIK, KBIS, and KXWA beginning this morning.
Expect ceilings and visibilities to lower to IFR levels,
especially at KDIK where snow will likely mix with the rain
by late morning. There is also a low chance for a thunderstorm
near KDIK early this morning. KMOT and KJMS will likely remain
dry with VFR ceilings. East-southeast winds will sustain around
15-20 kts through this afternoon.


&&

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

$$

SHORT TERM...Hollan
LONG TERM...Hollan
AVIATION...Hollan


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