Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Cheyenne Wyoming
906 PM MDT Tuesday Jul 29 2014
issued at 900 PM MDT Tuesday Jul 29 2014
Evening water vapor imagery depicted a upper level low over north
central Colorado. Regional radars detected widespread shower/isolated
tstorm coverage. Convection was propagating slowly to the east
and southeast due to weak steering winds aloft. Runoff from heavy
rainfall /one to two inches per hour/ with some of the more
persistent convection caused flooding in parts of Albany...Carbon
and Platte counties during the late afternoon and early evening.
Flash Flood Watch remains in effect through late tonight for much
of southeast Wyoming as any additional rainfall on already saturated
ground will result in further runoff. For the evening update...
tweaked probability of precipitation based on radar and hrrr model trends.
Short term...(tonight through Thursday
night) issued at 200 PM MDT Tuesday Jul 29 2014
Heavy rain event just starting with convection developing over
Albany County near Laramie and up near Shirley Basin at noon
today. Storms are very efficient rain producers. Please see Hydro
section below. Forecast challenges deal with timing/location of
heavy rainfall through Wednesday afternoon.
Currently...water vapor showing upper shortwave moving into
northeastern Utah this afternoon. Surface analysis showing
stationary front laying across southwestern Wyoming to just north
of Rawlins...then southeast by Saratoga...into northeastern
Colorado. Mosaic radar shows most of western Wyoming under
widespread rain with northern Colorado filling in the past few
images. Storms that have developed in the County warning forecast area are very slow
moving...with a general west to northwest movement of 5-10kt or
Guidance continues to fill in our western counties west of the
Laramie range this afternoon...spilling over into the east slopes
after 22z today. Precipitable waters here at Cheyenne continue to climb this
afternoon...up to 1.36 inches by middle evening. Very slow movement
on cells..under 10kt this afternoon and evening.
Very strong upsloping winds this evening. GFS and NAM showing 20kt
at 700mbs this evening. So we are going to have a pretty good
upsloping event for areas east of the Laramie range. Think that we
will see a fairly widespread stratiform event with moderate
rain...maybe some isolated thunderstorms embedded in the stratus.
Went ahead and upped probability of precipitation and continued heavy rain through
Wednesday afternoon. Strong shortwave moves through into Colorado
Wednesday night with rain ending from north to south. Rain to come
to an end by midnight Thursday morning.
Long term...(friday through tuesday)
issued at 310 PM MDT Tuesday Jul 29 2014
Long range models show little change to the overall weather
pattern into next week...with a broad upper level ridge over the
western third of the United States and a large scale trough over
the other two thirds of the country. With the exception of the
relatively strong trough over the eastern United States...this
pattern is typical for early August with monsoon flow over the
west. Therefore...can not rule out a daily chance of showers and
thunderstorms across most of the forecast area through the weekend.
Models show precipitable water/S increasing over one inch across the forecast
area this weekend with decent low level instability. Weak shortwave
energy may enhance the potential for showers and thunderstorms as well
Friday and again on Sunday. Kept pop between 20 to 40 percent
mainly across southeast Wyoming...with values below 20 percent
across western Nebraska due to dry air aloft. Temperatures will be
near or slightly below normal with highs in the 70s to middle 80s.
Sunday appears to be the warmest day ahead of a weak cool
front...with highs in the 80s to near 90 possible.
Precipitation and tstorm chances increase early next week with all
models showing a strong shortwave moving clockwise around the
upper level high. The axis of monsoon moisture will be right along
the Front Range by Monday and Tuesday. The European model (ecmwf) shows this
shortwave becoming nearly stationary across the area as well.
Continued to increase pop early next week with a favorable heavy
rainfall pattern with east to southeast upslope flow and good
instability along with plenty of moisture. A weak cool front will
push across the region...which will result in cooler than normal
temperatures over the area. Lowered temperatures on Tuesday...into
the middle to upper 70s.
Aviation...(for the 00z tafs through 00z Wednesday afternoon)
issued at 525 PM MDT Tuesday Jul 29 2014
Numerous showers and scattered thunderstorms...some producing heavy
rain...will continue tonight. The exception will be kcdr and kaia
which will likely be north of the heaviest rainfall. Thunderstorms
will become isolated after midnight. Expect MVFR to IFR conditions
due to heavy rain...with the possibility for fog and low ceilings
near the Laramie range. Showers will persist through Wednesday
morning. Low level winds will increase near the Laramie range as a
700mb low develops near the Colorado/Wyoming border tonight. This
may result in strengthening surface winds and wind driven rain
from kcys to krwl.
issued at 200 PM MDT Tuesday Jul 29 2014
Minimal fire weather concerns. A strong storm system combined
with monsoon moisture will give way to widespread showers and
thunderstorms on Tuesday and Wednesday. The greatest coverage will
be along and on either side of the Laramie range where a general 1
to 2 inches of rain can be expected. Cooler temperatures and new
moisture will mitigate any fire weather concerns through middle week.
Wyoming...Flash Flood Watch until 6 am MDT Wednesday for wyz103-106-112-