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Area forecast discussion 
National Weather Service Cheyenne Wyoming
627 am MDT Wednesday may 6 2015

Short term...(today through friday)
issued at 315 am MDT Wednesday may 6 2015

An area of light rain is currently located from Laramie County
northward into southern Niobrara and Converse counties. This is
associated with a deformation area on the northwest side of the
midlvl low that is over northeast Colorado. Not expecting this
band to move much through the this will be the focus
of the highest rainfall amounts through 18z. It will remain a
steady light rain with little chance of any thunderstorms...but still
could see widespread amounts of 0.25-0.50 inches along and east of
the Laramie range in southeast Wyoming. The upper level trough axis
will shift northeastward into northwest Nebraska by this afternoon.
Thus...this area of enhanced vertical ascent will shift into the
northern Nebraska Panhandle after 18z. We may actually see some
peaks of sunshine over southeast Wyoming by midday and both the NAM
and GFS show weak instability developing during the afternoon. Could
see an isolated thunderstorm with brief heavy rain. Front will push across
the region during the afternoon and evening with a drier airmass behind
it and precipitation chances will really decrease after the late evening.

With decreasing moisture levels and a generally stable
airmass...will see a decrease in shower coverage for Thursday and
Friday...most notable to the east of the Laramie range. Areas
along the Colorado border in southeast Wyoming will see the best
instability on Thursday afternoon (cape values of 300-600 j/kg) so scattered
storms will develop there. The combination of low pressure over
the Great Basin and higher pressures across the northern plains
will maintain northeast to east upslope along and east of the
Laramie range. Sref guidance suggests that areas of fog could be a
concern along the Cheyenne Ridge. Widespread and persistent low
clouds will keep highs in the middle 40s to low 50s. Once again...the
best chance of showers and thunderstorms will exist along and west of the
Laramie range through the afternoon...and then potentially spread
eastward to the plains by the evening.

Long term...(friday night through tuesday)
issued at 315 am MDT Wednesday may 6 2015

Confidence is growing that most of the area will be dealing with a
major winter storm this weekend. Forecast challenges deal with
snow levels and amounts along with timing of rain.

Starting off Friday night...area of surface low pressure will
continue to remain stationary along the Texas/Oklahoma
Panhandle...through central Colorado...up into western Wyoming.
Persistent southeast upslope winds continue to pump moisture into
the Front Range and into southeast Wyoming. The County warning forecast area is square in
the area of upper level diffluence at 500mbs ahead of an upper
level low positioned over southern Utah, we should be seeing
widespread rain Friday night into Saturday.

GFS and European model (ecmwf) remain very similar in the movement of the upper low
into western Colorado Saturday. Widespread moderate rainfall
appears likely for areas along and east of the Laramie range. Very
strong 700mb southeast upsloping winds Saturday afternoon on the
order of 40 to 45kts will enhance rain amounts along the eastern
foothills and possibly the eastern plains of southeast Wyoming.
Went ahead and added possibility of moderate rains in the forecast
for Saturday afternoon.

Colder 700mb temperatures remain on track for Saturday night into
Sunday European model (ecmwf) showing -4 to -6c across our northern zones at
00z...decreasing to -6 to -8c Sunday morning. Northeast 850mb
winds increase to 45-50kts on the European model (ecmwf) as the upper low tracks
into northeast Colorado. Added chances for moderate and heavy snow
into the forecast for Saturday night and Sunday as freezing levels
drop to 5500 feet and possibly lower during the day Sunday. Will
issue an Special Weather Statement on this upcoming event to alert customers of
significant winter weather.

Continued to cut guidance significantly on temperatures Sunday as
the snow will start Saturday night and envision it lasting most of
the day Sunday. Will probably struggle to get out of the 30s for
much of southeast Wyoming...with upper 30s/low 40s in the
Panhandle. Both the GFS and European model (ecmwf) very slow moving the upper level
low snow will continue into Sunday evening.

Forecast quantitative precipitation forecast guidance is scary...with both the GFS and European model (ecmwf)
showing 1.5-2.0 inches of liquid Saturday night through the day
Sunday. Even a 10 to 1 ratio on snow would produce significant
snowfall. Then add 35 to 40 miles per hour northeast winds on top of that and
we could be looking at a very dangerous condition.

By Monday morning...European model (ecmwf) has the upper low in eastern South
Dakota with 700mb temperatures of -8 to -10c over southeast
Wyoming. Gusty northwest winds appear likely Monday morning that
will continue to create areas of blowing snow.

Since the event is still 4 days out...conditions could change. The
low could track further north and then we would be dry slotted. If
current tracks though from the GFS and European model (ecmwf) continue as they have
the past several days believe this could be the
biggest snow storm that we have seen for quite a while here in
southeast Wyoming. Could be looking at blizzard conditions
Saturday night...all day Sunday and possibly continuing into
Monday morning.


Aviation...(for the 12z tafs through 12z Thursday morning)
issued at 626 am MDT Wednesday may 6 2015

Widespread IFR/LIFR to continue for much of our southeast Wyoming
and Nebraska Panhandle airports. Lack of available hrrr guidance
has US at less than high confidence. Persistence would indicate
low conditions to continue with southeast upslope flow continuing.
At least through the 18z timeframe.


Fire weather...
issued at 315 am MDT Wednesday may 6 2015

A cool and wet weather pattern will keep fire weather concerns very
low through the weekend. There will be a good chance of measurable
rainfall through this evening. A strong storm system will likely
impact the region by Saturday and Sunday with potentially widespread
and large precipitation amounts. Minimum relative humidity values
will be well above critical thresholds through the period.


Cys watches/warnings/advisories...


Short term...zf
long term...gcc
fire weather...zf

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