Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 241137
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
437 AM MST Fri Nov 24 2017

.SHORT TERM...(Today and tonight)
Issued at 317 AM MST Fri Nov 24 2017

High Wind Warnings remain in effect through 12 PM MST today. Winds
have been less than impressive so far, w/ most areas struggling to
achieve gusts of 50+ MPH. This may be resulting from a strong low-
level inversion or possibly only weak subsidence w/ better dynamic
support still lagging to the north. However, the latest GOES-16 WV
loop indicates a recent up-tick in mountain wave activity w/ quite
a few standing lenticulars developing along/E of the Laramie Range
and over the eastern slopes of the Snowies as well. The subsidence
situation should improve through 12-15z as the 120 knot 250 mb jet
pushes south. These factors suggest stronger winds should commence
shortly. In fact, we have seen a few gusts around 55 MPH in the S.
Laramie Range in the last hour due to the aforementioned mountain-
wave activity. Winds aloft remain very strong, upwards of 60 to 70
knots in the H7-H8 layer. In addition, forecast soundings continue
to show steep low-level lapse rates after 12z w/ 50-55 knots avail
to mix. Wave clouds may limit mixing, but any significant downward
momentum would easily yield 60+ MPH wind gusts. The tricky part of
the forecast here will be timing, as winds aloft are progged to be
decreasing significantly as low-lvl mixing improves. Gradients are
expected to decrease as well with the cold fropa. Warning-criteria
winds should end for all areas by 19z as 700 hpa flow decreases to
45 knots or lower.

A band of precipitation, likely in the form of rain is expected to
accompany the cold frontal passage during the morning and possibly
early afternoon hours. This may be enhanced somewhat by the upper-
level jet, so we might see some moderate rainfall over some of our
northern zones. H7 temperatures do drop considerably to -2C to -4C
over Converse/Niobrara counties, but temperatures upstream (behind
the front) are well above freezing at the surface. Snow should not
be a concern. Even if we do see a brief changeover, Douglas was 59
degrees at 11z so would expect minimal impacts.

.LONG TERM...(Saturday through Thursday)
Issued at 317 AM MST Fri Nov 24 2017

Amplified upper ridging will build over the Rockies through
Sunday. Relatively fast anticyclonic flow aloft will maintain
some lee troughing east of the Laramie Range and keep gradients
rather tight especially on Saturday. Could see windy conditions
continue in the wind prone areas on Saturday as CAG-CPR gradients
remain in the 40 to 50 meter range. It will be quite warm by
Sunday with H7 temps approaching 6-7C which is above the 95th
percentile for this time of year. Could see highs near or above
record levels once again on Sunday. Both the operational GFS and
EC continue to show lots of run to run inconsistency regarding the
evolution of the next deep upper trough progged to dig into the
western US early next week. Trend has been towards a more
elongated positively tilted trough with the energy splitting by
Monday evening and the southern end becoming more of a closed cut
off low over southern CO. This would keep the majority of the
precipitation south of the region as well as tone down the
strength of any bora wind event. Overall a pretty low confidence
forecast for Monday/Tuesday due to significant model variability.
With this said, there remains at least some potential for a
significant frontal passage with mountain snow and strong winds so
did not make too many changes to the long term forecast at this
time.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 1035 PM MST Thu Nov 23 2017

VFR will prevail through the period. Strong west winds will
continue over southeast Wyoming with gusts to 45kt likely between
12 and 18z for KCYS, KLAR, and KRWL. A cold front will cross the
area from the NW this afternoon bringing a chance of showers but
ceilings should remain VFR.


&&

.FIRE WEATHER...
Issued at 317 AM MST Fri Nov 24 2017

No fire weather concerns w/ humidity values remaining well outside
of critical thresholds. Periods of windy conditions will be common
over the next several days, however.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Warning until noon MST today for WYZ101-104>107-110-
     115>118.

NE...None.
&&

$$

SHORT TERM...CLH
LONG TERM...DEL
AVIATION...DEL
FIRE WEATHER...CLH



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