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Area forecast discussion 
National Weather Service Denver/Boulder Colorado
432 am MDT Tuesday may 5 2015

Short term...(today through tonight )
issued at 432 am MDT Tuesday may 5 2015

Concern today surrounds potential for heavy rainfall and flooding.
Last nights showers and storms were very efficient and some warm
rain processes were noted along the Front Range foothills. A small
area in north central Boulder County recorded 1.3 to 2.8 inches of
rain in low topped convection...and localized amounts over 1 inch
in central Jefferson County foothills with no lightning. Warm
cloud depth was as deep as 5000-5500 feet...and same expected for
today. There is an upslope east/southeast component most of the
day...and as soon as deeper moisture arrives with approach of
upper level trough and at least weak q-g lift should see showers
break out in earnest with a few embedded thunderstorms. Showers
were already increasing in southeast Colorado...and these should
reach Palmer Divide by middle morning and Denver/I-25 corridor late
morning. One factor that could limit rainfall amounts would be
convective clusters ahead of upper trough. A couple of these
already noted in West Texas and southeast New Mexico. Those could
keep threat of heavier rainfall on the eastern Colorado plains
into western Kansas. Showers and a few thunderstorms will taper
off overnight along the Front Range...but may continue over the
far northeast corner and the mountains as west/northwest flow
develops behind the trough. Finally...cant rule out an isolated
tornado threat with increasing low level wind shear and Denver
cyclone...but updrafts will be quite weak with limited cape less
than 500 j/kg. With regard to Flash Flood Watch...see further
meteorological discussion in hyrdo section below.

In the mountains...wet bulb zero levels suggest snow levels
averaging 10500-12000 feet today...lowering about a thousand feet
behind trough axis tonight. Several inches of snow expected over
the higher peaks above most roads...so no advisories at this time.

Areas of fog and drizzle will persist through at least middle
morning with moist easterly flow. Then deeper shower development
and mixing should erode fog toward late morning.

Long term...(wednesday through monday)
issued at 432 am MDT Tuesday may 5 2015

The abnormally wet weather pattern still looks like it will extend
into the weekend. After tonights negatively tilted upper trough
moves out onto the northern plains...Colorado will remain under
weak southwesterly flow aloft which will be open to the further
intrusion of moisture from the Gulf of Mexico. As the moist
airmass remains in place...daytime heating will be more than
sufficient to generate additional rounds of showers each day.
Medium range models show the 700 mb specific humidity levels
rising to a juicy 6 g/kg at times. Model soundings show modest
cape values around 500 j/kg on Wednesday and then almost 1000 j/kg
on Thursday. Low level winds will be out of the east and
southeast...helping to hold moisture near the foothills. All of
the ingredients appear to be in place for more rain across the
forecast area each day through Friday.

Over the weekend it looks like the next upper level closed low
will traverse across the state from the Desert Southwest. This
should draw up additional moisture ahead of the approaching low
and then generate areas of heavy rain at times on Saturday and
Sunday. 700 mb temperatures are forecast to drop to the -4 to -8
degree range...so this should bring the snow levels
down...especially Saturday night and Sunday morning. The higher
foothills could see accumulating snow which will also help out the
hydrology situation temporarily.

Temperatures will cooler than seasonal normals through the
forecast period...especially over the weekend. And probabilities
of precipitation will be quite high. The hydrological situation is
going to need to be monitored as areas receive repeated rainfall
and soil moisture levels begin approaching saturation. Next Monday
and Tuesday may be the break we are looking for as an upper ridge
is finally forecast to move back over the area...if only for a
couple days.

&&

Aviation...(for the tafs through 12z Wednesday morning)
issued at 432 am MDT Tuesday may 5 2015

Areas of fog and light drizzle with visibilities of 1/4sm to
1/2sm expected to remain in the area until 15z-16z. Kden and kbjc
will be more susceptible to this fog while kapa may very well be
on the edge with a slightly more southeasterly wind component
there. Even a switch to light southeast flow at kden could help
erode the fog. By 16z-17z expect rain showers to be developing
which will help mix the atmosphere and erode any lingering thicker
fog. Visibilities may hold in the 4-5sm range with rain and light
fog through the afternoon and early evening. Embedded
thunderstorms and heavier rain showers possible which would be
capable of lowering visibilities to around 2sm in heavy rain 19z-
24z. Expect a return of VFR conditions toward 03z-06z tonight as
low/middle level flow turns more southwesterly.

&&

Hydrology...
issued at 432 am MDT Tuesday may 5 2015

Forecast soundings are nearly saturated right up the moist
adiabat...so any weak heating or forcing mechanism should react
efficiently to produce heavy rain producing showers. Precipitable
water values are expected to climb from near 0.75 to 0.80 inch
along the Front Range this morning to 0.85 to 0.90 inch...all
above record levels for this date. Last nights rains have already
produced some minor stream rises...and grounds are becoming more
saturated. Therefore...we will be issuing a Flash Flood Watch for
portions of the forecast area. Right now...the highest threat will
be the foothills...burn scars...and Palmer Divide where the
heaviest rains fell last night. Showers and storms will certainly
be moving faster today but we expect several hours of moderate to
heavy rain given large scale support which may add to flooding
concerns.

&&

Bou watches/warnings/advisories...
Flash Flood Watch from noon MDT today until 9 PM this evening for
coz035-036-041-046.

&&

$$

Short term...barjenbruch
long term...dankers
aviation...barjenbruch
hydrology...barjenbruch

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