Area Forecast Discussion
Issued by NWS North Platte, NE

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FXUS63 KLBF 212317

Area Forecast Discussion
National Weather Service North Platte NE
617 PM CDT Tue Sep 21 2021

Issued at 330 PM CDT Tue Sep 21 2021

Early morning WV imagery had a decent mid/upper PV anomaly and
associated tropopause undulation moving from northeast CO into
northwest KS. 12Z LBF sounding data confirms the lowered tropopause
as the upper short wave moved overhead this morning. Further north,
another smaller circulation is visible in northern SD and these are
all moving eastward within the progressive upper long wave trough
across the northern Plains. Weak but efficient larger scale QG
forcing was able to take advantage of better lapse rates aloft to
generate scattered rain showers early this morning however these
have moved south of the forecast area through the morning hours.
GOES 17 full disk shows the progressive long wave pattern well,
punctuated by the deep cyclone near the tip of the Aleutian Islands
and a flat ridge immediately downstream, then the short wave and
building ridge over the northeast Pacific.


.SHORT TERM...(This evening through Wednesday night)
Issued at 330 PM CDT Tue Sep 21 2021

The main forecast issue in the short term is how far temps drop
overnight tonight prior to daybreak.

With the exiting short wave there will be mainly sunny skies through
the day. Northerly winds will lead to slightly drier low levels and
gradual isentropic descent along with larger scale subsidence as
height rises on the order of 100m slide through the western High
Plains overnight. Surface high pressure will slide southeast as well
by evening and will place the western third of NE back into
southerly return flow after midnight. The cumulus that developed
through the day should quickly errode by evening leaving a good
radiational cooling setup at least for the first half of the night.
With afternoon dew point temps in the 30s the stage will be set for
some areas of the Platte River valley to overnight lows dip into the
30s for the first time this fall. While patchy frost is certainly
possible in these areas, confidence is not high on spatial extent of
that. The lowest 5 percent min temperatures of all model ensemble
members would suggest low temps only in the 37/38 degree range
and very low probabilities of seeing temps as low as 35 in
selected locations within the Platte River valley. At this point
no headline is needed for frost or freeze, but the forecast will
lean heavily toward the lower (colder) end of the ensemble
envelope as early fall extremes are sometimes such outliers that
model guidance has difficulty identifying them.

Dry conditions continue for Wednesday, the first day of astronomical
fall. The aforementioned upper ridge currently located across the
PacNW will translate to a position along the spine of the Rockies
putting the western half of NE in northwest flow aloft and
isentropic downglide leading to mostlyclear skies. The return flow
from the exiting surface high pressure and lowering surface pressure
in the lee of the northern Rockies will create increasing southerly
flow and warm advection taking us back to near average for this time
of year.

Wednesday night will not be as cool as tonight owing to the building
ridge aloft and WAA as advertised by the good LLJ that develops just
off the surface Wednesday night.

.LONG TERM...(Thursday through Tuesday)
Issued at 330 PM CDT Tue Sep 21 2021

The long range remains progressive into the weekend though for
the most part dry as moisture availability looks limited. A longer
term trend of above average temperatures with dry conditions could
lead to fire weather concerns Thursday, then again this weekend
into next week.

As the next short wave moves through the northern Rockies Thursday a
thermal ridge will develop ahead of it bringing temps back above
average for this time of year once again. This breakdown of the
upper ridge will be followed by the passage of an associated surface
cold front Thursday afternoon and evening. Deterministic solutions
are mostly dry with this system as moisture availability will be
low, especially in the low levels with a downslope component to the
winds prior to frontal passage. This may instead be a potential fire
weather concern as the brekadown of an upper ridge, especially in
the transition seasons such as fall, is a locally studied pattern
favorable for low dew point temps and the potential for gusty
winds. Will elaborate on this in the next few shifts as details
become more clear but it is worth watching for sure. See Fire
weather section for more details.

The upper PV anomaly moves quickly through southern Canada and into
the Great Lakes this weekend, merging with the remains of the now
exiting system across the MO Valley, to carve out a more robust
negatively tilted long wave for the northeastern US to contend with.
For the Plains, the upper flow will transition briefly to zonal with
another ridge building in for the first of the week. All of this
points to a perpetuation of dry conditions across the area with
above average temps.


.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 617 PM CDT Tue Sep 21 2021

VFR conditions will continue the next 24 hours with a clear sky
tonight and Wednesday. Winds will become light south to southwest
later tonight, increasing to near 20012KT after 15Z Wednesday.


Issued at 330 PM CDT Tue Sep 21 2021

Thursday could be a potentially concerning day for fire weather
conditions as a thermal ridge builds into the area ahead of the
next advancing cold front which should move through the area
Thursday evening. As the upper ridge breaks down, west to
southwest winds will advect drier air into portions of fire zones
204/206 and possibly 209. Confidence is low with respect to wind
speeds increasing to above 25 mph at this time and also on the
overlap of higher winds with low relative humidities. However, as
mentioned, this pattern of the upper ridge breaking down favors
drier low levels, often moreso than models tends to pick up on
this far out. So this will be watched in the next day for more
details to emerge.




SHORT TERM...Stoppkotte
LONG TERM...Stoppkotte
FIRE WEATHER...Stoppkotte is the U.S. government's official web portal to all federal, state and local government web resources and services.