Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Cheyenne Wyoming
301 PM MDT Thursday Aug 21 2014
Short term...(this evening through Saturday night)
issued at 300 PM MDT Thursday Aug 21 2014
Current kcys radar loop shows a few showers across the area and no
thunderstorms at this time...which was expected due to increased
subsidence aloft. In addition...high clouds are moving northward
near the Colorado border which will suppress convection even more.
Kept pop near or below 20 percent for isolated showers and some
isolated thunder at times. Lowered pop to around 10 percent in the
lower valleys into this evening. Expect most of the thunderstorm
activity to be further north as a cool front drifts southward and
stalls across central Wyoming.
Models continue to show a change in the overall pattern across the
Rocky Mountain region for Friday and this weekend. All models
indicate a pretty strong cold front for this time of the year
pushing southward into southeast Wyoming. This front will stall
sometime on Friday as a wave of low pressure develops along the
front...and will be the main focus for scattered to numerous showers
and thunderstorms as an upper level trough from western Canada move
south and eventually eastward into Idaho. The main forecast
concern for Friday and Friday night is the potential for a steady
heavy rainfall as an upper level trough near the southern calif
coastline ejects northeastward across the area. The European model (ecmwf) and NAM
are the most aggressive with the feature showing widespread
showers and numerous thunderstorms developing across Colorado and
quickly moving northward into the forecast area. The GFS is the
least aggressive and shows the trough much weaker as it lift
northeast...and is barely noticeable by late Friday across the
plains. Increased pop up to 65 percent across the High
Plains...mostly over far eastern Wyoming and western Nebraska.
Rain showers and even isolated thunderstorms may continue well into the
overnight hours into early Saturday morning.
For Saturday...models show a brief break in the action by middle to
late morning as remnant shower activity moves into the Dakotas.
Shower and thunderstorms will quickly redevelop further west across
Wyoming/Utah and northwest Colorado as the main cold front and
pretty strong upper level trough moves eastward from Idaho into
Wyoming. Expect another round of showers and thunderstorms ahead and
along this front...which some decent rainfall amounts possible.
Increased pop between 40 to 60 percent across most areas Saturday
afternoon and evening. Clearing skies are expected after midnight
across southeast Wyoming with much cooler temperatures. A few
sheltered valleys may lower into the 30s Saturday night with
near freezing temperatures in the mountains.
Long term...(sunday through thursday)
issued at 330 PM MDT Thursday Aug 21 2014
Models very similar on movement of upper low into western Montana
early Sunday. Sunday could be a pretty breezy day as the low
placement in a favorable position for strong winds. In fact...the
European model (ecmwf) 700mb winds across southeast Wyoming are approaching
40kts...GFS a little lighter with 30-35kts. Should not be much in
the way of shower activity with downsloping winds off the Laramie
Next shortwave to impact the area comes in from the northwest
around that low Monday afternoon. Both the GFS and European model (ecmwf) showing
mountain showers initially. The European model (ecmwf) is forecasting a more
widespread showery event...though GFS does show an increase in
coverage Monday night. Pretty much kept what we had going for probability of precipitation
Monday with best chances in the mountains.
Discrepancies begin to increase after this shortwave as the European model (ecmwf)
slower in moving out this trough. By Tuesday afternoon...the European model (ecmwf)
shows the upper trough axis over extreme eastern Wyoming and the
GFS has it already moving into central Nebraska. With the slower
European model (ecmwf)...we really begin to see the discrepancies arise in 700mb
temperatures. GFS forecasting +4 to +8c over the County warning forecast area with the
European model (ecmwf) showing +10 to +12c. Did begin to go lower on temperatures
more in line with the GFS. Undercut extended procedure guidance by
3-4 degrees to come more in line with the GFS/mex guidance.
Looks fairy dry Wednesday afternoon and into Thursday with the
trough moving off to the east.
Aviation...(for the 18z tafs through 18z Friday morning)
issued at 1039 am MDT Thursday Aug 21 2014
Convection looks to be confined to extreme southern Wyoming and
the southern Nebraska Panhandle today. Used timing from latest
hrrr forecast that shows kcys and possibly kbff and ksny seeing some
convection late this afternoon. VFR conditions otherwise outside
issued at 205 PM MDT Thursday Aug 21 2014
Minimal fire weather concerns through early next week.
Subtropical moisture will remain over the area through the weekend
with showers and thunderstorms possible each day across southeast
Wyoming and western Nebraska. There is a good chance of wetting
rains on Friday through this weekend as showers and thunderstorm
coverage will be higher. Temperatures will trend cooler into this
weekend as a strong cold front for this time of the year moves
into the region late on Saturday. This unsettled weather is
expected to continue into early next week.