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Area Forecast Discussion (AFD)

Issued by Cheyenne, WY (CYS)

FXUS65 KCYS 201002

Area Forecast Discussion
National Weather Service Cheyenne WY
402 AM MDT Tue Aug 20 2019

.SHORT TERM...(Today through Wednesday night) 
Issued at 410 AM MDT Tue Aug 20 2019

Partly cloudy skies have been observed over the area, with some
fog and low clouds observed across the Nebraska panhandle early 
this morning. Thankfully, fog and low clouds have been pretty 
isolated and patchy so far. The primary forecast concern today 
will be the potential for severe thunderstorms east/along the
Laramie Range and I-25 corridor.

Surface cold front which is forecast to push into the area later
this afternoon is currently near the Montana/Wyoming border. 
Expect this front to slowly move southward at first, and then 
accelerate sometime this afternoon across the plains. In addition,
a weak upper level shortwave is currently near the four corners 
region based on midlevel watervapor imagery. Most models show this
feature lifting northeast into southeast Wyoming by this evening.
Both these features, along with plentiful llvl moisture, will be 
the primary triggers for late day or nocturnal convection today. 
Models continue to indicate MLCAPE between 2500 to 3500 j/kg along
and east of I-25, with even a few areas around 4000 j/kg. 0-6 
Bulk Shear sits between 30 to 45 knots through the late afternoon 
and evening hours. Models indicate widely scattered or scattered 
thunderstorms through late this evening, but are struggling on 
where they will develop and how they will propagate, mainly due to
limited confidence with available dynamic forcing present today 
as well as if the shortwave remains intact as it moves into 
Wyoming. Added severe weather wording to the forecast, especially 
far southeast Wyoming and most of the western Nebraska panhandle. 
00z model soundings showing a significant large hail threat where 
storms develop, with the potential for 2.0 inch diameter hail or 
larger. Strong winds and frequent cloud to ground lightning are 
also a threat. These thunderstorms may linger past midnight 
tonight across the eastern plains. Kept POP above 30 percent past 
midnight for scattered showers and thunderstorms. 

Cooler temperatures will move into southeast Wyoming and western
Nebraska for Wednesday with highs in the mid 70s to low 80s
outside of Carbon county (which will be warmer). There is a good
chance for additional thunderstorms on Wednesday even with the
cooler temperatures. Although the severe weather threat will be
limited, there is the potential for numerous thunderstorms in the
afternoon and evening hours as a stronger shortwave aloft moves
southeast into Colorado. Can not rule out some fog Wednesday
morning, but models are not in good agreement outside of the I-80
Summit at this time. 

.LONG TERM...(Thursday through next Monday night) 
Issued at 335 AM MDT Tue Aug 20 2019

Models continue to be in reasonable agreement through the weekend,
and are now coming into better agreement regarding a pretty strong
cold front for the last week of August arriving sometime next

Models continue to show the slow moving Pacific upper level trough
pushing east across Montana and northern Wyoming Thursday and
Thursday night. The GFS and Canadian are the slowest and show a
decent chance of thunderstorms across the mountains and the
eastern plains through the day and into the evening hours. Decided
to increase POP a bit for these areas into Thursday evening.
Thankfully, shear looks marginal so the severe weather threat is
pretty low. Once this Pacific system moves east, all models show
drier weather across the area during the weekend along with some
pretty warm temperatures in the 80s to low 90s. May need to
increase high temperatures even more on Sunday with most models
showing 700mb temperatures above 16c-18c. Further out, all models
are showing a strong cold front moving into the Pacific NW and the
northern plains, with 700mb temperatures dropping to near zero
early to mid next week. However, models are not in good agreement
regarding the timing of this front with the ECMWF 18 hours later
than the GFS and the Canadian is even later than that. In
addition, some models are showing some pretty cold 700mb
temperatures behind this front, with values as low as -5 across
northern Wyoming and Montana. This bares watching as there is
reasonable agreement with the ensemble forecasts 7 to 9 days out.


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 958 PM MDT Mon Aug 19 2019

Wyoming TAFS...VFR. Thunderstorms in the vicinity at Laramie and
Cheyenne after 01Z. Wind gusts to 23 knots from 15Z to 01Z.

Nebraska TAFS...VFR, except for IFR from 09Z to 15Z. Thunderstorms
in the vicinity after 01Z. Wind gusts to 27 knots from 15Z to 01Z.


Issued at 314 AM MDT Tue Aug 20 2019

Elevated fire weather concerns today with lowering concerns
through the rest of the week. A surface cold front will push 
across the eastern plains of southeast Wyoming and western 
Nebraska this afternoon, resulting in higher humidities and a good
chance for thunderstorms through tonight. This front is expected 
to stall across the mountains. West of the front, expected low 
relative humidities, but also for a few isolated dry thunderstorms
as well across mainly zones 303, 306, and 308 this evening.
Thankfully, winds are expected to be relatively light, so will
hold off on any last minute Red Flag headlines at this time. 
Higher humidities and a better chance for rainfall is expected 
Wednesday and Thursday.