Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 210112 AAA
AFDCYS

Area Forecast Discussion...UPDATED
National Weather Service Cheyenne WY
712 PM MDT Wed Sep 20 2017

.UPDATE...
Issued at 712 PM MDT Wed Sep 20 2017

Humidity values are climbing above 25 percent across east central
Wyoming and the northern Nebraska Panhandle with winds dropping
below 20 kts most locations. Therefore allowed the Red Flag
Warning to expire on time across those zones. Minor adjustments to
forecast parameters this evening. Otherwise expecting mostly clear
skies this evening to give way to some high clouds overspeading
the area toward morning.

&&

.SHORT TERM...(This evening through Friday)
Issued at 311 PM MDT Wed Sep 20 2017

Warm and dry pattern will be in place through much of the short
term as a deep upper trough digs across the Great Basin, and
increasing southwest flow aloft ahead of it over the CWA. Will
keep the Red Flag Warning going through this aftn with many
locations over fire weather zones 301 and 311 showing RH values of
15-20 percent and gusts to 25-30 kts. Temps will increase back to
above normal on Thursday as 700mb temps rise to 10-12C by the
aftn. It will be another precip free day with the main concern
being critical fire weather conditions in areas near the Colorado
Border. Southwest winds will be strongest over Albany/Converse
counties as 700 mb winds increase to 35-40 kts. With plenty of dry
air aloft and good mixing, min RH values will drop to 13-20
percent over most lower elevations. Decided to issue a Fire
Weather Watch for zones 308/309 on Thursday aftn. Will have to
watch winds across zone 310 also, but it looks like gusts should
mostly remain below 20 kts. Shortwave trough will lift
northeastward into western Montana on Friday. Will keep PoPs in
the 20-40 percent range for late Friday with the best ascent to
the northwest of the region.

.LONG TERM...(Friday night through Wednesday)
Issued at 311 PM MDT Wed Sep 20 2017

Large scale and anomalously deep upper trough digs into the
central/southern Rockies on Friday night. As the long wave feature
remains nearly stationary through Tuesday, significant shortwave
energy will eject out over the northern Rockies and high plains
through Monday. At the surface, a strong cold front will push
south across the area on Friday night, turning surface winds
northeasterly and inducing a cool wedge along and east of the
Laramie Range.

A strong shortwave will eject NE across the area Saturday into
Saturday night.  The presence of the low level baroclinic zone
associated warm advection/overrunning will provide excellent
support for significant precipitation across much of the area.  Snow
levels will be high enough so that most precipitation will fall as
rain except for the higher elevations of the Snowy and Sierra Madre
Ranges.  It should be noted that the NAM does have the low levels
much colder during this time frame along and just east of the
Laramie Range which would indicate more potential for snow. For now
am discounting that solution since it does not have much support
from the other guidance.  May eventually need some winter headlines
for the mountains however as snow seems a good bet there. After a
break on Sunday morning, models are in good agreement on yet another
strong shortwave traversing the area Sunday night into Monday
morning. Snow levels will slowly fall during the day Sunday and by
Monday morning will be at 6500 to 7500 feet so some snow will be
likely in the higher valleys of Southeast Wyoming possibly even into
the Cheyenne area. Still too early to pin down amounts or timing at
this time.

Models continue to show long wave troughing of some sort persisting
and possibly developing into a closed low over the SW US by Tuesday
into Wednesday which would favor the general cool weather pattern
continuing.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 537 PM MDT Wed Sep 20 2017

We are expecting the gusty southwesterly to southerly winds to
decouple and ease a bit most airports with the exception of
Chadron where the low level nocturnal jet will keep mixing going
late tonight and gusts to near 30 kts expected late evening into
the early morning. Otherwise VFR expected overnight with no
ceiling or visibility concerns.


&&

.FIRE WEATHER...
Issued at 311 PM MDT Wed Sep 20 2017

A Red Flag Warning remains in effect through the early evening
across fire weather zones 301 and 311.  An elevated to locally
critical fire weather threat will occur over the lower elevations of
southeast Wyoming on Thursday afternoon. Southwest winds will be
breezy with humidity values dropping to 15-20 percent.  A Fire
Weather Watch has been issued for fire weather zones 308 and 309.
Cooler temps along with increasing chances of precipitation will end
any fire weather concerns from Friday into early next week.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Fire Weather Watch from Thursday morning through Thursday
     evening for WYZ308-309.

NE...None.
&&

$$

UPDATE...JG
SHORT TERM...ZF
LONG TERM...DEL
AVIATION...JG
FIRE WEATHER...ZF



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