Area forecast discussion
National Weather Service Denver/Boulder Colorado
705 PM MDT Sat may 18 2013
Update...severe threat has shifted just east of Colorado into
Nebraska so Tornado Watch for the northeast corner of the state has
been cancelled. An outflow boundary was attempting to push back
into the Front Range from the east but will encounter downslope
flow off the Front Range which should slow/halt its progression.
Weak convection was developing behind this boundary and there is
some synoptic scale lift...so cant totally rule out a few
showers/storms lingering into the late evening hours along the
Front Range. Elsewhere including the mountains and eastern plains
will keep the chance of showers/isolated storms through the night.
Some snow already reported on top of Copper Mountain as cold
advection aloft kicks in. Could be snow showers all the way down
to valley floors late tonight.
&&
Aviation...isolated high based shower/storm threat will linger
until around 06z but if we do see a shower expect no restriction to
visibility. Only a very slight risk of a shower after 06z. Mainly
north/northwest winds will prevail but still some outflow
boundaries with possible variable winds until 06z. Then winds should
switch to light southwesterly after 06z before gusty northwest
winds up to 25 kts redevelop 16z- 18z Sunday.
&&
Hydrology...no concerns as heavy precipitation has ended.
&&
Previous discussion... /issued 310 PM MDT Sat may 18 2013/
Short term...deep upper trough will continue its slow eastward
progress across the Rocky Mountain region next 24 hours. A spoke of
middle-level energy/lift now rounding the southeast side of this
trough is prognosticated to swing northeast over the forecast area by
this evening. Convection associated with this shortwave becoming
more widespread upstream over south central Colorado. Closer to
home...dry line is presently snaking southward through extreme
northeast Weld...western central Morgan and southwest Washington
counties and down across Kit Carson County. 50s/lower 60s dewpoints
on its east side and 30s dewpoints to its west. Surface based convective available potential energy in
the higher dewpt air in the 1000 to 3000 j/kg range. Storm Prediction Center has most
of this area under a Tornado Watch. Beginning to see strong storms
firing along this dry line. Models show the dry line remaining
nearly stationary through about 00z when the shortwave trough
passes overhead. Should then see drier air up along the Front
Range spreading east across the plains through the evening.
Believe that/S when T-storms will become more numerous across the
far northeast corner of the state with the dry line pushing across
the area. Model are forecasting the best bouyancy near the Nebraska
border with convective available potential energy near 4000 j/kg early this evening. Vertical
shear profile not all that favorable for tornadic super
cells...although could a few spin ups along outflow boundaries.
Main severe weather will be hail up to 2.5 inches in diameter and
damaging straight line winds to around 60 miles per hour. Storm motions in
excess of 25kts should keep rainfall amounts low and any chance
for flooding even lower.
Elsewhere...atmosphere will be drier and less unstable. This will
limit T-storm intensity...but not there numbers. Expect to see
scattered showers and T-storms in the high country and isolated to
scattered T-storms on the plains adjacent to the Front Range. Low
dewpoints along the base of the foothills should result in the
precipitation chance for the County warning forecast area. However gusty winds and small hail still
a possibility in these areas.
Overnight...upper trough continues to shift over the region. Should
see storms winding down most areas by midnight...although storm may
linger till morning in the far northeast. Isolated showers also a
possibility throughout the night in the high country with the snow
level lowering to around 10500 feet by sunrise.
On Sunday...upper trough rotates across the state on Sunday with
winds shifting to the northwest on the back side of the trough
during the day. Should see scattered showers and afternoon T-storms
with lingering large scale lift and cold advection. However precipitation
amounts will be on the low side. Temperatures on Sunday some 8-12
degree f cooler than today. Highest elevations in line to see a mix of
rain and snow...with light accumulations possible on the highest
peaks.
Long term...closed upper level low will be centered near the
Nebraska pan handle Sunday evening. One short wave trough will be
over western Kansas and another will be near the Grand Canyon. Since
northeast Colorado will be between short waves...just expecting
isolated convection.
The short wave trough over the Grand Canyon will move into New
Mexico on Monday. At the surface...a cold front will push south
across Colorado bringing northerly winds. Increased moisture behind
the front combined with low and middle level lift will bring a good
chance for showers. A few thunderstorms will also be possible due to
daytime heating making the airmass slightly unstable. Convective available potential energy will be
less than 500 j/kg...so no threat of severe weather. Because of the
cooler airmass and cloudy skies...high temperatures will be Monday
will be around 60 degrees.
Northwest flow aloft will prevail Monday night and Tuesday as the
main low continues to move east. Showers will decrease Monday night
as the airmass stabilizes and lift decreases. For Tuesday...chances
for precipitation will be limited to the higher terrain and eastern
plains.
Expect Wednesday to be dry as an upper level ridge moves across the
state. Temperatures will start to rebound Wednesday with highs
climbing above 70 degrees.
The warming trend will continue into the later half of the week and
the weekend as a southwest flow aloft prevails. Models indicate low
level moisture will increase on Thursday and Friday. Because of
this...will have a slight chance of thunderstorms in the forecast.
For Saturday...will not mention any chance of thunderstorms because
of dry air that is forecasted to move into the area be the European model (ecmwf).
Because of the drier air...highs may be able to climb into the 80s.
Aviation...isolated thunderstorms in the Denver area through 03z
this evening will have the potential to produce hail up to a half
inch in diameter and wind gusts to around 40 kts. Moderate
rainfall with these northeastward moving storms could briefly
reduce visibilities to 4-5 miles. Winds in the Denver area will be
highly variable with the passing convection. After 03z...most of
this storm activity should move north and east of the Denver
metropolitan area. Surface winds are then expected to gradually transition
to a south-southwest wind of 7-14kts. On Sunday...the Denver area
could see a stray light rain shower in the area during the
morning...then scattered showers and isolated T-storms in the
afternoon. Storms not expected to be as intense with the cooler
airmass moving over the region.
Hydrology...no concerns at this time. Although showers and
thunderstorms through this evening could cause minor rises on
small creeks and streams in and near the mountains.
&&
Bou watches/warnings/advisories...
none.
&&
$$
Short term...barjenbruch
long term....meier
aviation...barjenbruch
