Area Forecast Discussion
Issued by NWS North Platte, NE

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FXUS63 KLBF 190852
AFDLBF

Area Forecast Discussion
National Weather Service North Platte NE
352 AM CDT Tue Sep 19 2017

.SYNOPSIS...
Issued at 350 AM CDT Tue Sep 19 2017

At 08z...A cold front has pushed east through Sheridan, Riverton
to Rock springs Wyoming. some scattered high clouds moving east
across western Nebraska. Temperatures were still near 65 across
the northwest sandhills, while upper 50s and patchy fog had
developed across the southeast.

&&

.SHORT TERM...(Today and tonight)
Issued at 350 AM CDT Tue Sep 19 2017

Some patchy fog will continue until 9am cdt, for areas southeast of
Ogallala through Bassett. Local visibilities could reach as low as a
quarter mile, especially in central Nebraska.

The cold front will advance eastward into the western Dakotas, and
extend southwest into southeast Wyoming by 18z, and should reach to
near Cody through Sidney in western Nebraska by 00z. This will be in
advance of an upper trough pushing into the Northern Plains. An
isolated shower or thunderstorm could develop in the northern
panhandle late this afternoon, to include northern Sheridan county.
In advance of the cold front, south to southwest winds will gust to
around 25 mph in some areas. Compressional heating will effectively
mix down 850 mb temperatures from 23 to 29C. Highs kept very near
previous forecast at 85 to 90 most areas, with low 90s south of I80.

For tonight, the upper trough will advance rather quickly east
across the Northern Plains, with the cold front expected to clear
western Nebraska by 06z. Northwest winds behind the front will gust
to 25 mph this evening, then diminish overnight. There remains a
slight chance for thunderstorms across far northern Nebraska this
evening. Organized thunderstorms expected across the eastern
Dakotas. On the tail end, a few strong storms could develop or move
into in portions of Keya Paha, Boyd or northern Holt county. Latest
mesoscale models keep this southern end of storms in southeast South
Dakota, just north of these counties. Lows from near 40 in Sheridan
county, to the low 50s far eastern zones.

.LONG TERM...(Wednesday through Monday)
Issued at 350 AM CDT Tue Sep 19 2017

A nearly meridional synoptic pattern aloft is in store for the next
several days as a trough digs across the Western U.S. and a high
remains anchored over the East. The GFS and ECM also close off a low
over the northern Rockies over the weekend. This places Nebraska in
persistent southwest flow. Meanwhile at the surface, a front stalls
across the state late Friday through Sunday. A high pressure follows
shortly thereafter, bringing a reinforcing shot of cool air.

The primary forecast challenges pertain to near critical fire
weather conditions Wednesday and Thursday (additional details in the
fire weather discussion) and the heavy rain potential over the
weekend.

Wednesday and Thursday... Downslope flow is replaced by
strengthening southerly flow Wednesday. Temperatures are not overly
impressive in the mid to upper 70s, but humidity will drop to near
or below 20 percent in the panhandle and western Sandhills. Winds
really pick up and highs soar to near 90F on Thursday ahead of the
cold front. Trended toward the warmer ECM guidance given plenty of
sunshine and proficient mixing in the PBL. Southwest Nebraska has
the greatest chance of seeing critical fire weather conditions,
where RH may drop to 15 percent.

Friday through Sunday... The cold front begins its descent into the
forecast area Friday, crossing the northwest third by 18z and most
of the Sandhills by 06z Sat. The front then stalls, essentially
bisecting the area from southwest to northeast. Enough elevated
instability and decent moisture ahead of the front support thunder
chances. Behind the front, left mainly as rain. Precip totals could
be rather impressive for areas east of Hwy 83 as moisture advection
continues all weekend. NAEFS climo ensembles indicate a prolonged
period (Fri PM - Sun AM) of PWAT 2+ SD above normal. Jet dynamics
also support convection with a 100+ kt streak nearby and very strong
deep layer shear (60+ kts in GFS soundings). Storm motion is also
parallel to the front, suggesting training cells. Will have to watch
for possible flood headlines, primarily Custer Co. and areas just
north where August was unusually wet. Highs take a dive with the
continued cloudiness and off/on precip, with upper 50s far NW to
upper 60s central for Sat and Sun.

Monday... Things dry out a bit, esp. northwest as a high settles
onto the northern Plains. Temps remain seasonably cool in the 60s.
Guidance suggest morning lows in the 30s northwest, which may
support another frost.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1130 PM CDT Mon Sep 18 2017

Satellite imagery shows mostly clear skies, except some high
clouds moving eastward across northern NEB. KLNX VWP winds (at
1136 PM CDT) at 250 m/~820 ft AGL and 500 m/~1640 ft AGL were 34
kts and 29 kts, respectively. Winds at low levels are expected to
increase attributable to a developing low-level jet across the
eastern Panhandle and far western NEB into western SD. This will
prompt LLWS conditions across portions of the north and far west.
LLWS conditions mentioned only in the the KVTN TAF at this time.
Upper height of the wind shear layer is forecast about 1500 ft AGL
with speeds at the top of the wind shear layer near 45 kts.

Fog still a concern, however, confidence has decreased some with
respect to impacts over southwest NEB. Current thinking is fog
will be patchy. Otherwise, stratus/fog is expected to develop
westward from northeast NEB into north central NEB and parts of
southwest NEB. Not as confident in impacts at the KLBF terminal
thus modified weather groups to lessen fog effects. Conditions are
anticipated to improve mid-late morning.

Otherwise, cold front advancing tomorrow late afternoon and
evening will prompt wind shift from southerly to northwesterly.
Slight chances for thunderstorms tomorrow, confined at this time
mainly to far north central NEB in the evening.


&&

.FIRE WEATHER...
Issued at 350 AM CDT Tue Sep 19 2017

Critical fire weather conditions expected across zones 204, 210, and
219 this afternoon and early evening. A Red Flag Warning for zone
210, was expanded to include 204 and 219. Southwest winds gusting to
20 mph, and possibly 25 mph at times will combine with low RH of 13
to 18 percent in zones 210 and 219. In zone 204, southwest winds
this afternoon will turn northwest late afternoon and early evening
with frequent gusts to around 25 mph. Lowest RH from 13 to 18
percent also expected.

On Wednesday, much drier air will extend across western Nebraska as
dewpoints range in the upper 20s west of highway 83. with cooler
highs from 74 to 78 degrees, minimum RH currently forecast from 17 to
20 percent near and west of highway 83. Winds do not look very
strong at 5 to 15 mph with gust to 20 mph possible northwest. If
lower RH or stronger winds develop, critical fire weather conditions
could still develop.

On Thursday, near critical RH is possible in zone 210 of southwest
Nebraska with south winds 5 to 15 mph.

&&

.LBF WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from 1 PM CDT /noon MDT/ this afternoon to 8 PM
CDT /7 PM MDT/ this evening for NEZ204-210-219.

&&

$$

SYNOPSIS...Roberg
SHORT TERM...Roberg
LONG TERM...Snively
AVIATION...ET
FIRE WEATHER...Roberg


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