Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 170624
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
124 AM CDT Sun Sep 17 2017

.UPDATE...

Issued at 120 AM CDT Sun Sep 17 2017

Low clouds lingering across much of the area, though there are
some signs of scattering out. High res models including the
namnest, glamp, and rap keep low cigs through 12-15Z so increased
sky cover based on their guidance and latest trends, which agree
with a more pessimistic approach. Lingering clouds will hinder
temperatures from dropping and thus far forecast mins have been
too cold so had to increase. No plans to modify frost/freeze
headlines this late in the game, though my confidence has
certainly decreased for some areas.

Second issue this overnight is fog. HRRR and RAP have been way too
aggressive with fog development, again due to lingering low cloud
cover. Kept fog wording as is for now but anticipate shrinking the
aerial coverage down for the next forecast issuance if clouds are
still slow to scatter and clear.



UPDATE Issued at 922 PM CDT Sat Sep 16 2017

Rain coverage has been diminishing over the past few hours so have
trimmed back chances throughout the area. Remainder of forecast
looks good.

UPDATE Issued at 546 PM CDT Sat Sep 16 2017

Broad area of rain continues to do a very gradual move to the
north. For early evening update have adjusted precipitation
chances in line with current movement of precipitation. Have also
delayed erosion of cloud cover a little bit as movement to the
north has been a bit slower than previous forecast. Remainder of
forecast is in good shape.

&&

.SHORT TERM...(This evening through Sunday)
Issued at 327 PM CDT Sat Sep 16 2017

Frost and freeze issues are the main concerns in the short term.
Rain ending from south to north tonight. Risk of frost across most
of western and parts of south central North Dakota tonight...with
freezing temperatures expected in the far southwest. A freeze
warning and frost advisory has been issued for late tonight into
Sunday morning.

The main upper low was over southern Alberta/Saskatchewan this
afternoon, with a secondary low along the border of Montana and
North Dakota. This secondary low/vigorous shortwave will continue
moving east/northeast and reach southern Manitoba by midnight
tonight. Expecting widespread rain to continue across northwest and
north central North Dakota this evening, with scattered to isolated
rain mainly along and north of I-94. After midnight, only lingering
scattered to isolated rain should be occurring across the far north
central.

Meanwhile, decreasing clouds are expected from southwest to
northeast later tonight. This will have some fog and frost/freeze
implications. Firstly, the widespread rains over the past couple of
days has resulted in wet surface soil conditions. Clearing skies and
falling temperatures, with plenty of low level soil moisture should
bring some patchy fog to western and central North Dakota. The
latest iterations of the short term high res models indicate that
areas south and west of the Missouri River would be more likely to
have fog. Thus have a mention of patchy fog all areas, with areas of
fog mentioned for the southwest.

The next issue is overnight low temperatures. H850 temperatures
around freezing with decreasing clouds and relatively light west
winds will allow radiational cooling tonight. The far southwest
(Slope, Bowman, Hettinger, Adams counties) is expected to see
freezing temperatures from around 30-32F. Surrounding areas south
and west of the Missouri River, as well as Divide and Williams
counties in the far northwest, should see at least some patchy
frost. Thus have issued a freeze warning for the 4 southwest
counties mentioned above, and a frost advisory for the remainder of
southwest and into northwestern North Dakota.

On Sunday, clearing skies with a southwest/southerly wind flow will
allow temperatures to rise into the 60s (around 60 for the Turtle
Mountains to the mid 60s southwest).

.LONG TERM...(Sunday night through Saturday)
Issued at 327 PM CDT Sat Sep 16 2017

Thunderstorms and possible severe thunderstorms Tuesday, then
unsettled weather for the remainder of next week highlight the long
term portion of the forecast.

Sunday night through Monday night: On Sunday the large upper level
low will have lifted northward into Manitoba, with another large low
pressure system moving into British Columbia. This will set up a
zonal flow aloft over North Dakota Sunday night, with a shortwave
ridge building on Monday as the large British Columbia low begins
moving across the Pacific Northwest/Rockies. A weak upper level
impulse moving through the zonal flow aloft Sunday night will bring
a slight chance of showers to mainly south central ND into the James
Valley Sunday night. A dry and warmer day Monday with highs in the
70s returning to our area.

Tuesday: The large low over the Pacific Northwest settles over the
southern Canadian Rockies and becomes quasi-stationary. As it spins
cyclonically, a significant shortwave develops in the base of the
low/trough over Idaho/Wyoming Tuesday, and sends a significant
shortwave eastward across the Dakotas Tuesday/Tuesday night before
lifting northeast and dumbelling around the main low. Southerly
winds will be increasing Monday night as a low level jet develops up
the Plains ahead of the system. A surface low/cold front will
develop associated with the ejecting upper level shortwave and move
east across the Dakotas. The NAM and ECMWF models position the cold
front over the western Dakotas by noon Tuesday, reaching the James
Valley by Tuesday evening, and nearing the Red River Valley after
midnight. The GFS depicts a faster cold front, positioned at
Minot/Bismarck around noon Tuesday, and just east of the James
Valley by Tuesday evening. Either scenario will set up a good chance
of thunderstorms across the state Tuesday, and Tuesday night over
the central/east or eastern North Dakota. In addition, increasing
CAPE and Bulk shear parameters will provide good ingredients for
severe thunderstorms. The position/movement of the cold front will
determine what areas are in the best position for heavy rains and
severe storms. Certainty has increased over the past couple of model
runs that the Storm Prediction Center has the central and eastern
Dakotas in its Severe Thunderstorm Outlook for Day 4 (Tuesday).

Wednesday through Saturday: The main Canadian Rockies low expands
further and develops a longwave trough southwestward towards
California, setting up a strong southwest flow aloft across the US
Rockies and the Dakotas through the rest of the week. The longwave
trough then begins moving east across the Rockies into the Plains
for next weekend.

This scenario will set up a chance for showers and thunderstorms
periodically Tuesday through Saturday, making for an unsettled
pattern, but with fairly seasonable temperatures for mid to late
September...with highs in the 60s and lows mainly in the 40s.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)

Issued at 120 AM CDT Sun Sep 17 2017

Aviation impacts expected this Taf period include vlifr/mvfr vsbys
through 14z Sunday at KISN/KDIK. Elsewhere, mvfr cigs will improve
to vfr through 12z Sunday. Northwest winds early in the period will
become southwest then southerly Sunday afternoon and evening, but
less than 10kt.


&&

.BIS WATCHES/WARNINGS/ADVISORIES...
Frost Advisory until 9 AM CDT /8 AM MDT/ this morning for NDZ001-
009-017>020-031>034-042-045.

Freeze Warning until 8 AM MDT this morning for NDZ040-041-043-
044.

&&

$$

UPDATE...NH
SHORT TERM...JV
LONG TERM...JV
AVIATION...KS


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