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Area forecast discussion 
National Weather Service Cheyenne Wyoming
846 PM MDT Wednesday may 6 2015

issued at 842 PM MDT Wednesday may 6 2015

Inherited gridded forecasts in nice shape for the rest of the
night. Made some minor changes based on radar reflectivity
and prognosticated 00z model guidance. Based on temperature and dewpoint
spreads...and forecast boundary layer relative humidities...
believe most locations will see areas of fog later tonight
especially where rain has fallen and we may see some areas of
dense fog over the Interstate 80 Summit between Laramie and


Short term...(this evening through Friday night)
issued at 300 PM MDT Wednesday may 6 2015

Afternoon water vapor loop portrayed a highly amplified large
scale pattern across the lower 48. Pesky upper trough over the
western High Plains continues to produce a wide variety of
inclement weather from the central rockies to northern and
Central Plains. A more significant upper trough was digging
south through the Pacific northwest. Most of the tstorm activity
has been well south and east of the County Warning Area today...with generally
cool and showery conditions. The surface analysis showed a quasi-
stationary front from southern Wyoming across South Dakota and northern Iowa. An
inverted surface trough extended north from a low over northeast
Colorado into the western Dakotas.

The 12z deterministic model guidance is in good agreement progging
the upper trough over the Central Plains to weaken as it lifts
northeast across the eastern Dakotas and upper Midwest through
Thursday night. Meanwhile...the upper trough over the northwest
intensifies as it digs further south into California during the next 24
hours...then gradually moves east toward The Four Corners Friday
night. The flow aloft will continue from the southwest...transporting
subtropical moisture into the County Warning Area. A series of weak upper level
disturbances will interact with the moisture to keep shower
chances going through the short term...with quantitative precipitation forecast amounts averaging
between 0.25" and 0.75". Afternoon/early evening tstorm activity
will remain isolated and generally confined to the southern
portion of the County Warning Area Thursday...and mainly west of the Laramie range
Friday. Fog potential will be minimized by a mixed boundary layer
tonight...but mentioned patchy fog from The Summit across the
southeast Wyoming plains and southern NE Panhandle. Near seasonal
temperatures will prevail through Thursday...then cool below
normal Thursday night and Friday with northeasterly flow.
Precipitation will begin to transition to a rain/snow mix Friday
night along and west of the Laramie range as snow levels drop to
around 8000 feet.

Long term...(saturday through wednesday)
issued at 300 PM MDT Wednesday may 6 2015

Attention will be focused on the upper low that will eject
northeast from The Four Corners area Saturday into the central and
northern plains on Sunday. By far main problems will be quantitative precipitation forecast and
snowfall amounts...and the snow levels.

Models continue to be in fairly good agreement with the evolution
of this system. Latest runs have nudged the system a bit faster
across the area but still very high quantitative precipitation forecast forecasted for much of the
County Warning Area. Lee cyclone will be deepening somewhere over eastern Colorado
Saturday with GFS farther north that the ec. Regardless...colder
air will be pulled south across the County Warning Area Saturday with rain likely
changing to snow at increasingly lower elevations during the day
as middle level temperatures fall with the approach of the upper low.
Precipitation should be all snow over much of southeast Wyoming Sat night
into Sunday as 700mb temperatures fall to -5 to -10c by Sunday morning...
with the Panhandle somewhat more delayed in changeover Sat night
as 700mb temperatures cool later. Gusty surface winds will likely add to
problems Sat night and Sunday. Snowfall amounts hard to project at
this time due to uncertainty of the track of the system and how
much quantitative precipitation forecast will occur, but amounts over a foot look a decent bet
over the mountains/Laramie ranges with even higher amounts quite
possible. Could very well be the most significant winter storm of
the may! Definitely looking like another white mothers
day for at least parts of the County Warning Area.

Snow likely to continue across the County Warning Area through much of Sunday as
the surface/upper lows edge slowly NE across Nebraska with precipitation
expected to end late Sunday as the system moves even farther away.
Mainly dry and cool Monday then moderating temperatures Tuesday and Wednesday
with some mainly mountain showers possible in association with a couple
weak impulses riding across the area.


Aviation...(for the 00z tafs through 00z Thursday evening)
issued at 541 PM MDT Wednesday may 6 2015

The main concern with this taf issuance is the potential for MVFR
ceilings tonight.

Latest water vapor loop was showing the upper level low spinning
across Nebraska with the surface low over the Nebraska Panhandle.
Meanwhile...high pressure has been trying to nose into southeast
Wyoming. This high pressure is inducing decent pressure rises
across this region as well. Latest radar loop was showing a
cluster of showers moving in from the Laramie range. This activity
should continue to spread east during the next couple of hours and
bring some rainfall and lower MVFR ceilings for the cys taf site
during the next 2 to 3 hours. Meanwhile...the lar/rwl taf sites
should see the ceilings climb to VFR through the remainder of the
evening. Meanwhile...the Panhandle taf sites will see the
scattered showers persist through the evening with the ceilings
fluctuating between MVFR and VFR. This shower activity should
begin to clear out Thursday morning before more shower activity
develops during the afternoon.


Fire weather...
issued at 300 PM MDT Wednesday may 6 2015

The cool and set weather pattern will continue through the end of
the week...with good chances for measurable rainfall. A strong
storm system will impact the districts this weekend with potentially
widespread and heavy precipitation well as gusty
northerly winds. This system will be cold enough...with snow levels
dropping to around 5500 feet. Minimum relative humidity values will
remain well above critical thresholds through the forecast period.
A trend toward warmer and drier conditions will commence next week.


Cys watches/warnings/advisories...


short term...mj
fire weather...mj

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