Area Forecast Discussion
Issued by NWS North Platte, NE

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FXUS63 KLBF 201725
AFDLBF

Area Forecast Discussion
National Weather Service North Platte NE
1225 PM CDT Wed Sep 20 2017

.SYNOPSIS...
Issued at 336 AM CDT Wed Sep 20 2017

At 08z...Northwest winds have diminished behind the cold front to 5
to 10 mph most locations. Drier air continues to push eastward with
a 31 dewpoint at Imperial. Temperatures remained fairly mild from 46
at North Platte to 57 at Broken Bow.

&&

.SHORT TERM...(Today and tonight)
Issued at 336 AM CDT Wed Sep 20 2017

As the upper trough lifts from the Northern Plains into South
Central Canada today, the associated cold air advection will end.
Westerly winds this morning will turn southwest this afternoon and
range fairly light at 5 to 15 mph. 850 mb temperatures also recover
to 17 to 20C, which equates to highs in the mid and upper 70ds. The
band of mid and high cloudiness extending across the northwest zones
will lift out of the area by around daybreak, leaving a sunny sky
for today.

For tonight, southwesterly flow aloft will persist across the
region, as an upper trough digs from the Pacific Northwest into the
Great Basin. A low level jet with winds reaching 35 to 50 knots will
develop across western Nebraska. Surface winds should remain light
and decoupled across southwest into central Nebraska. Across the
northwest, some mixing to the surface will cause south winds to
range from 15 to 25 mph at times overnight. Skies to remain mostly
clear, with lows from 50 to 55 north central, to the upper 40s
southwestern areas.

.LONG TERM...(Thursday through Tuesday)
Issued at 336 AM CDT Wed Sep 20 2017

The synoptic pattern aloft over the next week is dominated by a
large trough with a closed low over the Rockies and an amplified
ridge over the eastern third of the country. Nebraska is placed in
southwest flow during the entire period. At the surface, a cold
front approaches the Sandhills from the northwest late Thursday. The
front crosses the forecast area Friday and becomes quasi stationary
along the eastern periphery of the CWA over the weekend. High
pressure kicks the system out late Monday into Tuesday.

The primary concerns in this forecast period include critical fire
conditions Thursday, severe potential Friday, and heavy rain
Saturday and Sunday.

Thursday... The evening model suite is in agreement on strong
southerly flow ahead of the cold front. H85 winds approach 30kts
with sfc gusts around 20kts during the afternoon and evening. GFS
and NAM fairly similar in keeping the 50F+ dew points in eastern
Neb, but ECM brings better moisture toward the Hwy 83 corridor.
Regardless, critical fire conditions will likely be met across SW
Neb and have issued a Fire Weather Watch for Zone 210 (Frenchman
Basin - Chase, Hayes, Perkins, and SW Lincoln counties). Forecast
soundings indicate deep mixing to near 650mb and temps near 28C at
850mb, translating to highs in the lower 90s along and south of I-
80.

Friday... The model blend extends the front from VTN to OGA at 18z.
Better moisture advection takes place with dew points reaching the
lower 60s from LBF to ONL in the evening. Still looking at strong
southerly flow at H85 ahead of the front and decent WAA north
central Neb with highs in the upper 80s. GFS soundings suggest
MUCAPE over 1000j/kg and 0-6km shear around 50kts at 00z Sat. The
wind profile is nearly unidirectional above 850mb, and with the
storm motion parallel to the boundary, storm mode should primarily
be linear. The instability and shear parameters do support severe
storms, mainly a damaging wind threat. Training storms increase the
heavy rain threat.

Saturday through Monday... NAEFS continues to show an extended
period of PWATs 2+ S.D. above climo norms for parts of SW and
central Neb. The peak is Saturday evening with near record values
across north central Neb. The precip appears to come in three main
waves... the first being a round of convection Friday evening/night,
then an area of heavy rain moving northeast out of Kansas Saturday
evening followed by another wave Sunday evening. Preliminary
estimates show a swath of 4+ in. from BBW to ONL and 2+ in. as far
west as Oshkosh to Merriman. The persistent cloud cover and strong
CAA behind the front will result in highs in the 50s for Sheridan
and W. Cherry counties Sat. and generally the NW half of the CWA
Sunday. Moisture is relatively slow to exit Monday, and with
continued mid level fgen and low level lift, there will be some
remnant showers. Dry air really entrains into the system toward 00z
Tue, tapering most of the showers. H85 temps remain around 5C Monday
afternoon, resulting in another seasonably cool day in the 50s and
60s.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1215 PM CDT Wed Sep 20 2017

Clear skies will prevail through tonight. A few high clouds will
move into the area overnight into tomorrow morning. The main
concern will be winds, as wind speeds will increase after
midnight. Wind shear may be a factor as well as winds will gust
around 35 knots.

&&

.FIRE WEATHER...
Issued at 336 AM CDT Wed Sep 20 2017

This afternoon, much drier air will extend across western Nebraska
as dewpoints range in the mid 20s to lower 30s 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
northern Sheridan and northern Cherry county. While elevated fire
weather conditions are possible across northern portions of zone 204
and western portions of 208, critical fire weather conditions are
not expected.

On Thursday, deep southwesterly flow aloft will bring very warm 850
mb temperatures around 28C across southwest Nebraska with highs to
reach 90 to 95 degrees. Critical RH and wind is forecast to be
reached in zone 210. Minimum RH is expected to fall to near 15
percent with a with south winds 15 to 25 mph during the afternoon
and early evening. Therefore a Fire Weather Watch is in effect for
zone 210 from 18Z until 01Z. Elevated fire weather conditions are
expected northward to near highway 2. However, the combination of
low RH and wind gusts to 25 mph are not expected to be reached at
this time.

On Friday, very warm highs in the upper 80s to lower 90s will remain
across southwest Nebraska. Southerly winds of 15 to 25 mph are
forecast in portions of southwest Nebraska. Dewpoints are expected to
range in the upper 40s to mid 50s, keeping minimum RH values from 22
to 29 percent. If lower dewpoints would occur, elevated or critical
fire weather conditions could develop.

&&

.CLIMATE...
Issued at 336 AM CDT Wed Sep 20 2017

The system this weekend has the potential to bring near record
producing rainfall for portions of southwest and north central
Nebraska. Current forecasts of 3 to 4 inches east of Highway 83
would result in the second highest two day rain totals in
September at North Platte, Valentine, and Broken Bow.

&&

.LBF WATCHES/WARNINGS/ADVISORIES...
Fire Weather Watch from Thursday afternoon through Thursday
evening for NEZ210.

&&

$$

SYNOPSIS...Roberg
SHORT TERM...Roberg
LONG TERM...Snively
AVIATION...Gomez
FIRE WEATHER...Roberg
CLIMATE...Snively



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