Area Forecast Discussion
Issued by NWS North Platte, NE

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FXUS63 KLBF 210900
AFDLBF

Area Forecast Discussion
National Weather Service North Platte NE
400 AM CDT Thu Sep 21 2017

.SYNOPSIS...
Issued at 359 AM CDT Thu Sep 21 2017

H5 analysis tonight had a broad trough of low pressure across the
western half of the CONUS. An elongated area of low pressure
extended from northern Manitoba into Washington and Oregon. Broad
southwesterly flow aloft extended from southern California newd
into the central plains and upper midwest. High pressure was
located over the lower Ohio valley while TS Jose was located
roughly 200 miles southeast of Cape Cod MA. At the surface, high
pressure was located over northern Minnesota while a trough of low
pressure extended from eastern Wyoming into eastern Colorado. A
nice pressure gradient existed across the Nebraska panhandle
overnight and had led to gusty southerly winds of 15 to 25 MPH in
the eastern panhandle with higher wind gusts noted in the northern
panhandle. Readings as of 3 AM CDT ranged from 45 at Broken Bow,
to 61 at Gordon.

&&

.SHORT TERM...(Today and tonight)
Issued at 359 AM CDT Thu Sep 21 2017

Temperatures today and fire weather concerns are the main
forecast challenges over the next 24 hours. Low pressure will
deepen across northeastern Colorado this afternoon. Southerly
winds will increase with the highest speeds across the eastern
half of the forecast area. Across northwestern areas, a inverted
trough of low pressure will extend from northeastern Colorado into
the northwestern Sandhills. This will lead to lighter winds in
the northwestern CWA. Further south, gusty winds are expected
across southwestern Nebraska underneath of low level thermal
ridge. H85 temps in southwestern Nebraska will approach 29C this
afternoon with NAM12 forecast soundings showing a well mixed layer
up to 600 MB in swrn Nebraska. With 25 to 30 KT winds at this
level, sfc gust potential to 30 MPH looks probable across swrn
Nebraska. That being said, wit highs in the lower 90s, afternoon
dew points in the 30s and expected winds of 20 to 30 MPH, will
leave the inherited red flag warning intact for zone 210. There
were some initial concerns about adding zone 219 to the red flag
warning as well. However, minimum rh never reaches 15 percent, but
is closer to 20 percent later this afternoon in zone 219. That
being said, will leave out zone 219 from any fire headlines attm.
Will highlight this area in the HWO however, as near critical fire
weather conds are expected. Southerly winds tonight will persist
in the eastern forecast area and will lead to mild temperatures
overnight. Lows will range from the upper 40s in the west with
light winds, to around 70 in the far eastern forecast area, where
southerly winds of 10 to 20 MPH will persist into the overnight.

.LONG TERM...(Friday through Wednesday)
Issued at 359 AM CDT Thu Sep 21 2017

An H5 closed low drops south toward the Four Corners region this
weekend as a large trough digs across the West. The low fills
somewhat as it turns northeast, brushing the Black Hills by Tuesday.
Meanwhile, the amplified ridge over the Eastern U.S. holds its
ground through the weekend before slowly flattening. At the surface,
a cold front drops south across the Sandhills through the day on
Friday and stalls Saturday, generally lining up from MCK to OFK.
High pressure finally kicks it out of the area by Tuesday.

The main forecast challenges during the period include near critical
fire weather SW Neb Friday, severe potential NC Neb Friday evening,
and temperatures/heavy rain over the weekend.

Friday... The models are in agreement with the cold front bisecting
the forecast area generally from IML to ONL at 00z Sat. Along and
ahead of the front, increased wind speeds as H85 flow approaches 35
kts and soundings indicate a well mixed layer. Moisture really never
reaches far SW Neb invof IML, allowing RHs to drop to around 20% at
peak heating. Humidity is the limiting factor for Red Flag criteria
attm, but will definitely have to watch. Temperature wise, bumped
max temps up toward the warmer MAV guidance as ample sunshine is
expected early, veering low level winds indicate WAA, and NAEFS
ensembles suggest 97%ile H85 temps. Delayed PoP until 21z for
convective initiation along the front. Forecast soundings suggest
MUCAPE around 1000j/kg in the evening along with 45+ kts 0-6km
shear, capable of severe storms. The 110+ kt. H25 jet streak rounds
the base of the trough and ejects northeast across the Rockies
during this time. Veering continues in the 0-3km layer, however
water loading and storm motion (nearly parallel to the boundary)
would favor damaging winds as the primary threat over large hail,
especially later in the evening. Abnormally high PWAT and training
convection increase the heavy rain threat.

Saturday through Monday... The start of the heaviest rain has slowed
slightly, with the most widespread precip beginning after 00z Sun.
Still looking at impressive rain totals with a sfc high over the
southeast U.S. continuously pumping gulf moisture into the area.
NAEFS ensembles hit Sat. night the hardest over western/central Neb.
with PWAT values and water vapor transport >99%ile of norms. WPC now
includes a slight risk of excessive rainfall for areas east of the
panhandle Sat AM - Sun AM, and CPC continues to delineate a heavy
rain threat thereafter. Forecast totals remain largely unchanged
from previous forecast package with widespread 3+ in. east of Hwy
83. Southerly H85 flow still takes place Saturday across central Neb
along with temps near 15C, so highs will likely reach the 70s
despite the clouds/precip. Strong CAA takes over Sunday, dropping
highs into the 50s across the northwest half of the CWA and lower
60s elsewhere. Continued clouds and remnant showers, H85 temps only
around 5C, and brisk north winds will keep temps in the 50s for
virtually the entire area on Monday.

Tuesday and Wednesday... High pressure settles onto the northern
Plains as the upper flow turns quasi zonal. Temperatures moderate
slowly while precip chances remain low.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1125 PM CDT Wed Sep 20 2017

Latest satellite imagery shows a swath of high clouds over west
central NEB, otherwise skies are clear. Surface plot shows
generally light breezes occurring, strongest in the far west and
far north (up to 14 kts). VFR conditions will prevail through the
TAF period along with mostly clear skies. A modest low-level jet
overnight will prompt LLWS conditions across portions of western
NEB, strongest in far northern and northwest NEB. Upper height of
the wind shear layer is forecast about 1500 ft AGL with winds at
the top of the layer up to 45 kts. LLWS conditions diminishing by
mid morning. Increasing southerly winds tomorrow afternoon with
gusts up to 25 kts, strongest outside the eastern Panhandle.


&&

.LBF WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from noon today to 7 PM MDT this evening for
NEZ210.

&&

$$

SYNOPSIS...Buttler
SHORT TERM...Buttler
LONG TERM...Snively
AVIATION...ET



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