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Area forecast discussion 
National Weather Service Denver/Boulder Colorado
845 PM MDT Tuesday may 5 2015

Update...
issued at 845 PM MDT Tuesday may 5 2015

Based on radar and observations upper level low is over southern
Washington County at this time. A deformation band extends from
the Palmer Divide to the Nebraska pan handle west of the low.
Expect this band of rain to shift to the northwest as the low
lifts north-northwest across eastern Colorado this evening. Most
of the models show this band dissipating around 06z and a second
one forming later tonight and into Wednesday morning. Will keep
probability of precipitation up overnight with the highest probability of precipitation over northern Colorado.
Rainfall amounts under this band are expected to be around a
quarter inch or less.

Airmass will become unstable Wednesday afternoon with convective available potential energy up to
1000 j/kg. With a moisture airmass in place expected scattered to
numerous showers and thunderstorms Wednesday afternoon. Increased
probability of precipitation for this. Midnight shift may need to nudge up probability of precipitation a little
more.

Update issued at 547 PM MDT Tuesday may 5 2015

Satellite showing the upper level low is centered over eastern
central Colorado at this time. The heaviest rain associated with
it is over the eastern plains. Along the Front Range and Palmer
Divide...light rain has fallen with most locations receiving less
than a half inch. Additional rainfall is expected to light this
evening as the main lift moves north of the state. Threat for
heavy rain capable of producing flash flooding is very low...so
canceled the Flash Flood Watch. Expect the widespread rain to end
this evening as winds turn westerly along the Front Range as the
low lifts north of the state.

Many streams and rivers are running high and will continue to run
high for the next several days due to recent rainfall and snow
melt.

&&

Short term...(this evening through wednesday)
issued at 331 PM MDT Tuesday may 5 2015

Rain band continues to lift northward across northeast
Colorado...with scattered showers and a few thunderstorms forming
under the drier air aloft over southeast Colorado. So far rain on
the plains has been persistent but generally light. The heaviest
showers have been along a wind convergence line moving slowly
west...now just west of Interstate 25. This appears to be holding
together pretty well but weakening in intensity a bit...and peak
rainfall rates under this were about a half inch in an hour.
Greatest threats appear to be if this line were to flare up again
over the foothills...or if there is stronger convection in the dry
slot this evening. Still enough of a threat to hang on to the
Flash Flood Watch where we have it...as some areas could have
trouble with an inch of rain...but it appears that the threat of
heavy rain is diminishing.

The band will shift northward this evening...but there will still
be some showers rotating in from the west. These should be
weaker...but will hang on to clouds and some low probability of precipitation through the
night.

Airmass will be warmer and drier on Wednesday...but still unstable
enough for scattered showers. Could be a better chance of a few
thunderstorms in the afternoon with the warmer temperatures and
the remaining low level moisture. Bumped probability of precipitation up slightly for
Wednesday afternoon.

Long term...(wednesday night through tuesday)
issued at 331 PM MDT Tuesday may 5 2015

For Wednesday night the upper trough will shift to the northeast
with a weak front associated with a building surface high moving
into the Front Range from the north. Low level moisture will
spread back into the region with shallow upslope developing. As a
result...there could be a mix of light rain..drizzle or fog
Wednesday night into Thursday morning. On Thursday...the frontal
boundary will remain along the Front Range and Palmer Divide with
the surface high continuing to build over the northeast plains. A
closed will be dropping into central California with a diffluent
southwesterly flow aloft over Colorado. The best
convergence...lift and instability will be along the Front
Range/Palmer Divide with some elevated convection possible. Best
area will be over the Palmer Divide with surface based convective available potential energy of
600-800 j/kg...generally around 400 j/kg for the Front Range. Flow
aloft will go from a drier westerly component Thursday morning to
a moist southwesterly flow aloft in the afternoon and evening.
Overrunning type of profile will continue Thursday night and again
on Friday. Upslope/qg ascent will not be strong should be more of
a steady light rainfall Thursday night. Another surge of high
pressure may help to enhance precipitation along the Front Range/Palmer
Divide on Friday. The upper low at that time will be centered over
Southern California with a weak southwesterly flow aloft over
Colorado. Friday night into Saturday...the upper low will shift
northeast to near The Four Corners region by 12z Saturday.
Increasing middle and upper level qg ascent will occur...with
moderate to strong qg ascent by 18z Saturday. As a result...it
still looks like a widespread precipitation event for the area. Southerly
middle level flow Saturday morning become easterly Saturday
afternoon/evening then northerly Saturday night as the system
shifts northeast into Nebraska. Good cold air advection on Sunday
with the snow level dropping to around 7 thousand feet by 12z
Sunday. As a result...should see rain transition of snow or
rain/snow mix in the foothills Saturday night. Sunday and Sunday
night...wrap around moisture over northeastern Colorado on the
backside of the trough. Should start to see more sun vs clouds on
Monday with an upper ridge to the west and a drier northwesterly
flow over the County Warning Area.

&&

Aviation...(for the tafs through 06z Wednesday evening)
issued at 845 PM MDT Tuesday may 5 2015

Low clouds are expected to prevail most of the night with ceilings
mainly 1000 to 3000 feet. Ceilings are expected to improve
Wednesday...possibly above 6000 feet a time. There will be a good
chance for showers and thunderstorms Wednesday afternoon...after
18z with heavy rain and small hail possible. Northerly winds will
turn westerly overnight as the upper level low lift northeast
across eastern Colorado. On Wednesday...winds will be on the light
side...less than 15 knots.



&&

Bou watches/warnings/advisories...
none.
&&

$$

Update...meier
short term...gimmestad
long term...Cooper
aviation...rjk
hydrology...gimmestad

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