Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 271720
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
1220 PM CDT Thu Apr 27 2017

.UPDATE...
Issued at 1219 PM CDT Thu Apr 27 2017

Low clouds across southern ND have moved south for the most part,
with only a few residual clouds around the Hettinger area remaining.
The cumulus field that had developed a few hours ago between
Dickinson and Bismarck has begun to dissipate. We still may see some
scattered cumulus develop this afternoon, but mostly sunny skies
should be the main sky condition most areas. Regarding temperatures,
lowered expected high temperatures in eastern portions of Lamoure
and Dickey counties - where there were 2 to 4 inches of snow
accumulation yesterday. Melting snow there should act to lower
expected highs. Otherwise the forecast for the afternoon is on
track.

UPDATE Issued at 936 AM CDT Thu Apr 27 2017

Only changes this update was to refine cloud cover. Latest
iterations of the short term high res models do not expect low
clouds that are currently across far southern ND to creep northward.
Thus thinking is that some scattered late morning/afternoon cumulus
may develop...bringing a mostly sunny day across most of western and
central ND this afternoon. Minor hourly temperature trends were
adjusted but otherwise forecast on track.

UPDATE Issued at 648 AM CDT Thu Apr 27 2017

Little change with this update other than to further refine the
cloud cover forecast based on the latest trends in stratus from
satellite imagery, which shows clearing pushing south from Rolla
to Harvey, and a fragment of low cloud indeed pushing westward
across southwest ND through the Hettinger and Mott areas. The 11
UTC surface analysis also reveals temperatures of 14 to 16 F in
the Hazen and Glen Ullin areas where light winds and a clear sky
allowed for efficient radiational cooling in our dry air mass.

&&

.SHORT TERM...(Today and tonight)
Issued at 459 AM CDT Thu Apr 27 2017

Dry and quiet weather is expected today and tonight.

Synoptically-speaking, broadly cyclonic flow will persist across
the western and central U.S. today and tonight, with the primary
feature of interest an intense upper-level speed maximum denoted
by 300 mb wind speeds of at least 125 kt that will extend from the
Oregon coastline southeastward into the Texas Panhandle by 00 UTC.
The evolution of the jet streak and related subtle reconfiguration
of height fields aloft will result in the slow northeast movement
of the mid- and upper-level low located in northwestern MN early
this morning into central Ontario by late tonight. Locally, this
will yield weak height rises across western and central ND along
with weak warm air advection. The result should be a warmer day
with highs ranging from the lower 40s F in the Rolla and Jamestown
areas to around 50 F in western and south central ND. Note that we
did weight the forecast today slightly toward warmer MOS guidance
from the 00 UTC model cycle in respect to the trends observed in
western ND on Wednesday, and in recognition of the very dry air
mass that will be in place today in support of efficient diabatic
heating given sunshine. The dry air is statistically borne out by
the 00 UTC NAEFS guidance, which reveals that the precipitable
water values that will average near 0.20" are in the lowest 2.5
percent of values observed at this time of the year.

Note that despite the dry air mass, light wind fields through the
lower and middle atmosphere and an inversion layer based near 850
mb per 00 UTC NAM and GFS forecast soundings suggests vertical
mixing will be relatively weak. That should tend to keep humidity
values from bottoming out. These same characteristics will likely
allow for good radiational cooling tonight, so forecast lows are
once again in the lower to middle 20s F.

Finally, the stratus over south central and eastern ND early this
morning will pose some challenge to today`s forecast. The primary
uncertainty with this stratus is how far west it will ultimately
be able to build. Most guidance suggests only fragments of stratus
will work into western ND by morning, but we will be monitoring
later model cycles and satellite trends during the next few hours
to see how this progresses. Recent gridded LAMP cycles appear to
be capturing the extent of the stratus well, so we relied on them
to construct the sky forecast for today. That generally means we
expect the stratus to dissipate from west to east during the late
morning and early afternoon, lingering the longest (until late
afternoon or early evening) in the James River valley.

.LONG TERM...(Friday through Wednesday)
Issued at 459 AM CDT Thu Apr 27 2017

A warming trend is forecast during the long term forecast period
with highs generally in the upper 50s and lower 60s F for Saturday
through Wednesday per the 00 UTC multi-model consensus. Northwest
flow is forecast to prevail aloft as a mean 500 mb trough shifts
out of the western and into the central U.S., which should allow
for heights and thermal fields to recover from their low points
this week. The 00 UTC global models continue to suggest that a few
shortwave troughs embedded in the northwest flow may bring some
showers to the region from Monday through Wednesday, too.

The most interesting part of the forecast is the powerful upper-
level low consistently being forecast by the last several cycles
of the global deterministic and ensemble guidance to lift from
the southern plains into the upper Midwest early next week. This
low is simulated to contain strong dynamic forcing and low enough
850 mb temperatures in its cold sector to potentially yield a May
heavy snow event somewhere in the upper Midwest. The system is
forecast to stay southeast of the local area, but the 00 UTC ECMWF
did make a shift further northwest with its simulated deformation
band such that taken at face value its precipitation would indeed
impact the eastern part of ND. The ECMWF simulation did that as a
result of a deeper and faster northern-stream shortwave that
moves far enough eastward by Monday to create a weakness in the
height fields over ND, which the strong Midwestern cyclone then
takes advantage of in a regime marked by strong downstream 500 mb
ridging off the eastern seaboard. The amplitude and progression of
that northern-stream shortwave trough will be the key item to
watch in the coming days as it does afford a non-zero probability
of the forcing and resultant precipitation from the deep upper-
level trough extending further northwest.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1219 PM CDT Thu Apr 27 2017

VFR conditions are expected to prevail through the 18Z TAF period.

&&

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

$$

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



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